Design, Synthesis, Antifungal Activity and Molecular Docking of Thiochroman-4-one Derivatives

Abstract

N-Myristoyltransferase (NMT) has been validated pre-clinically as a target for treatment of fungal infections. Various substituted thiochroman-4-one derivatives have been synthesized by an efficient method. The synthesized compounds 7a–y and 8a–t were evaluated for their in vitro antifungal activity against the Canidia albicans, Cryptococcus neoformans, Epidermophyton floccosum, Mucor racemosus, Microsporum gypseum and Aspergillus nigerstrain. A series of compounds exhibited significant activity (minimal inhibitory concentrotion (MIC)=0.5–16 µg/mL) against Canidia albicans and Cryptococcus neoformans. The antifungal activity of compound 7b was reached to that of fluconazole, which can serve as a good starting point for further studies of structural diversity of the NMT inhibitors. The molecular docking studies revealed an interesting binding profile with very high receptor affinity for NMT of Canidia albicans.

The infection rate of fungi has increased significantly in the past decades.¹⁾ In fact, nasty infection of fungus has increased dramatically in recent years to become an important cause of morbidity and mortality in patients. Aspergillus and Candida spp. account for the majority of documented infections. Recent epidemiological trends indicate a shift towards infections by Aspergillus spp., non-albicans Candida spp. and previously uncommon fungi that often have diminished susceptibility to current antifungal agents.^2–6) Whereas, available antifungal drugs for such infections essentially have three molecular targets: 14α-demethylase (azoles), ergosterol (polyenes) and β-1,3-glucan synthase (echinocandins).⁷⁾ Unfortunately, none of them is ideal in terms of efficacy, antifungal spectrum or safety.⁸⁾ To overcome the deficiency of the current antifungal drugs and to obtain more high activity and low side-effect drugs, an antifungal drug having a novel mode of action should be developed.

Myristoyl-CoA : protein N-myristoyltransferase (NMT) is a cytosolic monomeric enzyme that catalyzes the transfer of the myristoyl group from myristoyl-CoA to the N-terminal glycine of a number of eukaryotic cellular and viral proteins.^9–12) NMT is a promising target enzyme^13,14) for the development of novel fungicidal drugs having a broad antifungal spectrum. Four kinds of NMT inhibitors have been reported (Fig. 1), peptidomimetics (SC58272),¹⁵⁾ benzothiazole (FTR1335),^16,17) tetrahydrocarbazole (compound 10c)¹⁸⁾ and benzofuran (R64).¹⁹⁾ Biological of R64 including quasi in vivo antifungal activity assay (IC₅₀=3.0 µM), in vitro antifungal activity assay (IC₅₀=0.12 µM), and CaNMT inhibitory activity (IC₅₀=0.0012 µM) were reported.¹⁹⁾ Structure of FTR1335 and R64 shares three important chemical elements (Fig. 1), a benzoheterocyclic core (B), a secondary amine at its side chain (A), and a hydrophobic group (C) through various linkers. According to these characteristics, Sheng et al.²⁰⁾ designed and synthesized a series NMT Inhibitors (compound 8f, Fig. 1).

Fig. 1. NMT Inhibitors

Thiochroman-4-one, which is heterocyclic compound containing sulfur atom has been reported to possess wide biological activity.^21–28) In an effort to investigate the importance of the benzoheterocyclic core on the antifungal activities, we designed and synthesized thiochroman-4-one derivatives as bioisostere of benzoheterocyclic NMT inhibitors. The double bond connection between ring B and ring C could the lost partial flexibility, but the molecular docking showed that the steric configuration of molecule is similar to R64 such as residues Phe240 produced Pi–Pi interaction with ring. Compared with R64 (C4-side chain), the location of side chain of thiochroman-4-one (C6-side chain) is different. It cannot be sure whether three or four carbons might be better for the chain length between hetero-cyclic core and amine side chain. Moreover, thiochroman-4-one with high-fat-soluble²⁹⁾ can easily cross cellular membranes and exert bioactivity. The structure–activity relationships (SARs) and molecular docking studies of the thiochroman-4-one derivatives were investigated.

Results and Discussion

Chemistry

According to the report,³⁰⁾ using the mercaptophenol (1) as the starting materials, the intermediate compound 3 (Chart 1) were synthesized via two steps. Compound 4 was obtained by aldol condensation of compound 3 with a substituted benzaldehyde. Compound 5 or 6 obtained by etherification reaction of 1,3-dibromopropane or 1,3-dibromobutane with compound 4.

Chart 1. Reagents and Conditions of the Synthetic Compounds

Reagents and conditions: (a) ClCH₂CH₂COOH, NaOH, H₂O, 100°C, 3h, 90–95%; (b) H₂SO₄, 12 h, 78–85%; (c) substituted benzaldehyde, KOH, EtOH, r.t., 65–85%; (d) 1,3-dibromopropane or 1,3-dibromobutane, K₂CO₃, 1,4-dioxane, reflux, 4–6 h, 25–30%; (e) benzylamine/phenethylamine/furfurylamine/morpholine/tetrahydroquinolines, KOH, 1,4-dioxane, 85°C, 6–9 h, 40–60%.

In the final, the preparation of compound 7a–y and 8a–t would be carried out by a suitable base. In order to study the optimum conditions of substitution reaction, taking (E)-3-benzylidene-6-(3-bromopropoxy)thiochroman-4-one (compound 7b) for instance, base, solvents and temperature were respectively investigated. As shown in Table 1, triethylamine as an organic weak base was not an efficient catalyst for this reaction, and the reaction was failed. Inorganic strong base such as NaOH or KOH can increase the yield to reach 40%. When the solvent was 1,4-dioxane, the yield was highest (51.2%). Optimum reaction temperature for the synthesis was 85°C and the yield was highest (58.3%). Compounds 7a–y, or 8a–t were obtained by treating compounds 5 or 6 with primary or secondary amine and KOH in 1,4-dioxane for 6–9 h at 85°C. The product was isolated and purified by silica gel column chromatography.

Table 1. The Effect of Different Conditions on Reaction e^a)

Entries	Base	Solvent	Temp (°C)	Time (h)	Isolated yield (%)
1	Et3N	Acetonitrile	70	—	—
2	K2CO3, KI	Acetonitrile	70	7.5	6.2
3	NaOH	Acetonitrile	70	8	40.3
4	KOH	Acetonitrile	70	8	46.8
5	NaH	Acetonitrile	70	6	11.5
6	KOH	Ethanol	70	—	—
7	KOH	Tetrahydrofuran	70	8	43.4
8	KOH	1,4-Dioxane	70	7.5	51.2
9	KOH	1,4-Dioxane	80	7.5	54.6
10	KOH	1,4-Dioxane	90	7	49.5
11	KOH	1,4-Dioxane	85	7.5	58.3

a) 3-Benzylidene-6-(3-bromopropoxy)thiochroman-4-one (1 mmol), phenethylamine (1.5 mmol), amount of base (200 mol%) and solvent (8 mL).

A nuclear Overhauser effect (NOE) enhancement (e.g. compound 7b) was observed (Fig. 2 where the arrow point) between the -S-CH₂-(δ_H 3.92) and -C=CH-(δ_H 8.22), indicating a trans-form double-bond configuration.

Fig. 2. NOESY Spectrum of Compound 7b

Biological Activity

The antifungal activity of each compound against Candida albicas (C. alb), Cryptococcus neoformans (C. neo) Mucor racemosa (M. rac), Microsporum gypseum (M. gyp), Epidermophyton floccosum (E. flo) and Aspergillus niger (A. nig) was expressed as the minimal inhibitory concentration (MIC). In vitro antifungal activity assay (Table 2) indicated that these compounds were more active against C. alb and C. neo than M. rac, M. gyp and E. flo. Compounds 8a–t (n=2) showed moderate antifungal activity against A. nig, while compound 7a–t (n=1) the marginal activity. In general, all compounds are more antifungal active against deep fungi than superficial except for E. flo. The compounds 7a–y (n=1) were more active than compounds 8a–t (n=2).

Table 2. Antifungal Activities of Compounds

Compd	R	R1	MIC (µg/mL)
			C. alb	C. neo	A. nig	M. rac	M. gyp	E. flo
7a	H	-NH-CH2-Ph	4	4	16	4	4	4
7b	H	-NH-CH2-CH2-Ph	0.5	1	>128	>128	8	16
7c	H	-NH-CH2-2-furan	4	8	>128	4	4	8
7d	H	-N-Morpholine	16	4	>128	>128	16	8
7e	3-CH2-O-CH2-4	-NH-CH2-Ph	2	2	>128	8	2	8
7f	3-CH2-O-CH2-4	-NH-CH2-CH2-Ph	8	8	>128	32	8	16
7g	3-CH2-O-CH2-4	-NH-CH2-2-furan	>128	32	>128	32	32	16
7h	3-CH2-O-CH2-4	-N-Morpholine	>128	32	>128	64	64	32
7i	3-CH2-O-CH2-4	-N-1,2,3,4-Tetrahydroquinoline	>128	>128	>128	>128	>128	32
7j	3,4-diOCH3	-NH-CH2-Ph	4	4	32	4	8	8
7k	3,4-diOCH3	-NH-CH2-CH2-Ph	4	4	>128	16	8	16
7l	3,4-diOCH3	-NH-CH2-2-furan	4	4	64	8	2	8
7m	3,4-diOCH3	-N-Morpholine	>128	16	>128	>128	128	32
7n	4-OCH3	-NH-CH2-Ph	4	16	>128	8	2	4
7o	4-OCH3	-NH-CH2-CH2-Ph	4	2	>128	8	8	8
7p	4-OCH3	-NH-CH2-2-furan	4	4	>128	8	8	4
7q	2,6-diCl	-NH-CH2-Ph	32	64	>128	64	>128	64
7r	2,6-diCl	-NH-CH2-CH2-Ph	16	32	>128	32	32	32
7s	2,6-diCl	-NH-CH2-2-furan	16	32	>128	32	64	32
7t	4-CH3	-NH-CH2-Ph	16	32	16	32	64	16
7u	4-CH3	-NH-CH2-CH2-Ph	64	>128	>128	>128	>128	>128
7v	4-CH3	-NH-CH2-2-furan	16	32	16	32	32	8
7w	4-F	-NH-CH2-Ph	16	64	>128	64	>128	2
7x	4-F	-NH-CH2-CH2-Ph	32	16	>128	32	>128	8
7y	4-F	-NH-CH2-2-furan	64	64	>128	64	64	8
8a	H	-NH-CH2-Ph	16	32	32	32	32	16
8b	H	-NH-CH2-CH2-Ph	16	>128	>128	>128	32	8
8c	H	-NH-CH2-2-furan	32	32	>128	32	64	64
8d	3-CH2-O-CH2-4	-NH-CH2-Ph	64	32	32	32	32	16
8e	3-CH2-O-CH2-4	-NH-CH2-CH2-Ph	32	32	>128	32	>128	4
8f	3-CH2-O-CH2-4	-NH-CH2-2-furan	64	128	>128	>128	>128	>128
8g	3,4-diOCH3	-NH-CH2-Ph	8	16	32	16	>128	16
8h	3,4-diOCH3	-NH-CH2-CH2-Ph	>128	>128	>128	>128	>128	>128
8i	3,4-diOCH3	-NH-CH2-2-furan	32	>128	>128	64	128	64
8j	4-OCH3	-NH-CH2-Ph	16	32	32	32	32	4
8k	4-OCH3	-NH-CH2-CH2-Ph	16	32	>128	32	16	32
8l	4-OCH3	-NH-CH2-2-furan	32	32	32	32	32	32
8m	2,6-diCl	-NH-CH2-2-furan	>128	>128	>128	>128	>128	>128
8n	2,6-diCl	-NH-CH2-CH2-Ph	16	32	>128	>128	>128	>128
8o	2,6-diCl	-NH-CH2-Ph	16	>128	>128	>128	>128	>128
8p	4-CH3	-NH-CH2-Ph	4	32	32	32	64	16
8q	4-CH3	-NH-CH2-CH2-Ph	8	8	16	16	16	2
8r	4-CH3	-NH-CH2-2-furan	>128	>128	>128	32	>128	16
8s	4-F	-NH-CH2-Ph	16	16	32	32	32	4
8t	4-F	-NH-CH2-CH2-Ph	4	8	16	32	32	4
Fcza)	—	—	16	32	>128	32	16	32
AmBb)	—	—	1	1	2	1	2	2

a) Fcz: Fluconazole, b) AmB: Amphotericin.

Compounds 7a–y (n=1) showed good potency inhibitor against C. alb and C. neo with MIC in the range of 0.5–16 µg/mL. For example, when the concentration of compound 7l was 4 µg/mL, C. alb was absolutely inhibited (Fig. 3c) as same as 1 µg/mL amphotericin B (Fig. 3d). Moreover, the antifungal activity of compound 7b against C. neo (MIC=1 µg/mL) and C. alb (MIC=0.5 µg/mL) had reached or exceeded the MIC of amphotericin B. As compared with C. alb, the activity of the compound 7a–y (n=1) against C. neo was decreased. To M. rac, compounds 7a, b, e, j, l, n, o and r still showed more active (MIC=4–8 µg/mL) than fluconazole but 2-fold less active than amphotericin B. Among these compounds, 8a–t (n=2) exhibited a moderate fungal activity (major MIC in the range of 32–128 µg/mL) against C. neo, A. nig, M. rac and M. gyp. As for C. alb, these compounds with major MIC in the range of 8–32 µg/mL were slightly better than other fungus. Remarkably, the activity of compound 8q against E. flo reached the level of amphotericin B (MIC=2 µg/mL). Interestingly, the activity of compound 9 against C. alb, A. nig and M. rac was lost, but it was meaningful antifungal activity against C. neo, M. gyp and E. flo. These results encouraged us to further research their antifungal mechanism in other ways.

Fig. 3. C. alb Was Inhibited in Different Concentrations by Compound 7l (a, 1 µg/mL; b, 2 µg/mL; c, 4 µg/mL) and Amphotericin B Was Used as Control (d, 1 µg/mL)

SARs of the Thiochroman-4-one Derivatives

In vitro antifungal estimate showed that when the benzofuran scaffold was replaced bythiochroman-4-one, the antifungal was retained. Various substituents on the C3-aromatic ring, C6-N-terminal linked groups and carbon in the C6-side, were investigated that various by representative antifungal activity of C. alb and C. neo.

The compounds 8a–t (n=2) were obvious less activity than compounds 7a–y (n=1). Antifungal activity of some compounds 8a–t (n=2) were lost, such as compounds 8k, m and r. Obviously, the length of the C6-side chain affect greatly on antifungal activity. But, the partial compounds 8a–t (n=2), like 8e, j, s, q and t, were more antifungal active against E. flo than compounds 7a–y (n=1). Compounds with electron-donating and hydrophobic group at the C3-phenyl group, such as -OCH₃, exhibited higher antifungal activity than those with electron-withdrawing groups such as -F and -Cl. Moreover, compounds 7j–m and 8g–i with di-OCH₃ substituents showed no advantage compared to -OCH₃ substituent. Among these electron-donating aryls, compounds 7b substituents showed the best antifungal activity (MIC=0.5 µg/mL). Replacing the benzyl and phenylethyl of C6-N-terminal with a morpholinyl (such as compounds 7d, h and m) and furfuryl (such as compounds 7g, p and s) resulted in an obvious decrease in activity. As for N-1,2,3,4-tetrahydroquinoline, compound 7i showed marginal antifungal activity. It suggested the C6-N-terminal nonheterocycle aromatic ring and secondary amine at its C6-side chain made a substantial contribution to the activity. Interestingly, the amine of compound 9 was lost, but the antifungal activity was partial retained against C. neo, M. gyp and E. flo. It showed that the thiochroman-4-one core with C6-side chain might be a key for antifungal selectivity.

Molecular Docking

For further understand the SARs observed in vitro antifungal activity assays, compounds 7b, e, 8t and original ligand R64 were selected to be performed molecular docking studies. The docking results were showed in Fig. 4 and Table 3. Obviously, compound 7b was absolutely embedded in NMT of C. alb. (CaNMT, PDB ID: 1IYL).¹³⁾

Fig. 4. Docking of Compounds 7b, e, 8t and Ligand R64

Table 3. Ligand–CaNMT Interactions of the Four Compounds

Compd	Binding free energy (kcal/mol)	Ligand efficiency	Interacting amino acids		Residues providing hydrophobic interaction or van der Waals contact without Pi–Pi interaction
			HBD	Pi–Pi
7b	−12.28	−0.40	Leu451, Tyr335, Tyr225	—	Leu450, Phe175, Phe117, Phe240, Phe339, Tyr119, Val390, Asn392, Val108, His227
7e	−11.88	−0.36	Leu451, Tyr354, Tyr107	A-Tyr107, B-Tyr354	Leu450, Phe117, Phe175, Phe176, Phe339, Tyr119, His227, Val390, Asn392, Val108, Tyr107
8t	−11.48	−0.32	Leu451	B-Tyr354, B-Tyr225	Asn392, Leu450, Phe117, Phe175, Phe176, Phe339, Phe240, Tyr119, Val390, Val108, His227
R64	−11.81	−0.35	Leu451, Tyr107, His227, Tyr119, Asn392	B-Tyr225, B-Phe240	Leu415, Leu450, Phe117, Phe175, Phe176, Phe339, Val390, Val108, His227, Tyr354

The thiochroman-4-one ring was located at the center of the active site, surrounded by some hydrophobic residues, such as Tyr225, His227 and Asn392. The compounds 7e and 8t formed Pi–Pi interaction with Tyr354 with benzene of ring B. The carbonyl of compounds 7b and e formed hydrogen bond interaction with residues Tyr225 and Tyr354, while compound 8t was non-interacting. Ring C of compounds 8t formed Pi–Pi interaction with residues Phe225. Ring A of compounds was surrounded by residues, such as Phe176, Tyr119, Val108 and Tyr107. Among them, compound 7b formed Pi–Pi interaction with residues Tyr107. The second amine group of all compounds produced hydrogen bond with the residue Leu451, which is an important functional residue in the catalytic cycle of CaNMT.¹³⁾ As for oxygen of chain between ring A and B, compounds 7b and e formed hydrogen bond interaction with residues Tyr335 and Tyr107, which agree with the binding free energy and antifungal activity.

The conformation of compounds 7b and 8t in the active site of CaNMT was similar to the original ligand R64, while compound 7e is different. As compared with original ligand R64, compound 7b has a C1-sulfur atom, which can form additional van der Waals interaction with CaNMT. The carbonyl group of ring B produced extra hydrogen bond interaction. As a result, although missing the nitrogen atom and two hydrogen bonds, compounds showed good binding free energy compared with R64.

It was worth mentioning that the docking data (Table 3) showed that the trend of the binding free energy was consistent with the antifungal activity of the compounds. It implied that the hydrogen and Pi–Pi bonding interactions between the ring 2 and CaNMT might be essential for locating the inhibitor to reasonable position and direction. It should be noted that the ligand efficiency of compounds 7b and 7e was better than original ligand R64 and might be a high-performance structure of NMT inhibitor. Overall, the molecular docking study could give a qualitative interpretation of the observed SARs in antifungal activity assay.

Conclusion

Overall, a series of novel thiochroman-4-one derivatives were designed and synthesized by an optimum reaction route. The importance of benzoheterocyclic core on the antifungal activity was investigated by the isostreric design of the thiochroman-4-one. In vitro antifungal activity assay indicated that the activity of thiochroman-4-one derivatives were good against four fungi but slightly lower than the benzofuran derivatives. The antifungal activity of compounds 7a–y (n=1) was better than compounds 8a–t (n=2). Compound 7b showed the best antifungal activity against C. alb (MIC=0.5 µg/mL) and C. neo (MIC=1 µg/mL), which was better than amphotericin B (MIC=1 µg /mL). The binding mode of the compounds 7b, e and 8t was indicated the importance of binding between these compounds and the residues (such as Leu451, Tyr354 and Try225) of CaNMT, and the binding model was consistent with original ligand R64. It clearly demonstrated that introduction of appropriate substituents on the C3-phenyl group of (E)-3-benzylidene-6-(3-(phenethylamino)propoxy)thiochroman-4 one (compound 7b) would lead to the more potent derivatives. It implied that compound 7b might be considered as new promising lead candidates for further design and synthesis of antifungal agents.

Experimental

General chemistry methods, synthesis procedures, spectral data, biological assays, molecular docking are given in Supplementary materials.

Acknowledgments

National Natural Science Foundation of China (21675039), Project funded by China Postdoctoral Science Foundation (2016M591401), Young Talent of Hebei Province, Hebei University Science Fund for Distinguished Young Scholars (2015JQ06) and Natural Science Foundation of Hebei Province (B2015201016). We gratefully acknowledge Dr. Carl LeBlond (Chemistry Department, College of Natural Sciences and Mathematics of IUP) for his helping for grammar and spelling of the paper.

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Materials

The online version of this article contains supplementary materials.

References

• 1) Garibotto F. M., Garro A. D., Masman M. F., Rodriguez A. M., Luiten P. G., Raimondi M. M., Zacchino S. A., Somlai C., Penke B., Enriz R. D., Bioorg. Med. Chem., 18, 158–167 (2010).
• 2) Groll A. H., Walsh T. J., Swiss Med. Wkly., 132, 303–311 (2002).
• 3) Groll A. H., Shah P. M., Mentzel C., Schneider M., Just-Nuebling G., Huebner K., J. Infect., 33, 23–32 (1996).
• 4) Viscoli C., Girmenia C., Marinus A., Collette L., Martino P., Vandercam B., Doyen C., Lebeau B., Spence D., Krcmery V., De Pauw B., Meunier F., Clin. Infect. Dis., 28, 1071–1079 (1999).
• 5) Wald A., Leisenring W., van Burik J. A., J. Infect. Dis., 175, 1459–1466 (1997).
• 6) Groll A. H., Walsh T. J., Clin. Microbiol. Infect., 7 (Suppl. 2), 8–24 (2001).
• 7) Georgopapadakou N. H., Expert Opin. Inv. Drug, 11, 1117–1125 (2002).
• 8) Masubuchi M., Ebiike H., Kawasaki K., Sogabe S., Morikami K., Shiratori Y., Tsujii S., Fujii T., Sakata K., Hayase M., Shindoh H., Aoki Y., Ohtsuka T., Shimma N., Bioorg. Med. Chem., 11, 4463–4478 (2003).
• 9) Sogabe S., Masubuchi M., Sakata K., Fukami T. A., Morikami K., Shiratori Y., Ebiike H., Kawasaki K., Aoki Y., Shimma N., D’Arcy A., Winkler F. K., Banner D. W., Ohtsuka T., Chem. Biol., 9, 1119–1128 (2002).
• 10) Rudnick D. A., McWherter C. A., Gokel G. W., Gordon J. I., Adv. Enzymol. Relat. Areas Mol. Biol., 67, 375–430 (1993).
• 11) Boutin J. A., Cell. Signal., 9, 15–35 (1997).
• 12) Knoll L. J., Johnson D. R., Bryant M. L., Gordon J. I., Methods Enzymol., 250, 405–435 (1995).
• 13) Sheng C., Zhang W. N., Curr. Med. Chem., 18, 733–766 (2011).
• 14) Zhao C., Ma S. T., ChemMedChem, 9, 2425–2437 (2014).
• 15) Devadas B., Sandra, K., Mark E., Hwang-Fun Lu., Srinivasan R., Nandini S., Jennifer K., David W., Charles A., Duttm V., Daniel P., Jeffrey I., James A., J. Med. Chem., 40, 2609–2625 (l997).
• 16) Yamazaki K., Kaneko Y., Suwa K., Ebara S., Nakazawa K., Yasuno K., Bioorg. Med. Chem., 13, 2509–2522 (2005).
• 17) Ebara S., Naito H., Nakazawa K., Ishii F., Nakamura M., Biol. Pharm. Bull., 28, 591–595 (2005).
• 18) Kawasaki K.-i., Masubuchi M., Morikami K., Sogabe S., Aoyama T., Ebiike H., Niizuma S., Hayase M., Fujii T., Sakata K., Shindoh H., Shiratori Y., Aoki Y., Ohtsuka T., Shimma N., Bioorg. Med. Chem. Lett., 13, 87–91 (2003).
• 19) Wang W. Y., Dong G. Q., Gu J. L., Zhang Y. Q., Wang S. Z., Zhu S. P., Liu Y., Miao Z. Y., Yao J. Z., Zhang W. N., Sheng C. Q., Med. Chem. Commun., 4, 353–362 (2013).
• 20) Sheng C., Xu H., Wang W., Cao Y., Dong G., Wang S., Che X., Ji H., Miao Z., Yao J., Zhang W., Eur. J. Med. Chem., 45, 3531–3540 (2010).
• 21) Hoettecke N., Rotzoll S., Albrecht U., Lalk M., Fischer C., Langer P., Bioorg. Med. Chem., 16, 10319–10325 (2008).
• 22) Bharkavi C., Kumar S. V., Ali M. A., Osman H., Muthusubramanian S., Perumal S., Bioorg. Med. Chem., 24, 5873–5883 (2016).
• 23) Liu Y., Luo W., Sun L., Guo C., Drug. Discov. Ther., 2, 216 (2008).
• 24) Song Y. L., Yang T., Dong Y. F., Wu F., Yang G. L., Chem. Lett., 43, 134–136 (2014).
• 25) Han X. Y., Zhong Y. F., Li S. B., Liang G. C., Zhou G., Wang X. K., Chen B. H., Song Y. L., Chem. Pharm. Bull., 64, 1411–1416 (2016).
• 26) Song Y. L., Liu X. M., Yang N., Yang G. L., Asian J. Chem., 25, 1849–1852 (2013).
• 27) Song Y. L., Wu F., Zhang C. C., Liang C. G., Zhou G., Yu J. J., Bioorg. Med. Chem. Lett., 25, 259–261 (2015).
• 28) Song Y. L., Dong Y. F., Yang T., Zhang C. C., Su L. M., Huang X., Zhang D. N., Yang G. L., Liu Y. X., Bioorg. Med. Chem., 21, 7624–7627 (2013).
• 29) Zhao Y., Li C., Suo H., Wang Y., Yang C., Ma Z., Liu Y., Glo. Adv. Res. J. Med. Med. Sci., 3, 240–250 (2014).
• 30) Bondock S., Metwally M. A., J. Sulfur Chem., 29, 623–653 (2008).