A recent update on new synthetic chiral compounds with antileishmanial activity.

Parasitic diseases, including malaria, leishmaniasis, and trypanosomiasis, affect billions of people and are responsible for almost 500,000 deaths/year. In particular, leishmaniasis, a neglected tropical disease, is considered a global public health problem because current drugs have several drawbacks including to toxicity, high cost, and drug resistance, which result in a lack of effective and readily available therapies. Therefore, the synthesis of new, safe, and effective molecules still requires the attention of the scientific community. Moreover, it is well known that chirality plays a crucial role in the antiparasitic activity of molecules, driving the design of their synthesis. Therefore, in this review we report a recent update on new chiral compounds with promising antileishmanial activity, focusing on synthetic approaches. Where reported, in most cases the enantiopure compound has shown better potency against the protozoa than its enantiomer or corresponding racemic mixture.

INTRODUCTION

Leishmaniasis is one of the most dangerous neglected diseases, second only to malaria in parasitic causes of death ; it is endemic in over 90 countries (in particular, South‐East Asia, East and North Africa, and Central America), involves more than 350 million people, and presents more than a million new cases per year. ,

The infection is caused by different species of leishmania protozoan parasite, which are transmitted to humans through the bite of infected sandflies. A specific sandfly, called phlebotominae, inoculates promastigotes into the skin of the host. In humans, these are taken up by macrophages or dendritic cells and transformed into flagellar amastigotes. The future course of the infection depends upon the strain of leishmania, and the type of immune response mounted by the host. , , There are three forms of Leishmaniasis: visceral leishmaniasis (VL), cutaneous leishmaniasis (CL), and mucocutaneous leishmaniasis (MCL). VL, the deadliest form, causes 20,000–40,000 casualties every year.

No vaccine is available to efficaciously prevent the disease. Moreover, commercial drugs (Figure 1) suffer various limitations including lack of safety and efficacy, high costs, high toxicity, difficulty in administration, long treatment duration, and drug resistance. , Current drug therapies cannot eliminate leishmaniasis completely.

FIGURE 1

Main commercial drugs against Leishmaniasis

Therefore, the discovery and development of new efficient, and safe molecules is still receiving great attention. , In recent years, several groups have come up with promising antileishmanial agents, some of them being chiral. , ,

In this review, we report recent (i.e., from 2017) syntheses of chiral compounds that show promising activity against leishmania. Different synthetic strategies to achieve the desired enantiopure compounds were utilized. From our investigation, the chiral pool approach, which essentially foresees the use of enantiopure reagents (e.g., amino acid), is highly preferred over asymmetric synthesis, in which the installation of the stereogenic center is generally driven by chiral catalysis. Moreover, other approaches, such as chiral resolution, and kinetic resolution were also utilized. In this update, only the most promising chiral molecules, in terms of showing good or excellent IC50/EC50 against several Leishmania strains, are reported. The review is ultimately divided by classifying the products. Initially, some classes of natural products (NPs) and their promising analogues are discussed, followed by amino acids containing compounds, with a particular focus on peptides. The last chapter is dedicated to nitroimidazoxazines and ‐oxazoles, which are promising agents against leishmania, some of them already drug candidates.

NATURAL PRODUCTS AND ANALOGUES

NPs are substances that are produced by a living organism and can be found in nature. Although they can be obtained by extraction, generally from plants, the possibility to develop new synthetic strategies for the formation of such molecules enables the selective modification of some of their properties to better respond to the desired requirements. Therefore, in this chapter, the synthesis of various NPs and their analogues that have shown interesting activity against leishmania are described.

Besides all, abietane‐type diterpenoids are characterized by a tricyclic ring system and have shown a wide range of chemical diversity and biological activity, including antitumor and anti‐infective properties. , Encouraged by these considerations, in 2019, González‐Cardenete et al. reported the first semisynthesis of the abietane diterpenoid (+)‐liquiditerpenoic acid A (abietopinoic acid) (5) together with several analogues with different C‐18 functional groups and oxidation pattern at C‐7. (Scheme 1). The compounds were synthesized starting from the methyl dehydroabietate (1), obtainable from commercially available (−)‐abietic acid.

SCHEME 1

Reagents and conditions: (i) acetyl chloride, anhydrous AlCl3, 1,2‐dichloroethane, 0°C to rt, 24 h, under Ar; (ii) m‐CPBA, TFA, CH2Cl2, 0°C to rt, 22 h; (iii) CrO3, AcOH, 0°C to rt, 20 h; (iv) K2CO3, MeOH, rt, 3.5 h; (v) LiI, 2,4,6‐collidine, reflux, 3 h, under argon, then H2O (0°C) and 6 N HCl (to pH = 1)

A Friedel–Crafts acetylation followed by Baeyer–Villiger oxidation afforded the key intermediate 3 (80% overall yield, two steps). From 3, the NP 5 is obtained through oxidation achieved by CrO3 at the benzylic C‐7 position, affording the ketone 4, followed by deacetylation and nucleophilic methyl ester cleavage, with 34% isolated yield (over 5 steps).

The antileishmanial activity of these molecules was evaluated against four promastigote Leishmania strains (Leishmania infantum, Leishmania donovani, Leishmania amazonensis, and Leishmania guyanensis) together with the cytotoxicity against J774 macrophages. The best results were obtained with 4, which showed a submicromolar IC50, 73‐fold more potent than miltefosine (0.65 μM vs. 47.7 μM) for L. amazonensis, 13 times (1.3 μM vs. 18.2 μM) for L. guyanensis. For L. infantum, 4 was also the most potent among the tested analogues, with an IC50 of 0.70 μM. However, the best balance of activity−selectivity was exhibited by compound 2 (Selectivity Index [SI] between 8.76 and 52.49) with reduced toxicity (CC50 = 129.6 μM) as compared to 4. Hence, analogue 2 was tested against amastigotes, showing IC50 values of 31.4 μM (L. amazonensis) and 37.2 μM (L. infantum). Moreover, molecular docking studies showed a comparatively good correlation between the in vitro activity of this compound in three L. pathogens with UDP‐glucose pyrophosphorylase (r 2 > 0.71) as potential targets for antileishmaniasis derivatives.

Florence et al., inspired by the activity of chamuvarinin against Trypanosoma brucei, , synthesized a series of triazole‐containing compounds, essentially by replacing the tetrahydrofuran ring with a 1,4‐triazole. , More recently, in 2017, they further continued this study introducing (hetero)aromatic rings on the side chain of these molecules and testing them also against Leishmania major.

An SN2 reaction of 6 with 6‐([triisopropylsilyl]oxy)hexan‐1‐ol on the mesylated 5‐hexen‐1‐ol afforded the olefin 7. The (S)‐epoxide 8 was obtained by kinetic resolution, with Jacobsen salen catalyst, of the racemic epoxide, synthesized via classical epoxidation with m‐CPBA. Successive treatment with homoallyl magnesium bromide and epoxidation of the double bond, generated, through an in situ cyclization, both diastereoisomers of the alcohol 9, which were then separated by chromatography column. “Click” reaction of the azide derived from the syn product 9 with the alkyne 11, furnished, after TIPS deprotection, the key intermediate 12. The latter was then functionalized to obtain different compounds showing the same scaffold. In particular, a coupling reaction of the primary amine 13 with aryl carboxylic acids, allowed the generation of the final desired amides bearing various five‐membered heteroaromatic rings. In Scheme 2, the synthesis for the most active furan compound 15 is reported. This was tested against L. major exhibiting micromolar activity (EC50 = 7.8 μM) with outstanding selectivity on human cell line.

SCHEME 2

Reagents and conditions: (i) MsCl, Et3N, dry CH2Cl2, 0°C to rt, 3 h; (ii) 6‐([triisopropylsilyl]oxy)hexan‐1‐ol, NaH (60% in mineral oil), dry THF, reflux, overnight; (iii) m‐CPBA, CH2Cl2, 0°C to rt, overnight; (iv) (S,S)‐Co‐salen catalyst, AcOH, THF, H2O, 0°C to rt, 16 h; (v) 3‐butenylmagnesium bromide, CuI, dry THF, −78°C to rt, 2 h; (vi) m‐CPBA, CH2Cl2, 0°C to rt, 2 h, then (±)‐CSA, 0°C to rt, 5 h (syn alcohol by flash column chromatography); (vii) (PhO)2P(O)N3, DIAD, PPh3, Et3N, 0°C to rt, overnight; (viii) 11, CuSO4·5H2O, sodium ascorbate, t‐BuOH:H2O = 1:1, rt, 16 h; (ix) (±)‐CSA, CH2Cl2,MeOH, rt; (x) NaN3, DMF, 40°C, overnight; (xi) PPh3, THF, rt, overnight then H2O; (xii) 14, EDC hydrochloride, DMAP, CH2Cl2, rt, overnight

N'Da and coworkers recently reported the in vitro anti‐infective potential of artemisinin derivatives, which are biologically active through the generation of reactive oxygen species (ROS). Similarly, the antileishmanial antimonials owe their pharmacological effects to the generation of ROS, resulting in oxidative stress and ultimately in parasite death. Starting from dihydroartemisinin (DHA), the authors synthesized and tested a plethora of artemisinin‐acridine hybrids and non‐hemiacetal artemisinin derivatives. Focusing on the latter, using commercially available DHA 16, the key intermediate aldehyde 19 was synthesized and reduced to the corresponding primary alcohol, which is esterified with various acyl chlorides obtaining the desired contracted artemisinin ester analogues 21a‐d (Scheme 3). It is worth mentioning that the stereochemistry of the new stereocenter, generated by the contraction of the six‐membered ring, is not controlled, being the products isolated as a mixture of epimers.

SCHEME 3

Reagents and conditions: (i) BF3·Et2O, Et2O, 0°C to r.t, 24–48 h; (ii) Br2, acetone, H2O, rt, 3 h; (iii) Et3N, CH2Cl2, rt, 3 h; (iv) NaBH4, dry MeOH, 0°C to rt, 2 h; (v) acyl chloride [i.e., 4‐nitrobenzoyl chloride, 4‐fluorobenzoyl chloride, (1,1′‐biphenyl)‐4‐carbonyl chloride, 4‐methylthiophene‐2‐carbonyl chloride], Et3N, CH2Cl2 dry, 0°C, 1 h, then rt, 18 h

The Leishmania antipromastigote activity of artemisinin analogues were studied in vitro in L. donovani and L. major stains. These molecules possessed high intrinsic activities (IC50 ≤ 10 μM), without a toxic profile and a selective action (SI ≥ 10) toward Leishmania pathogens. Regarding L. donovani strain, the best result was achieved with the ester containing biphenyl moiety 21c (IC50 = 2.80 μM), which was as much as 30‐fold more potent than clinical artemisinins. This molecule was therefore indicated as suitable for further investigation as possible intracellular antiamastigote hit.

Among NPs, both noscapine, , , , a natural phthalideisoquinoline alkaloid, and bile acids, , , , natural steroids produced by mammals, have shown interesting antiparasitic activity.

Starting from these scaffolds, Salehi and coworkers semi‐synthesized novel isothiocyanate derivatives (Scheme 4), considering that the SCN moiety was suggested as an interesting pharmacophore in inhibiting the Trypanothione reductase. Despite the differences in the starting materials, the authors pointed out the necessity to install a primary amine moiety. In particular, through a three‐component Strecker reaction, α‐amino nitrile derivatives 23 of noscapine were synthesized under acidic conditions. Then, the nitrile group was transformed into an amide, which was converted into the crucial primary amine 25 by utilizing NaBH4 and BF3·Et2O. Later, treatment of these molecules with CS2, followed by the use of Boc2O and catalytic DMAP, afforded the desired isothiocyanate derivatives 26a‐e. Regarding the bile acids derivatives 30a‐d, an amidation, using NH4Cl and TBTU, allows the obtainment of products 28a‐d, and the isothiocyanate derivatives were obtained according to the methods described above, again passing through formation of a primary amine.

SCHEME 4

Reagents and conditions: (A) (i) R‐CHO, AcOH, rt, 30 min, then KCN, rt, TLC monitored; (ii) H2O2 (30% v/v), K2CO3, MeOH:DMSO = 10:1, rt, 5–24 h; (iii) NaBH4, BF3·EtO2, dry THF, −5°C, 1 h, then rt, overnight; (iv) 1. CS2, Et3N, EtOH, rt, TLC monitored; 2. Boc2O, DMAP, EtOH, −5°C, 5 min, then rt, 2 h. (B) (i) NH4Cl, TBTU, Et3N, CH3CN, rt, 1–3 h. (ii) NaBH4, BF3·EtO2, dry THF, −5°C, 1 h, then rt, overnight; (iii) 1. CS2, Et3N, EtOH, rt, TLC monitored; 2. Boc2O, DMAP, EtOH, −5°C, 5 min, then rt, 2 h

The isothiocyanate compounds reported in Scheme 4 demonstrated excellent antileishmanial activities against L. donovani compared to miltefosine. In detail, 26a, 26b, 26c, 26e, 30b, 30c displayed IC50 between 0.4–1.0 μM and SI values varying from 1.7 to 18.4. The best activity was shown by isothiocyanate bile acid 30c (IC50 = 0.4 μM), which was two‐fold more active than miltefosine (IC50 = 0.7 μM). Isothiocyanates with noscapine scaffold 26a, 26b, were the most selective with SI of 18.4 and 17.5, respectively. Importantly, the introduction of the isothiocyanate group is crucial for the antileishmanial activity; in fact, no activity was observed in compound 25a‐e.

An enantioselective, modular, and convergent strategy for the synthesis of aporphines, another class of interesting natural alkaloids with interesting biological activities, , , , , , has been reported by Anderson et al. as described in Scheme 5.

SCHEME 5

Reagents and conditions: (i) 32, EDC hydrochloride, HOBt, NMM, DMF, 0°C to rt, 4 h; (ii) 1. Tf2O, 2‐chloropyridine, DCM, −78°C to 0°C, 15 min. 2. RuCl[(R,R)‐TsDPEN](p‐cymene), Et3N/HCO2H (2:5), 0°C to rt, 10 h; (iii) (a) Boc2O, (i‐Pr2)EtN, DMAP, CH2Cl2, rt, 2 h (34a to 35a) or (b) MeOCOCl, (i‐Pr2)EtN, DMAP, CH2Cl2, rt, 10 h (34b to 35b); (iv) Pd (OAc)2, (t‐Bu)2PMeHBF4, K2CO3, DMA, 130°C; (v) (c) ZnBr2, CH2Cl2, rt, 8 h (36a to 37a) or (d) LiAlH4, THF, 0°C to rt, 20 h (36b to 37b)

Phenylethylamines 31a,b are obtained from veratraldehyde and piperonal, through the respective β‐nitrostyrenes. A coupling with 2‐(2‐bromo‐4,5‐dimethoxyphenyl)acetic acid 32 afforded the amides 33a,b. Here, a Bischler‐Napieralsky cyclodehydratation in presence of Tf2O and 2‐chloropyridine, afforded the bezyldihydroisoquiniles, which were directly reduced by Noyori asymmetric transfer hydrogenation (AHT), obtaining the tetrahydroisoquinolines 34a and 34b in good yields with 93% ee and 94% ee respectively (S isomer). The reaction foresees the use of RuCl(p‐cymene)[(R,R)‐Ts‐DPEN] as catalyst and formic acid as the hydrogen source.

The amine moieties of these chiral molecules were then protected and a direct palladium‐catalyzed ortho‐arylation was next achieved using Pd (OAc)2 with bis (tert‐butyl)methylphosphine obtaining the aporphine carbamates 36a,b in moderate to good yields. The authors pointed out how the nature of the ligand is crucial in achieving such good results. Subsequently, deprotection of the Boc‐protected amines or reduction of the methyl carbamate formed the tested products 37a,b.

Key steps for achieving the reported transformation are therefore the Bischler‐Napieralski cyclization/Noyori asymmetric reduction to construct the (S)‐tetrahydroisoquinoline isomers and the successive Pd‐catalyzed arylation for ring closure.

These aporphine alkaloids were tested versus amastigote L. infantum. This study reported a reduction of toxicity in murine conjunctive cells (NCTC) with norglaucine 37a (CC50 = 71.3 μM) related to miltefosine (CC50 = 119.7 μM). On the other hand, dicentrine 37b was more bioactive (EC50 = 10.5 μM) but more toxic than 37a.

Dimeric tryptophan NPs were proofed as medicinally important molecules. , Among these, spiroindimicins are a class of chlorinated indole alkaloids characterized by three heteroaromatic rings structured around a congested spirocyclic stereocenter. In 2021, Smith et al. reported the first total synthesis of (+)‐spiroindimicin A. The main challenge associated with this total synthesis was the construction of a core quaternary spirocenter in an enantiocontrolled fashion. Hence, a synthetic route of nine steps starting from the commercially available 4‐nitroindole 38 was developed (Scheme 6). This, through a series of transformations, including Suzuki and Stille couplings with 40 and 42, respectively, was converted to the key triaryl compound 43. Then, after a screening on ligands and reaction conditions, the authors were able to reach the desired Pd‐catalyzed asymmetric spirocyclization using the chiral phosphoramidate ligand 44 attaining product 45 with high enantioselectivity. It is worth noting that the Boc deprotection and spirocyclization could be conducted as a one‐pot procedure. Finally, removal of the protecting benzenesulfonyl group on the starting indole nitrogen led to the spiroindimicin A (46). S‐46 and R‐46 antiparasitic activity were evaluated in L. amazonesis. Notably, both enantiomers did not exhibit significant cytotoxicity in RAW cells (CC50 > 10 μM), with respect to the racemic mixture. Regarding efficacy,46‐(S) showed lower EC50 than 46‐(R) (1.3 and 5.3 μM, respectively).

SCHEME 6

Reagents and conditions: (i) PhSO2Cl, (i‐Pr)2EtN, CH3CN, 80°C, 5 h, then Zn, AcOH, CH2Cl2, 0°C to rt, 2 h; (ii) NCS, DMF, 0°C, 1 h, then rt, 1 h; (iii) NaNO2, aq. HCl, H2O, 0°C, 2 h, then KI, H2O, 0°C, 2 h; (iv) 40, Pd (OAc)2/SPhos, K2CO3, THF, H2O, 75°C, 6 h; (v) N‐iodosuccinimide, p‐TsOH, CH2Cl2, 40°C, 6 h; (vi) 42, Pd‐PEPPSI‐IPr, Cs2CO3, 4 Å molecular sieves, dioxane, 90°C, 14 h; (vii) I2, KOH, DMF, rt, 6 h; (viii) neat, 150°C, 3 h, under Ar, then [Pd (allyl)Cl]2/44, Ag2CO3, Cs2CO3, toluene, 70°C, 12 h; (xi) Bu4NOH, THF, 80°C, 8 h

AMINO ACID‐BASED COMPOUNDS

Amino acids are preferred reagents for the synthesis of chiral compounds, due to the already present stereogenic center. Starting from natural (L)‐ or unnatural (D)‐amino acids, chiral pool strategies can be then easily projected. Moreover, due to their biological function, their presence can facilitate the interaction against a specific site of different enzymes.

Moreover, as shown with (+)‐spiroindimicin A, indole containing compounds have been widely studied against leishmania for their antiparasitic activities. , Recently, Nogueira, Sousa and co‐workers studied the activity of indoles having a pyrazino[2,1‐b]quinazoline‐3,6‐dione functionality against Plasmodium falciparum, T. brucei, and L. infantum.

The synthesis of the most active compound 53 is reported in Scheme 7, and essentially followed a Mazurkiewicz–Ganesan approach. The reaction of 2‐aminobenzoic acid 47 with methyl D‐tryptophanate 48, followed by a two‐phase Schotten–Baumann reaction with D‐leucine chloride 50 yielded the tripeptides 51. The addition of a dehydrating agent (i.e., Ph3P) led to the oxazoles 52 and the desired syn product 53 was obtained by cyclization after Fmoc‐deprotection using piperidine. Interestingly, employing a microwave‐assisted one‐pot strategy, similar to that reported by Liu et al., only the anti‐enantiomer was isolated.

SCHEME 7

Reagents and conditions: (i) 48, CH3CN, TBTU, Et3N, rt, 5 h; (ii) 50, CH2Cl2/aq. Na2CO3, rt, 3 h; (iii) CH2Cl2 dry, Ph3P, I2, (i‐Pr2)EtN, rt, overnight; (iv) piperidine in CH2CH2, rt, 12 min, then CH3CN, DMAP, reflux 19 h

Concerning the antileishmanial activity, compound 53 could represent an interesting hit against L. infantum with an IC50 of 2.6 μM. Notably, its configuration (1R,4R) is crucial for the activity; indeed the other stereoisomers were not biologically active. Moreover, in silico studies emphasized these molecules as possible inhibitors for prolyl‐tRNA synthetase. In particular, the indole scaffold is fundamental for the polar interactions with Ser323. Importantly, the appropriate geometry and chirality are essential for the interaction of this enzyme due to the tight recesses of the target active site.

Fytas and co‐workers studied the antileishmanial activity of lipophilic conformationally constrained spiro carbocyclic 2,6‐diketopiperazine‐1‐acetohydroxamic acid analogues, since similar compounds, reported by them, showed antitrypanosome activity. Besides all the tested molecules, chiral acetohydroxamic acids bearing the adamantane scaffold were the most active. Strecker reaction on 2‐adamantanone 54 in presence of the selected α‐amino acid (i.e., L‐alanine, L‐leucine, L‐methionine, L‐phenylalanine, and D‐phenylalanine) and successive amidation with NH3 of the formed carboxylic acid, afforded compounds 55a‐c. The spyrane, containing the piperazindione scaffold 56a‐c, was formed by cyclization reaction, followed by nucleophilic substitution on the benzyl 2‐bromoacetate. The successive formation of the acid 57a‐c allowed the coupling with the O‐benzylhydroxylamine to form O‐benzyl hydroxamates 58a‐c. Hydrogenolysis of the latter compounds led to the acetohydroxamic acid analogues 59a‐c (Scheme 8). ,

SCHEME 8

Reagents and conditions: (i) NaCN, α‐amino acid alkyl ester hydrochloride, DMSO/H2O 29:1 (v/v), rt, 48 h; (ii) 1. H2SO4 97%, CH2Cl2, rt, 24 to 48 h. 2. ice and then aq. NH3 26% to pH 7–8; (iii) 1. (Me3Si)2NK, THF, 0–5°C, then rt, 1 h, under Ar. 2. benzyl 2‐bromoacetate, DMF, rt, 48 h, under Ar; (iv) H2/Pd‐C 10%, EtOH‐EtOAc 3:2 (v/v), 50 psi, rt, 3 h; (v) 1. CDI, THF, 28°C, 1 h, under Ar. 2. O‐benzylhydroxylamine hydrochloride, Et3N, 28°C, 25 h, under Ar; (vi) H2/Pd‐C 10%, EtOH, 50 psi, rt, 3 h

These compounds demonstrated important activities against L. infantum promastigotes and intracellular amastigotes, with low micromolar IC50 values. C‐isobutyl analogue 59a, was active versus L. infantum promastigotes with IC50 of 7.23 μM. Nevertheless, L. infantum amastigote is more sensitive to this compound; in fact, it possesses higher efficiency (2.23 μM). An interesting activity was also shown with cyclooctenone scaffold instead of adamantone for this analogue. The C‐benzylated analogues 59b and 59c and racemate were efficient growth inhibitors on both forms of L. infantum, with low micromolar activity. Regarding these latter compounds, the (S)‐enantiomer 59b was the most potent and selective against L. infantum with similar antipromastigote (IC50 = 2.67 μM) and antiamastigote (IC50 = 2.60 μM) activities, being 1.4‐and 1.7‐fold more active than the (R)‐enantiomer 59c. Remarkably, 59c and its racemate were the only derivatives active against L. donovani promastigote (IC50 = 8.35 μM and IC50 = 9.73 μM). 59c also displayed an activity for the amastigotes (IC50 = 15.0 μM). Hence, it appears that efficacy against L. donovani of the (R)‐stereochemistry is favored over the (S) one. Importantly, these molecules show very low cytotoxicity against mammalian cells (CC50 > 200 μM), with the exception of 59b (CC50 = 29.2 μM), implying significant selectivity.

Dihydrofolate reductase is considered a key target; therefore, based on their previous work, Rashid et al. investigated the ability of dihydropyrimidine‐5‐carboxamide and 5‐benzyl‐2,4‐diaminopyrimidine‐based derivates to inhibit this enzyme for L. major. Compounds 64a‐d were synthesized through a Biginelli approach, utilizing differently p‐substituted benzaldehydes (60a‐d), urea, and compound 61. This step allows the formation of the 2‐oxo‐3,4‐dihydropyrimidine‐5‐carboxylates 62a‐d, which are converted into the respective acyl chloride. N‐Acylation with L‐glutamic acid 63 led to expected compounds 64a‐d (Scheme 9A). In Scheme 9B, a coupling with the Boc‐protected 4‐aminobenzoic acid 65, via acyl chloride formation, with the amino acid methyl esters 66a‐d, is followed by Boc‐deprotection. The subsequent reaction with product 68a, followed by hydrolysis yielded 69a‐d. Similarly, the trimethoprim‐based derivatives 70a‐d were essentially obtained by N‐alkylation of the opportune amino acid methyl ester 66a‐d with 68b and successive hydrolysis (Scheme 9C) Finally, 68a,b were previously synthesized by reaction of the hydroxy trimethoprim derivative , , , with 1,2‐dibromoethane or bromoacetyl bromide respectively.

SCHEME 9

Reagents and conditions: (A) (reaction time monitored by TLC) (i) 61, (NH2)2CO, SnCl2·2H2O. CH3CN, reflux; (ii) SOCl2, DMF (1 drop), CH2Cl2, reflux; (iii) 63, NMM, DMF, reflux. (B) (i) 65, SOCl2, DMF (1 drop), CH2Cl2, reflux, 18 h, under N2; (ii) L‐amino acid methyl esters 66a‐d, Et3N, DCM, 4 h; (iii) TFA:H2O = 1:1, CH2Cl2, 0°C, 2 h; (iv) 68a, CH2Cl2, reflux, 16 h; (v) 1‐M NaOH, MeOH, rt, overnight; (vi) 1‐M HCl (to pH 2–3). (C) (i) L‐amino acid methyl esters 66a‐d, CH2Cl2, reflux, 16 h; (ii) 1‐M NaOH, MeOH, rt, overnight then 1‐M HCl (to pH 2)

L. major DHFR inhibition activities of these compounds were evaluated. All compounds (except 69b) showed low micromolar and submicromolar inhibition of lmDHFR, emerging 70d as the more potent, with the IC50 = 0.10 μM. In addition, selectivity for lmDHFR over human DHFR was also measured. Analogues 70a‐d exhibited outstanding results for lmDHFR, resulting compounds 70a (SI = 84.5) and 70b (SI = 87.5) the most selective. Regarding antipromastigote activity, 64a‐d showed low micromolar concentration. Compounds 69a‐d demonstrated superb inhibitory effect against L. major and L. donovani strains submicromolar concentration, the best result obtained with 69d (IC50 = 0.19 μM vs. L. major and IC50 = 0.11 μM vs. L. donovani). Unfortunately, compounds 70a‐d do not show high potency against these pathogens.

Thiohydantoins have emerged as an important class of compounds due to their biological activity, , , , , , , with particular focus on the antiparasitic behavior. , , , Wowk and Pavanelli explored the antileishmanial (L. amazonensis) effect, determining the key mechanism presented in parasite death, of various thiohydantoins bearing an acetyl group at the N1‐position of the heterocyclic ring. Five L‐amino acids 71 (i.e., tryptophan, glutamine, methionine, leucine, and phenylalanine), in the presence of an equimolar amount of ammonium thiocyanate, were treated with acetic anhydride, affording the desired products with moderate to excellent yields (Scheme 10).

SCHEME 10

Reagents and conditions: (i) NH4SCN, Ac2O, 100°C, 30 min

Regarding the activity, acetyl‐thiohydantoin 72a and 72b inhibited the proliferation of promastigote form with an IC50 of 8 and 6 μM, respectively. Moreover, both induced cell cycle arrest at the G2/M phase, a reduction in the cell volume of the parasite, and caused morphological and ultrastructural changes, such as rounded shapes, reduced cell body size, cell surface roughness, plasma membrane damage, cytoplasmic content leakage, mitochondrial swelling, irregular flagellation, and nuclei alteration. Thiohydantoins exercised an apoptosis‐like mechanism on promastigote cells due to an increasing ROS production, phosphatidylserine revelation, plasma membrane permeabilization, and a loss of mitochondrial membrane potential, causing an accretion of lipid bodies and the formation of autophagic vacuoles on the pathogen. In intracellular amastigotes, a decreased of number of infected macrophages by enhancing ROS formation and decreasing TNF‐α amount were observed. Additionally, low cytotoxicity was measured in human monocytes (THP‐1), murine macrophages (J774), and sheep erythrocytes. Finally, molecular docking analyses of acetyl‐thiohydantoins were carried out on two important targets of L. amazonensis: arginase and TNF‐α converting enzyme. The results proposed the acetyl group and the thiohydantoin as plausible pharmacophoric groups thanks to possible hydrogen bond interactions with amino acid residues at the active site of these enzymes.

Amino acid derived compounds have also been synthesized and tested by Haldar et al. , In particular, in 2018 they investigated the potential antileishmanial activity of triazole‐based peptides, since this functional group is present in a variety of drugs for infective diseases. , , , , , Reaction of 2‐nitroaniline 73 with phthalic anhydride, afforded the corresponding 1,3‐isoindolinedione derivative 74. Successive reduction of the nitro group, azide formation, and amine deprotection furnished the 2‐azidoaniline 75, which was converted into the triazole derivate 76 by click chemistry using propiolic acid. , The final amidation with the selected L‐amino acid methyl ester 66a,b (leucine, phenylalanine) in presence of DCC and HOBt led to the final triazole‐based hybrid peptide 77a,b (Scheme 11).

SCHEME 11

Reagents and conditions: (i) phthalic anhydride, 215°C, 2 h, then AcOH, reflux, 30 min; (ii) Fe powder, AcOH, H2O, acetone, reflux, 8 h; (iii) NaNO2, AcOH:H2O = 1:1, 0°C, 3 h, then NaN3, 0°C, 30 min; (iv) NH2NH2·H2O, MeOH, rt, 1 h, then 1‐M NaOH, rt; (v) sodium ascorbate, CuSO4, propiolic acid, EtOH:H2O = 1:1, rt, 12 h; (vi) 66a or 66b, DCC, HOBt, dry CH2Cl2, rt, 48 h

Regarding activity, 77a and 77b showed IC50 values of 11 μg/ml (33 μM) and 21.2 μg/ml (58 μM) on L. major promastigotes, respectively. The better activity of 77a could be explained by the different lipophilicity. In fact, 77a containing leucine moiety (which is more lipophilic than 77b) can better pass through the leishmania cell membrane. Moreover, it showed 2–6‐fold more potency than the standard antileishmanial drugs like sodium stibogluconate (IC50 > 64 μg/ml or 70 μM), ketoconazole (IC50 = 72 μg/ml or 135 μM), or pentastam (IC50 > 64 μg/ml or 70 μM). , In vitro MTT based cell viability assay indicated that this peptide even at 2 × IC50 concentration did not have any toxic effect on macrophage cell line J774. Morphological changes of leishmanial cells were studied upon treatment with peptide 77a using the FESEM experiment, showing its “static” action versus leishmania. Indeed, cells treated with 77a, due to inhibition of metabolic processes, endured substantial cellular stress that may not kill the parasite but stopped the normal growth in vitro and resulted in morphological abnormality (especially shorter size and rounded shape).

More recently, Haldar and coworkers reported the synthesis of the m‐nitrocinnamic peptide 82 (Scheme 12). The synthesis foresees the initial formation of the 3‐(3‐nitrophenyl)‐acrylic acid 80 through Knoevenagel condensation reaction between 3‐nitrobenzaldehyde 78 and malonic acid 79. Solution‐phase peptide synthesis between 80 and the dipeptide H2N‐L‐Leu‐Aib‐OMe 81, using DCC and HOBt as coupling agents, gave the final product 82.

SCHEME 12

Reagents and conditions: (i) 79, piperidine, DMF:H2O = 20:3, 120°C, 6 h; (ii) H2N‐L‐Leu‐Aib‐OMe 81, DCC, HOBt, Et3N, CH2Cl2 dry, 0°C to rt, 48 h

This peptide had exhibited considerable growth inhibition activity on L. major promastigotes. Its higher lipophilicity may increase the possibility of reaching this intracellular parasite. This molecule exhibited at least 2–6‐fold higher potency (IC50 = 13 μg/ml or 32 μM) than the above mentioned standard drugs. , Moreover, in vitro MTT based cell viability assay points out that it can destroy the Leishmania promastigotes at a very low dose without a substantial destructive effect on the human‐type macrophage cell line.

In 2017, sugar amino acid‐based (SAA) linear lipopeptide analogues, having hybrid sequences of natural amino acids (AAs), unnatural (4R,5S)‐4‐amino‐5‐methylheptanoic acid (AMH), and a mannose‐derived sugar amino acid (MAA), were synthesized and examined by Chakraborty et al. to identify potential drug candidates to treat VL. Besides all, MMA‐permethylated analogues bearing longer hydrophobic chains at the terminal nitrogen, without AMH units, gave the better results, and only their synthesis is described here in detail (Scheme 13).

SCHEME 13

Reagents and conditions: (i) TFA, CH2Cl2, 0°C to rt, 30 min; (ii) 85, HOBt, EDC hydrochloride, CH2Cl2, 0°C, 10 min, then DIPEA, rt, 12 h; (iii) LiOH·H2O, THF:MeOH:H2O = 3:1:1, 0°C to rt, 1 h; (iv) EtOCOCl, Et3N, aq. NH3, THF, −20°C to 0°C, 1.5 h; (v) Ag2O, MeI, DMF, 0°C to rt, 12 h

Coupling of N‐Boc‐6‐amino‐2,5‐anhydro‐3,4‐di‐O‐methyl‐6‐deoxy‐D‐mannonate 83 (obtained in 11 steps from D‐mannitol , ) with L‐phenylalanine methyl ester and, successively, with the Boc‐Val‐Val unit, was performed by standard SPPS method using EDC and HOBt as coupling agents, obtaining the tetrapeptide 84. Boc deprotection and coupling with various acids bearing lipophilic side chains 85a‐c, under the already described peptide coupling conditions, gave the compounds 86a‐c. Finally, compounds 88a‐c were obtained by hydrolysis of the terminal methyl ester and successive amidation, forming compounds 87a‐c, followed by N‐permethylation.

It was observed that the permethylated SAA 88a‐c were more active against intra‐macrophagic amastigotes of L. donovani than the unmethylated parents 87a‐c, probably due to their enhanced membrane permeability. Furthermore, NMR and SAR studies highlighted that N‐methyl groups were essential for hampering the formation of any turn structure resulting in their increased activities. Particularly, 88a‐c showed moderate IC50 (10.10, 10.92 and 13.63 μM, respectively) and SI comparable to miltefosine.

Velázquez and co‐workers, encouraged by the results they obtained with linear and lactam‐bridged 13‐residue peptides against L. infantum, recently synthesized all‐hydrocarbon stapled α‐helical analogues of these peptides. This series of peptides were designed to improve the L. infantum trypanothione reductase (Li‐TryR) enzyme inhibition activity, the proteolytic stability, and the cell permeability of correspondent linear peptides which target the dimerization interface of Li‐TryR. Indeed, trypanothione reductase is another well‐known target for antileishmanial agents, since it maintains the cellular redox homeostasis in leishmania. The analogues were synthesized on Rink Amide‐MBHA polystyrene resin following the standard Fmoc/ Bu solid‐phase orthogonal protection strategy (SPPS), essentially foreseeing cycles of Fmoc deprotections and coupling until the desired sequence of amino acids is installed. In particular, the introduction of two units of Fmoc‐(S)‐α‐methyl‐α‐pentenylglycine at the suitable positions of the sequence during the chain elongation , allowed the synthesis of the linear precursors 89a,b. The key step for the formation of the hydrocarbon stapled into compound 90‐a,b (Scheme 14) is the ring‐closing metathesis (RCM), performed on‐resin. A complete conversion can be achieved using a Grubb's second‐generation Ru catalyst under microwave irradiation (75°C, 1 h) with essentially no control at the stereochemistry of the double bond. Eventually, after Fmoc deprotection and N‐terminus acetylation, the cleavage of the peptide from the resin, afforded the stapled hydrocarbons 90a,b. A polyarginine (R9 CPP) carrier was linked to the N‐terminal end of 90a to facilitate the delivery of the peptides into the parasites (obtaining peptide 91). , ,

SCHEME 14

Schematic representation for the synthesis of all‐hydrocarbon stapled α‐helical peptides 90 and 91

These new peptides maintained potent inhibitory activity against Li‐TryR, enhancing proteolysis resistance. Interestingly, 90a,b inhibit oxidoreductase activity in a different way, by stabilizing the TryR homodimer. These peptides were not able to cross the cell membrane, but the covalent binding of peptide 90a with R9 CPP promoted intracellular uptake, turning this molecule into a highly active compound (91) against both promastigotes and amastigotes of L. infantum parasite.

Notably, leishmanicidal activity versus axenic amastigotes of [R9]‐[stapled‐peptide] conjugate 91 was comparable to miltefosine (IC50 = 2.14 μM vs. 2.0 μM).

NITROIMADAZOXAZOLE AND ‐OXAZINE DERIVATIVES

One of the most promising drug candidates for the treatment of VL is the dihydroimidazoxazole VL‐2098 (99), , , In vitro biological activity highlighted the submicromolar IC50 values of 0.03 μM against L. donovani. Importantly, VL‐2098 showed better activity (IC50 = 0.17 μM) than the corresponding (S)‐enantiomer (IC50 = 0.33 μM) and was 4.5‐fold more potent than racemic (IC50 = 0.77 μM) against L. infantum. In the mouse model, VL‐2098 was remarkably superior to the (S)‐enantiomer and better than racemic itself (83%, 8%, and 64% inhibition at 3.13 mg/kg, respectively), and this trend was in parallel with the microsomal stability data (0.07 μg/ml). A relative evaluation in the CDRI hamster model proved the greatest activity of the (R)‐enantiomer (99) over the (S)‐form and racemic mixture at all dose levels. Moreover, dose–response evaluations in a similar L. infantum‐infected hamster model at LMPH also expressed an excellent in vivo efficacy of 99 (>99% inhibition in liver, spleen, and bone marrow at 25 mg/kg).

Recently, Pati et al. developed a kilogram scale process for the synthesis of 99 (Scheme 15). The authors, to make the process scalable, focused their attention on reducing safety hazards and facilitating separations, employing the in situ synthesis of some intermediates without their isolations.

SCHEME 15

Reagents and conditions: (i) (‐)‐diisopropyl‐D‐tartrate, Ti (Oi‐Pr)4, CH2Cl2 dry, −30°C, 1 h, under N2, then anhydrous TBHP, dry CH2Cl2, −30°C, 1 h then −5°C, 1 h, under N2; (ii) 94, K2CO3, MeOH, 60°C, 22 h; (iii) p‐nitrobenzenesulfonyl chloride, Et3N, CH2Cl2, 0°C to 30°C, 3 h; (iv) 12.5‐M NaOH, CH2Cl2 15°C, 1 h; (v) 97, (i‐Pr)2EtN, 115°C, 2 h; (vi) K2CO3, DMF, 60°C, 6 h

They began to investigate a small‐scale approach (25 g). A Sharpless asymmetric epoxidation, using Ti (OiPr)4 together with D‐(‐)‐diisopropyl tartrate as chiral ligand, of the β‐methallyl alcohol 92, defines the (R)‐configuration of the stereogenic center formed leading the epoxide 93, which, without isolation, was allowed to react with p‐trifluoromethoxyphenol 94, affording the diol 95. Next, the latter was subjected to a regioselective sulfonylation reaction on the primary hydroxyl group, obtaining the corresponding nosylate, which underwent a base‐catalyzed ring closure reaction to achieve the key intermediate oxirane 96 in high enantiomeric excess.

Treatment with 2‐bromo‐4‐nitroimidazole 97 in the presence of DIPEA gave 98, followed by a further base‐catalyzed annulation, and afforded the desired compound 99. The authors, addressing some issues due to the scale‐up of the reaction (e.g., maintaining anhydrous conditions on the Sharpless epoxidation), were eventually able to produce approximately 10 kg of 99 through their synthesis.

Another recent synthetic pathway proposed by Singh and coworkers differs from the previous method essentially for the initial approach to intermediate 95 formation. First, allylation of 4‐trifluoromethoxy phenol with 2‐methylallyl chloride, furnished the 1‐[(2‐methylallyl)oxy]‐4‐(trifluoromethoxy)benzene, which was subjected to a Sharpless asymmetric dihydroxylation using AD mix‐α (containing the chiral ligand [DHQ]2PHAL), leading to the (R)‐enantiomer diol key derivate 95 in high yield. Here, as already discussed, subsequent epoxidation, via mesylation, followed by successive coupling with 2‐bromo‐4‐nitroimidazole and treatment with base, gave 99 in 36% overall yield.

Inspired by the structural analogy with 99 (VL‐2098), Thompson et al. recently reported the synthesis of several substituted 2‐nitroimidazoxazine and evaluated their biological activity. , , They fulfilled an exceptional task, testing over a hundred racemic and enantiopure compounds against different types of leishmania strains. The most promising enantiopure candidates identified by the authors are reported in Figure 2.

FIGURE 2

Most promising 2‐nitroimidazoxazine derivatives

Mainly, a chiral pool approach to introduce the desired absolute configuration at the stereogenic center was applied.

They initially focused their attention on the synthesis of methyl‐O‐diaryl substituted 2‐nitroimidazoxazine (100–102).

Starting from the 2‐bromo‐4‐nitroimidazole 97, the racemic acetated compound 108 was synthesized and separated in the two enantiomers using preparative chiral HPLC separation. Subjecting both 109 to a standard alkylation and a Suzuki coupling, the expected benzyl ethers (R)‐100and (S)‐100 were eventually formed (Scheme 16 A). To obtain compounds (R)‐101, (S)‐101, (R)‐102, and (S)‐102, a coupling, using the appropriate optical isomer 111 [synthesis of (R)‐enantiomer is showed in Scheme 16B] with 2‐chloro4‐nitroimidazole112 gave the chiral acetal product 113, which, after hydrolysis and tosylation, gave compound 114. This latter was converted into the respective enantiopure epoxide. Subsequent opening by 6‐bromopyridin‐3‐ol, ring closure and Suzuki coupling with appropriate arylboronic acids ArB (OH)2 gave the enantiopure desired product (R)‐101 and (R)‐101 (Scheme 16B). More recently, they reported the synthesis of new analogues (R)‐103, (S)‐103, (R)‐104, (S)‐104, (R)‐105, (R)‐105 using a similar synthetic approach to that proposed for the synthesis of (R)‐101 and (S)‐101. Even in this case, starting from an enantiopure protected alcohol, a coupling with 2‐chloro‐4‐nitroimidazole, followed by the epoxidation and ring closure takes place to form the 2‐nitroimidazoxazine scaffold, which was eventually functionalized at the CH2OH position with different halo (hetero)cycles.

SCHEME 16

Reagents and conditions: (A) (i) preparative chiral HPLC (Chiral Pak IA: Amylose derivatives tris(3,5‐dimethylphenylcarbamate); (ii) K2CO3, aq MeOH, 20°C, 4 h; (iii) 1‐bromo‐4‐(bromomethyl)benzene, NaH, DMF, 0°C to rt, 7 h; (iv) [4‐(trifluoromethoxy)phenyl] boronic acid, toluene, EtOH, 2‐M Na2CO3, Pd (dppf)Cl2, 90°C, 1 h, under N2. (B) (i) 112, K2CO3, DMF, 70°C, 19–72 h; (ii) 1‐M HCl, MeOH, 0°C, 6 h; (iii) TsCl, pyridine, −10°C to rt, 14 h; (iv) DBU, CH2Cl2, 0°C, 9 h; (v) 6‐bromopyridin‐3‐ol, K2CO3, methyl ethyl ketone, 80°C, 19–42 h; (vi) NaH, DMF, 0°C, 3 h; (vii) (4‐fluorophenyl) boronic acid or [4‐(trifluoromethoxy)phenyl] boronic acid, DMF, (toluene, EtOH), 2‐M Na2CO3, Pd (dppf)Cl2, 80°C, 4 h, under N2. The (S)‐enantiomer can be obtained starting from (S)‐4‐(2‐iodoethyl)‐2,2‐dimethyl‐1,3‐dioxolane. (C) (i) 4‐(trifluoromethoxy)phenol, DEAD, PPh3, THF, 0–20°C, 60 h; (ii) TBAF, THF, 20°C, 0.5–18 h; (iii) I2, PPh3, imidazole, CH2Cl2, 20°C, 12–35 h; (iv) 97, K2CO3, DMF, 90°C, 64–111 h; (v) DDQ, CH2Cl2, 20°C, 10–28 h, then TsOH, MeOH, 20°C, 12 h; (vi) NaH, DMF, 0–20°C, 0.25–5.5 h. (D) (i) 1‐fluoro‐4‐(trifluoromethyl)benzene, NaH, DMF, 0–20°C, 0.25–5.5 h

Moreover, they also synthesized a series of 2‐nitroimidazoxazines linked to the (hetero)aryl moiety in position 6 by an etheric bond. Among all these, an extensive structure–activity relationship investigation was reported for (R)‐106 and (R)‐107, the synthesis of which is reported in Scheme 16C,D.

A Mitsunobu reaction of 4‐(trifluoromethoxy)phenol with the orthogonally deprotected triol 117 afforded the key intermediate 118 bearing the terminal alcohols protected by two different functional groups. The selective deprotection of these functionalities, followed by coupling with 2‐bromo‐4‐nitroimidazole and ring closure, furnished the desired enantiomer (R)‐106.

From the enantiopure chiral alcohol 121 the compound (R)‐107 was obtained via NaH‐catalyzed SNAr displacement with 2‐fluoro‐5‐(trifluoromethyl)pyridine.

Regarding in vitro antileishmanial activity, excellent results were achieved (in most cases with submicromolar values) as shown in Table 1. All compounds reported were non‐toxic against human lung fibroblast MRC‐5 cells. Confronting the activities of the couple of enantiomers, it appears that (R)‐isomers were more potent than the relative S‐enantiomer in both leishmania strains (L. donovani and L. infantum). In addition, microsomal stability of these series was evaluated (Table 2): an excellent percentage of remaining parent compounds (after 1 h incubation) was observed for (R)‐106, (R)‐107, (R)‐100, (S)‐100, (R)‐105, and (S)‐105. Successively, in vivo (mouse) efficacy against L. donovani at different doses was screened (Table 2): remarkable percentages of inhibition at 12.5 mg/kg were obtained with (R)‐106 and (R)‐103 (81% and 98% respectively), the latter showing a much more potent effect than the corresponding enantiomer. Lowering the dose to 6.25 mg/kg, the best results were achieved with (R)‐101 and (S)‐101 (93% and 85%). Moreover, compound (R)‐102 exhibited extraordinary efficacy at 1.56 mg/kg (84%), higher than the (S)‐enantiomer (57%). The most interesting compounds were orally dosed in the early curative L. infantum hamster model and the results are reported in Figure 3. Best results were attained with compound (R)‐101, which inhibited L. infantum in liver, spleen, and bone marrow at 12.5 mg/kg dose (99.5%, 99.4%, and 96.8%, respectively). Notably, also at 6.25 mg/kg, (R)‐101 showed exclusive inhibitory activity (91.0%, 91.6%, and 73.3%), still being more effective than the corresponding (S)‐enantiomer. All these data clearly highlight how chirality is essential for the antileishmanial activity of this class of drugs.

TABLE 1

In vitro antileishmanial activities of nitroimidazoxazines

Compound	IC50 a (μm)		MRC‐5
	L. don	L. inf
(R)‐100	0.24	1.3	>64
(S)‐100	1.3	1.3	>64
(R)‐101	(0.03) b	0.080	>64
(S)‐101	(0.08) b	0.22	>64
(R)‐102		0.11	>64
(S)‐102		0.13	>64
(R)‐103	0.06	0.098	>64
(S)‐103	0.37	0.17	>64
(R)‐104		0.29	>64
(S)‐104		0.75	>64
(R)‐105	0.44	1.3	>64
(S)‐105	0.74	2.3	>64
(R)‐106	(0.19) b	0.53	>64
(R)‐107	(0.15) b	1.1	>64

IC50 values of grown inhibition of leishmania stains in mouse macrophages and cytotoxicity toward human lung fibroblast (MRC‐5).

LMPH data.

TABLE 2

Microsomal stability and in vivo antileishmanial effect values of nitroimidazoxazines

Compound	Microsomal Stability a (% remaining at 1 h)			In vivo efficacy versus L. don b (% inhibition at dose in mg/kg)
	H	M	Ham	50	25	12.5	6.25	3.13	1.56
(R)‐100	86	79
(S)‐100	86	59
(R)‐101	58	69	34				93
(S)‐101	63	41	5				85
(R)‐102	50	53						>99	84
(S)‐102	52	46						52	57
(R)‐103	63	65	1,9		>99	98	49	51
(S)‐103	50	36	1,2		66	42	56	18
(R)‐104	73	61	0,4		84
(S)‐104	70	52	1,8		38
(R)‐105	86	89	57		72
(S)‐105	89	90	55		46
(R)‐106	81	79	19	>99	>99	81	42
(R)‐107	90	92	48	>99			30

Pooled human (H), CD‐1 mouse (M), or hamster (Ham) liver microsomes.

Dosing was oral, 1 daily/5 days; values are the mean percentage reduction of pathogen burden in the liver.

FIGURE 3

In vivo efficacy in the L. infantum hamster model

CONCLUSION

The syntheses of new chiral compounds with interesting antileishmanial activity reported during the last 5 years (literature update from 2017) are summarized in this review. Interestingly, from our survey, all of these active molecules presented at least one (hetero)cyclic scaffold. The most promising compounds, in terms of IC50/EC50 against different leishmania strains, were divided into three chapters by chemical classification: NPs and their analogues, amino acid‐containing compounds (e.g., peptides), and nitroimidazoxazines or ‐oxazoles. The syntheses and the antileishmanial activities are discussed in detail for each class of compounds. As might be expected, the installation of the stereogenic center(s) of the active molecule was mainly obtained through a chiral pool approach, although the asymmetric synthesis (i.e., chiral catalysis) and, in some cases, chiral resolution and kinetic resolution are also reported. Interestingly, no syntheses using a chiral auxiliary approach were reported. All the compounds presented good to excellent activities against the promastigotes and amastigotes of several leishmania strains and, in almost all cases, they showed less cytotoxicity against human cell lines when compared to the commercial drugs.

Moreover, it is important to notice that, at least in most of the described articles, the use of an enantiopure molecule greatly enhanced its activity toward the parasite compared to its pure enantiomer or the corresponding racemic mixture. Therefore, it appears clear that the development of new strategies for the synthesis of enantiopure active molecules against this disease is still fundamental.