Synthetic approaches and pharmaceutical applications of chloro-containing molecules for drug discovery: A critical review.

At present more than 250 FDA approved chlorine containing drugs were available in the market and many pharmaceutically important drug candidates in pre-clinical trials. Thus, it is quite obvious to expect that in coming decades there will be an even greater number of new chlorine-containing pharmaceuticals in market. Chlorinated compounds represent the family of compounds promising for use in medicinal chemistry. This review describes the recent advances in the synthesis of chlorine containing heterocyclic compounds as diverse biological agents and drugs in the pharmaceutical industries for the inspiration of the discovery and development of more potent and effective chlorinated drugs against numerous death-causing diseases.

Introduction

Chlorine in pharmaceuticals

Chlorine is one of the most vital industrial chemicals, which was utilized by various end-users of industries. And it has been tremendous sprite in pharmaceuticals as the major key ingredients in drugs to treat many diseases such as meningitis, cholera, plague, typhoid, bacterial skin infections, respiratory and nervous system problems etc., as per the Business Wire and A Berkshire Hathaway Company reports. The therapeutics and percentage of sales were presented to address the importance of chlorine chemistry in pharmaceutical drugs as what have been reported by HIS Applied Economics, Canada (Table 1 ). In United States, more than 88% of the pharmaceuticals were depended on chlorine chemistry including the drugs those have been used for the treatment of stomach ulcer, cancer, anemia, high cholesterol, depression, and epilepsy. As per statistics, the benefit from the chlorine chemistry was estimated to $450 billion per year. The net gain of the pharmaceuticals in the U.S. and Canada using chlorine is as high as $640 billion per year from the health care system reported by HIS Applied Economics, Canada [1]. According to the Business Wire and A Berkshire Hathaway Company reports, the estimating chlorine market for the period of 2018–2023 is approximately 4.8%. Some of the drugs presented with chemical structures containing chlorine (number of groups are different). One of the studies detailed that, 163 compounds among 233 approved drugs; nearly 73% of them contained single chlorine atom [2]. Of which, 23% of them possessed by two chlorines, 2.6% of them possessed by three chlorine atoms, 1.4% of them possessed by four chlorines, and 2.5% of them possessed by six chlorines in the compounds. Surprisingly, none of the drugs had been approved with five chlorine atoms yet. Also, among them, 98% were monosubstituted (CCl), only four were disubstituted (CCl2), and none of the approved drugs has trisubstituted (CCl3) groups. This interesting research gap suggests that, chlorine will further continue its industry ruler role to provide and benefit consumers of the pharmaceuticals in the future [1]. To improve the quality and advantage of chlorinated chemistry, scientists need to an advance understanding of the chlorine in the view of medicinal chemistry in the future (see Table 2).

Table 1

The chlorine based drugs against categories of the diseases.

Therapeutic category	Sales in billons
Cardiovascular disease	$4.7
Central nervous system	$4.9
Alimentary-metabolism	$26.5
Respiratory	$19.4
Anti-infectives	$9.5
Musculo-skeleta	$19.8
Genito-urinary	$2.7
Cytostatics	$12.7
Blood agents	$10.4
Hormones	$13.6
Antiretrovirals and miscellaneous	$8.3
Total, all categories	$168.5

Table 2

List out the FDA approved chlorine containing drugs in market (1949–2012).

Sl.No	Drug Name	Structure	Diseases	Approved Year
1	Chloromycetin	Image 2	AIN/DER/GUS/SEN	1949
2	pHisoHex	Image 3	Dermatological	1949
3	Tace	Image 4	Genito-Urinary and Sex hormone	1950
4	Chloro-Trimeton	Image 5	NES/RES	1950
5	Clonidine	Image 6	NER/CAR/SEN	1950
6	Ethchlorvynol	Image 7	Nervous System	1950
7	Phenoxybenzamine	Image 8	Cardiovascular	1953
8	Nesacaine	Image 9	Nervous System	1955
9	Ambenonium	Image 10	Nervous System	1956
10	Halothane	Image 11	Nervous System	1956
11	Atarax	Image 12	Nervous System	1956
12	Compazine	Image 13	Nervous System	1956
13	Leukeran	Image 14	Oncological	1957
14	Diuril	Image 15	Cardiovascular	1957
15	Thorazine	Image 16	Nervous System	1957
16	Trilafon	Image 17	Nervous System	1957
17	Diabinese	Image 18	Alimentary tract and metabolism	1958
18	Chlorprothixene	Image 19	Nervous System	1958
19	Parafon	Image 20	Musculo-skeletal	1958
20	Cytoxan	Image 21	Oncological	1959
21	Hydrochlorothiazide	Image 22	CAR/END	1959
22	Librium	Image 23	Nervous System	1960
23	Trancopal	Image 24	Musculo-skeletal	1960
24	Clomifene	Image 25	Genito-Urinary and Sex hormone	1960
25	Clomipramine		NER/END	1960
26	Methoxyflurane	Image 26	Nervous System	1960
27	Enduron	Image 27	Cardiovascular	1960
28	Daranide	Image 28	Sensory organ	1960
29	Grisactin	Image 29	Dermatological	1962
30	Valium	Image 30	Nervous System	1963
31	Alkeran	Image 31	Oncological	1964
32	Maolate	Image 32	Dermatological	1965
33	Serax	Image 33	Nervous System	1965
34	Enflurane	Image 34	Nervous System	1966
35	Lasix	Image 35	Cardiovascular	1966
36	Atromid-S	Image 36	CAR	1967
37	Haldol	Image 37	Nervous System	1967
38	Pathocil	Image 38	Anti-infective	1968
39	Cleocin	Image 39	AIN/DER/GUS	1970
40	Floxapen	Image 40	Anti-infective	1970
41	Dalmane	Image 41	Nervous System	1970
42	Ketalas	Image 42	Nervous System	1970
43	Lysodren	Image 43	Oncological	1970
44	Tranxene	Image 44	Nervous System	1972
45	Hyperstat	Image 45	Cardiovascular	1973
46	Sanorex	Image 46	Alimentary tract and Metabolism	1973
47	Closapen	Image 47	Anti-infective	1974
48	Monistat 7	Image 48	AIN/ALM/DER/GUS	1974
49	Klonopin	Image 49	Nervous system	1975
50	Lotrimin	Image 50	AIN/ALM/DER/GUS	1975
51	Loxitane	Image 51	Nervous system	1975
52	CeeNU	Image 52	Oncological	1976
53	Imodium	Image 53	Alimentary tract and Metabolism	1976
54	Lioresal	Image 54	Musculo-skeletal	1977
55	BiCNU	Image 55	Oncological	1977
56	Tavist	Image 56	RES/NER	1977
57	Cloderm	Image 57	Dermatological	1977
58	Ativan	Image 58	Nervous system	1977
59	Buclizine	Image 59	Respiratory System	1979
60	Ceclor	Image 60	Anti-infective	1979
61	Forane	Image 61	Nervous system	1979
62	Reglan	Image 62	Alimentary tract and Metabolism	1979
63	Adinazolam	Image 63	Nervous system	1980
64	Asendin	Image 64	Nervous system	1980
65	Halotex	Image 65	Dermatological	1980
66	Meclomen	Image 66	Musculo-skeletal	1980
67	Domperidone	Image 67	ALT/Met	1980
68	Xanax	Image 68	Nervous system	1981
69	Midamor	Image 69	Cardiovascular	1981
70	Emeyt	Image 70	Oncological	1981
71	Paxipam	Image 71	Nervous system	1981
72	Nizoral	Image 72	AIN/DER/GUS	1981
73	Restoril	Image 73	Nervous system	1981
74	Spectazole	Image 74	DER/GUS	1982
75	Halcion	Image 75	Nervous system	1982
76	Lozol	Image 76	Cardiovascular	1983
77	TZ-3	Image 77	DER/GUS	1983
78	Wellbutrin	Image 78	Nervous system	1985
79	Femstat	Image 79	Genito-Urinary and Sex hormone	1985
80	Versed	Image 80	Nervous system	1985
81	Progabide	Image 81	Nervous system	1985
82	Lamprene	Image 82	Anti-infective	1986
83	Tenex	Image 83	Cardiovascular	1986
84	Trazodone	Image 84	NER/END	1986
85	Zopiclone	Image 85	Nervous system	1986
86	Lopidine	Image 86	Sensory organ	1987
87	Mykrox	Image 87	Cardiovascular	1987
88	Elocon	Image 88	DER/RES	1987
89	Terazol 7	Image 89	Genito-Urinary and Sex hormone	1987
90	Voltaren	Image 90	MSK/SEN	1988
91	Ifex	Image 91	Oncological	1988
92	Oxistat	Image 92	DER/GUS	1988
93	Metahydrin	Image 93	Cardiovascular	1988
94	Clozaril	Image 94	Nervous system	1989
95	Prosom	Image 95	Nervous system	1990
96	Moclobemide	Image 96	Nervous system	1990
97	Plendil	Image 97	Cardiovascular	1991
98	Lorabid	Image 98	Anti-infective	1991
99	Zoloft	Image 99	NER/GUS	1991
100	Tilid	Image 100	Boold and blood forming organ	1991
101	Propulsid	Image 101	Alimentary tract and Metabolism	1993
102	Leustatin	Image 102	Oncological	1993
103	Lipidil	Image 103	CAR/END	1993
104	Claritin	Image 104	Respiratory system	1993
105	Lamictal	Image 105	Nervous system	1994
106	Zyrtec	Image 106	Respiratory system	1995
107	Cozaar	Image 107	Cardiovascular	1995
108	Astelin	Image 108	RES/SEN	1996
109	Zanaflex	Image 109	Musculo-skeletal	1996
110	Agrylin	Image 110	BBO/ONC	1997
111	Plavix	Image 111	BBO/END/CAR	1997
112	Corlopam	Image 112	Cardiovascular	1997
113	Meridia	Image 113	ALM/END	1997
114	Fareston	Image 114	END/ONC	1997
115	Sustiva	Image 115	Anti-infective	1998
116	Norvase	Image 116	Cardiovascular	1999
117	Atacand HCT	Image 117	Cardiovascular	2000
118	Clarinex	Image 118	Respiratory system	2001
119	Geodon	Image 119	Nervous system	2001
120	Abilify	Image 120	Nervous system	2002
121	Carbinoxamine	Image 121	Respiratory system	2003
122	Iressa	Image 122	ONC/RES	2003
123	Ertaczo	Image 123	Dermatalogical	2003
124	Clolar	Image 124	Oncological	2004
125	Lunesta	Image 125	Nervous system	2004
126	Nexavar	Image 126	Oncological	2005
127	Sprycel	Image 127	Oncological	2006
128	Fenofibric acid	Image 128	Cardiovascular	2007
129	Xyzal	Image 129	DER/RES	2007
130	Cleviprerx	Image 130	Cardiovascular	2008
131	Saphris	Image 131	Nervous system	2009
132	Bepreve	Image 132	Sensory organ	2009
133	Zipsor	Image 133	Nervous system	2009
134	Samsca	Image 134	Blood and blood forming organ	2009
135	Onfi	Image 135	Nervous system	2011
136	Daliresp	Image 136	Respiratory system	2011
137	Belviq	Image 137	Endocrime system	2012
138	Stivarga	Image 138	ALM/ONC	2012
139	Erivedge	Image 139	DER/ONC	2012

The pan class="Chemical">chlorine based drugs against categories of the diseases.

List out the FDA approved pan class="Chemical">chlorine containing drugs in mn>an class="Chemical">arket (1949–2012).

How important is the chlorine atom to improve the biological properties?

The presence of chlorine atom played a pivotal role in a number of natural products such as the antibiotics clindamycin [3], vancomycin [4], chloramphenicol [5], and griseofulvin. Over the course of time it has been found empirically that the introduction of a chlorine atom into one or more specific positions of a biologically active molecule may substantially improve the intrinsic biological activity [6]. The properties of the carbon-chlorine bond (C-Cl) in organochlorines have been analysed by Henschler [4,5]. However, in the low-molecular-weight chemicals investigated in that analysis, the electrophilic reactivity of the carbon centre adjacent to the chlorine atom, which facilitates displacement of chlorine by (bio)nucleophiles, determines the observed biological properties [4,5]. The increase of lipophilicity of the whole molecule by a chlorine substituent leads to a higher partitioning of a chlorinated compound into the lipophilic phase of a cell membrane or lipophilic domains of a protein. This causes a higher local concentration of the compound near a biological target site, but, not necessarily a higher biological activity. The most important effect of a non-reactive chlorine atom in the biological activity of many compounds comes from chlorine as a substituent on an aromatic, heteroaromatic or olefinic moiety.

The properties mentioned above will give rise steric and/or electronic effects of the chlorine substituents and lead to local electronic attraction or repulsion or to steric interference with any amino acid residue surrounding the position of the chlorine atom in the binding pocket of the protein. This in turn may cause a tighter interaction or a loosening of the contacts to the amino acids close to the chlorine or in other parts of the active molecule. Either one may affect the function of the target protein and cause an increase or decrease of biological activity. In other cases however a chlorine substituent may have no specific effect on the primary biological properties of the molecule to which it is attached [7]. Chlorinated compounds are not necessarily toxic or dangerous. Highly reactive chemicals or polychlorinated compounds cannot be compared with regard to toxicological properties with unreactive compounds having a low degree of chlorination. The chlorine atom, as one of many possible substituents used in synthetic organic chemistry, will remain in the future one of the important tools for probing structure-activity relationships in life science research and as a molecular component in commercialized compounds, in order to provide safer, more selective and more environmentally compatible products with higher activity for medicine and agriculture [7].

The application of chlorine in medicinal chemistry is one of the fastest growing hot areas in chemistry as its fascinating and instructive role of halogens distribution in the field of drug development. Surprisingly, among four halogens, chlorine (Cl) is the one which is more frequently found in drugs than others, even fluorine (F). Interestingly, in drugs, the elements of sulphur, chlorine, and fluorine were placed as 5–7 respectively after C, H, O, and N. The remaining phosphorous (P), bromine (Br), and iodine (I) are the rest of the top 10 elements in approved drugs, and remarkably, the Cl and F are the heavy hitters (Cl > F ≫ Br > I) [[8], [9], [10]].

Synthesis and biological applications of chlorinated analogues

Synthesis of chlorine containing antimicrobial agents

The problem of antibiotic resistance among pathogenic bacteria is as old as antibiotics itself [11]. The antibiotic resistance which accelerated by the use and misuse of antimicrobial drugs has been a major universal challenge for public health. Remarkable increase of human pathogenic bacteria was observed from the past decades due to their resistance to one or more antibiotics. A number of infections caused by resistant organisms not succeed at responding to the conventional treatment and even the last choice antibiotics were also lost their power [12]. After all, loss of effectiveness and resistance power of old antibiotics against new and upcoming bacterial pathogens prompt us to develop novel, less toxic and highly effective antimicrobial agents with diverse structures to fight with an emerging antibiotic-resistant bacterial infections. In this review article, we have focused on chlorine containing analogues as a core substituent of antibacterial agents for drug development.

Mujumdar and co-workers synthesized a novel class of sulfamate-containing natural products and screened for their in vitro antibacterial properties. Title compound 143 was synthesized according to the literature reported as shown in Scheme 1 . Commercially available 2-chloroadenosine 140 was reacted with tosylic acid under the optimal reaction condition to afford 2ʹ,3ʹ-O-isopropylidene protected adenosine precursors 141, which then reacted with chlorosulfonyl amine in the presence of DBU as base to obtain the corresponding 5ʹ-o-sulfamoyl adenosine 142 in moderate to good yields. The protecting group of compound 142 was removed by using a mixture solution of TFA and water (v/v = 4:2) to afford the target compound 143. In vitro antimicrobial activity evaluation showed that the compound 143 possessed highest antibacterial activity with a MIC of 5 μM against E. coli [13]. The structure-activity relationship (SAR) revealed that the presence of chlorine atom highly enhanced the antibacterial properties of compound 143. In addition, the presences of sulfonamide group also most favour to increases the antibacterial activity of potent compound 143. The compound 143 as the lead compound need to be designed and synthesized for further investigation.

Scheme 1

Synthesis of chlorine containing sulfamate-containing natural products as potent antibacterial agent.

Synthesis of pan class="Chemical">chlorine containing n>an class="Chemical">sulfamate-containing natural products as potent antibacterial agent.

Wang and co-workers developed the chlorine containing n>an class="Chemical">thiochrome derivatives and screened for their in vitro antimicrobial activity. Substituted thiophenols (144a-d) were reacted with KOH and ethanol in the presence of water at 60 °C to yield the compounds 145a-d. Compounds 145a-d were reacted with β-chloropropionic acid and then cyclized in the presence of concentrated H2SO4 to afford thiochroman-4-ones 146a-d via intra-molecular Friedel-Crafts reaction. Later, compounds 146a-d were treated under Vilsmeier-Haack conditions to produce the key intermediates of 147a-d. Then, compounds 147a-d were reacted with various substituted amines and treated with NaBH(OAc)3 in 1,2-dichloroethane at room temperature to yield the final target compounds 148a-d (Scheme 2 ) [14]. All the synthesized compounds were tested for in vitro antifungal activities against different fungal strains. Based on the in vitro antimicrobial results, two compounds (148a and 148b) were showed an excellent antifungal activities against C. albicans (MICs = 0.5–8 μg/mL) and C. neoformans (MIC = 0.25–1 μg/mL) respectively [15].

Scheme 2

Synthesis of potent chlorine containing thiochromenes as antifungal agents. Reagents and conditions: (i) KOH, H2O, EtOH, 60 °C; (ii) β-chloropropionic acid, 30% K2CO3 aqueous solution, reflux; (iii) H2SO4; (iv) POCl3, DMF, 50 °C, (v) NaBH(OAc)3, amine, DCE, N2, rt.

Synthesis of potent chlorine containing n>an class="Chemical">thiochromenes as antifungal agents. Reagents and conditions: (i) KOH, H2O, EtOH, 60 °C; (ii) β-chloropropionic acid, 30% K2CO3 aqueous solution, reflux; (iii) H2SO4; (iv) POCl3, DMF, 50 °C, (v) NaBH(OAc)3, amine, DCE, N2, rt.

Karthikeyan et al. synthesized the novel n>an class="Chemical">chlorine containing potent antimicrobial compounds 154a-f. From the beginning, starting material 149 was reacted with various substituted aldehydes (150a-f) in the presence of ethanol and Aq. KOH under reflux conditions to yield 1,3-diaryl-2-propen-1-ones (151a-f) in good yields. Then, compounds 151a-f underwent bromination in chloroform to give bromo substituted compounds 152a-f. Chalcone dibromides (152a-f) further treated with aryloxy acid hydrazides (153a-f) in the presence of triethylamine in absolute ethanol to furnish 1-aryloxy-3-aryl-5-hydroxy-5-aryl pyrazolines (154a-f, Scheme 3 ) in good yields. Compounds 154a (zone of inhibition was 22 mm against P. aeruginosa) and 154b (zone of inhibition was 21 mm against P. aeruginosa) turned out to be the most potent antibacterial agents. Compounds 154a (zone of inhibition was 21 mm against A. fumigatus) and 154b (zone of inhibition was 20 mm against P. marneffei) turned out to be the most potent antifungal agents [16].

Scheme 3

Synthesis of chlorine containing pyrazoline analogues as potent antimicrobial agents. Reagents and conditions: (i) EtOH, aq. KOH, (ii) Br2, CHCl3, (iii) EtOH, Et3N.

Synthesis of chlorine containing n>an class="Chemical">pyrazoline analogues as potent antimicrobial agents. Reagents and conditions: (i) EtOH, aq. KOH, (ii) Br2, CHCl3, (iii) EtOH, Et3N.

Very recently, Zha and co-workers synthesized benzo[d]thiazole-hydrazones and tested for their in vitro antimicrobial activities. The synthetic route was depicted in Scheme 4 . The intermediate 7-methylbenzo[d]thiazol-2-amine (156) was synthesized according to the literature of reported method [17,18]. Then, compound 156 was converted into hydrazides in the presence of hydrazine hydrate, catalytic amount of concentrated HCl and ethylene glycol under optimal reaction conditions for 3–4 h to afford the compound 157. The benzo[d]thiazole-hydrazones (158a-z) were obtained by the reaction of 157 with different aldehydes in the presence of catalytic amount of glacial acetic acid. All the derivatives were obtained in good to excellent yield. All the obtained derivatives were evaluated for in vitro antimicrobial activities against various bacterial and fungal pathogens. Compound 158a was found to be the most potent antifungal agent with 33 mm inhibition zone at 100 μg/mL against A. niger. The structure-activity relationship (SAR) revealed that, the presence of electron withdrawing groups (EWGs) (Cl, Br, NO2 and F) on the phenyl ring increased the antifungal properties and the presence of electron donating groups (EDGs) (OH and OCH3) diminished the antifungal properties [19]. Compound 158a may serve as new potential antibacterial candidate in the future.

Scheme 4

Synthesis of benzo[d]thiazole-hydrazones as potent antifungal agent. Reagents and conditions: (i) NH4SCN, Br2, glacial acetic acid, NH3, (ii) hydrazine hydrate, Con. HCl, ethylene glycol, 3–4 h, rt, (iii) R-CHO, catalytic amount of acetic acid, EtOH, reflux, 8–10 h.

Synthesis of benzo[d]thiazole-hydrazones as potent antifungal agent. Reagents and conditions: (i) n>an class="Chemical">NH4SCN, Br2, glacial acetic acid, NH3, (ii) hydrazine hydrate, Con. HCl, ethylene glycol, 3–4 h, rt, (iii) R-CHO, catalytic amount of acetic acid, EtOH, reflux, 8–10 h.

Very recently, Goa et al. have developed the potent antimicrobial aminothiazolyl berberine derivatives (162a-h) as illustrated in Scheme 5 . Initial compound 159 was reacted with hydrazinecarbothioamide in the presence of glacial acetic acid and ethanol under optimal reaction conditions to afford the compounds 160a-h in moderate to good yields. Subsequently, compounds 160a-h were reacted with 2-chloroacetaldehyde to yield 161a-h. Finally, compounds 161a-h were converted into aminothiazolyl berberine derivatives (162a-h) by using substituted benzyl chlorides under optimal reaction conditions. Among all the synthesized derivatives, compound 162a showed excellent antibacterial activity with MIC value of 2 mmol/mL against Gram-negative A. baumanii. The SAR revealed that the introduction of chlorine atom significantly improved the antibacterial effect. The EWGs (Cl) was useful to improve antibacterial activity. Molecular docking showed that hydrogen bonds existed in the supramolecular interaction between DNA gyrase and the active molecule 162a (Fig. 1 ). [20].

Scheme 5

Synthesis of chlorine containing aminothiazolyl berberine derivatives as potent antibacterial agents. Reagents and conditions: (i) substituted benzyl chlorides, K2CO3, DMF, 80 °C; (ii) hydrazinecarbothioamide, glacial acetic acid, EtOH, 80 °C; (iii) 2-chloroacetaldehyde, EtOH, 80 °C.

Fig. 1

Three-dimensional conformations of compound 162a docked in DNA gyrase B (PDB code: 4DUH).

Synthesis of chlorine containing n>an class="Chemical">aminothiazolyl berberine derivatives as potent antibacterial agents. Reagents and conditions: (i) substituted benzyl chlorides, K2CO3, DMF, 80 °C; (ii) hydrazinecarbothioamide, glacial acetic acid, EtOH, 80 °C; (iii) 2-chloroacetaldehyde, EtOH, 80 °C.

Three-dimensional conformations of compound 162a docked in pan class="Gene">DNA gyrase B (n>an class="Gene">PDB code: 4DUH).

The synthesis of novel class of antimicrobial quinoline bearing benzimidazole hybrids had been carried out by Garudachari et al. The targeted final compounds were synthesized through two steps. In the first step, isatin (163) was reacted with α-methylketone (164) in aqueous ethanol to yielded 4-carboxyquinoline (165) in good yield [[21], [22], [23]]. In the second step, 165 was reacted with various aromatic-1,2-diamines in polyphosphoric acid media to afford quinoline incorporated benzimidazole derivatives (166a-f, Scheme 6 ). Compound 166b was found to be potent antibacterial agent against S. aureus (12 mm inhibition zone) and compound 166a was found to be more potent against S. aureus (16 mm inhibition zone). The SAR results revealed that the presence of fused pyridine ring in benzimidazole moiety as well as 4-fluorophenyl group on second position of quinoline ring has some relationship with their antibacterial activity. The presence of two chlorine atoms on benzimidazole ring along with 4-fluorophenyl group on second position of quinoline ring account for the enhanced activity of compound 166b [24].

Scheme 6

Synthesis of chlorine containing potent antibacterial quinoline hybrids. Reagents and conditions: (i) 33% KOH, EtOH, (ii) Aromatic, 1-2-diamine, PPA.

Synthesis of chlorine containing potent antibacterial n>an class="Chemical">quinoline hybrids. Reagents and conditions: (i) 33% KOH, EtOH, (ii) Aromatic, 1-2-diamine, PPA.

Karthikeyan et al. have synthesized a new series of n>an class="Chemical">triazole substituted compounds which were acted as potent antimicrobial agents. Starting material 167 was reacted with substituted phenacyl bromides (168a-h) in the presence of base under optimal reaction conditions to yield 2-[5-(2,4-dichloro-5-fluorophenyl)-4H-1,2,4-triazol-3-yl]thio-1-(substituted phenyl) ethanone (169a-h) which underwent further cyclization in the presence of PPA to obtain a series of 2,4-dichloro-5-fluorophenyl containing thiazolotriazoles (170a-h) in good yields (Scheme 7 ). Compound 170a with triazole and chloro substituent was found to be the most active antibacterial agent (30 mm inhibition zone against P. aeruginosa) and antifungal agent (22 mm inhibition zone against A. niger) among all the synthesized series [25].

Scheme 7

Synthesis of chlorine containing triazole hybrids as potent antimicrobial agents.

Synthesis of chlorine containing n>an class="Chemical">triazole hybrids as potent antimicrobial agents.

Synthesis of chlorine derived anticancer agent

Development in the area of antin>an class="Disease">cancer therapeutic agents is one of the key challenges in medicinal chemistry. Cancer, a universal name for a group of diseases which characterized by uncontrolled cell proliferation, is found all over the world [26]. Cancer is the second major cause of death worldwide [27]. In the year of 2014 alone, it was estimated that about 585720 Americans were died from cancer, which refers to about 1600 deaths per day [28]. According to the assessment of the world health organization (WHO), global cancer rates could increase by 50% in the year 2020, which is approximately to 15 million. Cancers of the lung and bronchus, prostate, and colorectal continue to be the most common causes of cancer death [29].

A cancer consists of a group of cells that originated from a single cell with uncontrolled growth and rapn>id proliferation properties [30]. Presently, a wide range of cytotoxic drugs, either alone or in combination, are used to treat cancer, and several of these drugs are in different phases of clinical trials. These cytotoxic drugs suffer from several drawbacks and are not able to differentiate between cancerous and normal cell types; consequently, they can cause serious side effects that are often cumulative and dose-limiting. The anticancer drugs in recent clinical trials exhibited unnecessary organ toxicity, lack of cell specificity, short circulating half-life, and a noticeable tendency to induce resistance in the target cells [31]. Hence, in order to save the lives of millions of people globally, continuous efforts are being made to develop effective anti-cancer drug candidates with minimal side effects and less cost.

Zou et al. have prepared n>an class="Chemical">dihydropyridine substituted chalcones and screened for their in vitro anticancer activity against different cell lines. The synthetic protocol was illustrated in Scheme 8 . Initially, substituted aldehydes 171 were converted to esters 172 using Wittig-Horner reaction. Then, hydrolysis of 172 led to free acid 173 by using KOH, and compound 173 was reacted with pivaloyl chloride in DMF under the optimal reaction conditions to yield acylchloride 174. Without isolation, compound 174 directly treated with 6-chloro-5,6-dihydro- pyridin-2(1H)-one, base and n-butyllithium to afford compound 175 in good yields ( Scheme 8). Compound 175 showed excellent anticancer activity against HT-29 (IC50 = 0.92 μM), HCT-8 (IC50 = 1.79 μM) and HCT-116 (IC50 = 0.47 μM) cell lines. The SAR results revealed that, the presence of EWG, namely chlorine atom, on the dihydropyridine ring would contribute to the enhanced anticancer activity [32].

Scheme 8

Synthetic route of chlorine containing dihydropyridine substituted quinoline chalcones as potent anticancer agent. Reagents and conditions: (i) Wittig-Hornor reaction; (ii) KOH, 16 h; (iii) anhydrous DCM, pivaloyl chloride, one drop of DMF; (iv) anhydrous THF, TEA, n-butyllithium.

Synthetic route of chlorine containing n>an class="Chemical">dihydropyridine substituted quinoline chalcones as potent anticancer agent. Reagents and conditions: (i) Wittig-Hornor reaction; (ii) KOH, 16 h; (iii) anhydrous DCM, pivaloyl chloride, one drop of DMF; (iv) anhydrous THF, TEA, n-butyllithium.

Pn>an class="Chemical">ark et al. have developed the synthetic route of a large number of pyrazolopyrimidin derivatives from 176 through a two-step reaction. Firstly, compound 176 was reacted with hydrazine hydrate in methanol under reflux conditions for 3 h. After cooled to room temperature, HCl in dichloromethane was added into the reaction mixture at −2 °C and then stirred for 13–14 h to yield intermediate compound 177 in moderate yield. The benzylhydrazine 177 was treated with 2-amino-4,6-dichloropyrimi-dine-5-carbaldehyde in the presence of TEA to afford target compounds 178 and 179 (Scheme 9 ). All the synthesized compounds were screened for their in vitro anticancer activity as TRAP1 Inhibitors. Among them, compound 178 was found to be the most potent TRAP1 inhibitors with the IC50 values of 79 nM [33].

Scheme 9

Synthetic route of pyrazolopyrimidin derivatives as potent anticancer agents. Reagents and conditions: (i) NH2NH2·ΗCl, MeOH, 60 °C, 3 h; (ii) HCl, CH2Cl2, -2 °C, 13 h; (iii) 2-amino-4,6-dichloropyrimidine-5-carbaldehyde, TEA, CH2Cl2, −2 °C, 13 h.

Synthetic route of pyrazolopyrimidin derivatives as potent antin>an class="Disease">cancer agents. Reagents and conditions: (i) NH2NH2·ΗCl, MeOH, 60 °C, 3 h; (ii) HCl, CH2Cl2, -2 °C, 13 h; (iii) 2-amino-4,6-dichloropyrimidine-5-carbaldehyde, TEA, CH2Cl2, −2 °C, 13 h.

Solomon and co-workers reported the syntheses of N-alkylated 4-aminoquinoline compounds (183a-n) which were showed in Scheme 10 . An aromatic nucleophilic substitution of 4-chloro-7-substituted-quinoline (180) with excess amount of propanol amine in triethyl amine yielded the 3-((7-chloroquinoline-4-yl)amino)propane-ol (181) according to the previous reported method [34]. Compound 181 was reacted with sulfonyl chloride in the presence of base under the optimal reaction condition to yield mesylate (182) in high yield. The intermediate 182 was treated with various substituted amino components utilizing sodium hydride as the base to afford the final N-alkylated 4-aminoquinoline derivatives (183a-n) in moderate to good yields. All the synthesized compounds were evaluated for their in vitro anti-breast cancer activity. Among them, compound 183a was found to be the most potent analogues against tested two cancer cell lines MDA- MB221 (GI50 = 6.33 μM) and MCF-7 (GI50 = 4.99 μM). The bio assays result suggests that, compound 183a analogues could serve as potent anticancer agent for the development of a new group of effective cancer chemotherapeutics [35].

Scheme 10

Synthetic route of potent anticancer agents. Reagent and Conditions: (i) Triethyl amine, amino propanol, 120–130 °C for 6 h (ii) triethylamine, methane sulfonyl chloride, THF, RT, 4 h (iii) amino component, NaH, DMF reflux at 65 °C, 12 h.

Synthetic route of potent anticancer agents. Reagent and Conditions: (i) n>an class="Chemical">Triethyl amine, amino propanol, 120–130 °C for 6 h (ii) triethylamine, methane sulfonyl chloride, THF, RT, 4 h (iii) amino component, NaH, DMF reflux at 65 °C, 12 h.

Baijiao et al. have reported a simpn>le protocol for the synthesis of n>an class="Chemical">quinoline containing hybrids and evaluated for their in vitro anticancer activity against different cancer cell lines. Initially, starting material dichloroquinazoline (184) was reacted with substituted selenium containing analogues (185a-d) in the presence of hydrogen chloride in isopropanol to afford the target compounds 186a-d in good yields (Scheme 11 ). Among them, compound 186a was found to be the most potent anticancer agent against all tested cell lines with IC50 values ranging from 3.39 to 9.98 nM [36].

Scheme 11

Synthetic route of chloro containing quinoline hybrids as potent anticancer agents.

Synthetic route of chloro containing n>an class="Chemical">quinoline hybrids as potent anticancer agents.

Zolnowska and co-workers have reported a well-designed synthetic apn>proach towards the synthesis of functionalized guanidine containing derivatives (189a-d) and screened for their anticancer activity. The synthetic route was outlined in Scheme 12 . Initially, intermediate compounds 2-(2-arylmethylthio-4-chloro-5-methylbenzenesulfonyl)-1-(4,6-dichloro-1,3,5-triazin-2-ylamino)guanidine derivatives 188a-d were synthesized from the reaction of appropriate aminoguanidine (187a-d) with 2,4,6-trichloro-1,3,5-triazine in the presence of triethylamine under the optimal reaction conditions. Then intermediates 187a-d were treated with various substituted amines in the presence of DIPEA as base and argon atmosphere under reflux conditions for 3–11 h to yield corresponding guanidine substituted hybrids 189a-k in good to moderate yields. Compound 189a displayed excellent inhibitory potency against hCA IX with Ki values of 41.7 nM and showed prominent cytotoxic effect with IC50 of 17 μM against HeLa cancer cell line [37]. The SAR represented that the presence of two chlorine electron-withdrawing groups at aromatic rings highly enhanced the anticancer activity. On the other hand, the presence of sulfonyl and sulfonamide groups also increases the anticancer activity of compound 189a.

Scheme 12

Synthetic route of chlorine containing drugs as potent anticancer agents. Reagents and conditions: (i) 2,4,6-trichloro-1,3,5-triazine, TEA, anhydrous THF, 1 h at 0 °C, 2 h, rt (ii) X-NH2, DIPEA, 80 °C, argon atmosphere, 3–11 h.

Synthetic route of chlorine containing drugs as potent antin>an class="Disease">cancer agents. Reagents and conditions: (i) 2,4,6-trichloro-1,3,5-triazine, TEA, anhydrous THF, 1 h at 0 °C, 2 h, rt (ii) X-NH2, DIPEA, 80 °C, argon atmosphere, 3–11 h.

Pogorzelska et al. have evaluated their synthesized compounds for anticancer activity and found effective compounds. The syntheses of new series of target compounds 192a-w was showed in Scheme 13 . The starting materials 190a-l were reacted with phenylpropiolaldehyde diethyl acetal in the presence of PTSA in ethanol under reflux conditions to yield 2-(2-alkylthiobenzenesulfonyl)-3-(phenylprop-2-ynylideneamino)guanidines 191a-l in moderate to good yields. All the synthesized compounds 191a-l underwent CuI mediated electrophilic cyclizations of α, β-alkynic hydrazones, unfortunately, this step did not provided the desired derivatives but led to copper complexes with pyrazole moiety. In another way, treatment of compounds 191a-i with 20% PTSA under the optimal reaction conditions afforded the final compounds 192a-w. Compounds 192a-w were studied for their in vitro cytotoxic activity against MTT assays including three human cancer cell lines such as MCF-7, HCT-116 and HeLa cell lines. Compound 192a was found to be an excellent anticancer agent with IC50 value of 7 μM against HCT-116 and IC50 value of 3 μM against HeLa cell lines [38].

Scheme 13

Potent anticancer agents. Reagents and conditions: (i) phenylpropiolaldehyde diethyl acetal (1 eq.) or 4-phenylbut-3yn-2-one (1 eq.), PTSA (0.1 eq.), EtOH, reflux, 2–20 h, (ii) CuI, Et3N, MeCN, 82 °C, 1–4 h, (iii) 20% PTSA/MeCN, 1 h.

Potent antin>an class="Disease">cancer agents. Reagents and conditions: (i) phenylpropiolaldehyde diethyl acetal (1 eq.) or 4-phenylbut-3yn-2-one (1 eq.), PTSA (0.1 eq.), EtOH, reflux, 2–20 h, (ii) CuI, Et3N, MeCN, 82 °C, 1–4 h, (iii) 20% PTSA/MeCN, 1 h.

Nazarian and co-workers reported the synthesis of a series of novel antileishmanial activity compounds. Firstly, 5-chloro-2-hydroxybenzaldehyde 193 was reacted with acrolein in the presence of the base potassium carbonate in dioxane under reflux conditions to obtain intermediate compound chromene-3-carbaldehyde 194. Claisen-Schmidt condensation of compound 194 with various acetophenones in ethanolic solution of NaOH afforded 3-(6-chloro-2H-chromen-3-yl)propen-1-ones 195a–d ( Scheme 14 ). In another way, same starting material 5-chloro-2-hydroxybenzaldehyde 193 was reacted with methyl vinyl ketone in the presence of potassium carbonate base in dioxane under reflux conditions to afford 1-(6-chloro-2H-chromen-3-yl)ethanone 196. Then, Claisen-Schmidt condensation of compound 196 with different aldehydes in ethanolic solution of NaOH yielded the corresponding 1-(6-chloro-2H-chromen-3-yl)propen-1-ones 197a–d (Scheme 14). The target compounds were evaluated for their antileishmanial activity against the promastigote form of Leishmania major using MTT assay. Compounds 195a (IC50 = 1.22 μM) and 197a (IC50 = 1.33 μM) were showed excellent antileishmanial activity. Compound 195a was considered as a promising lead for the development of an effective agent for chemotherapy of leishmaniasis and other protozoan infections [39].

Scheme 14

Synthetic route of chlorine containing chalcones analogues as potent antileishmanial activity. Reagents and conditions: (i) acrolein, 1,4-dioxane, K2CO3, reflux (ii) appropriate acetophenone, NaOH, EtOH (iii) methyl vinyl ketone, 1,4-dioxane, K2CO3, reflux (iv) appropriate aldehyde, NaOH, EtOH.

Synthetic route of chlorine containing n>an class="Chemical">chalcones analogues as potent antileishmanial activity. Reagents and conditions: (i) acrolein, 1,4-dioxane, K2CO3, reflux (ii) appropriate acetophenone, NaOH, EtOH (iii) methyl vinyl ketone, 1,4-dioxane, K2CO3, reflux (iv) appropriate aldehyde, NaOH, EtOH.

Magar and co-workers synthesized a series of novel benzofuran substituted hybrids and screened for their in vitro anti-proliferative activity. Usually the target compound 203 was synthesized in three simple steps. In the first step, three different chlorine-substituted pyridinium iodide salts (199) were synthesized in good yields by the heating of corresponding aryl methyl ketones (198) in the presence of iodine and pyridine at 140 °C for 3 h. In the second step, Al2O3 catalyzed condensation reaction was applied to prepare six aryl benzofuran-3(2H)-ones 202 by condensing benzofuran-3(2H)-one (200) with aryl aldehydes 201 in methylene chloride for 3 h at room temperature. Finally, a modified Kröhnke pyridine synthetic method [40] was used to synthesize the target compounds 203. A mixture of aryl benzofuran-3(2H)-ones 202, pyridinium iodide salts (199) and NH4OAc in glacial acetic acid was heated at 100 °C for 12–24 h to afford compound 203 ( Scheme 15 ) in considerable yields. All the synthesized compounds were evaluated for in vitro topoisomerase I and II inhibition and anti-proliferative activity. Compound 203 exhibited more potent anti-proliferative activity than the positive control etoposide against HCT15 (IC50 = 5.47 μM) and T47D (IC50 = 7.47 μM) cell line. The SAR results suggested that, the chlorinated compounds showed better anti-proliferative activity than those of corresponding non-chlorinated compounds. The chlorination at the 2-phenyl ring is important to inhibit the topo I and II activity. These results provided useful information for the development of benzofuro[3,2-b]pyridine derivatives as a novel class of topoisomerase-targeted anticancer agents [41].

Scheme 15

Synthetic route of chlorine containing benzofuro[3,2-b]pyridine derivatives as potent anti-proliferative drugs. Reagents and conditions: (i) iodine, pyridine, 140 °C, 3 h, 76.7–93.0% yield; (ii) Al2O3, CH2Cl2, rt, 3 h, 55–72% yield; (iii) NH4OAc, glacial acetic acid, 100 °C, 12–24 h, and 16–54%.

Synthetic route of chlorine containing n>an class="Chemical">benzofuro[3,2-b]pyridine derivatives as potent anti-proliferative drugs. Reagents and conditions: (i) iodine, pyridine, 140 °C, 3 h, 76.7–93.0% yield; (ii) Al2O3, CH2Cl2, rt, 3 h, 55–72% yield; (iii) NH4OAc, glacial acetic acid, 100 °C, 12–24 h, and 16–54%.

Synthesis of new class of antiproliferative agent was carried out by Yao et al. The synthetic route was lengthy and included five steps started from 2-picolinic acid 204 (Scheme 16 ). Compound 204 was reacted with SOCl2 under reflux condition at 72 °C for 14 h to produce 4-chloropicolinoyl chloride 205 followed by the addition of methanol at room temperature for 1 h to generate methyl 4-chloropicolinate 206. Consequently, compound 206 was reacted with various substituted amines in the presence of CH3OH/THF at 2 °C for 3 h to yield 207a-e. Then, they were treated with 4-aminophenol to provide corresponding diaromatic ethers 208a-e in considerable yields. Finally, compounds 208a-e were treated with substituted diaromatic ethers 208a-e in DCM to afford the final targeted thiourea (209a-e) derivatives in considerable yields (Scheme 16). All the newly synthesized compounds were evaluated for their in vitro antiproliferative activities against HCT116 and MDA-MB-231 cell lines. Compound 209a was found to be the most potent antiproliferative agent with IC50 value of 9.15 μM against HCT-116 cell line [42].

Scheme 16

Synthetic route of thiourea substituted derivatives as potent anti-proliferative agents. Reagents and conditions: (i) SOCl2, reflux, 72 °C, 14 h, (ii) CH3OH, rt, 1 h; (iii) RNH2, CH3OH/THF, 2 °C, 3 h; (iv) 4-aminophenol, DMF, KOBu-t, 1 h; (v) substituted isocyanates, DCM, 0–5 °C, 2 h then 18 h at rt.

Synthetic route of thiourea n>an class="Chemical">substituted derivatives as potent anti-proliferative agents. Reagents and conditions: (i) SOCl2, reflux, 72 °C, 14 h, (ii) CH3OH, rt, 1 h; (iii) RNH2, CH3OH/THF, 2 °C, 3 h; (iv) 4-aminophenol, DMF, KOBu-t, 1 h; (v) substituted isocyanates, DCM, 0–5 °C, 2 h then 18 h at rt.

Zhao et al. have performed the synthesis of anti-proliferative compounds and tested their activity against four human cancer cell lines such as A549, MGC803, PC-3 and TE-1. The intermediates 211a-e were prepared by following the earlier reported procedure [43]. Then, the prepared intermediates 211a-e were treated with ethyl glyoxalate in EtOH under reflux condition for 2 h and led to the 6-chloro-2-(propylthio)-8,9-dihydro-7H-purine-8-carboxylate 212a-e in moderate yields, and then intermediates 212a-e were hydrolyzed into carboxyl in AcOH/H2O to yield 213a-e. Finally, compounds 213a-e were coupled with various substituted amines using coupling reagent EDCI/HOBt in dichloromethane at room temperature to afford final target compounds 214a-e ( Scheme 17 ). Compound 214a showed excellent anti-proliferative activity with IC50 values of 2.80 μM against A549 and 303.03 μM against GES-1, respectively. The SAR revealed that the presence of piperazine group highly enhanced the anti-proliferative activity, which suggested that the piperazine substitution was played an important role to enhance the anti-proliferative activity. The compound 214a, with a methyl substitution on the piperazine group, was found to be the most potent anti-proliferative agent in the series [44].

Scheme 17

Synthetic route of potent chlorine containing anti-proliferative agents. Reagents and conditions: (i) DMF, DIPEA, 100 °C, 12 h, (ii) EtOH, 80 °C, (iii) AcOH/H2O, 80 °C, (iv) 1-methylpiperazine, EDCI/HOBt, CH2Cl2.

Luo and co-workers developed a series of novel chlorine containing compounds with enhanced in vitro anti-tumour activity. Initially, commercially available 2-amino-4-chlorobenzoic acid 215 was condensed with formamide at 140–145 °C for 4.5 h to obtain intermediate 216, and then treated with SOCl2 in the presence of DMF under reflux conditions to produce intermediate 217. In addition, commercially available salicylaldehyde 218 was reacted with acetone in the presence of sodium hydride at room temperature to afford intermediate 219. Compounds 217 and 219 were reacted with K2CO3 in acetonitrile at 30–40 °C for 3.5 h to yield intermediates 220. Compound 220 was reacted with substituted aldehydes in the presence of anhydrous alcohol in acetone at room temperature to provide the final target compounds 221a-e in good yields (Scheme 18 ). Compound 221a was found to be the most potent antitumor agent with IC50 value of 1.96 μM against MGC-803 and 8.47 μM against Bcap-37 cell lines. Compound 221a could be considered as useful templates for the future development of more potent antitumor agents [45].

Scheme 18

Synthetic route of potent chlorine containing anti-proliferative agents. Reagents and conditions: (i) formamide, 140–145 °C, (ii) SOCl2, DMF, reflux, (iii) NaOH, H2O, rt (iv) CH3CN, K2CO3, 30–40 °C, (v) R-CHO, EtOH, NaOH.

Synthesis of chlorine containing anti-inflammatory agents

Inflammation demote to localised physical conditions causing swellness, n>an class="Disease">redness, heat with pain, which are mediated by the release of proinflammatory mediators like bradykinin and cytosine to increase the prostaglandin synthesis rate [46,47]. Non-steroidal anti-inflammatory drugs (NSAIDs), which existing in two isomeric forms, namely, constitutive form (COX-1) and an inducible form (COX-2), inhibit cyclooxygenases (COX) and further inhibiting the biosynthesis of prostaglandins (PGs) [48,49]. The role of COX-1 enzyme is to maintain the gastric integrity and kidney functioning whereas COX-2 may cause inflammation and pain [50,51].

Shantharam and co-workers have developed new series of imidazole hydrazones as potent anti-inflammatory agents. The synthesis of imidazole based hydrazones 225a-s wer performed in a manner outlined in Scheme 19 . Starting material 4-(2-hydroxypropan-2-yl)-2-propyl-1H-imidazole-5-carboxylic acid 222 was converted into ethylester using TMS-Cl in ethanol at room temperature followed by the addition of excess hydrazine hydrate afforded the corresponding imidazole hydrazide 224. Compound 224 was treated with various substituted aldehydes in ethanol at 80 °C for 8–10 h to yield final imidazole-hydrazones 225a-s in good yields. Compound 225a was found to be the most potent anti-inflammatory agent with IC50 value of 46 μM. The SAR revealed that the presence of electron withdrawing (-Cl) groups on the phenyl ring highly enhanced the anti-inflammatory activity [52].

Scheme 19

Synthetic route of imidazole-hydrazones as potent anti-inflammatory agents. Reagents and conditions: (i) TMS-Cl, MeOH, rt, 2–3 h, (ii) NH2NH2.H2O, EtOH, reflux, 78–80 °C, (iii) Ar-CHO, EtOH, reflux, 80 °C, 8–10 h.

Synthetic route of imidazole-hydrazones as potent anti-inflammatory agents. Reagents and conditions: (i) n>an class="Chemical">TMS-Cl, MeOH, rt, 2–3 h, (ii) NH2NH2.H2O, EtOH, reflux, 78–80 °C, (iii) Ar-CHO, EtOH, reflux, 80 °C, 8–10 h.

Continuation of their interest in drug development program, the research group of Rakesh identified the novel quinazolinone-hydrazones as potent in vitro anti-inflammatory activity. The quinazolinone starting materials 226 were methylated using TMS-Cl in methanol at room temperature followed by the addition of excess hydrazine hydrate afforded the corresponding quinazolinone hydrazides 228. The quinazolinone-hydrazones 229a-t were obtained by the reaction of 228 with different aromatic aldehydes in the presence of catalytic amount of glacial acetic acid (Scheme 20 ). Compounds 229a (IC50 = 84 μM/mL) and 229b (IC50 = 67 μM/mL) showed excellent anti-inflammatory activities with lower IC50 values than that of the standard compound aspirin (IC50 = 166 μM/mL). The SAR revealed that the presence of electron withdrawing (-Cl) groups on the phenyl ring highly enhanced the anti-inflammatory activity and the length of quinozolinone alkyl chain also played a major role to increases the activity. The presence of EDGs (OH and OMe) on the phenyl ring diminishes the anti-inflammatory activity [53].

Scheme 20

Synthetic route of quinazolinone-hydrazones as potent anti-inflammatory agents. Reagents and conditions: (i) TMS-Cl, MeOH, rt, 4 h, (ii) NH2NH2.H2O, EtOH, reflux, 16 h, (iii) EtOH, CH3COOH, reflux, 7–8 h.

Synthetic route of quinazolinone-hydrazones as potent anti-inflammatory agents. Reagents and conditions: (i) n>an class="Chemical">TMS-Cl, MeOH, rt, 4 h, (ii) NH2NH2.H2O, EtOH, reflux, 16 h, (iii) EtOH, CH3COOH, reflux, 7–8 h.

Abdellatif et al. have developed a novel class of imidazoles compounds and screened for their in vitro anti-inflammatory activity. Compound p-chlorobenzoylglycine 230 was synthesized by heating the mixture of glycine and 4-chlorobenzoyl chloride in sodium hydroxide solution (10%) as previously reported method [54]. Cyclocondensation of compounds 231a-h with substituted aldehydes in the presence of catalytic amount of sodium acetate in acetic anhydride under the optimal reaction conditions yielded 231a-h in good yield [54]. Then, compounds 231a-h were treated with sulfanilamide in glacial acetic acid to afford the compounds 232a-h in good yields (Scheme 21 ). Among all the synthesized compounds, compound 232a was found to be excellent anti-inflammatory activities with IC50 of 7.86 μM against COX-1 and IC50 of 0.86 μM against COX-2 which was more potent than that of the standard Celecoxib in the series [55].

Scheme 21

Synthesis of chlorine containing imidazole hybrids as potent anti-inflammatory agents. Reagents and conditions: (i) appropriate Ar-CHO, CH3COONa, acetic anhydride, 100 °C, 6 h, (b) H2NC6H4SO2NH2, CH3COONa, glacial acetic acid, reflux, 24 h.

Synthesis of chlorine containing n>an class="Chemical">imidazole hybrids as potent anti-inflammatory agents. Reagents and conditions: (i) appropriate Ar-CHO, CH3COONa, acetic anhydride, 100 °C, 6 h, (b) H2NC6H4SO2NH2, CH3COONa, glacial acetic acid, reflux, 24 h.

Abdelrahman and colleagues have reported a set of thirteen quinoline-2-carboxamide derivatives from p-chloroaniline 233. Firstly, condensation of p-chloroaniline 233 with diethyl oxalpropionate 234 afforded the corresponding imine 235, which cyclized in polyphosphoric acid to yielded quinoline-2-carboxylic acid ethyl ester 236 in excellent yield. The quinoline-2-carboxylate 236 was hydrolyzed into its carboxylic acid 237 under basic conditions, which then coupled with substituted amine using BOP in the presence of DIPEA to yield quinoline-2-carboxamides 238a-m (Scheme 22 ). All the developed hybrids were evaluated for their in vitro anti-inflammatory activities. Compounds 238a (IC50 = 7.9 μM against COX-1 and IC50 = 1.21 μM against COX-2) and 238b (IC50 = 8.6 μM against COX-1 and IC50 = 1.13 μM against COX-2) showed excellent anti-inflammatory activity compared to the reference drug Celecoxib. The docking study explained the results obtained from the in vitro COXs assays based on the structural features and their binding patterns inside the active sites. Obviously, the two active hits 238a and 238b adopted similar binding patterns and interactions to that of the co-crystallized bromocelecoxib, S-58 ligand where the benzimidazole and phenyl piperazine moieties in 238a and 238b, respectively assumed the same position of sulfonamide moiety of S-58 within the additional side pocket of COX-2 in Fig. 2 [56].

Scheme 22

Synthesis of the chlorine containing quinoline-2-carboxamides hybrids as potent anti-inflammatory agents. Reagents and reaction conditions: (i) acetic acid, ethanol, reflux, 48 h; (ii) PPA, 120 °C, 2 h; (iii) 5% NaOH, ethanol, 80 °C, 4 h; (iv) appropriate amine, BOP, DIPEA, DCM, overnight, rt.

Fig. 2

(A) Docking and binding pattern of compound 238b into COX-2 active site (PDB code: 1CX2). (B) The superimposition of the docked pose of 238b (red) and the co-crystallized S-58 (cyan) within active site of COX-2. (C) Docking and binding pattern of compound 238a into the same COX-2 binding pocket. (D) The superimposition of the docked pose of 238a (red) and the co-crystallized S-58 (cyan) within active site of COX-2. Dashed green lines represent hydrogen bonds. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Synthesis of the chlorine containing n>an class="Chemical">quinoline-2-carboxamides hybrids as potent anti-inflammatory agents. Reagents and reaction conditions: (i) acetic acid, ethanol, reflux, 48 h; (ii) PPA, 120 °C, 2 h; (iii) 5% NaOH, ethanol, 80 °C, 4 h; (iv) appropriate amine, BOP, DIPEA, DCM, overnight, rt.

A new series of eighteen benzo[d]imidazole-hydrazones were synthesized by Kumar et al. using commercially available compound 239 as starting material. Compound 239 was esterified by using con. H2SO4 in ethanol under reflux conditions to obtain compound 240 in good yield [57]. Subsequently, compound 240 was treated with propylamine and TEA to afford compound 241. Compound 242 was prepared from nitro reductive cyclization of 241 with 2,4-dichlorobenzaldehyde using sodium dithionite in DMSO as solvent at 90 °C. The one-pot reaction produced the compound 242 within 3 h in an excellent yield (94%). The ester group was then treated with hydrazine hydrate in ethanol medium under reflux condition for 6 h to provide hydrazide 243. Finally, the target hydrazones 244a-r were obtained by condensation of 243 with various substituted aldehydes in ethanol with catalytic amount of glacial acetic acid (Scheme 23 ). All the prepared analogues were screened for their in vitro anti-inflammatory activity. Compound 244a showed excellent anti-inflammatory activity with 71.97% inhibition compared to the reference drug indomethacin (69.34% inhibition). The SAR results revealed that, the presence of EWDs (Cl, Br and NO2) on the phenyl ring increased the anti-inflammatory activity and the presence of EDGs (OH and OCH3) on phenyl ring reduced the anti-inflammatory activity [58].

Scheme 23

Synthetic route of chlorine containing benz[d]imidazole hybrids as potent anti-inflammatory agents. Reagents and conditions: (i) ethanol, con. H2SO4, reflux; (ii) propylamine TEA, THF, rt; (iii) Na2S2O4, DMSO, 90 °C, 3 h; (iv) hydrazine hydrate, ethanol, 78–80 °C, reflux, 8 h; (v) Ar-CHO, ethanol, cat. glacial acetic acid, reflux, 10 h.

Synthetic route of chlorine containing n>an class="Chemical">benz[d]imidazole hybrids as potent anti-inflammatory agents. Reagents and conditions: (i) ethanol, con. H2SO4, reflux; (ii) propylamine TEA, THF, rt; (iii) Na2S2O4, DMSO, 90 °C, 3 h; (iv) hydrazine hydrate, ethanol, 78–80 °C, reflux, 8 h; (v) Ar-CHO, ethanol, cat. glacial acetic acid, reflux, 10 h.

Synthesis of chlorine bearing anti-tuberculosis drugs

Tuberculosis is a highly infectious chronic deadly disease caused by Mycobacterium tuberculosis (MTB). This disease threatens the human life by affecting lungs primarily (pulmonary TB) distant from other vital organs. Drug-resistant TB (DR-TB), multidrug-resistant TB (MDR-TB), extensively drug-resistant TB (XDR-TB) and totally drug resistant TB (TDR) emerging nowadays are completely resistant for the action of presently available standard drugs [59]. The infection of TB is so common that it has caused around 1.4 million deaths and 10.4 million clinical cases all over globe as reported in 2015 [60,61]. However the treatment of TB with drugs such as Isoniazid (INH), Ethambutol (EMB0, Rifampicin (RIF) and Pyrazinamide (PZA) is proved to be highly effective. Discovery of Rifampicin (RIF) helped in developing handful of anti-TB drug compounds to the humans. However, there is still a number of derivatives awaiting to be explored to stop the activity of bacteria and further spreading of TB [62,63].

Sun et al. have developed a new series of sixteen chiral piperidinol derivatives and screened for their in vitro anti-tuberculosis activity. The synthesis of the chiral piperidinol compounds 251a-p or 252a-p were first performed by reacting optically active epoxide intermediate (S)-(+)-epichlorohydrin (245) or (R)-(−)-epichlorohydrin (246) with a substituted phenol or thiophenol (247) in acetonitrile under reflux condition in the presence of cesium carbonate to afford chiral epoxide derivative 248 or 249 [64,65]. Subsequently, the crude product 248 or 249 was reacted with 4-[4-chloro-3-(trifluoromethyl)-phenyl]-4-piperidinol (250) in ethanol under reflux condition to afford alcohol diols 251a-p or 252a-p in moderate yields (Scheme 24 ). Compound 251a was found to be potent anti-tuberculosis activity with MIC values of 1.4 μg/mL, which could be attributed to the presence of electron-withdrawing groups (CF3 and Cl) [66].

Scheme 24

Synthesis of chlorine containing piperidinol derivatives as potent anti-tuberculosis agents. Reagents and conditions: (i) CS2CO3, MeCN, reflux, overnight, (ii) EtOH, reflux, overnight.

Synthesis of chlorine containing n>an class="Chemical">piperidinol derivatives as potent anti-tuberculosis agents. Reagents and conditions: (i) CS2CO3, MeCN, reflux, overnight, (ii) EtOH, reflux, overnight.

In 2012, Kratky and co-workers have developed a novel class of sulfonamides as potent antimycobacterial activity against M. tuberculosis 331/88, M. avium (330/88) and two strains of M. kansasii (235/80 and 6509/96). The target compounds of sulfonamide derivatives were synthesized in a simple and single step. Sulfonamide 254a-d and substituted aldehydes 253a-d reacted in ethanol under reflux conditions for 3 h and stirred at room temperature for another 12 h to yield final compounds 255a-d at 4 °C in good yields (Scheme 25 ). Compound 255a was found to be the most potent antimycobacterial activity against M. tuberculosis 331/88, M. avium (330/88) and two strains of M. kansasii (235/80 and 6509/96) with MIC values ranging between 32 and 62.5 μmol/L [67]. The SAR predicted that the presence of electron-donating group (OH) on the phenyl ring highly enhanced the antimycobacterial activity. Furthermore, the introduction of electron-withdrawing chloro group to the same phenyl ring increases the activity.

Scheme 25

Synthetic route to potent anti-tuberculosis agents.

Synthetic route to potent anti-pan class="Species">tuberculosis agents.

Shah and co-workers have developed a new series of active anti-tuberculosis agents in one pot synthesis. The final target compounds N-arylamino biquinoline derivatives 259a-x were synthesized from the reaction of 2-chloro-3-formyl quinolines 256, malononitrile 257 with appropriate enhydrazinoketones 258 in absolute ethanol in the presence of base piperidine under optimal reaction conditions (Scheme 26 ). All the prepared hybrids were evaluated for their in vitro antituberculosis activity against M. tuberculosis H37Rv. Among them, compound 259a exhibited the most potent anti-tuberculosis inhibition (87%) against M. tuberculosis H37Rv [68].

Scheme 26

Synthetic route to N-arylamino biquinoline derivatives as potent anti-tuberculosis agents.

Synthetic route to pan class="Chemical">N-arylamino biquinoline derivatives as potent anti-n>an class="Species">tuberculosis agents.

Synthesis of chlorine containing α-glucosidase agents

Diabetes is one of the insistent diseases rising in the world. According to the estimated data obtained in 2010, n>an class="Chemical">around 285 million peoples were suffered from diabetes all over the world and it may increase to 439 million by 2030 [69,70]. Blood glucose changing due to the insulin resistance is regarded as the feature of being diabetic in 95% of the cases [71] which give raise to several problems like high blood pressure, heart problem, kidney failure, stroke and blindness [72]. Consequently, the inhibition of α-glucosidase (EC. 3.2.1.20), a key carbohydrate hydrolyzing enzyme, could serve as an effective methodology in both preventing and treating diabetes through controlling the postprandial glucose level and suppressing postprandial hyperglycemia [73]. α-Glucosidase specifically performs the hydrolysis of α-glucopyranoside bond, resulting in the production of α -d-glucose from the non-reducing end of the sugar [74]. Several α-glucosidase inhibitors like acarbose, voglibose, and miglitol, have appeared in clinic for the treatment of type II diabetes mellitus [75], however, number and intensity of side effects call for the development of potent, structurally diverse, safe and efficacious drugs for the effective treatment of diabetes mellitus.

Very recently, Javid et al. have developed the synthesis and SAR study of a series of novel thiosemicarbazide compounds. The targeted thiosemicarbazide compounds were synthesized in simple and three steps. First, equimolar amount of commercially available p-chlorobenzaldehyde 260 was treated with thiosemicarbazide 261 in methanol in the presence of catalytic amount of HCl under reflux condition for 3–4 h to yield compound 262. Then compound 262 was cyclized in the presence of iodine and potassium carbonate in 1,4-dioxane to afford compound 263. Next, compound 263 was reacted with dichloro benzaldehyde in methanol in the presence of catalytic amount of conc. HCl to yield final compound 264 in good yield (Scheme 27 ). Compound 264 was found to be excellent α-glucosidase inhibitory agent with IC50 value of 4.70 μM. The presence of electron withdrawing group (Cl) on the phenyl ring highly enhanced the α-glucosidase activity [76].

Scheme 27

Synthesis of chlorine containing drugs as potent α-glucosidase activity.

Synthesis of pan class="Chemical">chlorine containing drugs as potent α-glucosidase activity.

In 2010, Pirotte et al. have synthesized a series of new 6-chloro-substituted-3-alkylamino/cycloalkylamino-4H-1,2,4-benzothiadiazine 1,1-dioxides analogues and tested for their in vitro α-glucosidase activity. The starting material aniline 265 was reacted with chlorosulfonyl isocyanate under the optimal reaction conditions to yield 6-chloro-substituted 3-oxo-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide 266. Subsequent thionation of the oxo derivatives 266 with phosphorus pentasulfide in pyridine led to the corresponding compound 267. In the next step, compound 268 was prepared from the reaction of thioxo compound 267 with methyl iodide in the presence of sodium hydrogenocarbonate. Finally, compound 268 was treated with isopropyl amine under optimal reaction conditions to obtain final product 269 [77] (Scheme 28 ). The compound 269 was found to be the most potent glucose-induced insulin secretion with RIS value of 13 μM per 1 μM concentration. The position of the chlorine atom on the benzene ring strongly affected the activity on insulin-secreting cells. Taken as a whole, the rank order of potency of 3-isopropylamino-substituted compounds on pancreatic β-cells was found to be 6-chloro = 6,7-dichloro > 7-chloro > 8-chloro > 5-chloro [78].

Scheme 28

Synthesis of chlorine containing drugs as potent α-glucosidase activity. Reagents and conditions: (i) (a) CISO2NCO, CH3NO2; (b) AlCl3; (ii) P2S5, pyridine; (iii) CH3I, NaHCO3, CH3OH/H2O; (iv) R3NH2.

Synthesis of chlorine containing drugs as potent α-glucosidase activity. Reagents and conditions: (i) (a) n>an class="Chemical">CISO2NCO, CH3NO2; (b) AlCl3; (ii) P2S5, pyridine; (iii) CH3I, NaHCO3, CH3OH/H2O; (iv) R3NH2.

Taha et al. have synthesized novel imidazole-pyridine hyn>an class="Chemical">brids and screened for their in vitro biological activities. These compounds were prepared from commercially available starting materials 5-chloropyridine-2,3-diamine 270. Compound 270 was reacted with substituted aldehydes 271 in the presence of Na2S2O5 with DMF as solvent under reflux conditions to afford desired products 272a-z in high yields (Scheme 29 ). All the newly synthesized derivatives were tested for their in vitro biological activities such as antioxidant, antiglyacation and β-glucuronidase activities. Among them, compound 272a (IC50 = 240.12 μM) was found to be the most potent antiglyacation agent, compound 272b (IC50 = 29.25 μM) showed excellent β-glucuronidase activities and compound 272c (IC50 = 72.50 μM) exhibited promising antioxidant activity [79].

Scheme 29

Synthesis of chlorine containing drugs as potent α-glucosidase activity.

Synthesis of pan class="Chemical">chlorine containing drugs as potent α-glucosidase activity.

Synthesis of chlorine derived anti-malarial agents

Malaria refers to parasitic infection which spreads worldwide and caused serious problems in the tropical and subtropical parts of Asia, Central and South America, Africa and Middle East [80,81]. A parasitic class called Plasmodium which carried by the female Anopheles mosquito is the cause of this disease which get entered into human bloodstream. The treatment and management of this disease is irrationally high not only because of medication but also due to low production [82]. The complexities in controlling malaria lies in growing resistance of malaria parasite to most of the antimalarial drugs are used [83].

A series of phthalazine containing imidazole derivatives with different substitution pattern have been synthesized and evaluated for their in vitro anti-malarial activity extra and intracellular forms of T. cruzi. The starting material 1,4-dichlorophthalazine 273 treated with 3-(imidazole-1-yl)propylamine 274 in the presence of K2CO3 under reflux conditions to provide compound 275 in 48% yield (Scheme 30 ). Compound 275 was found to be most active in vitro against extra and intracellular forms of T. cruzi (IC50 value is 8.8 μM against epimastigote form) and less toxic against Vero cells which was better than the standard drug benznidazole. Furthermore, the study of antiparasitical activity of compound 275 was at a higher level for developing a new drug in future [84].

Scheme 30

Synthetic route of chlorine containing potent anti-malarial agent.

Synthetic route of pan class="Chemical">chlorine containing potent anti-pan class="Disease">malarial agent.

In 2005, Joshi et al. have designed, synthesized and evaluated a series of compounds for their antimalarial activity. Starting material guanidine nitrate 276 reacted with malononitrile 277 in the presence of sodium alkoxide in dry ethanol or methanol under optimal reaction conditions to yield 2,4,6-triaminopyrimidine 278. Compound 278 was condensed with 2,4-dichlorobenzoic acid 279 in the presence of activated copper bronze powder at 180–190 °C to yield N-(2,4-diamino-6-pyrimidino)-4-chloroanthranilic acid 280 which further cyclized using concentrated sulfuric acid, yielded 5-oxo-(10H)-2,4-diamino-8-chloropyrimido-[4,5-b]quinoline 281. Compound 281 was reacted with phosphorous oxychloride to obtain 2,4-diamino-5,8-dichloropyrimido-[4,5-b]quinoline 282, which stirred with liquid ammonia at room temperature to yield 2,4,5-triamino-8-chloropyrimido-[4,5-b]quinoline 283 (Scheme 31 ). The synthesized compounds were screened using Rane's test for blood schizonticidal activity in mice infected by P. berghei. Based on the results, three compounds possessed antimalarial activity comparable to chloroquine and compound 283 was most active one [85].

Scheme 31

Synthetic route to potent anti-malarial agents. Reagents and conditions: (i) sodium alkoxide, reflux, 6 h, (ii) activated Cu bronze powder, 180–190 °C, 12 h (iii) Con. H2SO4, 100 °C, 8 h, (iv) POCl3, 135–140 °C, (v) liq. NH3, rt, 1hr.

Synthetic route to potent anti-malarial agents. Reagents and conditions: (i) n>an class="Chemical">sodium alkoxide, reflux, 6 h, (ii) activated Cu bronze powder, 180–190 °C, 12 h (iii) Con. H2SO4, 100 °C, 8 h, (iv) POCl3, 135–140 °C, (v) liq. NH3, rt, 1hr.

A library of novel triazines substituted hybrids have been developed by Kumar et al. and were evaluated for in vitro anti-malarial activity against P. falciparum. Compound 4,7-dichloroquinoline 282 was stirred with excess of 2-aminoethanol in n-butanol under optimal reaction conditions to give the 2-(7-chloro-quinolin-4-ylamino)-ethanol 283 in moderate to good yield [86]. Chemoselective o-mesylation was synthesized in pyridine at 0 °C for 5 h to yield themethanesulfonic acid 2-(7-chloro-quinolin-4-ylamino)-ethyl ester 284 [87]. Compound 284 was subjected to nucleophilic substitution with trisubstituted triazines to yield the corresponding targeted compounds 285a-s under microwave condition (Scheme 32 ). Compound 285a displayed more than 99% suppression activity after four days against in-vitro model of P. falciparum and showed high 99.11% suppression against chloroquine resistant strain N-67 of P. yoelii in an in vivo assay [88].

Scheme 32

Synthetic route of triazine substituted hybrids as potent anti-malarial agents. Reagents and conditions: (i) 1-amino ethanol, n-butanol, 90 °C, 8 h, (ii) methanesulfonyl chloride, pyridine, 0 °C, 3hr, (iii) tri-substituted triazines, MW, NMP, 30 s.

Synthetic route of triazine n>an class="Chemical">substituted hybrids as potent anti-malarial agents. Reagents and conditions: (i) 1-amino ethanol, n-butanol, 90 °C, 8 h, (ii) methanesulfonyl chloride, pyridine, 0 °C, 3hr, (iii) tri-substituted triazines, MW, NMP, 30 s.

A new series of anti-malarial active compounds has been developed by Madrid et al. [89]. A simple two-step synthesis method was used to prepare ring substituted 4-chloroquinolines derivatives 289 allowing analysis of the effect of ring substitutions on inhibition of growth of chloroquine sensitive and resistant strains of P. falciparum. The first step in the sequence was the condensation of substituted aniline 286 with Meldrum's acid and trimethylorthoformate to give eneamine 287 intermediates, which then sealed into a glass reaction tube with a small volume of phenyl ether as solvent and subjected to microwave irradiation for 5 min at 300 °C (Scheme 33 ). The synthesized compounds were evaluated for their growth inhibition activity against P. falciparum with the help of fluorescent-active cell sorting (FACS) assay. For in vitro activity, two strains i.e. drug-sensitive and a drug-resistant were employed. Form this study it was found that substitutions with small electron withdrawing groups were found most active for anti-malarial activity.

Scheme 33

Synthesis of chloroquinolines as potent anti-malarial agents. Reagents and conditions: (i) Meldrum's acid, CH(OCH3)3, DMF, reflux, 2hr, (ii) diphenylether, 300 °C, 300 W, 5min, (iii) POCl3, reflux, 3hr.

Chlorine containing drugs as miscellaneous applications

Alzheimer's disease (AD) is neurodegenerative disorder featured with cognitive dysfunction and memory lapse which accounts for the major dementia cases. According to the present estimation, about 45 million people are going through this disease worldwide and it may reach up to 131 million by 2050 if the condition left untreated [[90], [91], [92]]. Though actual etiology for the AD progression is not known yet, a number of pathophysiology factors are believed to be responsible for the progression of this disease. Deficits of acetylcholine (Ach), inflammation, β-amyloid (Aβ) deposits, oxidative stress, dyshomeostasis of biometals, tau-protein aggregation are considered to be such pathophysiological factors [[93], [94], [95]]. Unfortunately the medicines for the cure of AD and its progression have not been developed yet. But certain medicines were approved and prescribed for the AD patients for the temporary relief [96,97].

Wang et al. have synthesized a new series of selenium containing hydrochloride derivatives and tested for their in vitro multi-target activity against Alzheimer's disease. Firstly, Na2Se2 reacted with the corresponding benzenediazonium salts, which were prepared from commercially available substituted 2-amino benzoic acids 290a-p and HNO2 to yield 2,2-diselenobisbenzoic acids 291a-p. Subsequently, SOCl2 reacted under reflux conditions to obtain 292a-p, which then reacted with corresponding chloro substituted amines in anhydrous CH2Cl2 to afford final target compounds 293a-p. In final step, the Boc group was removed from compounds 293a-p using HCl gas to yield de-protected hydrochloride compounds 294a-p (Scheme 34 ). Compound 294a showed excellent Alzheimer's disease inhibition activities against tested all multi-targeted enzymes. Mainly, compound 294a was found to be the most potent Aβ1-42 aggregation agent with IC50 value of 9.6 μM [98].

Scheme 34

Synthesis route of potent Alzheimer's agents. Reagents and conditions: (i) H2O, NaNO2, HCl, 0–5 °C, 30 min, Na2Se2, 50 °C; (ii) SOCl2, reflux; (iii) anhydrous CH2Cl2, triethylamine, rt; (iv) CH2Cl2, HCl (g), 0–5 °C.

Synthesis route of potent Alzheimer's agents. Reagents and conditions: (i) n>an class="Chemical">H2O, NaNO2, HCl, 0–5 °C, 30 min, Na2Se2, 50 °C; (ii) SOCl2, reflux; (iii) anhydrous CH2Cl2, triethylamine, rt; (iv) CH2Cl2, HCl (g), 0–5 °C.

Viruses are infectious agents affecting the life forms. They n>an class="Chemical">are responsible for various dangerous diseases like human immunodeficiency virus (HIV), hepatitis B and C viruses (HBV and HCV, respectively), severe acute respiratory syndrome (SARS), corona viruses (Middle east respiratory Syndrome, MERS; influenza (seasonal, pandemic), viral haemorrhagic fevers (Ebola), dengue, and chikungunya etc. These diseases have caused adverse impact on human health by leading to unexpected illnesses and deaths and troubling day-to-day normal life activities. Viruses are the major cause for the emergence of newer pandemics e.g. H1N1 influenza, Ebola, and Zika virus etc. threatening the public health [99,100].

Likewise, more than 60 antiviral drugs of diverse chemical classes have been approved by the FDA, mainly for the treatment of HIV, the hepatitis B and C, herpes and influenza A and B viruses and still many molecules are in various stages of clinical trials. But there is still a pressing need for the development of new drugs acting through several mechanisms and combat the viral resistance as viruses [101]. However it is always challenging for the medicinal chemists to develop newer drugs focus on the unique biological features of viruses and treat the emerging viral disease in one or the other way without harming the host cells [102].

In 2012, Regina and co-workers have designed and synthesized a series of novel imidazole derivatives in a simple and single step. All synthesized derivatives were evaluated for their in vitro anti-HIV activity against MT-4 cells. Initially, carboxylic group free imidazole 295 was reacted with various substituted amines in the presence of BOP and TEA in DMF at room temperature to yield compound 296 (Scheme 35 ). Compound 296 exhibited the most potent anti-HIV activity against MT-4 cells with IC50 values of 2 nM. Compound 296 emerged as a useful lead compound for the development of new therapeutic tools for EFV-based HIV-1 therapies which showed the L100I and K103 N mutations [103]. The SAR revealed that the presence of pyridine group at R position highly enhanced the activity. Moreover, the electron-withdrawing chloro group was also played a crucial role in increasing anti-HIV activity.

Scheme 35

Synthesis of imidazole analogues as potent anti-HIV agents.

Synthesis of pan class="Chemical">imidazole analogues as potent anti-n>an class="Species">HIV agents.

Boland and co-workers have synthesized a novel series of potent candidate of PDE4 inhibitors in six steps. Starting from the commercially available 3,4-dihydroxybenzaldehyde 297. Difluoromethylation of 297 in DMSO using sodium chlorodifluoroacetate as a source of difluorocarbene provided the compound 298. Alkylation of the remaining hydroxyl group of 298 was performed with alkyl dibromides to yield intermediates 299. In the next step, the aldehyde compound was oxidized into the corresponding benzoic acids 300, using sodium chlorite (Lindgren oxidation) in combination with sulfamic acid as chlorine dioxide scavenger. Compound 300 coupled with 3,5-dichloropyridin-4-amine to provide intermediates 301. Further, compound 301 underwent nucleophilic substitution with 3-amino-sulfanyldihydrofuranone to afford final compound 302 (Scheme 36 ). The final synthesized compounds were screened for their PDE4 inhibition activity against two PDEB1 and PDE4D2. Compounds 302 (IC50 = 5.4 nM against PDEB1 and 0.7 nM against PDE4D2) and 303 (IC50 = 4.0 nM against PDEB1 and 66 nM against PDE4D2) showed excellent PDE4 inhibition activity compared to standard drug Rolipram (IC50 = 52 nM against PDEB1 and 130 nM against PDE4D2) [104].

Scheme 36

Synthetic route of chlorine containing hybrids as potent PDE4 inhibitors. Reagents and conditions: (i) ClF2CCOONa, NaOH, DMF, H2O, 120 °C, 2 h; (ii) Br(CH2)nBr, K2CO3, ACN, reflux, 4 h; (iii) H2NSO3H, NaClO2, AcOH, H2O, 10 °C, 30 min; (iv) (1) SOCl2, toluene, 90 °C, 1.5 h; (2) 3,5-dichloro-4-aminopyridine 60% NaH, THF, 10 °C, 1 h; (v) 2-oxo-3-amino-oxolane, K2CO3, ACN, reflux, 1.5 h.

Synthetic route of chlorine containing hyn>an class="Chemical">brids as potent PDE4 inhibitors. Reagents and conditions: (i) ClF2CCOONa, NaOH, DMF, H2O, 120 °C, 2 h; (ii) Br(CH2)nBr, K2CO3, ACN, reflux, 4 h; (iii) H2NSO3H, NaClO2, AcOH, H2O, 10 °C, 30 min; (iv) (1) SOCl2, toluene, 90 °C, 1.5 h; (2) 3,5-dichloro-4-aminopyridine 60% NaH, THF, 10 °C, 1 h; (v) 2-oxo-3-amino-oxolane, K2CO3, ACN, reflux, 1.5 h.

A series of novel cannabinoid type 1 receptor antagonists were synthesized by Szabo et al. In the beginning, cycloalkyl-benzenes 304 reacted with the acyl-chloride in trichloroethylene under the optimal reaction conditions to provide phenones 305. Claisen condensation of 305 with imidazole-1-yl-oxo-acetic acid ethyl ester was reacted under optimal reaction condition at −78 °C to yield diketone esters 306 in good yields. The synthesis of acids 308 involved basic hydrolysis of the corresponding esters 307, which in turn were prepared by condensing diketone esters 306 with suitably substituted phenyl hydrazines. The intermediates 308 were then converted into their acid chlorides by using thionyl chloride in refluxing toluene and these intermediates were reacted with commercially available 1-aminopyrrolidine in dichloromethane at room temperature to afford compounds 309 (Scheme 37 ). Compound 309 was found to be the most potent cannabinoid Type 1 receptor antagonist with Ki values of 4 nM. The SAR studies suggest that, the pyrazole substituents and the presence of chloro groups on the phenyl ring and pyrrolidine ring enhanced the activity of these novel CB1 antagonists [105].

Scheme 37

Synthetic route of pyrrolidine containing hybrids as potent cannabinoid Type 1 receptor antagonists. Reagents and conditions: (i) R1CH2COCl, AlCl3, trichloroethylene, −50 °C; (ii) LiHMDS, THF, Imidazole-1-yl-oxo-acetic acid ethyl ester, −78 °C, 1 N HCl; (iii) 2,4-dichlorophenylhydrazine hydrochloride, EtOH, reflux; (iv) 2.5 N KOH, MeOH, reflux; (v) (COCl)2, DMF, CH2Cl2 rt; (vi) 1-aminopyrrolidine, CH2Cl2, rt.

Synthetic route of pyrrolidine containing hyn>an class="Chemical">brids as potent cannabinoid Type 1 receptor antagonists. Reagents and conditions: (i) R1CH2COCl, AlCl3, trichloroethylene, −50 °C; (ii) LiHMDS, THF, Imidazole-1-yl-oxo-acetic acid ethyl ester, −78 °C, 1 N HCl; (iii) 2,4-dichlorophenylhydrazine hydrochloride, EtOH, reflux; (iv) 2.5 N KOH, MeOH, reflux; (v) (COCl)2, DMF, CH2Cl2 rt; (vi) 1-aminopyrrolidine, CH2Cl2, rt.

Lan et al. have reported a series of aminoalkoxy pyrimidine derivatives as potent neuropathic pain agents. The targeted compounds were synthesized in three steps from starting material aryl- or alkyl-amidine 310. Compound 310 was reacted with ethyl 2-chloro-3-oxobutanoate under optimal reaction conditions to yield intermediate 311, and then compound 311 was treated with Br(CH2)3Br under reflux conditions to provide compound 312. Compound 1,3-dibromopropane 312 was reacted with piperidine in the presence of Cs2CO2 in acetonitrile to afford aminoalkoxy pyrimidine derivatives 313a-h (Scheme 38 ). Compound 313a was found to be the most potent in vitro neuropathic pain agents with Kiσ1 of 1.06 nmol and Kiσ2 of 1425 nmol. Moreover, compound 313a exhibited good safety, acceptable pharmacokinetic properties and good selective profile to some specific targets. Thus, compound 313a may facilitate the development of a novel class drugs for the treatment of neuropathic pain. Further studies of compound 313a and evaluation of these series of derivatives are currently underway in their laboratory and will be reported in due course [106].

Scheme 38

Synthetic route for aminoalkoxy pyrimidine derivatives as potent neuropathic pain agents. Reagents and conditions: (i) t-BuOK, MeOH, reflux; (ii) Br(CH2)3Br, K2CO3, acetone, reflux; (III) piperidine, Cs2CO3, acetonitrile, reflux.

Synthetic route for aminoalkoxy pyrimidine derivatives as potent n>an class="Disease">neuropathic pain agents. Reagents and conditions: (i) t-BuOK, MeOH, reflux; (ii) Br(CH2)3Br, K2CO3, acetone, reflux; (III) piperidine, Cs2CO3, acetonitrile, reflux.

In 2016, Cao and co-workers have designed and synthesized a new series of pyridazinone substituted analogues and evaluated for their in vitro antineuropathic pain activity. Starting material phenylhydrazine hydrochloride 314 was reacted with maleic anhydride 315 in the presence of conc. HCl at reflux conditions to obtain cyclization product 316. Then, compound 316 alkylated with 1,3-dibromopropane to yield 317, and then reacted with the piperidine to afford the final compounds 318 in moderate yields (Scheme 39 ). Compound 318 showed potent σ1 receptor affinity (Kiσ1 = 1.4 nM) and excellent selectivity over not only σ2 receptor (1366-fold). These profiles suggested that compound 318 may be a novel class of candidate drugs for treatment of neuropathic pain [107].

Scheme 39

Synthetic route for chlorine containing pyridazinone substituted analogues as potent antineuropathic pain agents. Reagents and conditions: (i) H2O, conc. HCl, reflux; (ii) Br(CH2)3Br, K2CO3, acetone, reflux; (iii) piperidine, K2CO3, KI, acetonitrile, reflux.

Synthetic route for chlorine containing n>an class="Chemical">pyridazinone substituted analogues as potent antineuropathic pain agents. Reagents and conditions: (i) H2O, conc. HCl, reflux; (ii) Br(CH2)3Br, K2CO3, acetone, reflux; (iii) piperidine, K2CO3, KI, acetonitrile, reflux.

In 2007, Nencka and co-workers have developed a series of novel substituted-6-chlorouracils analogues as potent inhibitory agent against recombinant human TP expressed in V79 Chinese hamster cells. Commercially available 4-chloro-2,6-dimethoxypyrimidine 319 underwent direct ortho-lithiation with butyllithium at −78 °C to afford intermediate compound 320. Then compound 320 was reacted with acetone at −78 °C until the temperature reached to room temperature to afford 321. In the final step, compound 321 was treated with con. HCl in dioxane under reflux conditions to obtain target compound 322 in good yield (Scheme 40 ). The most effective inhibitor is compound 322, which inhibited the enzyme expression in V79 cells competitively with K i of 0.20 μM and the enzyme purified from placenta with Ki of 0.29 μM. In this manner, this study changes the traditional view on uracil-based TP inhibitors and provides a novel lead for further research [108].

Scheme 40

Synthesis of substituted-6-chlorouracils analogues as potent inhibitory activity. Reagents and Conditions: (i) n-BuLi, THF, −78 °C; (ii) ketone, −78 °C to rt; (iii) concd HCl, dioxane, reflux.

Synthesis of substitutedn>an class="Chemical">-6-chlorouracils analogues as potent inhibitory activity. Reagents and Conditions: (i) n-BuLi, THF, −78 °C; (ii) ketone, −78 °C to rt; (iii) concd HCl, dioxane, reflux.

Recently, Tzvetkov and co-workers have reported indole substituted analogues as potent monoamine oxidase B inhibitors. The targeted compounds were synthesized in single step from commercially available indole carboxylic acid 323. Compound 323 was coupled with dichloro amine 324 using coupling reagent EDC.HCl in methanol at room temperature to give compound 325 in good yield (Scheme 41 . Compound 325 was screened for its monoamine oxidase B inhibition activity and showed potent activity with IC50 values of 0.227 nM against human MAO-B and 1300 nM against human MAO-A enzymes. Future efforts will be directed toward further improving the compounds’ drug-like properties with regard to water-solubility, bioavailability, metabolism, and toxicity and to evaluate the new MAO-B inhibitors in relevant animal models [109].

Scheme 41

Synthesis of chlorine containing indole analogues as potent monoamine oxidase B inhibitors.

Synthesis of chlorine containing n>an class="Chemical">indole analogues as potent monoamine oxidase B inhibitors.

Very recently, Kadayat and co-workers developed 2,4-diphenyl-5H-indeno[1,2-b]pyridines as potent topoisomerase inhibitor. In the first step, 1-indanone 326 was condensed with aryl aldehydes 327 to prepare indanone intermediates 328 in the presence of 5% aqueous NaOH in ethanol using Claisen-Schmidt condensation reaction [110]. Then, six pyridinium iodide salts 330 were synthesized by refluxing acetophenones 329 with iodine in pyridine. Finally, using modified Kröhnke synthesis [111], indanone intermediates 328 and pyridinium iodide salts 330 were reacted in the presence of dry ammonium acetate in methanol or acetic acid to yield compounds 331a-e in the moderate yields (Scheme 42 ). All the synthesized eighteen new chlorinated compounds were assessed for topoisomerase inhibitory activity and cytotoxicity against HCT15, T47D, and HeLa cancer cell lines. Among them, compounds 331a (IC50 = 0.05 nM against HCT-15 and IC50 = 1.08 nM against T47D cell lines) and 331b (IC50 = 0.11 nM against HCT-15 and IC50 = 1.16 nM against T47D cell lines) showed the most potent topoisomerase activity which is better than the reference drugs. All of the chlorinated compounds displayed significant cytotoxic effect, revealing potent anticancer activity against T47D breast cancer cells [112].

Scheme 42

Synthesis of chlorine containing hybrids as potent topoisomerase inhibitor. Reagents and conditions (i) aq. NaOH (5%), EtOH, 1–12 h, room temperature; (ii) pyridine, iodine, 3 h, 140 °C, (iii) NH4OAc, methanol or glacial acetic acid, 24–36 h, 100 °C.

Synthesis of chlorine containing hyn>an class="Chemical">brids as potent topoisomerase inhibitor. Reagents and conditions (i) aq. NaOH (5%), EtOH, 1–12 h, room temperature; (ii) pyridine, iodine, 3 h, 140 °C, (iii) NH4OAc, methanol or glacial acetic acid, 24–36 h, 100 °C.

Synthesis of new class of acaricidal and insecticidal agents was carried out by Kadayat and co-workers. The synthetic route was medium length and included three chemical steps. In the first step, 5-chloropyrazole-4-carbaldehyde 332 was reacted with substituted phenols in the presence of KOH under the optimal reaction conditions to obtain intermediate compound 333 [113]. Further, the compound 333 was reacted with hydroxylamine hydrochloride in methanol or ethanol medium to yield compound 5-aryloxy pyrazole oximes 334 in good yields. The treatment of intermediate 334 with 2-chloromethyl-5-alkoxy-1,3,4-thiadiazole in acetonitrile medium using potassium carbonate as alkali produced corresponding pyrazole oximes containing a substituted 1,3,4-thiadiazole moiety 335a-z (Scheme 43 ). All the newly synthesized compounds were evaluated for their in vitro acaricidal and insecticidal activities. Compound 335a showed the most potent insecticidal activities against P. xylostella with LC50 value of 9.78 μM which was better than the control compound Pyridalyl (LC50 = 17.40 μM) [114].

Scheme 43

Synthesis of thiadiazole containing pyrazole oxime derivatives as potent acaricidal and insecticidal agents. Reagents and conditions: (i) substituted phenols, KOH, DMF or DMSO, 45 °C, 2 h, 110 °C, 6–22 h; (ii) NH2OH.HCl, KOH, CH3OH or CH3CH2OH, reflux, 5–20 h; (iii) substituted 2-chloromethyl-5-alkoxy-1,3,4-thiadiazole, K2CO3, CH3CN, reflux, 8–17 h.

Synthesis of thiadiazole containing n>an class="Chemical">pyrazole oxime derivatives as potent acaricidal and insecticidal agents. Reagents and conditions: (i) substituted phenols, KOH, DMF or DMSO, 45 °C, 2 h, 110 °C, 6–22 h; (ii) NH2OH.HCl, KOH, CH3OH or CH3CH2OH, reflux, 5–20 h; (iii) substituted 2-chloromethyl-5-alkoxy-1,3,4-thiadiazole, K2CO3, CH3CN, reflux, 8–17 h.

Very recently, Taha and co-workers have developed a novel class of indole derivatives as potent α-amylase inhibition agent. The synthetic route was very simple and it included only two steps. In the first step, indole ethyl ester 336 was converted into indole-hydrazide 337 using hydrazine hydrate in ethanol under reflux condition for 6 h. The indole hydrazide 337 was reacted with various aromatic isothiocyanate in chloroform and stirred for 3 h to yield final target indole derived products 338a-s in good yields (Scheme 44 ). All the synthesized compounds were tested for their α-amylase inhibitory activity. Compounds 338a and 338b displayed the most potent α-amylase inhibitory activity with IC50 values of 2.10 μM and 2.03 μM respectively. The SAR revealed that the presence of electron withdrawing groups (Cl and F) on the phenyl ring highly enhanced the α-amylase activity. Compounds having substituents on para-position are more active than their ortho and meta counterpart [115].

Scheme 44

Synthesis of chlorine containing indole derivatives as potent α-amylase inhibitory agents. Reagents and condition: (i) NH2-NH2, ethanol, reflux, 6 h; (ii) arylisothiocyanatee CHCl3, stir.

Synthesis of chlorine containing n>an class="Chemical">indole derivatives as potent α-amylase inhibitory agents. Reagents and condition: (i) NH2-NH2, ethanol, reflux, 6 h; (ii) arylisothiocyanatee CHCl3, stir.

Chourey and co-workers have designed and synthesized a novel eicosatetraenoic acid as potent OXE receptor agent. Chloro substituted indole analog 339 was reacted with (R)-methyl 5-chloro-3-methyl-5-oxopentanoate under optimal reaction conditions to obtain intermediate 340 in very good yield. Then methyl ester group of intermediate 340 was converted into free carboxylic acid group using LiOH·H2O in THF/H2O (4:1) at room temperature to obtain final compound 341 in good yield (Scheme 45 ). The synthesized compound was found to be the most potent OXE receptor with IC50 value of 120 pM. These new highly potent OXER antagonists may provide a novel strategy for the treatment of eosinophilic disorders like asthma. The SAR revealed that the addition of a phenyl group at the end of the hexyl side chain of the OXE-R antagonist S-230 contribute to dramatic increasement in both in vitro potency and half-life in the circulatory system when administered orally to monkeys [116].

Scheme 45

Synthesis of eicosatetraenoic acid as potent OXE receptor agent. Reagents and conditions: (i) Me2AlCl, CH2Cl2, rt, 3 h, 93%; (b) LiOH·H2O, THF/H2O (4:1), MeOH, rt, 16 h, 95%.

Synthesis of eicosatetraenoic acid as potent OXE recepn>tor agent. Reagents and conditions: (i) n>an class="Chemical">Me2AlCl, CH2Cl2, rt, 3 h, 93%; (b) LiOH·H2O, THF/H2O (4:1), MeOH, rt, 16 h, 95%.

In 2011, Kumar et al. have designed and synthesized a novel series of n>an class="Chemical">quinolinyl amines and screened for their in vitro antidepressant activity. The intermediate product 2-chloro-3-(chloromethyl)-8-methylquinoline 344 was prepared in two steps from 2-chloro-3-formyl-8-methylquinoline 342 via its reduction with NaBH4 followed by chlorination with SOCl2. The quinolinyl amines 345a-p were prepared by nucleophilic substitution reaction of 344 with various amines in absolute ethanol in the presence of base triethylamine (Scheme 46 ). Compounds 345a and 345b showed promising antidepressant activities. The preliminary SAR of quinolinyl amines suggested that compound with electron withdrawing groups (F and Cl) showed good antidepressant activity [117].

Scheme 46

Synthesis of chlorine containing quinolinyl amines as potent antidepressant agent. Reagent and conditions: (i) NaBH4, methanol, stirring rt; (ii) SOCl2, dry benzene, 80 °C, reflux. (iii) Various amine, ethanol, TEA, reflux.

Synthesis of chlorine containing n>an class="Chemical">quinolinyl amines as potent antidepressant agent. Reagent and conditions: (i) NaBH4, methanol, stirring rt; (ii) SOCl2, dry benzene, 80 °C, reflux. (iii) Various amine, ethanol, TEA, reflux.

In 2010, Bouloc and co-workers have developed pyrazine substituted hybrids and tested their in vitro Aurora kinase inhibitory activity. The starting material 6,8-Dibromoimidazo[1,2-a]pyrazine 346 was chlorinated at the C-3 position in the presence of NCS under the optimal reaction conditions to afford intermediate compound 347. Then, intermediate 374 underwent aromatic nucleophilic substitution at the C-8 position to provide compound 348 followed by a chemoselective Suzuki cross-coupling at C-6 position to afford final compound 349 (Scheme 47 ). Compound 349 was found to be the most potent aurora inhibitory agent with IC50 of 0.190 ± 0.138 μM [118].

Scheme 47

Synthesis of pyrazine substituted hybrids as potent Aurora kinase A inhibitory agent. Reagents and conditions: (i) NCS, CH3CN/DCE (3:1), reflux, (ii) 4-(4-morpholino)aniline, DIPEA, Dioxane, 180 °C, reflux, (iii) 3-pyridylboronic acid, Pd(PPh3)2Cl2, Na2CO3, CH3CN, 150 °C, 30min.

Synthesis of pyrazine n>an class="Chemical">substituted hybrids as potent Aurora kinase A inhibitory agent. Reagents and conditions: (i) NCS, CH3CN/DCE (3:1), reflux, (ii) 4-(4-morpholino)aniline, DIPEA, Dioxane, 180 °C, reflux, (iii) 3-pyridylboronic acid, Pd(PPh3)2Cl2, Na2CO3, CH3CN, 150 °C, 30min.

In 2018, Karthik and co-workers have developn>ed n>an class="Chemical">piperazine-1-carbothioamide chitosan silver nanoparticles (P1C-Tit*CAgNPs) as potent anti-hemolytic agents. The starting materials 350 and p-chloro isocyanates 351 were reacted with the base TEA in dichloromethane under room temperature to obtain compound (N-(4-chlorophenyl)-4-(2,3dihydrobenzoic[b]1,4]dioxine-2-carbonyl)piperazine-1carbothioamide 352 in good yield (Scheme 48 ). The final compound 352 was converted to silver nanoparticles (P1C-Tit*CAgNPs) and the final silver coated nanoparticles were evaluated for their in vitro anti-inflammatory activity. Compound 352 showed potent anti-hemolytic activity with IC50 value of 55 μg/mL. Compound 352 also showed potent phospholipase A2 (PLA2) enzyme inhibitory activity with IC50 value of 18 μg/mL, which was much better than the reference drug Diclofenac (70 μg/mL) [119].

Scheme 48

Synthesis of chlorine containing piperazine-1-carbothioamide chitosan silver nanoparticles (P1C-Tit*CAgNPs) as potent anti-hemolytic agents.

Synthesis of chlorine containing n>an class="Chemical">piperazine-1-carbothioamide chitosan silver nanoparticles (P1C-Tit*CAgNPs) as potent anti-hemolytic agents.

Very recently, Jalaja et al. have prepared n>an class="Chemical">triazoles derived natural products as potent pancreatic lipase inhibitors. The starting material propargylated labdane 353 was reacted with various substituted benzyl and phenacyl azides 354a-q at room temperature to provide 1,2,3-triazole appended labdane derivatives 355a-q in good to excellent yields (Scheme 49 ). All the synthesized natural product derived labdane appended triazoles were evaluated for their pancreatic lipase inhibitor activity. Among them, compound 355a was found to be the most potent pancreatic lipase inhibitor agents with IC50 value of 0.77 μM, which was much better than the reference drug Orlistat (IC50 = 0.8 ± 0.03 μM) [120].

Scheme 49

Synthesis of triazoles derived natural products as potent pancreatic lipase inhibitors.

Synthesis of pan class="Chemical">triazoles derived natural products as potent pancreatic lipn>ase inhibitors.

In 2018, Hangeland and co-workers synthesized potent lipase inhibitors from starting material Ethyl 1-ethyl-4-hydroxy-5-oxo-2,5-dihydro-1H-pyrrole-3-carboxylate 356 [121,122]. Compound 356 was first o-methylated with trimethylsilyldiazomethane in the presence of DIPEA [123], followed by saponification of the ester. The free carboxylic acid 357 was coupled with amines using the corresponding acid chloride which was generated from oxalyl chloride. The resulting amides were demethylated with boron trichloride to yield final products 358a-d in moderate yields (Scheme 50 ). All the produced analogues were screened as a small set of nonselective lipase inhibitors against endothelial lipase (EL) to identify a potent and reversible inhibitor. Compound 358a (EL_HDL IC50 = 218 nM) showed good inhibitory activity against the tested endothelial lipase [124].

Scheme 50

Synthesis of chlorine containing hybrids as potent lipase inhibitors. Reagents and conditions: (i) TMSCH2N2, DIPEA, Et2O, rt, 48 h; (ii) NaOH, MeOH/H2O (1:1), 70 °C, 1 h; (iii) oxalyl chloride, CH2Cl2, DMF(cat), (iv) R3-NH2, DIPEA, CH2Cl2, rt, 3 h; BCl3, CH2Cl2, rt, 5 h.

Synthesis of chlorine containing hyn>an class="Chemical">brids as potent lipase inhibitors. Reagents and conditions: (i) TMSCH2N2, DIPEA, Et2O, rt, 48 h; (ii) NaOH, MeOH/H2O (1:1), 70 °C, 1 h; (iii) oxalyl chloride, CH2Cl2, DMF(cat), (iv) R3-NH2, DIPEA, CH2Cl2, rt, 3 h; BCl3, CH2Cl2, rt, 5 h.

A novel class of pyridopyrimidinones derivatives have been synthesized by Yu et al. and evaluated the enzyme activity against PI3K and mTOR. In addition, reaction of 7-bromo-4H-pyrido[1,2-a]pyrimidin-4-one 359 with NCS under the optimal reaction conditions provided intermediate 360. Subsequently, chlorine-substituted 360 reacted with pyridineboronic acid pinacol ester 361 to afford final product 362 (Scheme 51 ). Compound 362 was evaluated as a novel class of efficacious dual PI3K/mTOR inhibitors. Compound 362 showed good enzyme activity against PI3K and mTOR (IC50 (PI3kα/mTOR) = 3.4/4.7 nM) and potent suppression of Akt and p70s6k phosphorylation in cellular assays. Furthermore, compound 362 also demonstrated significant in vivo efficacy in a PC-3M tumour xenograft model [125].

Scheme 51

Synthetic route of chlorine containing pyridopyrimidinones derivatives as potent enzyme activity agents. Reagents and conditions: (i) NSC, DMF, rt, (ii) 3, Pd(dppf)2Cl2, K2CO3, dioxane/H2O, 100 °C.

Synthetic route of chlorine containing n>an class="Chemical">pyridopyrimidinones derivatives as potent enzyme activity agents. Reagents and conditions: (i) NSC, DMF, rt, (ii) 3, Pd(dppf)2Cl2, K2CO3, dioxane/H2O, 100 °C.

A novel class of sulfonamide derivatives have been synthesized by Kindon et al. and evaluated as CCR4 receptor antagonists. The syntheses of target compounds was lengthy and it includes six chemical steps starting from 2-aminopyrazine 363. Commercially available 2-aminopyrazine 363 was diboration by selective displacement of the bromine in the 3-position with sodium methoxide to obtain 2-amino-5-bromo-3-methoxypyrazine 364. The bromine at 5-position was then removed by hydrogenation and coupling of the 2,3-dichlorophenylsulphonyl chloride with 2-amino-3-methoxypyrazine 365 to yield 366. A highly selective 5-position nitration of 366 followed by reduction of the nitro group 367 gave an amino-pyrazine 368 which was diazotized in the presence of hydrofluoric acid to give 369 in good yield (Scheme 52 ). The synthesized final compound and intermediates were screened for their CCR4 receptor inhibition activity. Compounds 366 (pIC50 7.8) and 369 (pIC50 8.6) were showed good potency against Human CCR4 receptor [126].

Scheme 52

Synthesis of sulfonamides derivatives as potent CCR4 receptor antagonists. Reagents and conditions: (i) Br2, dichloromethane, 2,6-lutidine; (ii) NaOMe, MeOH, reflux (iii) H2, Pd/C, ethanol, rt (iv) ArSO2Cl, KOtBu, 0–25 °C, THF, (v) HNO3, AcOH, 70–85 °C, (vi) H2, Pd/C, AcOH, 60 °C, (vii) HBF4, CH3CN, 0–5 °C, NaNO2 or HF/pyridine/NaNO2.

Synthesis of sulfonamides derivatives as potent n>an class="Gene">CCR4 receptor antagonists. Reagents and conditions: (i) Br2, dichloromethane, 2,6-lutidine; (ii) NaOMe, MeOH, reflux (iii) H2, Pd/C, ethanol, rt (iv) ArSO2Cl, KOtBu, 0–25 °C, THF, (v) HNO3, AcOH, 70–85 °C, (vi) H2, Pd/C, AcOH, 60 °C, (vii) HBF4, CH3CN, 0–5 °C, NaNO2 or HF/pyridine/NaNO2.

At last, very recently, Filipski and co-workers have developed potent Sodium-Phosphate Cotransporter NaPi2a (SLC34A1) inhibitors. Pyrrole derivative 370 [127] were reacted with enol tosylate 371 [128] to form 372 and then DBU was used to effect pyridine ring cyclization to give azaindole 373. Conversion of 7-hydroxyazindole 373 to the corresponding 7-chloroazaindole 374 was accomplished using phosphorus oxychloride. N-Chlorosuccinimide chlorination of the 3-position yielded 374 and S_NAr with (S)-morpholin-2-ylmethanol 375 provided the desired 376 (Scheme 53 . The compound 376 was screened for their Sodium-Phosphate Cotransporter NaPi2a (SLC34A1) inhibitor activity and compound 376 (NaPi2a IC50 = 380 nM) was found to be good NaPi2a inhibitor. Compound 376 (PF-06869206) was the first orally bioavailable selective NaPi2a inhibitor and represented a pharmacological tool to probe the functional effects of selective NaPi2a inhibition in vivo [129].

Scheme 53

Synthesis of chlorine containing hybrids as potent NaPi2a (SLC34A1) inhibitors. Reagents and conditions: (i) i-Pr2NEt, CH3CN, −10 to −5 °C, 87%; (ii) DBU, CH3CN, 90 °C, 70%; (iii) POCl3, CH3CN, 60 °C, 90%; (iv) NCS, DMF, 30 °C, 76%; (v) i-Pr2NEt, CH3CN, 80 °C, 86%.

Synthesis of chlorine containing hyn>an class="Chemical">brids as potent NaPi2a (SLC34A1) inhibitors. Reagents and conditions: (i) i-Pr2NEt, CH3CN, −10 to −5 °C, 87%; (ii) DBU, CH3CN, 90 °C, 70%; (iii) POCl3, CH3CN, 60 °C, 90%; (iv) NCS, DMF, 30 °C, 76%; (v) i-Pr2NEt, CH3CN, 80 °C, 86%.

Conclusion

Chlorine based drugs in medicinal chemistry is an attractive and useful to study the exact frequency, drug target, distribution, and diverse biological application ways in U.S. FDA approved pharmaceuticals. Additionally, we hope that the synthetic organic chemistry researchers could improve the ways to synthesize potent chlorine-based drugs as well as underrepresented and non-existent drugs. In this review, we mainly focused on the synthesis of chlorinated drugs, chlorinated drugs as a function of approval date, disease condition, chlorine attachment and structure-activity relationship. We believe that this review article will be useful for inspiring the structural design and developments of less toxic and powerful chlorine-based drugs against the numerous death-causing diseases.