An efficient synthetic route to construct diverse pyrazole-based chalcones from 1-phenyl-1H-pyrazol-3-ols bearing a formyl or acetyl group on the C4 position of pyrazole ring, employing a base-catalysed Claisen-Schmidt condensation reaction, is described. Isomeric chalcones were further reacted with N-hydroxy-4-toluenesulfonamide and regioselective formation of 3,5-disubstituted 1,2-oxazoles was established. The novel pyrazole-chalcones and 1,2-oxazoles were characterized by an in-depth analysis of NMR spectral data, which were obtained through a combination of standard and advanced NMR spectroscopy techniques.
An efficient synthetic route to construct diverse pyrazole-based chalcones from 1-phenyl-1H-pyrazol-3-ols bearing a formyl or acetyl group on the C4 position of pyrazole ring, employing a base-catalysed Claisen-Schmidt condensation reaction, is described. Isomeric chalcones were further reacted with N-hydroxy-4-toluenesulfonamide and regioselective formation of 3,5-disubstituted 1,2-oxazoles was established. The novel pyrazole-chalcones and 1,2-oxazoles were characterized by an in-depth analysis of NMR spectral data, which were obtained through a combination of standard and advanced NMR spectroscopy techniques.
Chalcones (or 1,3-diaryl-2-propen-1-ones) are widely distributed in naturally occurring products produced by bacteria, fungi, and numerous plant species. Chalcones do not accumulate in natural sources and serve as intermediates for flavanoid biosynthesis [1,2,3]. Chalcone-rich sources are highly valued, as they possess beneficial biological properties [1]. For example, licochalcone A demonstrated antibacterial effects against B. subtilis, human pathogenic Mycobacteria and Legionella species [4,5] and inhibited the growth of both Leishmania major and Leshmania donovani promastigotes and amastigotes or P. falciparum strains [6,7]. Isobavachalcone showed antifungal effects against Candida albicans and Cryptococcus neoformans [8]. Moreover, xanthohumol demonstrated antiviral properties against bovine viral diarrhoea virus, HSV-1 (herpes simplex virus) and HSV-2, CMV (cytomegalovirus) [9], and coronaviruses [10] and showed anti-HIV-1 activity [11]. Xanthohumol caused a dose-dependent decrease in the growth of human breast cancer (MCF7) [12,13], colon cancer (HT-29) [14,15], ovarian cancer (A-2780) [16] and prostate cancer cells in vitro [17].The structural simplicity and therapeutic potential have motivated the design and development of synthetic chalcones with enhanced activity and potency [18].Chalcones are usually synthesized from aromatic aldehydes and aliphatic aldehydes or ketones via the Claisen–Schmidt condensation reaction in the presence of base or acid catalysts [19,20,21,22]. Other procedures were efficiently employed for the synthesis of chalcones, including the Pd-catalysed Suzuki cross-coupling reaction between the appropriate cinnamoyl chloride and phenylboronic acid or benzoyl chloride and phenylvinylboronic acid or Heck coupling reaction between aryl iodide and an unsaturated ketone [23,24,25]. The Wittig olefination reaction of triphenylbenzoylmethylene phosphoranes and benzaldehydes and the Julia–Kocienski olefination technique of heteroaryl-sulfonyl phenylethanones and benzaldehydes were also applied to give chalcones in efficient yields [26].Molecular modeling studies of chalcones using DFT methods indicated that 1,3-diaryl-2-propen-1-ones have two isomers, the (E)-isomer being thermodynamically more stable than the (Z)-isomer [27]. The (E)-chalcone derivatives are synthesized far more easily than the (Z)-isomer, and there have been only a few reports concerning the synthesis of the (Z)-isomers [28,29]. However, Yoshizawa et al. reported the synthesis of (Z)-chalcones from 1,3-diaryl-2-propynyl silyl ethers by a catalytic reaction using potassium tert-butoxide under acid treatment, in high yields and stereoselectivity [29]. Rajakumar et al. induced the photochemical E- to Z- isomerization of chalcone derivatives [30].Among the synthetic chalcone derivatives, heterocyclic chalcones are important for medicinal chemistry, as most biologically active chemical entities contain a heterocyclic scaffold [31,32]. For example, Pd(II) or Pt(II) complexes containing chalcone I displayed good anticancer and antimicrobial activities [33], while the structure of pyridine-chalcone derivative II was developed as a potential anti-tubulin agent, with antiproliferative activity against a panel of cancer cell lines (Figure 1) [34]. The anticancer activity was also reported for indolizinyl compound III, with the potential to induce the caspase-dependent apoptosis of human lymphoma cells [35] or quinoxalinyl derivative IV, which was active against MCF-7-cell lines [36], and thiophen-2-yl derivative V, which was active against colorectal carcinoma cells by causing apoptosis [37]. Chalcone VI showed remarkable inhibition potency against AChE and MAO-B enzymes and, therefore, can be further developed as a novel, phenothiazine-based, dual-targeting inhibitor for neurogenerative diseases [38], while compound VII acts as a tissue transglutaminase inhibitor [39]. Compound VIII and analogues were designed as promising anti-tubercular agents by combining in silico design, QSAR-driven virtual screening, synthesis, and experimental evaluation. The synthesized nitroaromatic chalcone derivatives were also active against Mycobacterium tuberculosis strains resistant to isoniazid or rifampicin [40]. In addition, indole-based chalcone IX was reported to act as a nonselective COX-1 and COX-2 inhibitor and showed anti-inflammatory and antioxidant activities in vivo [41]. Moreover, pyrazole-based chalcone derivatives X–XII were also synthesized and investigated. Compound X was evaluated for its anti-inflammatory activity, and compounds XI and XII showed potential activity as chemotherapeutic agents for the treatment of hepatocellular carcinoma (HCC), as they caused cell cycle arrest at the G2/M phase and induced apoptotic cell death [42,43].
Figure 1
Selected examples of heterocyclic chalcone derivatives for some biological applications.
Among heterocycles, pyrazoles are considered privileged scaffolds in medicinal chemistry [32]. Pyrazole derivatives are known to exhibit anti-inflammatory, analgesic, anticancer, antimicrobial, anti-infective and other activities [44,45,46,47]. In recent publications, we have reported the synthesis and antimitotic activity of 2,4- or 2,6-disubstituted- and 2,4,6-trisubstituted-2H-pyrazolo[4,3-c]pyridines [48], the antiproliferative activity of 2,4,6,7-tetrasubstituted-2H-pyrazolo[4,3-c]pyridines [49], the photodynamic properties in the human skin melanoma cell line G361 of pyrazole-indole hybrids [50] and N-aryl-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridin-7-amines [51] and the anthelmintic activity of benzopyrano[2,3-c]pyrazol-4(2H)-ones [52].In continuation of our interest in the efficient synthesis of pyrazole-containing polycyclic systems starting from easily accessible 3-hydroxy-1-phenyl-1H-pyrazole, we report herein the synthesis and structural elucidation of novel, diverse pyrazole-chalcone derivatives via the base-catalysed, Claisen–Schmidt condensation reaction of 4-formyl or 4-acetyl-1-phenyl-1H-pyrazol-3-ols and appropriate acetophenones or carbaldehydes.
2. Results and Discussion
The synthesis of various (E)-3-(3-alkoxy-1-phenyl-1H-pyrazol-4-yl)-1-phenylprop-2-en-1-ones 4a–k was carried out, as depicted in Scheme 1. It started with the easily accessible 1-phenyl-1H-pyrazol-3-ol 1, which was converted to 3-methoxy-, 3-propoxy-, 3-(2-methoxyethoxy)- and 3-benzyloxy-1-phenyl-1H-pyrazoles 2a–d and corresponding 4-carbaldehydes 3a–d via O-alkylation and Vilsmeier–Haack formylation procedures in a similar manner to what we described earlier [48,53,54,55,56]. The obtained compounds 3a–d were then subjected to a Claisen–Schmidt condensation reaction with variously 4′-substituted acetophenones in the presence of ethanolic sodium hydroxide. The heating reaction mixture at 55 °C for 30 min afforded chalcones 4a–k in fair to excellent yields (58–97%). A similar synthetic approach towards pyrazole-chalcones employing alcoholic NaOH-catalysed, Claisen–Schmidt condensation of 3-aryl-1H-pyrazole-4-carbaldehydes and acetophenones was also demonstrated by Aneja et al. in the course of pyrazolylpyrazolines [57] or by Baytas et al. for the preparation of 1,3-diarylpyrazoles [43].
Scheme 1
Reagents and conditions: (i) NaH, DMF (abs.), 0 °C; alkylhalide, 70 °C, 1 h; (ii) POCl3, DMF, −10 °C to 70 °C, 1 h; (iii) acetophenone, NaOH, EtOH, 55 °C, 30 min.
An in-depth analysis of NMR spectral data, which were obtained through a combination of standard and advanced NMR spectroscopy techniques, such as 1H-13C HMBC, 1H-13C HSQC, 1H-13C H2BC, 1H-15N HMBC, 1H-15N LR-HSQMBC, 1H-1H TOCSY, 1H-1H COSY, 1H-1H NOESY and 1,1-ADEQUATE experiments, provided the key information in the establishment of structural assignments and predominant configuration, due to conformations in a solvent of novel pyrazole-chalcones. The synthesis and biological activity of the related compounds has been reported in previous works, but no data on conformational analysis supported by NMR experiments were given [58,59,60,61].In the case of compound 4a, the key information for structure elucidation was obtained from the 1H-13C HMBC, 1H-13C H2BC, 1H-13C HSQC and 1H-15N LR-HSQMBC spectral data (Figure 2). For instance, the pyrazole 5-H proton (singlet, δ 7.96 ppm) was easily distinguished, as it exhibited not only long-range HSQMBC correlations with neighboring N-1 “pyrrole-like” (δ −183.7 ppm) and N-2 “pyridine-like” (δ −118.7 ppm) nitrogen atoms, but also HMBC correlations with the quaternary carbons C-3 (δ 163.3 ppm) and C-4 (δ 107.0 ppm), respectively. The 1H-13C HSQC spectrum indicated that the aforementioned proton had a one-bond connectivity with carbon C-5 (δ 129.0 ppm), thus completing our assignment of the 1H-pyrazol-4-yl moiety. Moreover, these findings unambiguously confirmed the connectivity with the neighboring 1-phenylprop-2-en-1-one fragment via long-range HMBC correlations of the olefinic Ha proton and the aforementioned pyrazole ring carbons. The E-configuration at the C=C double bond unequivocally follows from the magnitude of the vicinal coupling between the olefinic protons Ha (δ 7.74 ppm) and Hb (δ 7.60 ppm), which exhibited an AB-spin system and appeared as two sets of doublets (3JHa,Hb = 15.5 Hz). As expected, the 1H-13C HMBC spectrum revealed distinct, long-range correlations between these olefinic protons and the phenyl group 2″(6″)-H protons (δ 8.02–8.03 ppm), with the characteristic signal of a carbonyl carbon (δ 190.7 ppm). The 1H-1H NOESY spectrum of 4a further elucidated the connectivities based on through-space correlations. In this case, distinct NOEs were exhibited between the pyrazole ring proton 5-H and the olefinic proton Ha, while the phenyl group 2″(6″)-H protons displayed correlation with the olefinic proton Hb, thus allowing different structural fragments to be joined.
Figure 2
Relevant 1H-13C HMBC, 1H-13C H2BC, 1H-15N LR-HSQMBC and 1H-1H NOESY correlations and 1H NMR (italics), 13C NMR and 15N NMR (bold) chemical shifts of compound 4a.
The obtained (E)-3-[3-(benzyloxy)-1-phenyl-1H-pyrazol-4-yl]-1-phenylprop-2-en-1-ones 4g–i were further treated with trifluoroacetic acid for debenzylation. (E)-3-(3-Hydroxy-1-phenyl-1H-pyrazol-4-yl)-1-phenylprop-2-en-1-ones 5a–c were obtained in fair to good yields (63–83%), as outlined in Scheme 2. As expected, the cleavage of the OCH3 group (compound 5b) under the given conditions was not observed.
Scheme 2
Reagents and conditions: (i) TFA, toluene, rt, 24 h.
Furthermore, we investigated the applicability of pyrazol-3-ol 1 as a starting material for the synthesis of (E)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)-3-phenylprop-2-en-1-ones 8a–l (Scheme 3). Compound 1 was converted to pyrazol-3-yl acetate 6, which was further subjected to Fries rearrangement reaction conditions, as previously described [48,56]. The obtained 4-acyl-3-hydroxy-1-phenyl-1H-pyrazole 7 was reacted with various (het)aromatic carbaldehydes under the Claisen–Schmidt reaction conditions to form targeted chalcones 8a–l. Stirring the reaction mixture of pyrazole 7 and benzaldehyde in EtOH in the presence of NaOH at 55 °C [57] led to the formation of chalcone 8a with a 88% yield. The same reaction conditions were applied to synthesize a series of novel (2E)-1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)-3-phenylprop-2-en-1-ones 8b–l. Most chalcones were obtained in fair to excellent yields (58–95%); when 4-(dimethylamino)benzaldehyde, pyridine-3-, pyridine-4- and thiophene-2-carbaldehydes were utilized for the condensation, lower yields of the appropriate products 8f,h,i, and k were obtained (25–48%).
Scheme 3
Reagents and conditions: (i) Ac2O, 100 °C, 0.5 h in accordance with ref. [56]; (ii) AlCl3, CS2, reflux, 3 h in accordance with ref. [56]; (iii) appropriate carbaldehyde, NaOH, EtOH, 55 °C, 3–5 h; (iv) NaH, DMF (abs.), 0 °C; alkylhalide, 70 °C, 1 h.
To obtain (E)-1-(3-alkoxy-1-phenyl-1H-pyrazol-4-yl)-3-(het)arylprop-2-en-1-ones 9a–i, appropriate pyrazol-3-ols 8 were further O-alkylated using appropriate methyl-, propyl- and 2-methoxyethylhalides to produce alkoxy derivatives 9a–i in fair to excellent yields (40–96%).A representative 3-phenylprop-2-en-1-one 9a showed distinct, long-range correlations in the 1H-13C HMBC, 1H-13C H2BC and 1H-15N HMBC spectra, which in combination with the data from the 1,1-ADEQUATE experiment, allowed us to provide unambiguous assignments of the 1H, 13C and 15N NMR resonances (Figure 3). For example, the pyrazole 5-H proton (singlet, δ 8.42 ppm) exhibited not only long-range HMBC correlations throughout the 1H-pyrazol-4-yl moiety, but also a strong correlation with the most downfield 13C resonance, which confidently was assigned to a carbonyl carbon (δ 183.3 ppm). This finding, in combination with the data from the 1,1-ADEQUATE experiment, allowed the adjacent protonated carbon to be assigned to the signal at δ 124.3 ppm, which showed a sole correlation with the aforementioned carbonyl carbon. Moreover, the protonated carbon also shared a correlation with an adjacent olefinic carbon that resonated at δ 142.6 ppm. With this information, the 1H-13C HSQC spectral data were applied to identify olefinic protons Ha (δ 7.63 ppm) and Hb (δ 7.82 ppm), which exhibited an AB-spin system and appeared as two sets of doublets (3JHa,Hb = 15.7 Hz). The 1H-1H NOESY spectrum of 9a exhibited NOEs between the phenyl group 2″(6″)-H protons and both olefinic protons Ha and Hb, while proton Ha also had an NOE with a pyrazole 5-H proton, which confirms their proximity in space.
Figure 3
Relevant 1H-13C HMBC, 1H-13C H2BC, 1H-15N HMBC, 1H-1H NOESY and 1,1-ADEQUATE correlations and 1H NMR (italics), 13C NMR and 15N NMR (bold) chemical shifts of compound 9a.
To expand the structural diversity of the pyrazole-based, chalcone derivatives, we also employed Claisen–Schmidt reaction conditions for the synthesis of 1-(1-phenyl-1H-pyrazol-4-yl)-3-phenylprop-2-en-1-ones with pyridin-3-yl or pyridin-4-yl substituents on the third position of the pyrazole ring (Scheme 4). We have previously demonstrated that 1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)ethan-1-one can be efficiently converted to various 1-(3-aryl-1-phenyl-1H-pyrazol-4-yl)ethan-1-ones via O-triflation and Pd-catalysed Suzuki, Sonogashira or Heck reaction sequences [56]. In this research, 4-acetyl-1-phenyl-1H-pyrazol-3-yl trifluoromethanesulfonate 10 was subjected to a Pd-catalysed, Suzuki cross-coupling reaction with pyridin-3-yl and pyridin-4-yl boronic acids. When the cross-coupling reaction of triflate 10 and pyridin-3-yl boronic acid was performed under conventional heating in the presence of Pd(PPh3)4 and K3PO4 in refluxing dioxane, the starting materials decomposed. Refluxing the reaction mixture of coupling partners in EtOH in the presence of Pd(OAc)2 and Cs2CO3 led to the formation of product 11a with only a 20% yield. The best result of Suzuki cross-coupling was accomplished using Pd(PPh3)4 as a catalyst and Cs2CO3 as a base and by performing the reaction at 80 °C with a microwave irradiation potency of 150 W in EtOH. 1-[1-Phenyl-3-(pyridinyl)-1H-pyrazol-4-yl]ethan-1-ones 11a,b were obtained in 63–64% yields. The reaction was carried out in the presence of KBr, which is known to suppress triflate reduction by stabilizing the cationic (σ-aryl)-palladium transition state [62]. 4-Acetylpyrazoles 11a,b were further employed in the Claisen–Schmidt condensation reaction with different carbaldehydes. The reaction was performed under the above-described conditions in the presence of ethanolic NaOH at 55 °C. As a result, when 1-[1-phenyl-3-pyridinyl-1H-pyrazol-4-yl]ethan-1-ones 11a,b were reacted with benzaldehyde, 4-methyl- or 4-(trifluoromethoxy)benzaldehyde E- and Z-chalcones (12a–f and 13a–f, respectively) were obtained in fair to good total yields (51–70%). The NMR spectra of inseparable mixtures showed the presence of both isomers in different ratios, with a predominance of the E-isomer. In contrast, compounds 12g–j were obtained only as pure E-isomers. This latter observation can be explained by the strong electron-donor capacity of the 4-methyloxy group based on the resonance structure, which results in increased electron density of the enone moiety, while the electronegativity of the 4-trifluoromethyl group has a significance for decreased electron density of the enone moiety. It is known in most cases that the E-isomer is more stable from the perspective of thermodynamics, which makes it the predominant configuration among the chalcones [27].
Scheme 4
Reagents and conditions: (i) Tf2O, TEA, DCM, rt, 1 h in accordance with ref. [56]; (ii) pyridin-3- or pyridin-4-yl boronic acid, Pd(PPh3)4, Cs2CO3, KBr, EtOH, 80 °C, 150 W, 10 min; (iii) appropriate carbaldehyde, NaOH, EtOH, 55 °C, 10 min.
It is widely accepted that a large and constant difference in the magnitudes of the 3JHH coupling constants of the olefinic protons in E-Z isomers can be used for structural elucidation, which in our case were larger by approximately 3 Hz for the predominant E-isomer (15.6–15.7 Hz), while the minor Z-isomer provided significantly lower coupling constant values (12.7–12.8 Hz). Moreover, the 1D selective NOESY experimental data clearly showed that upon irradiation of the olefinic protons of the minor Z-isomer, the expected NOEs between them were observed. In the case of the major E-isomer, the olefinic protons exhibited only appropriate correlations with neighboring aromatic protons, therefore, unambiguously confirming the correct configuration (Figures S1–S3).As expected, the NMR spectral data of compound 12g revealed a distinct difference in chemical shifts in the 1H-pyrazol-4-yl moiety compared with the other series of pyrazolo-chalcones, due to the pyridin-3-yl substituent on the third position of the pyrazole ring (Figure 4). The key information for structure elucidation of the pyridin-3-yl moiety was obtained from the 1H-1H TOCSY spectrum. The results clearly showed a spin system of four protons, which were mostly downfield. Moreover, a comparison between the 1H-1H COSY spectra and the 1H-1H TOCSY spectra showed a complete absence of COSY cross-peaks between one of the protons, with the remainder from the aforementioned spin system. This finding strongly hinted at a neighboring quaternary carbon at site 3′′′, which was unambiguously assigned from 1,1-ADEQUATE spectral data, where the protonated pyridine carbons C-2′′′ (δ 149.0 ppm) and C-4′′′ (δ 135.9 ppm) showed a sole correlation with C-3′′′ at δ 127.5 ppm. The remainder of the protonated pyridine carbons were easily assigned from the appropriate correlations in the 1H-13C H2BC spectrum. The 3-(pyridinyl)-1H-pyrazol-4-yl heterocyclic system contains three nitrogen atoms. The chemical shifts of the N-1 and N-2 atoms of compound 12g were δ −161.8 and δ −78.2 ppm, respectively. The pyridin-3-yl substituent nitrogen resonated at δ −70.6 ppm.
Figure 4
Relevant 1H-13C HMBC, 1H-13C H2BC, 1H-15N LR-HSQMBC, 1H-1H NOESY and 1,1-ADEQUATE correlations and 1H NMR (italics), 13C NMR and 15N NMR (bold) chemical shifts of compound 12g.
Chalcones are versatile synthons for the synthesis of five- and six-membered nitrogen heterocycles, such as pyrazoles, pyrazolines, isoxazoles, isoxazolines, pyridines, pyrimidines, and others [63]. This was briefly demonstrated in this work by the treatment of compounds 4a and 9a with N-hydroxy-4-toluenesulfonamide (TsNHOH), in the presence of NaOH in EtOH/H2O (9:1 v/v) [64,65,66] (Scheme 5). When a chalcone 4a was used as a substrate, a regioselective formation of 3-(1H-pyrazol-4-yl)-5-phenyl-1,2-oxazole 14 was observed in 56% yield, while 5-(1H-pyrazol-4-yl)-3-phenyl-1,2-oxazole 15 was formed in a lower yield (36%), using chalcone 9a as a starting material. The efforts to obtain pyrazole-isoxazoles from chalcones using more prevalent reaction conditions reported in the literature [67,68], i.e., treating compound 9a with hydroxylamine in the presence of NaOH in MeOH/H2O (95/5 v/v), led to a mixture of 1,2-oxazoles 14 and 15 with a poor total yield of 23%. The formation of intermediate reaction products, such as isoxazolines or oximes, could be also identified by HPLC/MS data (HPLC data of crude reaction mixture are provided in Figure S194, followed by MS data in Figures S195–S198).
Scheme 5
Conversion of chalcones 4a and 9a to 1,2-oxazoles 14 and 15.
The regioselective formation of pyrazole-isoxazoles 14 and 15 was confirmed by NMR studies (Figure 5). As expected, the 1H, 13C and 15N NMR chemical shifts and the relevant correlations in the two-dimensional NMR spectra of these two isomeric 1,2-oxazoles were highly similar. The unambiguous formation of 1,2-oxazole (isoxazole) moiety was easily deduced from 1H-15N HMBC spectral data, as it clearly showed a distinct long-range correlation between the isoxazole methine 4′-H proton and nitrogen N-2′, which resonated at δ −18.6 and −19.6 ppm for compounds 15 and 14, respectively, and this is in good agreement with the data reported in the literature [69]. The 2 Hz optimized 1H-15N HMBC spectra hinted in favor of these structures. For instance, the conversion of chalcone 4a provided an 1,2-oxazole derivative, in which the pyrazole 5-H proton (singlet, δ 8.33 ppm) exhibited not only long-range HMBC correlations throughout the 1H-pyrazol-4-yl moiety, but also a weak correlation with the oxazole N-2′ nitrogen at δ −19.6 ppm was observed. Meanwhile, the 1,2-oxazole derivative obtained from chalcone 9a was assigned to structure 15, due to the correlation with the neighboring protons 2″(6″)-H (δ 7.88 ppm) from the phenyl moiety. The aforementioned protons from the pyrazole and phenyl moieties in the 1H-13C HMBC spectrum showed three-bond connectivities with the appropriate isoxazole quaternary carbons C-3′ and C-5′, which allowed us to confirm the correct structure assignments afterwards via the analysis of JCN couplings.
Figure 5
Relevant 1H-15C HMBC and 1H-15C HMBC correlations and 1H NMR (italics), 13C NMR and 15N NMR (bold) chemical shifts of compound 14, 15.
Then, in order to avoid any ambiguity in the structure assignment of regioisomeric 1,2-oxazoles, the 15N-labeled pyrazole-isoxazoles 16 and 17 were synthesized by analogy to 14 and 15. The treatment of chalcone 9a with 15N-hydroxylamine hydrochloride produced an inseparable mixture of regioisomers 16 and 17 in a ratio of about 8:1 (Scheme 6). The selective 15N-labeling in azaheterocycles is an important method for studying molecular structures, which significantly expands the possibilities of using standard NMR methods [70]. The 15N-labeled aromatic heterocyclic structures typically have well-resolved 1H-15N (JHN) and 13C-15N (JCN) coupling constants, including additional splitting of the corresponding signals in the standard proton decoupled 1D 13C NMR and 1D 1H NMR spectra [71,72].
Scheme 6
Synthesis of 15N labeled 1,2-oxazoles 16 and 17.
In the case of 15N-labeled pyrazole-isoxazoles 16 and 17, the analysis of 1H-15N (JHN) coupling constants 3JH4′-N2′ did not provide significant information regarding the correct structure confirmation, and were 1.23 Hz and 1.31 Hz for major and minor regioisomers, respectively. As expected, the unambiguous structure assignment of regioisomeric 1,2-oxazoles was achieved after a careful analysis of the 13C-15N (JCN) coupling constants, which were obtained from a 13C NMR spectrum. The 13C-15N spin–spin interaction was observed for the signals of the major regioisomer C-3′ (1JC3′-N2′ = 2.89 Hz), C-4′ (2JC4′-N2′ = 1.23 Hz) and C-5′ (2JC5′-N2′ = 1.39 Hz) from the 1,2-oxazole moiety, as well as the 2JCN and 3JCN couplings from the adjacent phenyl ring. The minor regioisomer provided similar data, where the 1JCN coupling constants were higher than 2JCN coupling constants, C-3′ (1JC3′-N2′ = 2.25 Hz), C-4′ (2JC4′-N2′ = 1.11 Hz), and C-5′ (2JC5′-N2′ = 1.52 Hz) in the 1,2-oxazole moiety, which is in good agreement with the data reported in the literature [73]. Moreover, the 2JCN and 3JCN couplings were observed for the signals from the pyrazole fragment. These 13C-15N spin–spin interactions with adjacent phenyl and pyrazole moieties were an additional criterion to confirm the final structures of the pyrazole-isoxazoles 16 and 17.
3. Materials and Methods
3.1. General Information
All starting materials were purchased from commercial suppliers and were used as received. Microwave reactions were conducted using a CEM Discover synthesis unit (CEM Corp., Matthews, NC, USA) and performed in glass vessels (capacity: 10 mL), sealed with a septum. The pressure was controlled by a load cell connected to the vessel. The temperature of the contents of the vessel was monitored using a calibrated infrared temperature controller, mounted under the reaction vessel. All experiments were performed with stirring. Flash column chromatography was performed on silica gel, 60 Å (230–400 µm, Merck). Thin-layer chromatography was carried out on silica gel plates (Merck Kieselgel 60 F254) and visualized by UV light (254 nm). The melting points were determined on a Büchi M-565 melting point apparatus (Büchi Labortechnik AG, Flawil, Switzerland) and were uncorrected. The IR spectra were recorded on a Bruker Vertex 70v FT-IR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) using neat samples or on a Bruker Tensor 27 (Bruker Optik GmbH, Ettlingen, Germany) spectrometer using KBr pellets and were reported in frequency of absorption (cm−1). Mass spectra were obtained on a Shimadzu LCMS-2020 (ESI+) spectrometer (Shimadzu Corporation, Kyoto, Japan). High-resolution mass spectra were measured on a Bruker MicrOTOF-Q III (ESI+) apparatus (Bruker Daltonik GmbH, Bremen, Germany). The 1H, 13C and 15N NMR spectra were recorded in CDCl3 or DMSO-d6 solutions at 25 °C on a Bruker Avance III 700 (700 MHz for 1H, 176 MHz for 13C, and 71 MHz for 15N) spectrometer (Bruker BioSpin AG, Fallanden, Switzerland), equipped with a 5 mm TCI 1H-13C/15N/D z-gradient cryoprobe and a Bruker Avance III 400 (400 MHz for 1H, 101 MHz for 13C, and 40 MHz for 15N) spectrometer (Bruker BioSpin AG), using a 5 mm directly detecting BBO probe. The chemical shifts (δ), expressed in ppm, were relative to tetramethylsilane (TMS). The 15N NMR spectra were referenced to neat, external nitromethane (coaxial capillary). Full and unambiguous assignment of the 1H, 13C and 15N NMR resonances was achieved using a combination of standard NMR spectroscopic techniques [74], such as DEPT, COSY, TOCSY, NOESY, ROESY, gs-HSQC, gs-HMBC, H2BC, LR-HSQMBC and 1,1-ADEQUATE experiments [75]. The following abbreviations are used in reporting NMR data: Ph, phenyl; Pz, pyrazole; Pyr, pyridine; Naph, naphtalene; Quin, quinoline; Th, thiophene; Ox, 1,2-oxazole. 1H-, 13C-, and 1H-15N HMBC NMR spectra, and HRMS data of the new compounds are provided in the Supplementary Materials as Figures S4–S193.
3.2. Chemistry
3.2.1. General Procedure for the Synthesis of 2b,c
To a solution of 1-phenyl-1H-pyrazol-3-ol (1) (320 mg, 2 mmol) in abs. DMF (20 mL), cooled to 0 °C under an inert atmosphere, NaH (60% dispersion in mineral oil, 80 mg, 2 mmol) was added portion wise [76]. After stirring the mixture for 15 min, iodopropane (for 2b) or 1-chloro-2-methoxyethane (for 2c) (2.4 mmol) was added dropwise. The reaction mixture was stirred at 60 °C for 1 h, poured into water (20 mL) and extracted with ethyl acetate (3 × 20 mL). The organic layers were combined, washed with brine, dried over Na2SO4, and filtrated, and the solvent was evaporated. The residue was purified by column chromatography (SiO2, eluent: ethyl acetate/n-hexane, 1:7, v/v) to produce pure 2b,c.
3.2.2. General Procedure for the Synthesis of 3b,c
Phosphoryl chloride (0.37 mL, 4 mmol) was added dropwise to DMF (0.31 mL, 4 mmol) at −10 °C. Then, 2b,c (1 mmol) was added to the Vilsmeier–Haack complex, and the reaction mixture was heated at 70 °C for 1 h. After the neutralization with 10% aq NaHCO3, the precipitate was filtered off and recrystallized from DCM to produce pure 3b,c.
3.2.3. General Procedure for the Synthesis of 4a–k
To a solution of appropriate 3-(alkyloxy)-1-phenyl-1H-pyrazole-4-carbaldehyde (3a–d) (1 mmol) in EtOH (96%, 5 mL), NaOH (0.2 g, 5 mmol) and appropriate acetophenone (1.1 mmol) were added. The reaction mixture was stirred at 55 °C for 30 min., cooled to room temperature, diluted with H2O, and extracted with EtOAc (3 × 10 mL). The organic layers were combined, dried with sodium sulphate, filtrated off and concentrated. The obtained residue was purified by column chromatography (SiO2, eluent: ethyl acetate/n-hexane, 1:6, v/v) to provide the desired product 4a–k.
3.2.4. General Procedure for the Synthesis of 5a–c
To a solution of 4g–j (1 mmol) in toluene (3 mL), TFA (3mL) was added. The reaction mixture was stirred at room temperature for 24 h. Toluene and trifluoroacetic acid were evaporated. The residue was recrystalized from ACN to produce pure 5a–c.
3.2.5. General Procedure for the Synthesis of 8a–l
To a solution of 1-(3-hydroxy-1-phenyl-1H-pyrazol-4-yl)ethan-1-one (7) (2.02 g, 10 mmol) in EtOH (96%, 80 mL), NaOH (2 g, 50 mmol) and appropriate aldehyde (20 mmol) were added. The reaction mixture was stirred at 55 °C for 3–5 h, cooled to room temperature and neutralized to pH = 7 using 6N HCl. The precipitate was filtered off, washed with water and cold ether and recrystallized from ACN to produce pure 8a–l.
3.2.6. General Procedure for the Synthesis of 9a–i
To a solution of an appropriate compound 8 (1 mmol) in abs. DMF (3 mL), NaH (60% suspension in mineral oil, 0.04 g, 1 mmol) and an appropriate alkylhalide (1.1 mmol) were added [76]. The reaction mixture was stirred at room temperature for 1–2 h, diluted with KHSO4 aq. (10 mL) and extracted with EtOAc (2 × 10 mL). The organic layers were combined, washed with H2O (4 × 20 mL), dried with sodium sulphate, filtrated off and concentrated. The residue was purified by column chromatography (SiO2, eliuent: hexane/ethyl acetate 9/1) to produce pure 9a–i.
3.2.7. General Procedure for the Preparation of 3-pyridinyl- and 4-pyridinyl-1-phenyl-1H-pyrazol-4-ylethanones (11a,b)
To the solution of triflate 10 (320 mg, 1 mmol) in abs. EtOH (2.5 mL), 4-pyridinyl- or 3-pyridinylboronic acid (123 mg, 1 mmol), Cs2CO3 (652 mg, 2 mmol), KBr (36 mg, 0.3 mmol) and Pd(PPh3)4 (116 mg, 0.1 mmol) were added, and the reaction mixture was irradiated (150 W) at 80 °C temperature for 10 min. EtOH was evaporated, the mixture was diluted with water (10 mL) and extracted with ethyl acetate (3 × 10 mL). The organic layers were combined, washed with brine, dried over Na2SO4, filtrated off, and the solvent was evaporated. The residue was purified by flash column chromatography (SiO2, eluent: ethyl acetate/n-hexane, 1:4, v/v) to provide the desired compounds 11a,b.
3.2.8. General Procedure for the Synthesis of 12a–j
To a solution of an appropriate pyrazole ethanone (11a,b) (50 mg, 0.19 mmol) in EtOH (96%, 2 mL), NaOH (75 mg, 1.9 mmol) and appropriate benzaldehyde (0.47 mmol) were added. The reaction mixture was heated at 55 °C for 10 min. After the completion of the reaction as monitored by TLC, EtOH was evaporated, the mixture was diluted with water (10 mL) and extracted with ethyl acetate (3 × 10 mL). The organic layers were combined, washed with brine, dried over sodium sulphate, filtrated off, and the solvent was evaporated. The residue was purified by flash column chromatography (SiO2, eluent: ethyl acetate/n-hexane, 1:2, v/v) to provide the desired compounds 12a–j.
3.2.9. General Procedure for the Preparation of 3-(3-methoxy-1-phenyl-1H-pyrazol-4-yl)-5-phenyl-1,2-oxazole (14) and 5-(3-methoxy-1-phenyl-1H-pyrazol-4-yl)-3-phenyl-1,2-oxazole (15)
To a solution of N-hydroxy-4-toluenesulfonamide (1.56g, 8.35 mmol) in EtOH/H2O (9:1 v/v, (25 mL)), NaOH (0.4 g, 10 mmol) was added. A solution of appropriate chalcone 4a or 9a (0.3 g, 1 mmol) in EtOH (3 mL) was added to a mixture. The reaction mixture was stirred at 40 °C for 48 h; the progress of the reaction was monitored by TLC. An additional amount of NaOH (0.4 g, 10 mmol) was added and the reaction mixture was stirred at 55 °C for another 24 h. Upon completion, the reaction mixture was diluted with EtOAc (30 mL). Then, the mixture was washed with water (3 × 30 mL) and brine (30 mL). The organic layers were dried over sodium sulphate, filtrated and concentrated. The residue was purified by column chromatografy (SiO2, eliuent: hexane/ethyl acetate, 3/1, v/v) to produce pure 14 or 15.
3.2.10. Procedure for the Preparation of Mixture of 3-(3-methoxy-1-phenyl-1H-pyrazol-4-yl)-5-phenyl-15N-1,2-oxazole (16) and 5-(3-methoxy-1-phenyl-1H-pyrazol-4-yl)-3-phenyl-15N-1,2-oxazole (17)
To a solution of 15N hydroxylamine hydrochloride (139 mg, 2 mmol) and potassium hydroxide (160 mg, 4 mmol) in EtOH (96%, 15 mL), chalcone 9a was added (304 mg, 1 mmol). The reaction mixture was stirred at 80 °C for 3 h, poured into water (30 mL) and extracted with EtOAc (3 × 50 mL). The organic layers were washed with brine (30 mL) and dried over sodium sulphate, filtrated and concentrated. The residue was purified by column chromatografy (SiO2, eliuent: hexane/ethyl acetate, 3/1, v/v) to produce an inseparable mixture of compounds 16 and 17 with a 23% yield. The inseparable mixture of regioisomers 16 (major) and 17 (minor) was obtained in a ratio of about 8:1.Yellow solid; yield 23% (73 mg mixture); 1H NMR (700 MHz, CDCl3): δH ppm 4.15 (s, 3H, CH3 of minor regioisomer), 4.16 (s, 3H, CH3 of major regioisomer), 6.77 (d, J = 1.23 Hz, 1H, Ox 4-H of major regioisomer), 6.93 (d, J = 1.31 Hz, 1H, Ox 4-H of minor regioisomer), 7.27–7.29 (m, 1H, NPh 4-H of both regioisomers), 7.45–7.49 (m, 5H, NPh 3,5-H, CPh 3,5-H, CPh 4-H of both regioisomers), 7.67–7.69 (m, 2H, NPh 2,6-H of both regioisomers), 7.84–7.86 (m, 1H, NPh 2,6-H of minor regioisomer), 7.87–7.89 (m, 2H, CPh 2,6-H of major regioisomer), 8.24 (s, 1H, Pz 5-H of major regioisomer), 8.33 (s, 1H, Pz 5-H of minor regioisomer). 13C NMR (176 MHz, CDCl3): δC ppm 56.62 (CH3 of minor regioisomer), 56.73 (CH3 of major regioisomer), 97.51 (d, 2J = 1.23 Hz, Ox C-4 of major regioisomer), 98.45 (d, 2J = 1.11 Hz, Ox C-4 of minor regioisomer), 98.82 (d, 2J = 8.40 Hz, Pz C-4 of minor regioisomer), 98.96 (d, 3J = 0.66 Hz, Pz C-4 of major regioisomer), 118.12 (NPh C-2,6 of minor regioisomer), 118.20 (NPh C-2,6 of major regioisomer), 125.53 (Pz C-5 of major regioisomer), 125.88 (CPh C-2,6 of minor regioisomer), 126.03 (d, 3J = 1.47 Hz, Pz C-5), 126.06 (NPh C-4 of minor regioisomer), 126.27 (NPh C-4 of major regioisomer), 126.91 (d, 3J = 2.28 Hz, CPh C-2,6 of major regioisomer), 127.59 (CPh C-1 of minor regioisomer), 128.85 (CPh C-3,5 of major regioisomer), 128.94 (NPh C-3,5 of minor regioisomer), 129.28 (d, 2J = 7.11 Hz, CPh C-1 of major regioisomer), 129.50 (CPh C-3,5 of minor regioisomer), 129.54 (NPh C-3,5 of major regioisomer), 129.89 (CPh C-4 of major regioisomer), 130.06 (CPh C-4 of minor regioisomer), 139.51 (NPh C-1 of major regioisomer), 139.68 (NPh C-1 of minor regioisomer), 155.35 (d, 1J = 2.25 Hz, Ox C-3 of minor regioisomer), 161.09 (Pz C-3 of major regioisomer), 162.27 (d, 3J = 1.92 Hz, Pz C-3 of minor regioisomer), 162.77 (d, 1J = 2.89 Hz, Ox C-3 of major regioisomer), 162.84 (d, 2J = 1.39 Hz, Ox C-5 of major regioisomer), 169.61 (d, 2J = 1.52 Hz, Ox C-5 of minor regioisomer). 15N NMR (71 MHz, CDCl3): δN ppm −184.8 (Pz N-1 of minor regioisomer), −184.5 (Pz N-1 of major regioisomer), −119.9 (Pz N-2 of minor regioisomer), −119.7 (Pz N-2 of major regioisomer), −19.5 (Ox N of minor regioisomer), −18.6 (Ox N of major regioisomer). HRMS (ESI+) for C19H1515NN2NaO2 ([M + Na]+) calcd 340.1056, found 340.1061.
4. Conclusions
In conclusion, we have synthesized novel diverse pyrazole-chalcone derivatives, starting with easily accessible 3-hydroxy-1-phenyl-1H-pyrazole via the Claisen–Schmidt condensation reaction of intermediate 4-acetyl or formyl-1-phenyl-1H-pyrazoles. The condensation of 4-formyl-1-phenyl-1H-pyrazoles with various acetophenones, as well as the reaction of 4-acetyl-3-hydroxy-1-phenyl-1H-pyrazoles with different carbaldehydes, produced appropriate (E)-chalcones. The reaction of 4-acetyl-3-pyridinyl-phenyl-1H-pyrazoles with acetophenones caused the formation of mixtures of compounds with predominant (E)-chalcones and less stable (Z)-chalcones in different ratios. Extensive NMR spectroscopic studies have been undertaken using standard and advanced methods to unambiguously determine the structure and configuration of novel, pyrazole-chalcone regioisomeric 3(5)-(1H-pyrazol-4-yl)-5(3)-phenyl-1,2-oxazole derivatives.
Authors: C L Miranda; J F Stevens; A Helmrich; M C Henderson; R J Rodriguez; Y H Yang; M L Deinzer; D W Barnes; D R Buhler Journal: Food Chem Toxicol Date: 1999-04 Impact factor: 6.023
Authors: Victor E Buckwold; Richard J H Wilson; Aysegul Nalca; Brigitte B Beer; Thomas G Voss; Jim A Turpin; Robert W Buckheit; Jiayi Wei; Michelle Wenzel-Mathers; Eve M Walton; Robert J Smith; Melanie Pallansch; Priscilla Ward; Jay Wells; Lara Chuvala; Sandra Sloane; Robin Paulman; Julie Russell; Tracy Hartman; Roger Ptak Journal: Antiviral Res Date: 2004-01 Impact factor: 5.970
Figure 1
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Scheme 6