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Abstract

New series of phenolic azomethine compounds in addition to 5-arylidene thiazolidinones are synthesized and screened for their anticancer activity against the brain cancer cell line SNB-75 and non-small lung cancer cells HOP-92. The azomethine derivative 12b is the most active compound against SNB-75 displaying an IC₅₀ value of 0.14 μM. Compounds 7b, 16a and 27d display submicromolar activity against the HOP-92 cell line with IC₅₀ values of 0.73, 0.74 and 0.81 μM, respectively. Moreover, studying the cytotoxic effects of the most active compounds against normal lung cells WI-38 revealed that compounds 7b, 16a and 27d showed high safety profiles as anticancer agents.

Keywords

anticancer azomethines phenolic compounds synthesis thiazolidinones

Introduction

Cancer is one of the most feared diseases worldwide affecting around 14 million people every year.¹ Specifically, lung cancer is a major cause of death with a survival rate of only about 18%.² Although, chemotherapy remains one of the most effective strategies among all cancer treatments, resistance to chemotherapeutic agents is the major challenge.³ Thus, the discovery of new antitumor agents with promising biological activity and safety profiles remains a necessity for scientific research.

In addition, it is well established that phenolic azomethine derivatives display biological activity as anticancer, antibacterial and anti-inflammatory agents.^4,5 In this context, many phenolic azomethine–based molecules featured with different aryl/heteroaryl moieties were reported to exert antitumor effects.⁴ Luo and co-workers reported the synthesis of salicylaldehyde-o-phenylenediamine Schiff base I, which displayed antitumor activity against the leukaemia cell lines K562 and HEL with IC₅₀ values of 11.95 and 9.75 μM, respectively (Figure 1).⁵Moreover, the phenolic azomethine derivatives II and III exhibited good anticancer activity against the colon cancer cell line HCT-116.^6,7

Figure 1.

Structures of some reported anticancer compounds bearing phenolic/2-hydroxy-1-naphthyl azomethines, and the design of the proposed target compounds.

Furthermore, there is evidence that azomethine derivatives obtained from the condensation of 2-hydroxy-1-naphthaldehyde (fused phenolic azomethines) showed promising antitumor activity.^8,9 In 2018, 1-[(4-fluorobenzylimino)methyl]naphthalene-2-ol (IV) was described by Devi et al.⁸ to exert antiproliferative activity against the alveolar adenocarcinoma A459, breast cancer MCF7 and prostate cancer DU145 cell lines (Figure 1). Moreover, cytotoxic studies showed that the phenolic azomethine derivative V elicited promising cytotoxic activity against five cell lines: HepG2, MGC80-3, T-24, SK-OV-3 and HL-7702.⁹ Lara and co-workers¹⁰ discovered the iminomethyl-2-hydroxy-1-naphthaldehyde derivative salermide VI that displayed anticancer activity against the leukaemia cell lines MOLT4 and KG1A.

Based on the aforementioned findings, the phenolic azomethine moiety was selected to build up the final target compounds. Therefore, two series with the phenolic azomethine moiety (scaffold A), namely, 2-hydroxy-1-phenylazomethines 3a, 7a,c, 12a, 16a, 20a and 24a and 2-hydroxy-1-naphthyl-azomethines 3b, 7b,d, 12b, 16b, 20b and 24b, were designed to compare the lipophilic effect on the obtained antiproliferative activity (Figure 1). The phenolic azomethine moiety was extended by a phenyl core linked to a pendent aryl group (either phenyl 3a,b, 7a–d, 16a,b, 20a,b and 24a,b or 4,6-dimethylpyrimidine (12a,b)) through different linkers to ensure variable degrees of flexibility between the phenyl core and the side chain.

Literature reviews showed that compounds containing a thiazole core, for example, compound VII, or a thiazolidinone core, for example, compound VIII, displayed potent antitumor activity against several cell lines (Figure 1).^11,12 Consequently, a new approach was adopted through bioisosteric replacement of the azaphenyl core in 7a–d with a thiazolidinone ring (scaffold B) as in compounds 27a,b,d,e. Finally, the phenolic moiety in the latter congeners was replaced by a 2-thienyl ring in 27c,f to prove the importance of the phenolic moiety on the cytotoxic activity.

The cytotoxic activity of all the target compounds was tested against the lung cancer cell line HOP-92 and the brain cancer cell line SNB-75. Subsequently, the growth inhibitory activity of the most active compounds against the human normal lung fibroblast cell line WI-38 was tested.

Results and discussion

Chemistry

The target compounds bearing a phenyl core 3a,b, 7a–d, 12a,b, 16a,b, 20a,b and 24a,b were synthesized adopting the chemical pathways outlined in Schemes 1 –5. According to the reported procedures, 3-aminobenzoic acid 1 was employed as a suitable precursor for the synthesis of the intermediate azomethines 2a,b.^13,14 The amide derivatives 3a,b were prepared by reacting the 3-aminobenzoic acid derivatives 2a,b with carbonyldiimidazole (CDI) in dry dichloromethane/dimethylformamide (DMF) at 0°C followed by reaction with benzylamine at room temperature (Scheme 1).

Scheme 1.

Synthetic pathway to compounds 3a,b. Reagents and conditions: (a) aldehyde, EtOH, glacial CH₃COOH, 70°C, 6 h, yield 77%–85%; (b) CDI, benzylamine, CH₂Cl₂/DMF, RT, 24 h, yield 52%–76%.

Scheme 2.

Synthetic pathway to compounds 7a–d and 12a,b. Reagents and conditions: (a) benzoyl chloride/phenylacetyl chloride, Et₃N, CH₂Cl₂, reflux, 5 h, yield 69%–86%; (b) SnCl₂·2H₂O, conc. HCl, absolute EtOH, RT, 48 h, yield 45%–80%; (c) appropriate aldehyde, glacial CH₃COOH, absolute EtOH, 70°C, 6 h, yield 65%–86%; (d) ClCH₂COCl, CH₂Cl₂, reflux, 5 h, yield 88%; and (e) anhydrous K₂CO₃, absolute EtOH, reflux, 6 h, yield 74%.

Scheme 3.

Synthetic pathway to compounds 16a,b. Reagents and conditions: (a) benzaldehyde, NaOH, EtOH, RT, 2 h, yield 92%; (b) SnCl₂·2H₂O, conc. HCl, absolute EtOH, reflux, 4 h, yield 56%; and (c) appropriate aldehyde, absolute EtOH, glacial CH₃COOH, 70°C, 12–15 h, yield 74%–79%.

Scheme 4.

Synthetic pathway to compounds 20a,b. Reagents and conditions: (a) benzylamine, triethylamine, CH₂Cl₂, RT, 2 h, yield 77%; (b) SnCl₂·2H₂O, absolute EtOH, RT, overnight, yield 58%; and (c) appropriate aldehyde, glacial CH₃COOH, absolute EtOH, 70°C, 5 h, yield 81%–87%.

Scheme 5.

Synthetic pathway to compounds 24a,b. Reagents and conditions: (a) PhNCO, CH₂Cl₂, Et₃N, RT, 4 h, yield 92%; (b) (COCl)₂/CH₂(COCl)₂, dioxane, RT, 48 h, yield 79%–83%; (c) SnCl₂·2H₂O, HCl, EtOH, reflux, 2 h, yield 20%–25%; and (d) appropriate aldehyde, absolute EtOH, glacial CH₃COOH, 70°C, 4 h, yield 58%–65%.

Starting from 3-nitroaniline (4), derivatives 6a,b were synthesized adopting methods described in the previous literature.^15,16 Synthesis of the required Schiff bases 7a–d was performed via reaction of the aryl amino intermediates 6a,b with the appropriate aldehyde in refluxing ethanol in the presence of a catalytic amount of glacial acetic acid (Scheme 2). Meanwhile, intermediate 10 was synthesized by heating the 2-mercapto-4,6-dimethylpyrimidine hydrochloride (9) in ethanol in the presence of potassium carbonate to liberate the free base, followed by the addition of chloro-acylated compound 8. Reduction of the nitro derivative 10 using SnCl₂/HCl afforded the corresponding amino counterpart 11, which in turn reacted with the appropriate aldehyde to yield the desired Schiff bases 12a,b (Scheme 2).

Moreover, derivatives 14 and 15 were synthesized from 3-nitroacetophenone (13) according to the reported methods.^16,17 The target azomethine derivatives 16a,b were prepared via the reaction of the aryl amino intermediate 15 as mentioned previously (Scheme 3).

To afford the 3-nitrobenzenesulfonamide derivative 18, 3-nitrobenzenesulfonyl chloride (17) was treated with benzylamine, followed by reduction with SnCl₂/HCl to yield the corresponding amino derivative 19. The target final compounds 20a,b were prepared by reacting compound 19 with the corresponding aldehydes in refluxing ethanol (Scheme 4).

In attempts to prepare new compounds with a rigidified linker between the phenyl core and the side chain, the intermediate imidazolidine-trione derivative 22a and pyrimidine-trione derivative 22b were prepared through the reaction of N,N′-disubstituted urea derivative 21 with oxalyl chloride or malonyl chloride in dioxane (Scheme 5). Upon reduction of the nitro group in intermediates 22a,b with SnCl₂/HCl, reductive cleavage of the heterocyclic rings occurs affording the urea derivative 23. The ¹H NMR spectrum of 23 revealed the presence of three exchangeable protons at 5.11, 8.36 and 8.54 ppm correlating with the protons of the primary aromatic amine and protons of the secondary amino groups of urea, respectively, confirming reductive cleavage of the heterocyclic ring of the intermediates 22a,b. Moreover, reduction of the nitro derivatives 22a,b with Fe/NH₄Cl or Fe/acetic acid did not afford the target amino derivatives. The final compounds 24a,b were obtained by the reaction of the amino derivative 23 with the appropriate aldehyde. The ¹H NMR spectra of the analogues 24a,b showed the presence of two singlets at 8.94, 9.66 and 13.08, 15.66 ppm corresponding to the azomethine protons and OH protons, respectively. Moreover, the ¹³C NMR spectra of compounds 24a,b showed signals at 159.2–160.7 and 163.8–171.1 corresponding to the C=N and C=O carbons, respectively, in addition to aromatic signals confirming their carbon skeletons.

The target compounds bearing a thiazolidinone core 27a–f were synthesized according to the pathway depicted in Scheme 6. According to the reported procedure, 2-amino-thiazolidin-4-one (25) was prepared by reacting thiourea with ethyl chloroacetate.¹⁸ Reaction of the precursor 25 with benzoyl chloride or phenylacetyl chloride afforded the intermediates 26a,b. Condensation of compounds 26a,b with different aldehydes gave the target analogues 27a–f. The structures of the target compounds were confirmed by elemental analysis and spectral data. ¹H NMR and ¹³C NMR spectra of some of the final compounds are given as supplementary materials.

Scheme 6.

Synthetic pathway to compounds 27a–f. Reagents and conditions: (a) appropriate acid chloride, pyridine, reflux, 4 h, yield 60%–78%; (b) appropriate aldehyde, absolute EtOH, CH₃COONa, reflux, 4 h, yield 62%–85%.

Biological evaluation

In vitro cytotoxic activity against SNB-75 and HOP-92 cell lines

The prepared compounds 3a,b, 7a–d, 12a,b, 16a,b, 20a,b, 24a,b and 27a–f were assayed in vitro for their cytotoxicity utilizing the MTT assay against the brain cancer cell line SNB-75 and the non-small lung cancer cell line HOP-92 using sorafenib as a positive control. The synthesized compounds can be classified into derivatives with a phenyl core 3a,b, 7a–d, 12a,b, 16a,b, 20a,b and 24a,b and derivatives with a thiazolidinone core 27a–f. As shown in Table 1, almost all the synthesized compounds exhibited mild to moderate cytotoxic activities against both cell lines. Moreover, compound 12b displayed submicromolar activity against the brain cancer cell line SNB-92. On the other hand, derivatives 7b, 16a and 27d displayed submicromolar activity against the non-small lung cancer cell line HOP-92.

Table 1.

Cytotoxic activity of the synthesized compounds (IC₅₀, μM) against the brain cancer cell line SNB-75 and the non-small lung cancer cell line HOP-92.


Compound	Ar	n	IC₅₀ (μM)
Compound	Ar	n	SNB-75	HOP-92
3a	2-HO-C₆H₄	0	2.86 ± 0.12	1.62 ± 0.03
3b	2-HO-C₁₀H₆	0	8.90 ± 0.38	5.11 ± 0.17
7a	2-HO-C₆H₄	0	1.56 ± 0.07	1.08 ± 0.09
7b	2-HO-C₁₀H₆	0	2.27 ± 0.13	0.73 ± 0.04
7c	2-HO-C₆H₄	1	7.09 ± 0.33	6.66 ± 0.33
7d	2-HO-C₁₀H₆	1	13.71 ± 0.82	2.91 ± 0.12
12a	2-HO-C₆H₄	0	25.46 ± 1.71	8.54 ± 0.39
12b	2-HO-C₁₀H₆	0	0.14 ± 0.02	5.69 ± 0.21
16a	2-HO-C₆H₄	0	4.89 ± 0.25	0.74 ± 0.04
16b	2-HO-C₁₀H₆	0	47.64 ± 2.81	1.97 ± 0.07
20a	2-HO-C₆H₄	0	3.25 ± 0.22	16.49 ± 0.95
20b	2-HO-C₁₀H₆	0	8.55 ± 0.61	3.45 ± 0.08
24a	2-HO-C₆H₄	0	4.73 ± 0.25	11.35 ± 0.05
24b	2-HO-C₁₀H₆	0	3.15 ± 0.15	3.98 ± 0.12
27a	2-HO-C₆H₄	0	1.59 ± 0.07	3.26 ± 0.19
27b	2-HO-C₁₀H₆	0	5.30 ± 0.19	6.38 ± 0.36
27c	2-Thienyl	0	8.02 ± 0.36	9.36 ± 0.47
27d	2-HO-C₆H₄	1	1.39 ± 0.07	0.81 ± 0.07
27e	2-HO-C₁₀H₆	1	2.32 ± 0.09	6.38 ± 0.25
27f	2-Thienyl	1	8.98 ± 0.25	14.31 ± 0.81
Sorafenib			14.86 ± 0.43	6.87 ± 0.41

Regarding the brain cancer cell line SNB-75, derivatives with a phenyl core, 3a,b, 7a–d, 12a,b, 16a,b, 20a,b and 24a,b, displayed a wide range of anticancer activity with IC₅₀ values ranging from 0.14 to >20 μM. Compounds 3a, 7a,b, 12b, 20a, 24b, 27a,d and 27e (IC₅₀ range = 0.14–3.25 μM) presented higher antiproliferative activity than the positive control sorafenib (IC₅₀ = 14.86 μM). In particular, compound 12b showed superior potency with an IC₅₀ value of 0.14 μM. With respect to the effect of the substitution pattern on the derivatives with a phenyl core, compounds 3a,b, 7a–d, 12a,b, 16a,b, 20a,b and 24a,b, and the 2-hydroxyphenyl derivatives 3a, 7a, 7c, 16a and 20a exhibited higher anticancer activity than the corresponding 2-hydroxynaphthyl derivatives 3b, 7b, 7d, 16b and 20b, except for derivatives bearing ureido linker 24a,b and compounds featuring a mercaptodimethylpyrimidine side chain 12a,b.

Dealing with the linker length, compounds 7a,b with an amide linker showed good anticancer activity with IC₅₀ values of 1.56 and 2.27 μM, respectively. Increasing the spacer length by attaching a benzyl group to the amide linkage as in 7c,d decreased the activity more than four times. Meanwhile, replacing the amide linkage in the derivatives 7c,d with a reversed amide linkage 3a,b or sulfamoyl linkage 20a,b resulted in compounds with comparable antiproliferative activity. Replacing the acetamide linkage in the derivatives 7c,d with a ureido linkage as in 24a,b increased the anticancer effect of the 2-hydroxynaphthyl derivative. Conversely, an α,β-unsaturated ketone linkage as in 16a,b resulted in a sharp decrease in the antiproliferative activity of the 2-hydroxynaphthyl derivative 16b.

Furthermore, the mercaptodimethylpyrimidine derivative 12b with the longest linker displayed superior anticancer activity against the SNB-92 cancer cell line with IC₅₀ = 0.14 μM. On the other hand, replacing the 2-hydroxynaphthyl moiety in 12b with a 2-hydroxyphenyl moiety as in the congener 12a (IC₅₀ = 25.46 μM) was detrimental to the antiproliferative activity.

Concerning the thiazolidinone derivatives 27a–f, compounds with an acetamide linker between the thiazolidinone ring and the pendent phenyl ring, that is, 27d–f, displayed comparable cytotoxic activity to the corresponding analogues with an amide linker (27a–c). The 2-hydroxyphenyl derivatives 27a,d displayed the highest anticancer activity with IC₅₀ values of 1.59 and 1.39 μM. Replacing the 2-hydroxyphenyl moiety in 27a,d with a 2-thienyl moiety as in compounds 27c,f decreased the antiproliferative activity, which highlights the impact of the 2-hydroxyphenyl moiety on the anticancer activity.

With regard to the non-small lung cancer cell line HOP-92, compounds with a phenyl core, 3a,b, 7a–d, 12a,b, 16a,b, 20a,b and 24a,b, displayed a wide range of anticancer activity with IC₅₀ values ranging from 0.73 to 16.49 μM. Compounds 3a, 7a,b, 16a and 27d (IC₅₀ = 0.74–1.62 μM) presented higher anticancer activity than the positive control sorafenib (IC₅₀ = 6.87 μM). In contrast with the SNB-75 cancer cell line, the 2-hydroxynaphthyl derivatives 7b, 7d, 12b, 20b and 24b showed higher antiproliferative activity than the corresponding 2-hydroxyphenyl derivatives 7a, 7c, 12a, 20a and 24a, with the exception of compounds bearing the reversed amide linkage (3a,b) and an α,β-unsaturated ketone linkage (16a,b).

Studying the effect of the spacer, derivatives 7a,b with an amide linker between the phenyl core and the pendent phenyl showed good anticancer activity with IC₅₀ values of 1.08 and 0.73 μM, respectively. Increasing the spacer length by attaching a benzyl group to the amide linkage (7c,d) decreased the antiproliferative activity by six- and fourfold, respectively. Meanwhile, replacing the acetamide linkage in the analogues 7c,d with a reversed acetamide linkage as in 3a,b or an α,β-unsaturated ketone linkage as in 16a,b resulted in an increase in the antiproliferative activity of the 2-hydroxyphenyl derivatives; whereas, replacing the acetamide linkage in the derivatives 7c,d with a urea linkage (24a,b), a sulfamoyl linkage (20a,b) or increasing the linker length as in the mercaptodimethylpyrimidine derivatives (12a,b) did not have a marked effect on the anticancer activity of the corresponding derivatives.

Concerning the compounds with a thiazolidinone core 27a–f, they displayed a wide range of anticancer activity with IC₅₀ values ranging from 0.81 to 14.31 μM. The 2-hydroxyphenyl derivatives 27a,d displayed the highest anticancer activity with IC₅₀ = 3.26 and 0.81 μM, whereas the thienyl analogues 27c,f exhibited the lowest antiproliferative activity (IC₅₀ = 9.36 and 14.31 μM). As for the influence of the linker length, replacing the amide linker in compounds 27a–c by an acetamide linker as in 27d–f afforded congeners with comparable activity except for the 2-hydroxyphenyl derivative 27d which showed enhanced cytotoxicity (IC₅₀ = 0.81 μM).

In vitro cytotoxic activity towards non-tumorigenic human WI-38 cells

The cytotoxic effects of the most active compounds 7b, 12b, 16a and 27d were examined against the non-tumorigenic human lung fibroblast cell line WI-38 to investigate their potential safety towards normal cells. The results in Table 2 showed that the tested compounds 7b, 12b, 16a and 27d exhibited non-significant cytotoxic impact towards the normal cell line WI-38. In addition, the derivatives 7b, 16a and 27d displayed good selectivity indices, thereby providing good safety as anticancer agents.

Table 2.

Cytotoxic activity of the most active compounds against normal WI-38 cells and their selectivity indices.

Compound	IC₅₀ (μM)		Selectivity index
Compound	WI-38	HOP-92	Selectivity index
7b	30.82 ± 1.72	0.73 ± 0.04	42
12b	21.88 ± 1.61	5.69 ± 0.21	4
16a	29.90 ± 2.04	0.74 ± 0.04	40
23d	24.55 ± 1.38	0.81 ± 0.07	30

Conclusion

In the present work, new series of 2-hydroxy-1-phenyl/2-hydroxy-1-naphthyl azomethine compounds 3a,b, 7a–d, 12a,b, 16a,b, 20a,b and 24a,b in addition to 5-arylidene thiazolidinones 27a–f were synthesized and screened for their anticancer activity against the brain cancer cell line SNB-75 and the non-small lung cancer cell line HOP-92. The target compounds 7b, 12b, 16a and 27d displayed submicromolar activity against both cell lines. Moreover, studying the cytotoxic effects of the most active compounds 7b, 16a and 27d against normal lung cells WI-38 showed good selectivity indices ranging from 30 to 42; thus, these compounds provide high safety profiles as anticancer agents.

Materials and methods

Chemistry

Starting materials and solvents were purchased from commercial suppliers and were used without further purification. Melting points were determined using an Electrothermal Stuart SMP3 digital melting point apparatus and are uncorrected. Elemental microanalyses were performed at the Regional Centre for Mycology and Biotechnology, Al-Azhar University. The infrared (IR) spectra were recorded on a Shimadzu FTIR 8400S, Faculty of Pharmacy, Cairo University. The proton nuclear magnetic resonance ¹H NMR spectra were performed in CDCl₃ or DMSO-d₆ using a Bruker, 400 MHz NMR spectrometer, Microanalytical Unit, Faculty of Pharmacy, Cairo University. ¹³C NMR spectra were recorded using a Bruker, 100 MHz NMR spectrometer, Microanalytical Unit, Faculty of Pharmacy, Cairo University. Mass spectra were obtained using a Finnigan SSQ 7000 GC/MS, at the Micro Analytical Centre, Faculty of Science, Cairo University. Reactions were followed by thin-layer chromatography (TLC), using silica gel TLC plates with a fluorescence indicator (F254) obtained from Merck and the spots were visualized using a Vilber Lourmat ultraviolet lamp at λͅ = 254 nm. The purity of the newly synthesized compounds was assessed by TLC. Compounds 2a,¹⁹ 2b,¹⁴ 5a,²⁰ 5b,²¹ 6a,¹⁶ 6b,²¹ 8,²² 9,²³ 14,²⁴ 15,¹⁶ 18,²⁵ 19,²⁵ 21,²⁶ 25¹⁸ and 26a²⁷ were prepared according to the reported procedures.

General procedure for the preparation of compounds 3a,b

A solution of the appropriate 3-aminobenzoic acid derivative 2a,b (2 mmol) in dry dichloromethane containing 5%–20% volume DMF was cooled to 0°C in an ice bath. CDI (0.49 g, 3 mmol) was added, and the mixture was stirred for 1 h. Next, benzylamine (0.65 mL, 6 mmol) was added, and the mixture was stirred at room temperature for 24 h. The reaction mixture was diluted with dichloromethane and washed with 1M HCl and brine, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure.

N-Benzyl-3-[(2-hydroxybenzylidene)amino]benzamide (3a): The product was separated as yellow crystals, m.p. 46–48°C, yield 0.34 g (52%). IR ν (cm⁻¹): 3396 (OH), 3059–3029 (CH aromatic), 2922 (CH aliphatic), 1631 (C=O), 1582 (C=N), 1496–1456 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 4.82 (s, 2H, CH₂), 6.88–6.94 (m, 3H, aromatic H), 7.28–7.40 (m, 9H, aromatic H and NH), 7.48 (d, J = 7.56 Hz, 2H, aromatic H), 8.72 (s, 1H, CH=N), 13.46 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 62.4 (CH₂), 116.9, 119.1, 119.2, 127.7, 128.3, 129.1, 132.2, 132.9, 139.1 (aromatic carbons), 161.0 (C=N), 167.0 (C=O). Anal. calcd for C₂₁H₁₈N₂O₂ (330.39): C, 76.34; H, 5.49; N, 8.48; found: C, 76.01; H, 5.63; N, 8.67%.

N-Benzyl-3-{[(2-hydroxynaphthalen-1-yl)methylidene]amino}benzamide (3b): The title compound was separated as yellow crystals and purified by preparative TLC using chloroform as the mobile phase, m.p. 73–75°C, yield 0.58 g (76%). IR ν (cm⁻¹): 3398 (OH), 3290 (NH), 3059–3034 (CH aromatic), 2922 (CH aliphatic), 1632 (C=O), 1614 (C=N), 1595–1541 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 4.89 (d, J = 3.6 Hz, 2H, CH₂), 6.77 (d, J = 9.32 Hz, 1H, aromatic H), 7.22 (t, J = 7.56 Hz, 1H, aromatic H), 7.32 (t, J = 6.60 Hz, 1H, aromatic H), 7.38–7.48 (m, 10H, aromatic H and NH), 7.66 (d, J = 7.72 Hz, 1H, aromatic H), 7.75 (d, J = 9.32 Hz, 1H, aromatic H), 8.14 (d, J = 8.36 Hz, 1H, aromatic H), 9.32 (s, 1H, CH=N), 14.42 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 55.4 (CH₂), 106.6, 119.1, 122.8, 125.5, 125.9, 128.2, 128.1, 128.4, 129.2, 129.4, 134.6, 137.6, 138.3 (aromatic carbons), 160.0 (C=N), 176.9 (C=O). MS (EI): m/z (%) = 380 (M⁺, 0.01), 91 (100). Anal. calcd for C₂₅H₂₀N₂O₂ (380.45): C, 78.93; H, 5.30; N, 7.36; found: C, 78.70; H, 5.41; N, 7.64%.

General procedure for the preparation of the Schiff bases 7a–d, 12a,b, 16a,b, 20a,b and 24a,b

A solution of the appropriate amine 6a,b, 11, 15, 19, 23 (2.35 mmol) in absolute ethanol (20 mL) and salicylaldehyde (0.28 mL, 2.59 mmol) or 2-hydroxy-1-naphthaldehyde (0.41 g, 2.35 mmol) was heated under reflux for 4–15 h at 70°C in the presence of a catalytic amount of glacial acetic acid. The reaction mixture was left to cool, the precipitated crystals were filtered and recrystallized from an appropriate solvent(s).

N-(3-((2-Hydroxybenzylidene)amino)phenyl)benzamide (7a): The title compound separated as yellow crystals, recrystallized from n-butanol, m.p. 175–177°C, yield 0.61 g (83%). IR ν (cm⁻¹): 3445 (OH), 3308 (NH), 3055–3032 (CH aromatic), 1647 (C=O), 1620 (C=N), 1595–1558 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 6.98–7.01 (m, 2H, aromatic H), 7.19 (d, J = 7.96 Hz, 1H, aromatic H), 7.42–7.47 (m, 2H, aromatic H), 7.54–7.62 (m, 3H, aromatic H), 7.69–7.74 (m, 2H, aromatic H), 7.88 (s, 1H, aromatic H), 7.99 (d, J = 7.36 Hz, 2H, aromatic H), 8.98 (s, 1H, CH=N), 10.38 (br s, 1H, NH, D₂O exchangeable), 13.06 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 114.1, 116.6, 117.1, 119.3, 119.70, 119.74, 128.1, 128.9, 130.1, 132.2, 133.0, 133.9, 135.2, 140.7, 148.9, 160.8 (aromatic carbons), 163.8 (C=N), 166.2 (C=O). MS (EI): m/z (%) = 316 (M⁺, 54.45), 105 (100). Anal. calcd for C₂₀H₁₆N₂O₂ (316.36): C, 75.93; H, 5.10; N, 8.86; found: C, 76.14; H, 5.23; N, 9.15%.

N-(3-(((2-Hydroxynaphthalen-1-yl)methylene)amino)phenyl)benzamide (7b): The product separated as yellow crystals, recrystallized from n-butanol, m.p. 221–223°C, yield 0.74 g (86%). IR ν (cm⁻¹): 3460 (OH), 3323 (NH), 1653 (C=O), 1622 (C=N), 1593–1531 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.03 (d, J = 9.20 Hz, 1H, aromatic H), 7.37 (t, J = 8.16 Hz, 1H, aromatic H), 7.49–7.63 (m, 6H, aromatic H), 7.76 (br s, 1H, aromatic H), 7.81 (d, J = 7.92 Hz, 1H, aromatic H), 7.94 (d, J = 9.16 Hz, 1H, aromatic H), 7.96 (s, 1H, aromatic H), 8.01 (d, J = 7.48 Hz, 2H, aromatic H), 8.48 (d, J = 8.44 Hz, 1H, aromatic H), 9.66 (s, 1H, CH=N), 10.40 (br s, 1H, NH, D₂O exchangeable), 15.78 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 108.9, 113.3, 115.4, 118.8, 120.6, 122.8, 124.0, 127.1, 128.1, 128.7, 128.9, 129.5, 130.4, 132.2, 133.6, 135.2, 137.6, 140.8, 144.5, 155.8 (aromatic carbons), 166.2 (C=N), 171.4 (C=O). MS (EI): m/z (%) = 366 (M⁺, 89.97), 59 (100). Anal. calcd for C₂₄H₁₈N₂O₂ (366.42): C, 78.67; H, 4.95; N, 7.65; found: C, 79.01; H, 5.12; N, 7.88%.

N-(3-((2-Hydroxybenzylidene)amino)phenyl)-2-phenylacetamide (7c): The title compound was obtained as yellow crystals, recrystallized from n-butanol, m.p. 195–197°C, yield 0.61 g (78%). IR ν (cm⁻¹): 3450 (OH), 3260 (NH), 3047–3026 (CH aromatic), 2993–2920 (CH aliphatic), 1661 (C=O), 1622 (C=N), 1589–1533 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 3.67 (s, 2H, CH₂), 6.97–7.00 (m, 2H, aromatic H), 7.12 (d, J = 7.84 Hz, 1H, aromatic H), 7.26 (t, J = 6.40 Hz, 1H, aromatic H), 7.32–7.44 (m, 6H, aromatic H), 7.49 (d, J = 8.20 Hz, 1H, aromatic H), 7.67–7.71 (m, 2H, aromatic H), 8.92 (s, 1H, CH=N), 10.32 (br s, 1H, NH, D₂O exchangeable), 13.02 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 43.8 (CH₂), 112.8, 116.2, 117.1, 118.0, 119.6, 119.7, 127.0, 128.8, 129.6, 130.2, 133.0, 133.8, 136.3, 140.7, 149.0, 160.7 (aromatic carbons), 163.8 (C=N), 169.8 (C=O). MS (EI): m/z (%) = 330 (M⁺, 100). Anal. calcd for C₂₁H₁₈N₂O₂ (330.39): C, 76.34; H, 5.49; N, 8.48; found: C, 76.49; H, 5.67; N, 8.67%.

N-(3-{[(2-Hydroxynaphthalen-1-yl)methylidene]amino}phenyl)-2-phenylacetamide (7d): The product was obtained as yellow crystals, recrystallized from n-butanol, m.p. 194–196°C, yield 0.72 g (80%). IR ν (cm⁻¹): 3441 (OH), 3287 (NH), 3055–3028 (CH aromatic), 2943 (CH aliphatic), 1662 (C=O), 1627 (C=N), 1589–1535 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 3.69 (s, 2H, CH₂), 7.01 (d, J = 9.16 Hz, 1H, aromatic H), 7.26–7.42 (m, 8H, aromatic H), 7.51–7.55 (m, 2H, aromatic H), 7.78–7.81 (m, 2H, aromatic H), 7.94 (d, J = 9.24 Hz, 1H, aromatic H), 8.45 (d, J = 8.48 Hz, 1H, aromatic H), 9.60 (s, 1H, CH=N), 10.35 (br s, 1H, NH, D₂O exchangeable), 15.71 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 43.8 (CH₂), 108.8, 112.0, 115.0, 117.6, 120.5, 122.8, 124.0, 127.0, 127.1, 128.71, 128.74, 128.8, 129.5, 129.6, 130.5, 133.5, 136.2, 137.6, 140.8, 144.5, 155.7 (aromatic carbons), 170.0 (C=N), 171.6 (C=O). MS (EI): m/z (%) = 380 (M⁺, 100). Anal. calcd for C₂₅H₂₀N₂O₂ (380.45): C, 78.93; H, 5.30; N, 7.36; found: C, 78.70; H, 5.49; N, 7.68%.

2-((4,6-Dimethylpyrimidin-2-yl)thio)-N-(3-((2-hydroxybenzylidene)amino)phenyl) acetamide (12a): The title compound separated as yellow crystals, crystallized from methanol, m.p. 139–140°C, yield 0.60 g (65%). IR ν (cm⁻¹): 3479 (OH), 3263 (NH), 3082 (CH aromatic), 2970 (CH aliphatic), 1666 (C=O), 1605 (C=N), 1577–1543 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 2.34 (s, 6H, 2 CH₃), 4.07 (s, 2H, CH₂), 6.96–7.00 (m, 3H, aromatic H), 7.13 (d, J = 7.72 Hz, 1H, aromatic H), 7.38–7.47 (m, 3H, aromatic H), 7.67–7.70 (m, 2H, aromatic H), 8.92 (s, 1H, CH=N), 10.39 (br s, 1H, NH, D₂O exchangeable), 13.00 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 23.8 (2 CH₃), 36.0 (CH₂), 112.9, 116.2, 116.5, 117.1, 118.0, 119.6, 119.7, 130.3, 133.0, 133.8, 140.5, 149.0, 160.7, 163.8, 167.3, 167.4 (aromatic carbons), 169.7 (C=N), 192.0 (C=O). MS (EI): m/z (%) = 392 (M⁺, 44.38), 181 (100). Anal. calcd for C₂₁H₂₀N₄O₂S (392.48): C, 64.27; H, 5.14; N, 14.28; found: C, 64.49; H, 5.26; N, 14.49%.

2-((4,6-Dimethylpyrimidin-2-yl)thio)-N-(3-(((2-hydroxynaphthalen-1-yl)methylene) amino)phenyl)acetamide (12b): The obtained product separated as yellow crystals, crystalized from n-butanol, m.p. 204–206°C, yield 0.70 g (67%). IR ν (cm⁻¹): 3444 (OH), 3294 (NH), 3059–3032 (CH aromatic), 2997–2920 (CH aliphatic), 1666 (C=O), 1620 (C=N), 1578–1535 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 2.35 (s, 6H, CH₃), 4.09 (s, 2H, CH₂), 6.98 (s, 1H, aromatic H), 7.01 (d, J = 9.20 Hz, 1H, aromatic H), 7.34 (t, J = 7.20 Hz, 1H, aromatic H), 7.44–7.47 (m, 3H, aromatic H), 7.55 (t, J = 7.08 Hz, 1H, aromatic H), 7.76 (br s, 1H, aromatic H), 7.80 (d, J = 7.48 Hz, 1H, aromatic H), 7.94 (d, J = 9.20 Hz, 1H, aromatic H), 8.45 (d, J = 8.44 Hz, 1H, aromatic H), 9.61 (s, 1H, CH=N), 10.41 (br s, 1H, NH, D₂O exchangeable), 15.70 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 23.7 (2 CH₃), 35.9 (CH₂), 108.8, 112.0, 115.0, 116.6, 117.6, 120.5, 122.8, 124.1, 127.1, 128.8, 129.5, 130.6, 133.5, 137.7, 140.5, 144.4, 155.6, 167.5, 169.6 (aromatic carbons), 171.7 (C=N), 193.4 (C=O). MS (EI): m/z (%) = 442 (M⁺, 70.74), 153 (100). Anal. calcd for C₂₅H₂₂N₄O₂S (442.54): C, 67.85; H, 5.01; N, 12.66; found: C, 67.53; H, 5.24; N, 12.48%.

(2E)-1-{3-[(2-Hydroxybenzylidene)amino]phenyl}-3-phenylprop-2-en-1-one (16a): The title compound separated as pale yellow crystals on cooling the reaction mixture, recrystallized from 90% ethanol, m.p. 115–117°C, yield 0.61 g (79%). IR ν (cm⁻¹): 3419 (OH), 3062–3022 (CH aromatic), 1662 (C=O), 1618 (C=N), 1607–1574 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.02 (t, J = 8.12 Hz, 2H, aromatic H), 7.44–7.49 (m, 4H, aromatic H), 7.65–7.74 (m, 3H, aromatic H), 7.81 (d, J = 15.60 Hz, 1H, ethylenic H), 7.93–7.95 (m, 2H, aromatic H), 8.03 (d, J = 15.60 Hz, 1H, ethylenic H), 8.09 (d, J = 7.44 Hz, 1H, aromatic H), 8.18 (s, 1H, aromatic H), 9.08 (s, 1H, CH=N), 12.90 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 117.1, 119.7, 119.8, 121.6, 122.5, 126.7, 127.2, 129.2, 129.4, 129.5, 130.4, 131.2, 133.2, 134.0, 135.1, 139.3, 144.9, 149.3, 160.8, 165.1 (aromatic carbons), 189.3 (C=N), 192.3 (C=O). MS (EI): m/z (%) = 327 (M⁺, 100). Anal. calcd for C₂₂H₁₇NO₂ (327.38): C, 80.71; H, 5.23; N, 4.28; found: C, 80.53; H, 5.60; N, 4.32%.

(2E)-1-(3-{[(2-Hydroxynaphthalen-1-yl)methylidene]amino}phenyl)-3-phenylprop-2-en-1-one (16b): The obtained product was filtered while hot and crystallized from n-butanol as yellow crystals, m.p. 203–205°C, yield 0.66 g (74%). IR ν (cm⁻¹): 3414 (OH), 3059–3028 (CH aromatic), 1655 (C=O), 1620 (C=N), 1605–1574 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.07 (d, J = 9.12 Hz, 1H, aromatic H), 7.38 (t, J = 8.00 Hz, 1H, aromatic H), 7.49–7.53 (m, 3H, aromatic H), 7.58 (t, J = 8.00 Hz, 1H, aromatic H), 7.70 (t, J = 8.00 Hz, 1H, aromatic H), 7.81–7.84 (m, 2H, ethylenic H and aromatic H), 7.95–7.99 (m, 4H, aromatic H), 8.04 (d, J = 15.52 Hz, 1H, ethylenic H), 8.05 (d, J = 7.92 Hz, 1H, aromatic H), 8.33 (s, 1H, aromatic H), 8.56 (d, J = 8.48 Hz, 1H, aromatic H), 9.78 (s, 1H, CH=N), 15.63 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 109.2, 121.0, 121.1, 122.3, 122.6, 124.1, 125.6, 126.8, 127.3, 128.7, 129.1, 129.4, 129.5, 130.6, 131.3, 133.6, 135.1, 137.6, 138.9, 139.5, 145.0, 145.4, 157.5, 170.4 (aromatic carbons), 189.4 (C=N), 193.3 (C=O). MS (EI): m/z (%) = 377 (M⁺, 100). Anal. calcd for C₂₆H₁₉NO₂ (377.44): C, 82.74; H, 5.07; N, 3.71; found: C, 82.41; H, 5.39; N, 3.98%.

N-Benzyl-3-((2-hydroxybenzylidene)amino)benzenesulfonamide (20a): The title product was obtained as pale yellow crystals, recrystallized from methanol/chloroform mixture (5:1), m.p. 153–154°C, yield 0.70 g (81%). IR ν (cm⁻¹): 3475 (OH), 3302 (NH), 3063 (CH aromatic), 2980 (CH aliphatic), 1620 (C=N), 1610 (NH bending), 1570 (C=C), 1350, 1150 (SO₂). ¹H NMR (DMSO-d₆, 400 MHz): δ 4.05 (d, J = 5.48 Hz, 2H, CH₂), 7.00–7.04 (m, 2H, aromatic H), 7.20–7.30 (m, 5H, aromatic H), 7.45 (t, J = 7.76 Hz, 1H, aromatic H), 7.65–7.66 (m, 2H, aromatic H), 7.72–7.74 (m, 3H, aromatic H), 8.23 (t, J = 5.72 Hz, 1H, NH, D₂O exchangeable), 8.97 (s, 1H, CH=N), 12.62 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 46.6 (CH₂), 111.6, 117.2, 117.7, 119.7, 125.6, 127.9, 128.1, 128.6, 128.7, 129.9, 130.9, 133.0, 136.9, 137.9, 142.5, 149.4, 160.6, 165.2 (aromatic carbons), 192.5 (C=N). MS (EI): m/z (%) = 366 (M⁺, 100). Anal. calcd for C₂₀H₁₈N₂O₃S (366.44): C, 65.56; H, 4.95; N, 7.65; found: C, 65.80; H, 5.12; N, 7.83%.

N-Benzyl-3-(((2-hydroxynaphthalen-1-yl)methylene)amino)benzenesulfonamide (20b): The product separated as yellow crystals, recrystallized from n-butanol, m.p. 182–184°C, yield 0.85 g (87%). IR ν (cm⁻¹): 3417 (OH), 3244 (NH), 3059–3028 (CH aromatic), 2935 (CH aliphatic), 1620 (C=N), 1574 (C=C), 1350, 1150 (SO₂). ¹H NMR (DMSO-d₆, 400 MHz): δ 4.07 (d, J = 6.32 Hz, 2H, CH₂), 7.09 (d, J = 9.16 Hz, 1H, aromatic H), 7.19–7.27 (m, 5H, aromatic H), 7.40 (t, J = 7.20 Hz, 1H, aromatic H), 7.60 (t, J = 8.32 Hz, 1H, aromatic H), 7.66–7.73 (m, 2H, aromatic H), 7.83–7.90 (m, 3H, aromatic H), 8.00 (d, J = 9.16 Hz, 1H, aromatic H), 8.23 (t, J = 6.32 Hz, 1H, NH, D₂O exchangeable), 8.53 (d, J = 8.48 Hz, 1H, aromatic H), 9.69 (s, 1H, CH=N), 15.39 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (CDCl₃, 100 MHz): δ 47.4 (CH₂), 109.1, 118.4, 119.3, 121.1, 123.9, 124.6, 125.5, 127.9, 128.0, 128.4, 128.7, 129.4, 130.4, 132.9, 136.0, 137.0, 141.7, 147.3, 157.2, 168.1 (aromatic carbons), 193.3 (C=N). MS (EI): m/z (%) = 416 (M⁺, 100). Anal. calcd for C₂₄H₂₀N₂O₃S (416.50): C, 69.21; H, 4.84; N, 6.73; found: C, 69.03; H, 4.97; N, 6.94%.

1-{3-[(2-Hydroxybenzylidene)amino]phenyl}-3-phenylurea (24a): The title compound separated as yellow crystals, recrystallized from ethanol, m.p. 227–229°C, yield 0.51 g (65%). IR ν (cm⁻¹): 3290 (OH), 3200, 3125 (2 NH), 3059–3035 (CH aromatic), 1732 (C=O), 1647 (NH bending), 1620 (C=N), 1593–1558 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 6.98–7.05 (m, 4H, aromatic H), 7.27–7.31 (m, 3H, aromatic H), 7.38 (t, J = 7.88 Hz, 1H, aromatic H), 7.41–7.48 (m, 3H, aromatic H), 7.59 (s, 1H, aromatic H), 7.69 (d, J = 7.60 Hz, 1H, aromatic H), 8.73 (br s, 1H, NH, D₂O exchangeable), 8.81 (br s, 1H, NH, D₂O exchangeable), 8.94 (s, 1H, CH=N), 13.08 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 111.8, 114.8, 117.1, 118.8, 119.7, 122.5, 127.1, 129.3, 129.8, 130.3, 133.0, 133.8, 136.9, 141.2, 149.1, 153.0 (aromatic carbons), 160.7 (C=N), 163.8 (C=O). MS (EI): m/z (%) = 331 (M⁺, 10.43), 93 (100). Anal. calcd for C₂₀H₁₇N₃O₂ (331.38): C, 72.49; H, 5.17; N, 12.68; found: C, 72.68; H, 5.40; N, 12.50%.

1-(3-{[(2-Hydroxynaphthalen-1-yl)methylidene]amino}phenyl)-3-phenylurea (24b): The title compound separated as yellow crystals, recrystallized from ethanol, m.p. 262–265°C, yield 0.56 g (58%). IR ν (cm⁻¹): 3541 (2 NH), 3398 (OH), 3067 (CH aromatic), 1732 (C=O), 1628 (C=N), 1593–1535 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.04 (d, J = 9.16 Hz, 1H, aromatic H), 7.18 (t, J = 9.16 Hz, 1H, aromatic H), 7.39–7.43 (m, 3H, aromatic H), 7.51–7.57 (m, 3H, aromatic H), 7.81–7.90 (m, 4H, aromatic H), 7.96 (d, J = 9.20 Hz, 1H, aromatic H), 8.07 (s, 1H, aromatic H), 8.46 (d, J = 8.44 Hz, 1H, aromatic H), 9.66 (s, 1H, CH=N), 10.90 (br s, 1H, NH, D₂O exchangeable), 10.97 (br s, 1H, NH, D₂O exchangeable), 15.66 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 108.9, 113.2, 116.8, 118.8, 120.6, 121.0, 122.6, 124.1, 125.2, 127.2, 128.7, 129.3, 129.6, 130.5, 133.6, 137.6, 138.0, 139.2, 144.8, 156.1, 158.9 (aromatic carbons), 159.2 (C=N), 171.1 (C=O). MS (EI): m/z (%) = 410 (M^±1, 9.74), 92 (100). Anal. calcd for C₂₄H₁₉N₄O₃ (411.44): C, 75.57; H, 5.02; N, 11.02; found: C, 75.79; H, 5.13; N, 11.34%.

2-[(4,6-Dimethylpyrimidin-2-yl)sulfanyl]-N-(3-nitrophenyl)acetamide (10): A suspension of 2-mercapto-4,6-dimethylpyrimidine hydrochloride (9) (1.76 g, 10 mmol) and potassium carbonate (2.76 g, 20 mmol) in absolute ethanol (20 mL) was heated under reflux for 30 min. Then, 2-Chloro-N-(3-nitrophenyl)acetamide (8) (2.15 g, 10 mmol) was added, and the mixture was heated under reflux for another 3 h. The obtained residue was filtered and washed several times with water (3 × 10 mL) and recrystallized from ethanol. The product separated as yellowish-white crystals, m.p. 138–139°C, yield 2.36 g (74%). IR ν (cm⁻¹): 3259 (NH), 3094–3024 (CH aromatic), 2978–2924 (CH aliphatic), 1678 (C=O), 1585–1535 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 2.32 (s, 6H, 2 CH₃), 4.09 (s, 2H, CH₂), 6.95 (s, 1H, aromatic H), 7.61 (t, J = 8.20 Hz, 1H, aromatic H), 7.90–7.93 (m, 2H, aromatic H), 8.62 (s, 1H, aromatic H), 10.73 (br s, 1H, NH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 23.8 (2 CH₃), 36.0 (CH₂), 113.6, 116.6, 118.2, 125.5, 130.7, 140.6, 148.4, 167.4, 167.9 (aromatic carbons), 169.6 (C=O). MS (EI): m/z (%) = 318 (M⁺, 5.77), 181 (100). Anal. calcd for C₁₄H₁₄N₄O₃S (318.35): C, 52.82; H, 4.43; N, 17.60; found: C, 52.69; H, 4.65; N, 17.87%.

N-(3-Aminophenyl)-2-[(4,6-dimethyl pyrimidin-2-yl)sulfanyl]acetamide (11): A suspension of 10 (1.02 g, 3.23 mmol) in ethanol (9 mL) was slowly added to a solution of stannous chloride dihydrate (SnCl₂·2H₂O) (2.55 g, 11.31 mmol) and 37% hydrochloric acid (3 mL) while cooling in an ice bath. After 30 min, the reaction mixture allowed to reach room temperature and stirred for a further 48 h. After completion of the reaction, the solution was diluted with water (100 mL) and neutralized with ammonia. The resulting solution was extracted with chloroform, washed with water and dried over anhydrous sodium sulfate. The obtained aniline derivative was crystallized from ethanol.

General procedure for the preparation of compounds 22a,b

To a suspension of the diphenylurea derivative 21 (0.5 g, 1.94 mmol) in dioxane (10 mL), the appropriate acid chloride (1.94 mmol) was added dropwise while cooling in an ice bath. The reaction mixture was stirred at room temperature for 48 h. The obtained solution was poured onto ice/water and stirred for 30 min. The residue was filtered, washed with water, dried.

1-(3-Nitrophenyl)-3-phenylimidazolidine-2,4,5-trione (22a): The title product was separated while hot when the residue was boiled with ethanol as white crystals, m.p. 350–353°C, yield 0.50 g (83%). IR ν (cm⁻¹): 3024 (CH aromatic), 1732 (C=O), 1593–1535 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.47–7.54 (m, 3H, aromatic H), 7.59–7.63 (m, 2H, aromatic H), 7.90–7.97 (m, 2H, aromatic H), 8.36–8.39 (m, 2H, aromatic H). Anal. calcd for C₁₅H₉N₃O₅ (311.25): C, 57.88; H, 2.91; N, 13.50; found: C, 58.12; H, 3.17; N, 13.79%.

1-(3-Nitrophenyl)-3-phenylpyrimidine-2,4,6(1H,3H,5H)-trione (22b): The title product was separated as buff crystals, recrystallized from ethanol, m.p. 163–165°C, yield 0.50 g (79%). IR ν (cm⁻¹): 3093 (CH aromatic), 2970 (CH aliphatic), 1763 (2 C=O), 1686 (C=O), 1570–1531 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 4.09 (s, 2H, CH₂), 7.30–7.38 (m, 2H, aromatic H), 7.42–7.52 (m, 3H, aromatic H), 7.80–7.86 (m, 2H, aromatic H), 8.28–8.36 (m, 2H, aromatic H). Anal. calcd for C₁₆H₁₁N₃O₅ (325.28): C, 59.08; H, 3.41; N, 12.92; found: C, 59.26; H, 3.56; N, 13.21%.

1-(3-Aminophenyl)-3-phenylurea (23): A suspension of 22a,b (3.23 mmol) in ethanol (9 mL) was slowly added to a solution of stannous chloride dihydrate (SnCl₂·2H₂O) (2.55 g, 11.31 mmol) and 37% hydrochloric acid (3 mL) while cooling in an ice bath. After 30 min, the reaction mixture had reached room temperature and was then refluxed for 2 h. After completion of the reaction, the solution was diluted with water (100 mL) and neutralized with ammonia solution. The resulting solution was extracted with chloroform, washed with water and dried over anhydrous sodium sulfate. The obtained aniline derivative was used without purification. The title product separated as a yellow oil, yield 0.15–0.18 g (20%–25%). IR ν (cm⁻¹): 3441 and 3329 (NH₂), 3136 (NH), 3059 (CH aromatic), 1720 (C=O), 1630 (NH bending), 1597–1539 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 5.11 (s, 2H, NH₂), 6.19 (dd, J = 7.92, 1.16 Hz, 1H, aromatic H), 6.56 (dd, J = 8.84, 0.96 Hz, 1H, aromatic H), 6.78 (s, 1H, aromatic H), 6.90 (t, J = 7.96 Hz, 1H, aromatic H), 6.96 (t, J = 7.32 Hz, 1H, aromatic H), 7.27 (t, J = 8.16 Hz, 2H, aromatic H), 7.44 (d, J = 7.68 Hz, 2H, aromatic H), 8.36 (s, 1H, NH), 8.54 (s, 1H, NH). Anal. calcd for C₁₃H₁₃N₃O (227.27): C, 68.70; H, 5.77; N, 18.49; found: C, 68.94; H, 5.83; N, 18.71%.

N-(4-Oxo-4,5-dihydro-1,3-thiazol-2-yl)-2-phenylacetamide (26b): To a continuously stirred and cold suspension of pseudothiohydantoin 21 (5.8 g, 50 mmol) in anhydrous pyridine (20 mL), phenylacetyl chloride (5.83 mL, 50 mmol) was added dropwise. The mixture was heated for 4 h under reflux. The solution turned yellow and changed to a clear orange solution, which after being cooled to room temperature was poured onto finely crushed ice. After 12 h, the dark-brown crystals which had separated out were filtered, dried and recrystallized from ethanol/charcoal. The product was obtained as reddish brown crystals, m.p. 205–206°C, yield 7.02 g (60%). IR ν (cm⁻¹): 3410 (NH), 3028 (CH aromatic), 2932 (CH aliphatic), 1713 (C=O), 1666 (C=O), 1624 (NH bending), 1549 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 3.83 (s, 2H, CH₂), 3.86 (s, 2H, CH₂), 7.27–7.36 (m, 5H, aromatic H), 12.82 (br s, 1H, NH). Anal. calcd for C₁₁H₁₀N₂O₂S (234.27): C, 56.40; H, 4.30; N, 11.96; found: C, 56.71; H, 4.53; N, 12.19%.

General procedure for the preparation of compounds 27a–f

To a mixture of the appropriate aldehyde (2.27 mmol) and thiazolidinone derivative 22a,b (2.27 mmol) in absolute ethanol (15 mL), fused sodium acetate (0.18 g, 2.27 mmol) was added. The reaction mixture was heated under reflux for 4 h, then transferred into a beaker and left to cool. The obtained precipitate was filtered, washed, dried and recrystallized from ethanol.

N-[5-(2-Hydroxybenzylidene)-4-oxo-4,5-dihydro-1,3-thiazol-2-yl]benzamide (27a): The title compound was obtained as pale brown crystals, m.p. 243–244°C, yield 0.46 g (62%). IR ν (cm⁻¹): 3387 (OH), 3155 (NH), 3062 (CH aromatic), 1701 (2 C=O), 1628 (NH bending), 1597–1539 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 6.99–7.04 (m, 2H, aromatic H), 7.35 (t, J = 7.24 Hz, 1H, aromatic H), 7.51–7.59 (m, 3H, aromatic H), 7.67 (t, J = 7.32 Hz, 1H, aromatic H), 8.07 (s, 1H, CH=C), 8.18 (d, J = 7.44 Hz, 2H, aromatic H), 10.59 (br s, 1H, OH, D₂O exchangeable), 13.08 (br s, 1H, NH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 116.7, 120.2, 120.7, 122.9, 129.1, 129.6, 129.8, 130.0, 133.1, 133.8, 158.0 (aromatic carbons), 169.0, 176.0 (2 C=O). MS (EI): m/z (%) = 324 (M⁺, 1.38), 77 (100). Anal. calcd for C₁₇H₁₂N₂O₃S (324.35): C, 62.95; H, 3.73; N, 8.64; found: C, 63.11; H, 3.86; N, 8.88%.

N-{5-[(2-Hydroxynaphthalen-1-yl)methylidene]-4-oxo-4,5-dihydro-1,3-thiazol-2-yl}benzamide (27b): The product was obtained as dark-brown crystals, m.p. 180–182°C, yield 0.62 g (73%). IR ν (cm⁻¹): 3483 (OH), 3314 (NH), 3067 (CH aromatic), 1697 (2 C=O), 1628 (NH bending), 1578–1531 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.28 (d, J = 9.08 Hz, 1H, aromatic H), 7.41 (t, J = 8.00 Hz, 1H, aromatic H), 7.55–7.67 (m, 2H, aromatic H), 7.72 (t, J = 7.72 Hz, 1H, aromatic H), 7.89–7.97 (m, 2H, aromatic H), 8.05 (d, J = 7.92 Hz, 1H, aromatic H), 8.14 (d, J = 7.20 Hz, 1H, aromatic H), 8.20–8.22 (m, 2H, aromatic H), 9.27 (s, 1H, CH=C), 10.94 (br s, 1H, NH), 12.89 (br s, 1H, OH). ¹³C NMR (DMSO-d₆, 100 MHz): δ 113.2, 118.6, 123.7, 123.8, 127.8, 128.3, 128.6, 128.9, 128.9, 129.0, 129.1, 129.6, 129.7, 129.8, 132.1, 132.5, 133.4, 135.7, 135.9 (aromatic carbons), 154.5, 172.5 (2 C=O). Anal. calcd for C₂₁H₁₄N₂O₃S (374.41): C, 67.37; H, 3.77; N, 7.48; found: C, 67.12; H, 3.89; N, 7.63%.

N-[4-Oxo-5-(thiophen-2-ylmethylidene)-4,5-dihydro-1,3-thiazol-2-yl]benzamide (27c): The title compound was obtained as yellowish-green crystals, m.p. 224–226°C, yield 0.61 g (85%). IR ν (cm⁻¹): 3391 (NH), 3028 (CH aromatic), 1724 (C=O), 1697 (C=O), 1647 (NH bending), 1589–1551 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 7.33 (t, J = 4.32 Hz, 1H, thiophene H), 7.58 (t, J = 7.44 Hz, 2H, aromatic H), 7.68 (t, J = 7.32 Hz, 1H, aromatic H), 7.75 (d, J = 3.36 Hz, 1H, thiophene H), 8.07–8.09 (m, 2H, CH=C and thiophene H), 8.20 (d, J = 7.32 Hz, 2H, aromatic H), 13.13 (br s, 1H, NH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 122.2, 127.6, 129.2, 129.5, 129.8, 133.9, 134.3, 135.0, 135.9, 137.9 (aromatic carbons and thiophene carbons), 168.7, 176.0 (2 C=O). Anal. calcd for C₁₅H₁₀N₂O₂S₂ (314.38): C, 57.31; H, 3.21; N, 8.91; found: C, 57.60; H, 3.49; N, 9.12%.

N-[5-(2-Hydroxybenzylidene)-4-oxo-4,5-dihydro-1,3-thiazol-2-yl]-2-phenylacetamide (27d): The product was obtained as light brown crystals, m.p. 267–268°C, yield 0.53 g (69%). IR ν (cm⁻¹): 3263 (OH), 3128 (NH), 3032 (CH aromatic), 2927 (CH aliphatic), 1724 (C=O), 1663 (C=O), 1582–1555 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 3.89 (s, 2H, CH₂), 6.94–6.98 (m, 2H, aromatic H), 7.27–7.37 (m, 6H, aromatic H), 7.45 (d, J = 7.68 Hz, 1H, aromatic H), 8.08 (s, 1H, CH=C), 10.54 (br s, 1H, NH, D₂O exchangeable), 13.02 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 43.5 (CH₂), 116.7, 120.2, 120.9, 124.9, 127.5, 128.9, 129.3, 129.9, 130.0, 130.8, 133.0, 134.4 (aromatic carbons), 158.0, 174.0 (2 C=O). Anal. calcd for C₁₈H₁₄N₂O₃S (338.38): C, 63.89; H, 4.17; N, 8.28; found: C, 63.71; H, 4.38; N, 8.05%.

N-{5-[(2-hydroxynaphthalen-1-yl)methylidene]-4-oxo-4,5-dihydro-1,3-thiazol-2-yl}-2-phenylacetamide (27e): The title compound was obtained as dark-brown crystals, m.p. 200–201°C (decomposition), yield 0.55 g (63%). IR ν (cm⁻¹): 3395 (OH), 3186 (NH), 3062–3032 (CH aromatic), 2981 (CH aliphatic), 1717 (2 C=O), 1631 (NH bending), 1593–1535 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 3.91 (s, 2H, CH₂), 7.22–7.37 (m, 3H, aromatic H), 7.46–7.53 (m, 2H, aromatic H), 7.62–7.74 (m, 2H, aromatic H), 7.87–7.90 (m, 2H, aromatic H), 8.11–8.19 (m, 2H, aromatic H), 9.02 (s, 1H, CH=C), 10.15 (br s, 1H, NH, D₂O exchangeable), 13.15 (br s, 1H, OH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 43.5 (CH₂), 112.0, 112.6, 114.0, 125.1, 127.1, 127.6, 127.7, 128.3, 128.7, 128.8, 128.9, 129.1, 129.7, 129.9, 130.0, 130.3, 131.1, 131.8, 133.3, 135.2 (aromatic carbons), 150.2, 178.9 (2 C=O). Anal. calcd for C₂₂H₁₆N₂O₃S (388.44): C, 68.03; H, 4.15; N, 7.21; found: C, 68.21; H, 4.33; N, 7.49%.

N-[4-Oxo-5-(thiophen-2-ylmethylidene)-4,5-dihydro-1,3-thiazol-2-yl]-2-phenylacetamide (27f): The product was obtained as brown crystals, m.p. 280–281°C, yield 0.55 g (74%). IR ν (cm⁻¹): 3113 (NH), 3028 (CH aromatic), 2908 (CH aliphatic), 1720 (C=O), 1686 (C=O), 1589–1558 (C=C). ¹H NMR (DMSO-d₆, 400 MHz): δ 3.90 (s, 2H, CH₂), 7.28–7.36 (m, 6H, aromatic H and thiophene H), 7.71 (d, J = 3.40 Hz, 1H, thiophene H), 8.01 (d, J = 4.92 Hz, 1H, thiophene H), 8.06 (s, 1H, CH=C), 13.11 (br s, 1H, NH, D₂O exchangeable). ¹³C NMR (DMSO-d₆, 100 MHz): δ 43.5 (CH₂), 124.1, 127.5, 128.4, 128.9, 129.6, 129.9, 130.0, 133.8, 134.3, 135.8, 138.5 (aromatic carbons and thiophene carbons), 172.9, 180.4 (2 C=O). Anal. calcd for C₁₆H₁₂N₂O₂S₂ (328.40): C, 58.52; H, 3.68; N, 8.53; found: C, 58.74; H, 3.85; N, 8.79%.

Biological evaluation

In vitro MTT cytotoxic activity screening

All target compounds were screened for their cytotoxic activity, at the Confirmatory Diagnostic Unit in VACSERA-Egypt, against the brain SNB-75 and the lung HOP-92 cancer cell lines as well as normal WI-38 fibroblasts using the MTT assay according to the reported procedure.²⁸ Sorafenib was used as a positive control.

Supplemental Material

Supplementary_Material – Supplemental material for Synthesis of new phenolic compounds and biological evaluation as antiproliferative agents

Supplemental material, Supplementary_Material for Synthesis of new phenolic compounds and biological evaluation as antiproliferative agents by Marwa A Ibrahim, Riham F George, Sahar M Abou-Seri and Samir M El-Moghazy in Journal of Chemical Research

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

ORCID iD

Marwa A Ibrahim

Supplemental material

Supplemental material for this article is available online.

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