Synthesis and in vitro antibacterial, antifungal, anti-proliferative activities of novel adamantane-containing thiazole compounds

Abstract

A series of (Z)-N-(adamantan-1-yl)-3,4-diarylthiazol-2(3H)-imines (5a-r) was synthesized via condensation of 1-(adamantan-1-yl)-3-arylthioureas (3a-c) with various aryl bromomethyl ketones (4a-f). The structures of the synthesized compounds were characterized by ¹H NMR, ¹³C NMR and by X-ray crystallography. The in vitro inhibitory activities of the synthesized compounds were assessed against a panel of Gram-positive and Gram-negative bacteria, and pathogenic fungi. Compounds 5c, 5g, 5l, 5m, and 5q displayed potent broad-spectrum antibacterial activity, while compounds 5a and 5o showed activity against the tested Gram-positive bacteria. Compounds 5b, 5l and 5q displayed potent antifungal activity against Candida albicans. In addition, the synthesized compounds were evaluated for anti-proliferative activity towards five human tumor cell lines. The optimal anti-proliferative activity was attained by compounds 5e and 5k which showed potent inhibitory activity against all the tested cell lines. Molecular docking analysis reveals that compounds 5e and 5k can occupy the positions of NAD cofactor and the histone deacetylase inhibitor EX527 at the active site of SIRT1 enzyme.

Introduction

Adamantane cage is an interesting core of several drugs possessing diverse pharmacological properties^1–4. The chemotherapeutic effectiveness of adamantane-based compounds was early explored after the discovery of amantadine^5,6 and rimantadine⁷ as efficient medications for the control of Influenza A viral infections. Tromantadine was also developed as potent antiviral drug for the treatment of skin infection caused by herpes simplex virus⁸. Adamantane cage constitutes an important moiety of currently employed anticancer drugs. Adaphostin is a tyrosine kinase inhibitor that displays anti-proliferative activity in leukemia, non-small cell lung-cancer, and prostate cancer⁹. Adarotene, a IκB kinase-β inhibitor, was also developed as potent anticancer drug for the treatment of lymphoma, leukemia and prostate cancer¹⁰. The adamantane-based synthetic retinoid CD437 was also reported as a promising anticancer agent acting by inhibition of DNA polymerase¹¹, while Opaganib is a recently approved adamantane-based anticancer drug for the treatment of advanced solid tumors¹². Opaganib induces its anticancer activity via inhibition of sphingosine kinase (SK). Recently, opaganib proved to be a promising candidate for the treatment of severe COVID-19 pneumonia¹³. Other adamantane-based derivatives have demonstrated potent activity against pathogenic bacteria, mycobacteria and fungi. The adamantane-diamine hybrid derivative SQ109 was discovered for treatment of drug-resistant tuberculosis¹⁴, while the structurally-related dipiperidine analogue SQ609 was developed as a lead compound with potent action against Mycobacterium tuberculosis¹⁵ (Fig. 1).

Figure 1.

Adamantane-based chemotherapeutic drugs and drug candidates.

On the other hand, thiazole heterocycle represents the bioactive core of numerous drugs^16–18. Several thiazole-based compounds were found to be effective chemotherapeutic agents with anticancer^19,20, antifungal^21–23, antibacterial^24,25, antiviral²⁶ and anti-leishmanial²⁷ activities. The thiazole-based natural product largazole was discovered as potent anticancer drug acting via inhibition of histone deacetylases (HDACs)^28,29. Dasatinib³⁰, dabrafenib³¹ and alpelisib³² are currently-used drugs for the treatment of acute lymphoblastic leukemia, advanced melanoma and breast cancer, respectively (Fig. 2). Besides, the thiazole-based natural products patellamide A³³, ixabepilone³⁴ and epothilones³⁵ are clinically useful for the treatment of resistant prostate cancer, metastatic breast cancer, ovarian and rectal cancers.

Figure 2.

Thiazole-based anticancer drugs.

In light of the previously reported observations and as a continuation of an ongoing interest in the chemotherapeutic activities of adamantane-based derivatives^36–38, we report herein the synthesis, characterization, and biological evaluation of a series of adamantane-linked thiazole derivatives as potential antibacterial, antifungal and anti-proliferative agents.

Results and discussion

Chemical synthesis

Adamantan-1-amine 1 was allowed to react with phenyl isothiocyanate 2a, 4-fluorophenyl isothiocyanate 2b and 4-chlorophenyl isothiocyanate 2c via heating in ethanol under reflux for 4 h to yield the corresponding 1-(adamantan-1-yl)-3-arylthiourea derivatives 3a-c in good yields^39–41. The thiourea derivatives 3a-c were subsequently reacted with different aryl bromomethyl ketones 4a-f via prolonged heating in ethanol followed by addition of sodium acetate to yield the corresponding (Z)-N-(adamantan-1-yl)-3,4-diarylthiazol-2(3H)-imine derivatives 5a-r rather than the isomeric (Z)-3-(adamantan-1-yl)-4,N-diarylthiazol-2(3H)-imines 6a-r. The structures of the diarylthiazol-2(3H)-imines 5a-r were assigned based on ¹H NMR, ¹³C NMR and elemental analysis. Single crystal X-ray analysis of compounds 5d and 5f. was also performed to assign their configuration and to exclude the formation of compounds 6a-r (Fig. 3). The ¹H NMR spectra of compounds 5a-r showed the adamantane protons (15H) as three singlets (condensed multiplets) at δ 1.65–1.76 (6H), 1.84–1.99 (6H) and 2.06–2.17 (3H) ppm. The thiazole CH protons were shown as distinguished singlets at δ 5.78–6.04 ppm. Meanwhile, the ¹³C NMR spectra of compounds 5a-r showed the adamantane carbons as four characteristic peaks at δ 29.64–30.12, 36.60–37.08, 40.63–41.17 and 53.49–54.19 ppm. In addition, the C5, C4 and C2 carbons of the thiazole ring were shown at δ 96.11–99.09, 148.47–150.28 and 158.89–160.08 ppm, respectively. The aromatic protons and carbons and their substituents were properly shown in their ¹H NMR and ¹³C NMR spectra (see “Materials and methods” section).

Figure 3.

Synthetic outline of the target compounds 5a-r.

The molecular and crystal structures of compounds 5d and 5f. were determined by single crystal X-ray analysis to explore the configuration of compounds 5a-r and to exclude the formation of compounds 6a-r. The crystal data and refinement parameters of the compounds are summarized in Table S1.

Compound 5d crystallizes in the monoclinic system with the space group I2/a. The asymmetric unit contains one molecule of the compound as cyclohexane solvate (Fig. 4a). The atoms C27 and C30 of the cyclohexane molecule lie on two-fold axis. The six-membered rings of the adamantane moiety exhibit a chair conformation, whereas a cyclohexane solvent adopts twist-boat conformation. The central thiazole ring is oriented at an angle of 47.30° with the mean plane of the p-tolyl group, whereas the same thiazole unit makes 53.66° angle with the phenyl ring attached at the third position of the thiazole ring. In the solid state, molecules of 5d and cyclohexane solvent observed in the crystal lattice forming in a columnar fashion (Fig. 4b) as observed in many adamantane derivatives. No classical hydrogen bonds observed in the crystal structure. However, a weak intermolecular C–H…π (involving the tolyl proton, H6 and centroid of the thiazole ring; C6–H6…Cg1 = 149°; H6…Cg1 = 2.72 Å, C6…Cg1 = 3.572(3); symmetry element: − x, y + 1/2, − z + 1/2).

Figure 4.

(a) ORTEP plot of compound 5d and a cyclohexane solvent in the crystalline lattice (symmetry element (a) − x + 1/2, y, − z). The displacement ellipsoid is drawn at the 50% probability level along atom-labelling scheme and (b) columnar packing mode of compound 5d along the crystallographic ac plane. The lattice cyclohexane solvent is shown in red. All H atoms have been omitted for the sake of clarity.

This interaction links the molecules into a chain that runs parallel to the crystallographic b axis (Fig. 5a). The cyclohexane solvent establishes a pair of weak H…H contacts with the protons of the adamantane and phenyl moieties (Fig. 5b).

Figure 5.

(a) Molecular chain formed by a weak C–H…π interaction in the solid state of compound and (b) cyclohexane…adamantane/phenyl interactions in compound 5d.

Compound 5f. crystallizes in the triclinic system with the P-1 space group. The asymmetric unit contains one molecule (Fig. 6a). This compound has a naphthalene ring instead of the p-tolyl position in compound 5d. In compound 5f., the central thiazole ring is oriented at an angle of 42.03° with the mean plane of the naphthalene ring. However, the mean plane of the phenyl ring makes 60.25° with the thiazole core.

Figure 6.

(a) ORTEP representation of compound 5f. with atom-labelling scheme, (b) crystal packing of compound 5f. projected along bc plane and the π-stacking interaction between phenyl rings is shown as red dashed lines, (c) structural superposition of compounds 5d and 5f. (cyclohexane solvent has been omitted for clarity) and (d) molecular chain is generated via a weak intermolecular C–H…S interaction.

The six-membered rings of the adamantane cage exhibits chair conformation as observed in related structures. In the crystalline state, the molecules pack in a columnar fashion (Fig. 6b). Figure 6c shows the structural overlay between 5d and 5f. structures and one can see from this figure that the central thiazole and adamantane cage overlap well and the aryl rings show twist orientations. The crystal structure of 5f. is stabilized primarily by a weak π-stacking interaction formed between phenyl rings in adjacent columns (centroid–centroid = 3.848 (2), symmetry element: 1-x, 1-y, 1-z). In addition, a weak C–H…S interaction (C27-H27…S1 = 161°, H27…S1 = 3.02 Å and C27…S1 = 3.959 Å; symmetry element: x-1, y, z) links the neighboring molecules into a chain that runs parallel to the crystallographic a axis as shown in Fig. 6d.

In vitro antibacterial and antifungal activities

The in vitro antibacterial and antifungal activity of newly designed compounds 5a-r was assessed against a panel of standard pathogenic strains of the Institute of Fermentation of Osaka (IFO) namely; Staphylococcus aureus IFO 3060 (SA), Bacillus subtilis IFO 3007 (BS), Micrococcus luteus IFO 3232 (ML) (Gram-positive bacteria), Escherichia coli IFO 3301 (EC), Pseudomonas aeruginosa IFO 3448 (PA) (Gram-negative bacteria ) and the pathogenic fungi Candida albicans IFO 0583 (CA), Aspergillus oryzae IFO 4177 (AO) and Aspergillus niger IFO 4414 (AN). The initial screening was carried out using the semi-quantitative agar disc-diffusion method using Müller-Hinton agar medium⁴².

The results of the initial antimicrobial screening of compounds 5a-r (200 µg/disc); the antibacterial antibiotics Ampicillin trihydrate, Ciprofloxacin (100 µg/disc) and the antifungal drug Fluconazole (100 µg/disc) are shown in Table 1. The results showed varying degrees of inhibition against the tested microorganisms. In general, potent antibacterial activity was displayed by compounds 5a, 5c, 5e, 5g, 5h, 6j, 5l, 5m, 5n, 5o, 5p and 5q, which produced growth inhibition zones ≥ 18 mm against one or more of the tested bacterial strains. Meanwhile, compounds 5b, 5d, 5f., 5i, 5k and 5r displayed moderate antibacterial activity (growth inhibition zones 14–17 mm). In addition, the Gram-positive bacteria Staphylococcus aureus, Bacillus subtilis and to a lesser extent Micrococcus luteus are considered the most sensitive among the tested bacterial strains. The inhibitory activity against the tested Gram-negative bacteria was generally lower than the Gram-positive bacteria; compounds 5c, 5g, 5l, 5m and 5q showed potent inhibitory activity against Escherichia coli, while only compounds 5c, 5l and 5m were strongly active against Pseudomonas aeruginosa. The optimum antibacterial activity was attained by compounds 5c, 5g, 5l, 5m and 5q which displayed potent broad spectrum antibacterial activity.

Table 1.

Antibacterial and antifungal activities of compounds 5a-r (200 μg/8 mm disc), the broad-spectrum antibacterial drugs Ampicillin trihydrate, Ciprofloxacin and the antifungal drug Fluconazole (100 μg/8 mm disc) against the Gram-positive bacteria Staphylococcus aureus IFO 3060 (SA), Bacillus subtilis IFO 3007 (BS) and Micrococcus luteus IFO 3232 (ML), the Gram-negative bacteria Escherichia coli IFO 3301 (EC) and Pseudomonas aeruginosa IFO 3448 (PA) and the pathogenic fungi Candida albicans IFO 0583 (CA), Aspergillus oryzae IFO 4177 (AO) and Aspergillus niger IFO 4414 (AN).

Comp. No	X	Ar	Diameter of Growth Inhibition Zone (mm)*
			SA	BS	ML	EC	PA	CA	AO	AN
5a	H	C6H5	25	26	20	12	-	-	-	-
5b	H	4-ClC6H4	14	17	10	-	-	18	-	-
5c	H	4-BrC6H4	28	23	24	25	19	10	-	-
5d	H	4-CH3C6H4	17	14	15	14	12	-	-	-
5e	H	4-CH3OC6H4	18	15	15	-	-	-	-	-
5f.	H	2-Naphthyl	14	15	12	11	10	-	-	-
5g	F	C6H5	21	17	21	21	15	-	13	-
5h	F	4-ClC6H4	18	15	17	16	14	12	10	-
5i	F	4-BrC6H4	12	14	12	12	11	-	-	-
5j	F	4-CH3C6H4	18	17	15	15	12	14	-	-
5k	F	4-CH3OC6H4	14	14	12	14	10	17	-	-
5l	F	2-Naphthyl	25	28	22	22	18	18	-	-
5m	Cl	C6H5	22	20	18	26	20	-	-	-
5n	Cl	4-ClC6H4	18	17	15	15	12	-	-	-
5o	Cl	4-BrC6H4	26	22	18	14	12	-	-	-
5p	Cl	4-CH3C6H4	18	15	10	12	-	-	-	-
5q	Cl	4-CH3OC6H4	24	22	22	18	15	19	14	-
5r	Cl	2-Naphthyl	14	10	12	-	-	-	-
Ampicillin trihydrate	28	30	25	24	22	NT	NT	NT
Ciprofloxacin	34	38	32	38	36	NT	NT	NT
Fluconazole	NT	NT	NT	NT	NT	21	22	24

NT not tested.

*-Inactive (inhibition zone < 10 mm).

Significant values are in bold.

The antifungal activity of the compounds 5a-r was generally lower than their antibacterial activity. Compounds 5b, 5l and 5q displayed high activity against the yeast-like pathogenic fungus Candida albicans, while compounds 5j and 5k were moderately active and compounds 5c and 5h exhibited marginal activity (growth inhibition zones 10–13 mm). Compounds 5g, 5h and 5q were almost weakly against Aspergillus oryzae, and all the compounds were totally inactive against Aspergillus niger.

In general, the antibacterial and antifungal potency and spectrum are greatly influenced by the nature of the aryl groups at positions 3 and 4 of the core thiazole nucleus. In the 3-phenyl series 5a-f, the 3,4-diphenyl analogue 5a displayed potent antibacterial activity against the tested Gram-positive bacteria and lacked activity against the tested Gram-negative bacteria and pathogenic fungi. Replacement of the 4-phenyl group with 4-(4-chlorophenyl) substituent (compound 5b) deteriorated the antibacterial activity and greatly enhanced the activity against Candida albicans. Meanwhile, the 4-(p-tolyl) 5d and 4-(2-naphthyl) 5f. analogues retained moderate activity against the tested Gram-positive bacteria, with only marginal activity against the tested Gram-negative bacteria, and no antifungal activity. In addition, the 4-(4-methoxyphenyl) derivative 5e displayed good activity against Staphylococcus aureus and moderate activity against Bacillus subtilis and Micrococcus luteus with no activity against the tested Gram-negative bacteria and fungi. The optimum antibacterial activity was attained in the 4-bromophenyl analogue 5c which displayed excellent broad-spectrum antibacterial activity.

In the 3-(4-fluorophenyl) series 5g-l, the 4-phenyl, 4-(4-chlorophenyl) and 4-(p-tolyl) derivatives 5g, 5h and 5j showed marked broad-spectrum antibacterial activity and weak antifungal activity against Candida albicans and/or Aspergillus oryzae. Unlike the antimicrobial activity in the 3-phenyl derivatives 5a-f, the antibacterial activity of the 4-bromophenyl analogue 5i was found inferior compared to the majority of the 3-(4-fluorophenyl) derivatives 5g-l, and with no antifungal activity. Meanwhile, the 4-(p-tolyl) 5j and 4-(4-methoxyphenyl) 5k analogues showed almost moderate or weak antibacterial activity and moderate activity against Candida albicans. In contrary to the antimicrobial activity of the 3-phenyl derivatives 5a-f, the 4-(2-naphthyl) analogue 5l displayed excellent broad-spectrum antibacterial activity in addition to potent antifungal activity against Candida albicans.

In the 3-(4-chlorophenyl) series 5m-r, the optimum antibacterial activity was shown by the 4-phenyl 5m and 4-(4-methoxyphenyl) 5q derivatives which almost displayed potent broad-spectrum antibacterial activity, while the 4-(4-bromophenyl) derivative 5o retained potent and almost weak activity against the tested Gram-positive and Gram-negative bacteria, respectively. The antibacterial activity of the 4-(4-chlorophenyl) 5n and 4-(p-tolyl) 5p derivatives was found similar, but to lower extent, to the activity of the 4-(4-bromophenyl) derivative 5o. The 4-(2-naphthyl) analogue 5r only retained moderate or marginal activity against the tested Gram-positive bacteria. Potent and moderate antifungal activity against Candida albicans and Aspergillus oryzae, respectively, was displayed by the 4-(4-methoxyphenyl) 5q derivative.

Taken together, the broad spectrum antibacterial activity was exhibited by monohalogenated derivatives rather than the dihalogenated ones. This is illustrated by the activity 5c, 5l, 5m and 5q. The same results were clearly demonstrated by the antifungal activity of compounds 5b, 5l and 5q. Upon comparing the antibacterial activity of the two isomeric structures 5b and 5m, it has been discovered that the halogen substitution pattern is more beneficial at the N-3 position than the C-4 position.

The minimal inhibitory concentrations (MIC) and the minimal biocidal concentrations (MBC, bactericidal and fungicidal) for seven of the most active compounds (5a, 5c, 5g, 5l, 5m, 5o and 5q) against the same microorganisms used in the primary screening were carried out using the micro-dilution susceptibility method in Müller-Hinton Broth^43–45. The MIC and MBC values of compounds 5a, 5c, 5g, 5l, 5m, 5o, 5q, Ampicillin trihydrate, Ciprofloxacin and Fluconazole are depicted in Table 2. The MIC of the tested compounds were in accordance with the results obtained in the primary screening. According to the antimicrobial standards, an agent is usually considered as bactericidal or fungicidal if the MBC/MIC ratio is not more than 4⁴⁶. The MBC/MIC ratio for the tested compounds (Table 2) were found to be around 2. According to the MBC/MIC values of the tested compounds, it could be concluded that the (Z)-N-(adamantan-1-yl)-3,4-diarylthiazol-2(3H)-imine derivative 5a, 5c, 5g, 5l, 5m, 5o and 5q could be considered as potential antibacterial candidates for further studies. Though these compounds are of inferior activity compared to the reference drugs, they have displayed MBC/MIC ratio equivalent to that of the references.

Table 2.

The Minimal Inhibitory Concentrations (MIC) and Minimal Biocidal Concentrations (MBC, bactericidal and fungicidal) of compounds 5a, 5c, 5g, 5l, 5m, 5o and 5q in comparison with the broad spectrum antibacterial drugs Ampicillin trihydrate, Ciprofloxacin and the antifungal drug Fluconazole against the Gram-positive bacteria Staphylococcus aureus IFO 3060 (SA), Bacillus subtilis IFO 3007 (BS) and Micrococcus luteus IFO 3232 (ML), the Gram-negative bacteria Escherichia coli IFO 3301 (EC) and Pseudomonas aeruginosa IFO 3448 (PA) and the pathogenic fungus Candida albicans IFO 0583 (CA).

Comp. No	MIC/MBC (μg/mL)*
	SA	BS	ML	EC	PA	CA
5a	3.25/7.20	3.5/8.25	7.0/15.5	ND/ND	ND/ND	ND/ND
5c	2.0/5.20	4.0/7.50	3.0/6.20	6.5/11.50	10.25/ND	ND/ND
5g	6.50/10.25	ND/ND	6.50/12.0	8.0/16.20	ND/ND	ND/ND
5l	3.25/8.50	2.25/5.0	4.0/9.50	7.20/15.50	ND/ND	8.50/16.20
5m	6.0/12.50	8.50/15.50	ND/ND	4.25/8.75	8.50/ND	ND/ND
5o	3.20/8.25	3.75/8.0	ND/ND	ND/ND	ND/ND	ND/ND
5q	3.50/8.20	3.75/7.50	4.20/7.80	ND/ND	ND/ND	7.25/13.50
Ampicillin	2.0/3.5	1.0/3.0	2.5/4.0	ND/ND	ND/ND	ND/ND
Ciprofloxacin	0.75/1.50	0.50/1.0	1.0/2.0	0.25/1.0	ND/ND	ND/ND
Fluconazole	ND/ND	ND/ND	ND/ND	ND/ND	ND/ND	6.25/12.0

ND not determined.

*Data shown are the mean values of three experiments.

The adamantane-linked thiazoles 5a, 5c, 5l and 5o which displayed the best antibacterial activity against Gram-positive bacteria were selected for further assessment of their bacterial biofilm inhibitory activity using the crystal violet staining method⁴⁷. Bacterial biofilms are clusters of bacteria that are attached to a surface and/or to each other and embedded in a self-produced matrix. The biofilm matrix mainly consists of proteins and polysaccharides. In addition to the protection offered by the matrix, bacteria in biofilms can employ several survival strategies to evade the host defense systems^48,49. The minimal biofilm inhibitory concentrations (IC₅₀) of compounds 5a, 5c, 5l, 5o and the broad-spectrum antibiotic Erythromycin towards the Gram-positive bacteria Staphylococcus aureus and Micrococcus luteus are shown in Table 3. The IC₅₀ (the lowest concentrations of tested compounds that exhibited 50% inhibition on the biofilm formation) ranged from 1.6 to 2.5 and 3.4 to 7.4 μg/mL against Staphylococcus aureus and Micrococcus luteus, respectively.

Table 3.

Anti-biofilm activity (IC₅₀, μg/mL) of compounds 5a, 5c, 5l, 5o and the broad-spectrum antibiotic Erythromycin against Staphylococcus aureus IFO 3060 (SA) and Micrococcus luteus IFO 3232 (ML).

Comp. No	Anti-biofilm activity (μg/mL)*
	SA	ML
5a	2.4 ± 0.12	4.8 ± 0.20
5c	1.6 ± 0.25	6.5 ± 0.25
5l	2.5 ± 0.15	3.4 ± 0.15
5o	2.0 ± 0.15	7.4 ± 0.21
Erythromycin	0.45 ± 0.15	0.62 ± 0.21

*Results are the mean values of three experiments ± SD.

In vitro anti-proliferative activity

The in vitro anti-proliferative activity of the adamantane-linked thiazoles 5a-r was assessed against five human tumor cell lines namely; PC-3 (human prostate cancer), HCT-116 (human colorectal cancer), HepG-2 (human hepatocellular cancer), HeLa (human cervical cancer) and MCF-7 (human breast cancer) using the 3-[4,5-dimethylthiazoyl-2-yl]-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay⁵⁰. The obtained results, together with that of the anticancer drug doxorubicin, are shown in Table 4.

Table 4.

In vitro anti-proliferative activity of compounds 5a-r and the anticancer drug Doxorubicin toward human prostate cancer (PC-3), colorectal carcinoma (HCT-116), hepatocellular carcinoma (HepG-2), epithelioid cervical carcinoma (HeLa) and mammary gland breast cancer (MCF-7) cell lines.

Comp. No	X	Ar	IC50 (µM)a
			PC-3	HCT-116	HepG-2	HeLa	MCF-7
5a	H	C6H5	67.26 ± 3.6	59.23 ± 3.3	48.32 ± 2.9	41.81 ± 2.4	51.26 ± 2.9
5b	H	4-ClC6H4	87.82 ± 4.1	85.51 ± 4.4	65.29 ± 3.4	71.48 ± 3.7	78.62 ± 3.9
5c	H	4-BrC6H4	78.97 ± 3.9	80.25 ± 4.2	56.43 ± 3.2	83.86 ± 4.0	72.31 ± 3.7
5d	H	4-CH3C6H4	53.19 ± 2.9	58.63 ± 3.2	38.24 ± 2.5	49.83 ± 2.7	45.66 ± 2.6
5e	H	4-CH3OC6H4	12.85 ± 1.0	11.96 ± 0.9	16.30 ± 1.4	9.87 ± 0.7	6.90 ± 0.4
5f.	H	2-Naphthyl	> 100	94.26 ± 4.8	81.62 ± 4.2	> 100	86.49 ± 4.4
5g	F	C6H5	16.73 ± 1.3	28.27 ± 2.1	20.87 ± 1.6	30.67 ± 2.1	19.64 ± 1.4
5h	F	4-ClC6H4	23.56 ± 1.9	39.74 ± 2.5	22.07 ± 1.8	18.11 ± 1.5	32.78 ± 2.2
5i	F	4-BrC6H4	> 100	> 100	88.57 ± 4.5	> 100	91.18 ± 4.8
5j	F	4-CH3C6H4	90.04 ± 4.6	73.86 ± 3.9	70.28 ± 3.8	52.53 ± 2.9	66.28 ± 3.5
5k	F	4-CH3OC6H4	9.71 ± 0.8	6.47 ± 0.5	11.27 ± 0.9	7.09 ± 0.5	3.51 ± 0.2
5l	F	2-Naphthyl	> 100	89.05 ± 4.5	77.21 ± 4.0	93.62 ± 4.7	84.03 ± 4.3
5m	Cl	C6H5	33.59 ± 2.2	36.42 ± 2.3	29.53 ± 2.2	21.59 ± 1.8	26.04 ± 1.9
5n	Cl	4-ClC6H4	40.70 ± 2.4	46.11 ± 2.6	31.98 ± 2.3	25.65 ± 2.0	39.21 ± 2.5
5o	Cl	4-BrC6H4	61.39 ± 3.4	24.50 ± 1.8	47.38 ± 2.8	15.28 ± 1.2	13.72 ± 1.2
5p	Cl	4-CH3C6H4	55.86 ± 3.1	62.79 ± 3.5	43.65 ± 2.7	37.26 ± 2.3	57.39 ± 3.2
5q	Cl	4-CH3OC6H4	42.08 ± 2.6	17.58 ± 1.4	35.18 ± 2.5	10.49 ± 0.9	8.53 ± 0.7
5r	Cl	2-Naphthyl	75.45 ± 3.8	68.35 ± 3.7	51.79 ± 3.1	56.40 ± 3.3	63.54 ± 3.4
Doxorubicin			8.87 ± 0.6	5.23 ± 0.3	4.50 ± 0.2	5.57 ± 0.4	4.17 ± 0.2

^aIC₅₀ Values presented as the mean ± SD of three separate determinations.

Significant values are in bold.

Compounds 5a-r displayed variable degrees of inhibitory activity against the tested cancer cell lines. In general, compounds 5e, 5g, 5h, 5k, 5o and 5q displayed potent anti-proliferative activity (IC₅₀ < 25 μg/mL) against at least three of the tested cancer cell lines, compounds 5a, 5d and 5n showed moderate activity (IC₅₀ > 25 to 50 μg/mL), compounds 5b, 5c, 5j and 5r showed weak activity (IC₅₀ > 50 to 75 μg/mL) while compounds 5f., 5i and 5l were inactive (IC₅₀ > 75 μg/mL). In addition, the optimal anti-proliferative activity was attained by compounds 5e and 5k which showed excellent activity against all the tested cell lines.

The anti-proliferative potency was found to be mainly dependent on the nature of the aryl groups at positions 3 and 4 of the N-(adamantan-1-yl)-thiazol-2(3H)-imine core. Within the 3-phenyl derivatives 5a-f, the 3,4-diphenyl analogue 5a exhibited moderate activity against HepG-2 and HeLa cell lines with weak activity against PC-3, HCT-116 and MCF-7 cell lines. Replacement of the 4-phenyl group with 4-(4-chlorophenyl), 4-(4-bromophenyl) or 4-(p-tolyl) substituents greatly deteriorated the anti-proliferative activity against PC-3 and HCT-116 cell lines with almost weak or marginal activity against HepG-2, HeLa and MCF-7 cell lines. The 4-(4-methoxyphenyl) substituent (compound 5e) is optimistic for anti-proliferative activity against all the tested cell lines. In contrary, a 4-(2-naphthyl) substituent (compound 5f.) dramatically declined the anti-proliferative activity.

In the 3-(4-fluorophenyl) derivatives 5g-l, the 4-phenyl analogue 5g displayed potent activity against PC-3, HepG-2 and MCF-7 cell lines and moderate activity against HCT-116 and HeLa cell lines, while the 4-(4-chlorophenyl) analogue 5h exhibited high activity against PC-3, HepG-2 and HeLa cell lines and moderate activity against HCT-116 and MCF-7 cell lines. In addition, the 4-(4-bromophenyl), 4-(p-tolyl) and 4-(2-naphthyl) derivatives 5i, 5j and 5l were almost inactive against all the tested cell lines. The 4-(4-methoxyphenyl) analogue 5k was found to the most active among the 3-(4-fluorophenyl) derivatives 5g-l which resembles the anti-proliferative activity of Doxorubicin.

The anti-proliferative activity of the 3-(4-chlorophenyl) derivatives 5m-r was rather lower than the 3-(4-fluorophenyl) derivatives 5g-l. the 4-phenyl analogue 5m displayed potent activity against HeLa cell lines and moderate activity against PC-3, HCT-116, HepG-2 and MCF-7 cell lines, while the 4-(4-chlorophenyl) analogue 5n showed moderate activity against all the tested cell lines. In contrary to the activity of the 3-phenyl and 3-(4-fluorophenyl) derivatives (5c and 5i), the 4-(4-bromophenyl) analogue 5o showed potent activity against HCT-116, HeLa and MCF-7 cell lines, moderate activity against HepG-2 and week activity against PC-3 cell lines. As in case of the 3-phenyl and 3-(4-fluorophenyl) derivatives (5e and 5k). the anti-proliferative activity of the 4-(4-methoxyphenyl) analogue 5q was found to the most active among the 3-(4-chlorophenyl) derivatives 5m-r as it in displayed potent activity against HCT-116, HeLa and MCF-7 cell lines, moderate activity against HepG-2 and week activity against PC-3 cell lines.

In view of the available results, it can be concluded that the presence of a 4-methoxyphenyl group at C-4 of the thiazole ring is optimal for the anti-proliferative activity, regardless of the nature of the C-3 substitution. Other C-4 substitutions diminish such activity, especially the 2-naphthyl group that impaired activity against all tested cell lines.

Prediction of activity spectra and molecular docking studies

The online PASS (Prediction of Activity Spectra) structure–activity relationship tool⁵¹ was utilized to predict a suitable target of the designed (Z)-N-(adamantan-1-yl)-3,4-diarylthiazol-2(3H)-imine compounds 5a-r. The PASS analysis results indicated that the predicted highest probability of biological activity (Pa) was for histone deacetylase (HDAC) SIRT1 enzyme (Table S2). The PASS prediction results are supported by the reported HDAC inhibitory activity of various adamantane^52–54 and thiazole^{28,29,55–59} derivatives.

HDACs are capable of catalyzing the removal of N-acetyl group from acetylated lysine residues in histones or some non-histone proteins. The overexpression of HDACs has been linked to a variety of solid and hematological malignancies, neurological disorders, inflammatory diseases and metabolic disorders^60–62.

As shown in Table 4, compounds 5e and 5k showed excellent anti-proliferative activity against all the tested cell lines. In order to understand the bioactivity of these compounds, we carried out molecular docking against the SIRT1 enzyme. Before docking of 5e and 5k with SIRT1, we carried out a positive control docking simulation for the NAD cofactor and EX527 inhibitor⁶³ (PDB ID: 4I5) with SIRT1. The result showed that there is an excellent agreement between experimental conformations and the predicted poses of NAD cofactor and EX527 (Fig. 7). The binding affinity for the NAD ligand was calculated to be − 13.2 kcal mol⁻¹ and − 11.0 kcal mol⁻¹ for EX527. The same docking parameters were used for compounds 5e and 5k and the results suggest that both compounds showed similar binding affinities (5e: − 7.0 and 5k: − 7.3 kcal mol⁻¹).

Figure 7.

Experimental conformations (yellow) and predicted poses (magenta) of NAD cofactor and EX527 inhibitor are in stick representation at the active site of SIRT1. The top ranked predicted pose of 5e (green) and 5k (blue) at the active site is also shown.

As can be seen from Fig. 7, compounds 5e and 5k bound in a similar manner at the active site of the SIRT1. Moreover, the adamantane cage of these compounds occupies the position of the cycloheptanecarboxamide moiety of the nanomolar inhibitor EX527⁶⁴. The five-membered ring overlaps with the position of the 5-chlorophenyl of EX527. The methoxyphenyl and the substituted phenyl moieties of 5e and 5k are closer to the position of the nicotinamide and pentose sugar moieties of the NAD cofactor. The predicted pose of 5e and 5k clearly suggest that they could act as potential inhibitors.

As shown in Fig. 8, the interacting residues of the active site of SIRT1 are identical for both compounds 5e and 5k because the binding poses of 5e and 5k are similar. The adamantane moiety establishes hydrophobic interactions with Ala 262, Ile 270, Asn 346, Ile 347 and Asp 348. The substituted phenyl also participated in the hydrophobic interaction with Gln 345 and Val 445. It is interesting to note that His 363 establishes π-stacking interaction with the substituted phenyl ring and the methoxy phenyl ring in compounds 5e and 5k. The active site residue Phe 414 has participated in a hydrophobic interaction with the methoxy phenyl ring.

Figure 8.

Interactions between SIRT1 (a) compound 5e and (b) compound 5k. Small spheres represent the centroid of the respective rings.

Conclusions

Eighteen new adamantane-linked thiazole derivatives were synthesized and their structures were confirmed by ¹H NMR, ¹³C NMR data in addition to single crystal X-ray crystallography. The in vitro antibacterial, antifungal activities of the synthesized compounds were assessed against a panel of pathogenic Gram-positive, Gram-negative bacteria and fungi. Compounds 5c, 5g, 5l, 5m, and 5q displayed potent broad-spectrum antibacterial activity and compounds 5a and 5o showed potent activity against the tested Gram-positive bacteria, while compounds 5b, 5l and 5q displayed potent antifungal activity against Candida albicans. The anti-proliferative activity of the synthesized compounds was evaluated towards five human tumor cell lines. Compounds 5e and 5k showed potent inhibitory activity against all the tested cell lines. Molecular docking simulation analysis demonstrated that the potent compounds 5e and 5k bind to the active site of the SIRT1 enzyme, occupying the positions of the NAD cofactor and the known histone deacetylase inhibitor EX527. The adamantane moiety of these compounds establishes extensive hydrophobic interactions with the active site residues in addition to other interactions.

Materials and methods

Melting points (℃, uncorrected) were measured in open glass capillaries using Stuart SMP30 electro-thermal melting point apparatus. NMR spectra were obtained on a Bruker 400 Avance III at 400.20 MHz for ¹H and 100.63 MHz for ¹³C; and Jeol ECA 500 III at 500.16 MHz for ¹H and 125.77 MHz for ¹³C using CDCl₃ as solvent. The chemical shifts are expressed in δ (ppm) downfield from tetramethylsilane (TMS) as internal standard; coupling constants (J) are pressed in Hz. The splitting patterns were designated as: s (singlet), d (doublet), t (triplet), q (quartet) and m (multiplet). Monitoring the reactions and checking the purity of the final products were carried out by thin layer chromatography (TLC) using silica gel pre-coated aluminum sheets (60 F₂₅₄, Merck), petroleum ether/ethyl acetate (8:2) as eluent and visualization with ultraviolet light (UV) at 365 and 254 nm. Compounds 3a-c were prepared via condensation of adamantan-1-amine 1 with the corresponding aryl isothiocyanate according to the reported procedures^39–41. Elemental analyses (C, H, N & S) were in agreement with the proposed structures within ± 0.3% of the theoretical values (Table S3). The standard experimental methods for evaluation of the antimicrobial and anti-proliferative activities^{42–45,47,50} are presented in the supplementary data. The reference drugs Ampicillin trihydrate (CAS 7177-48-2), Ciprofloxacin (CAS: 85721-33-1), Fluconazole (CAS: 86386-73-4), Erythromycin (CAS: 114-07-8) and Doxorubicin (CAS 23214-92-8) were purchased from Sigma-Aldrich Chemie GmbH, Germany.

General procedure for the synthesis of (Z)-N-(adamantan-1-yl)-3,4-diarylthiazol-2(3H)-imines (5a-r)

A mixture of the appropriate 1-(adamantan-1-yl)-3-arylthiourea 3a-c (2.0 mmol) and the appropriate aryl bromomethyl ketone 4a-f (2.0 mmol), in ethanol (10 mL), was heated under reflux for 24 h. On cooling, sodium acetate (1.0 g) and crushed ice (100 g) were added and the mixture was stirred for 30 min. and allowed to stand at room temperature for 3 h. The precipitated crude product was filtered, washed with water, dried and crystallized from EtOH/CHCl₃ or EtOH to yield the target compounds (5a-r) as fine colorless crystals.

(Z)-N-(Adamantan-1-yl)-3,4-diphenylthiazol-2(3H)-imine 5a. Yield: 78%, m.p.: 198–200 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₆N₂S (386.56). ¹H NMR (500.16 MHz): δ 1.68 (s, 6H, Adamantane-H), 1.89 (s, 6H, Adamantane-H), 2.09 (s, 3H, Adamantane-H), 5.88 (s, 1H, Thiazole-H), 7.05–7.07 (m, 2H, Ar–H), 7.11–7.23 (m, 8H, Ar–H). ¹³C NMR (125.77 MHz): δ 29.64, 36.60, 40.63, 53.49 (Adamantane-C), 96.97 (Thiazole C-5), 126.02, 127.60, 127.77, 127.90, 129.05, 132.52, 138.84, 139.08 (Ar–C), 148.98 (Thiazole C-4), 160.02 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(4-chlorophenyl)-3-phenylthiazol-2(3H)-imine 5b. Yield: 62%, m.p.: 176–178 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₅ClN₂S (421.0). ¹H NMR (400.20 MHz): δ 1.68 (s, 6H, Adamantane-H), 1.88 (s, 6H, Adamantane-H), 2.08 (s, 3H, Adamantane-H), 5.89 (s, 1H, Thiazole-H), 6.98 (d, 2H, Ar–H, J = 8.4 Hz), 7.10–7.18 (m, 5H, Ar–H), 7.22–7.28 (m, 2H, Ar–H). ¹³C NMR (100.63 MHz): δ 29.83, 36.77, 40.82, 53.90 (Adamantane-C), 98.14 (Thiazole C-5), 123.99, 124.59, 126.65, 128.28, 128.45, 129.21, 133.82, 137.98 (Ar–C), 149.30 (Thiazole C-4), 159.34 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(4-bromophenyl)-3-phenylthiazol-2(3H)-imine 5c. Yield: 55%, m.p.: 179–181 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₅BrN₂S (465.45). ¹H NMR (400.20 MHz): δ 1.68 (s, 6H, Adamantane-H), 1.88 (s, 6H, Adamantane-H), 2.08 (s, 3H, Adamantane-H), 5.89 (s, 1H, Thiazole-H), 6.93 (d, 2H, Ar–H, J = 8.4 Hz), 7.10–7.18 (m, 3H, Ar–H), 7.22–7.30 (m, 4H, Ar–H). ¹³C NMR (100.63 MHz): δ 29.78, 36.65, 40.75, 54.19 (Adamantane-C), 99.09 (Thiazole C-5), 120.69, 122.28, 127.12, 128.57, 129.12, 129.58, 131.44, 138.22 (Ar–C), 148.90 (Thiazole C-4), 159.97 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-phenyl-4-(p-tolyl)thiazol-2(3H)-imine 5d. Yield: 88%, m.p.: 158–160 °C (EtOH), Mol. Formula (Mol. Wt.): C₂₆H₂₈N₂S (400.58). ¹H NMR (400.20 MHz): δ 1.70 (s, 6H, Adamantane-H), 1.92 (s, 6H, Adamantane-H), 2.11 (s, 3H, Adamantane-H), 2.28 (s, 3H, CH₃), 5.85 (s, 1H, Thiazole-H), 6.97–6.99 (m, 4H, Ar–H), 7.14–7.17 (m, 3H, Ar–H), 7.22–7.28 (m, 2H, Ar–H). ¹³C NMR (100.63 MHz): δ 21.29 (CH₃), 29.89, 36.83, 40.86, 53.78 (Adamantane-C), 96.56 (Thiazole C-5), 123.90, 126.27, 127.98, 128.89, 129.36, 130.0, 137.78, 139.19 (Ar–C), 149.42 (Thiazole C-4), 160.08 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(4-methoxyphenyl)-3-phenylthiazol-2(3H)-imine 5e. Yield: 85%, m.p.: 156–158 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₆H₂₈N₂OS (416.58). ¹H NMR (400.20 MHz): δ 1.67 (s, 6H, Adamantane-H), 1.89 (s, 6H, Adamantane-H), 2.08 (s, 3H, Adamantane-H), 3.73 (s, 3H, OCH₃), 5.78 (s, 1H, Thiazole-H), 6.67 (d, 2H, Ar–H, J = 8.8 Hz), 6.97 (d, 2H, Ar–H, J = 8.8 Hz), 7.11–7.15 (m, 3H, Ar–H), 7.20–7.26 (m, 2H, Ar–H). ¹³C NMR (100.63 MHz): δ 29.87, 36.82, 40.86, 53.80 (Adamantane-C), 55.22 (OCH₃), 96.13 (Thiazole C-5), 113.56, 125.12, 126.38, 128.10, 129.39, 138.88, 139.25, 157.71 (Ar–C), 149.77 (Thiazole C-4), 159.23 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(naphthalen-2-yl)-3-phenylthiazol-2(3H)-imine 5f. Yield: 80%, m.p.: 177–179 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₉H₂₈N₂S (436.62). ¹H NMR (400.20 MHz): δ 1.76 (s, 6H, Adamantane-H), 1.99 (s, 6H, Adamantane-H), 2.17 (s, 3H, Adamantane-H), 6.04 (s, 1H, Thiazole-H), 7.08–7.26 (m, 6H, Ar–H), 7.47- 7.50 (m, 2H, Ar–H), 7.59 (d, 1H, Ar–H, J = 8.4 Hz), 7.74- 7.78 (m, 3H, Ar–H). ¹³C NMR (100.63 MHz): δ 29.85, 36.78, 40.84, 53.94 (Adamantane-C), 98.21 (Thiazole C-5), 125.55, 126.38, 126.42, 126.51, 127.15, 127.56, 127.64, 128.08, 128.19, 129.21, 132.66, 133.02, 139.03, 139.18 (Ar–C), 150.02 (Thiazole C-4), 159.02 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-fluorophenyl)-4-phenylthiazol-2(3H)-imine 5g. Yield: 90%, m.p.: 193–195 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₅FN₂S (404.55). ¹H NMR (500.16 MHz): δ 1.65 (s, 6H, Adamantane-H), 1.85 (s, 6H, Adamantane-H), 2.06 (s, 3H, Adamantane-H), 5.85 (s, 1H, Thiazole-H), 6.86–6.89 (m, 2H, Ar–H), 7.03–7.08 (m, 4H, Ar–H), 7.15–7.17 (m, 3H, Ar–H). ¹³C NMR (125.77 MHz): δ 29.88, 36.84, 40.93, 53.77 (Adamantane-C), 97.24 (Thiazole C-5), 114.86, 128.02, 128.29, 130.76, 132.54, 135.25, 138.90, 161.73 (Ar–C), 149.27 (Thiazole C-4), 159.78 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(4-chlorophenyl)-3-(4-fluorophenyl)thiazol-2(3H)-imine 5h. Yield: 92%, m.p.: 194–196 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₄ClFN₂S (438.99). ¹H NMR (500.16 MHz): δ 1.66 (s, 6H, Adamantane-H), 1.85 (s, 6H, Adamantane-H), 2.07 (s, 3H, Adamantane-H), 5.87 (s, 1H, Thiazole-H), 6.89–6.93 (m, 2H, Ar–H), 6.96–6.99 (m, 2H, Ar–H), 7.04–7.07 (m, 2H, Ar–H), 7.14–7.16 (m, 2H, Ar–H). ¹³C NMR (125.77 MHz): δ 29.84, 36.76, 40.88, 53.99 (Adamantane-C), 98.20 (Thiazole C-5), 115.19, 128.62, 128.80, 129.11, 130.74, 134.09, 137.87, 161.95 (Ar–C), 149.76 (Thiazole C-4), 159.99 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(4-bromophenyl)-3-(4-fluorophenyl)thiazol-2(3H)-imine 5i. Yield: 84%, m.p.: 221–223 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₄BrFN₂S (483.44). ¹H NMR (400.20 MHz): δ 1.66 (s, 6H, Adamantane-H), 1.85 (s, 6H, Adamantane-H), 2.07 (s, 3H, Adamantane-H), 5.88 (s, 1H, Thiazole-H), 6.89–6.95 (m, 4H, Ar–H), 7.04–7.08 (m, 2H, Ar–H), 7.29 (d, 2H, Ar–H, J = 8.4 Hz). ¹³C NMR (100.63 MHz): δ 29.85, 36.81, 40.93, 53.80 (Adamantane-C), 97.90 (Thiazole C-5), 115.03, 122.14, 129.46, 130.71, 131.44, 135.04, 137.73, 162.04 (Ar–C), 148.72 (Thiazole C-4), 159.60 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-fluorophenyl)-4-(p-tolyl)thiazol-2(3H)-imine 5j. Yield: 64%, m.p.: 144–146 °C (EtOH), Mol. Formula (Mol. Wt.): C₂₆H₂₇FN₂S (418.57). ¹H NMR (400.20 MHz): δ 1.69 (s, 6H, Adamantane-H), 1.89 (s, 6H, Adamantane-H), 2.10 (s, 3H, Adamantane-H), 2.29 (s, 3H, CH₃), 5.84 (s, 1H, Thiazole-H), 6.90–7.01 (m, 6H, Ar–H), 7.09–7.12 (m, 2H, Ar–H). ¹³C NMR (100.63 MHz): δ 21.23 (CH₃), 29.82, 36.76, 40.85, 53.78 (Adamantane-C), 96.68 (Thiazole C-5), 114.84, 127.95, 128.94, 130.77, 135.16, 137.94, 138.95, 161.96 (Ar–C), 149.56 (Thiazole C-4), 159.52 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-fluorophenyl)-4-(4-methoxyphenyl)thiazol-2(3H)-imine 5k. Yield: 94%, m.p.: 179–181 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₆H₂₇FN₂OS (434.57). ¹H NMR (400.20 MHz): δ 1.68 (s, 6H, Adamantane-H), 1.87 (s, 6H, Adamantane-H), 2.08 (s, 3H, Adamantane-H), 3.74 (s, 3H, OCH₃), 5.78 (s, 1H, Thiazole-H), 6.69 (d, 2H, Ar–H, J = 8.8 Hz), 6.88–6.98 (m, 4H, Ar–H), 7.06–7.10 (m, 2H, Ar–H). ¹³C NMR (100.63 MHz): δ 29.86, 36.80, 40.90, 53.80 (Adamantane-C, 55.24 (OCH₃), 96.11 (Thiazole C-5), 113.67, 114.88, 124.85, 129.44, 130.85, 135.15, 138.68, 161.99 (Ar–C), 149.76 (Thiazole C-4), 159.55 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-fluorophenyl)-4-(naphthalen-2-yl)thiazol-2(3H)-imine 5l. Yield: 60%, m.p.: 155–157 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₉H₂₇FN₂S (454.61). ¹H NMR (500.16 MHz): δ 1.68–1.69 (m, 6H, Adamantane-H), 1.89–1.00 (m, 6H, Adamantane-H), 2.09 (s, 3H, Adamantane-H), 5.98 (s, 1H, Thiazole-H), 6.85–6.89 (m, 2H, Ar–H), 7.01–7.02 (m, 1H, Ar–H), 7.11–7.15 (m, 2H, Ar–H), 7.43–7.46 (m, 2H, Ar–H), 7.57 (d, 1H, Ar–H, J = 8.5hz), 7.68 (s, 1H, Ar–H), 7.71–7.75 (m, 2H, Ar–H). ¹³C NMR (125.77 MHz): δ 29.86, 36.78, 40.85, 54.04 (Adamantane-C), 98.18 (Thiazole C-5), 115.10, 125.58, 126.58, 127.37, 127.74, 128.14, 129.79, 130.77, 130.84, 132.77, 133.06, 134.91, 139.13, 161.88 (Ar–C), 150.28 (Thiazole C-4), 159.93 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-chlorophenyl)-4-phenylthiazol-2(3H)-imine 5m. Yield: 76%, m.p.: 190–192 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₅ClN₂S (421.0). ¹H NMR (500.16 MHz): δ 1.66–1.68 (m, 6H, Adamantane-H), 1.86–1.87 (m, 6H, Adamantane-H), 2.08 (s, 3H, Adamantane-H), 5.87 (s, 1H, Thiazole-H), 7.04–7.07 (m, 4H, Ar–H), 7.15–7.20 (m, 5H, Ar–H). ¹³C NMR (125.77 MHz): δ 30.12, 37.08, 41.17, 54.08 (Adamantane-C), 97.97 (Thiazole C-5), 128.25, 128.34, 128.44, 128.64, 130.67, 132.74, 138.09, 138.87 (Ar–C), 149.10 (Thiazole C-4), 159.09 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3,4-bis(4-chlorophenyl)thiazol-2(3H)-imine 5n. Yield: 77%, m.p.: 197–199 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₄Cl₂N₂S (455.44). ¹H NMR (400.20 MHz): δ 1.66 (s, 6H, Adamantane-H), 1.85 (s, 6H, Adamantane-H), 2.06 (s, 3H, Adamantane-H), 5.88 (s, 1H, Thiazole-H), 6.97 (d, 2H, Ar–H, J = 8.4 Hz), 7.03 (d, 2H, Ar–H, J = 8.8 Hz), 7.15–7.20 (m, 4H, Ar–H). ¹³C NMR (100.63 MHz): δ 29.81, 36.74, 40.86, 53.99 (Adamantane-C), 98.48 (Thiazole C-5), 128.45, 128.68, 129.18, 130.34, 134.08, 137.54 (Ar–C), 148.55 (Thiazole C-4), 159.02 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-4-(4-bromophenyl)-3-(4-chlorophenyl)thiazol-2(3H)-imine 5o. Yield: 82%, m.p.: 210–212 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₅H₂₄BrClN₂S (499.90). ¹H NMR (400.20 MHz): δ 1.66 (s, 6H, Adamantane-H), 1.84 (s, 6H, Adamantane-H), 2.07 (s, 3H, Adamantane-H), 5.89 (s, 1H, Thiazole-H), 6.91 (d, 2H, Ar–H, J = 8.0 Hz), 7.03 (d, 2H, Ar–H, J = 8.4 Hz), 7.18 (d, 2H, Ar–H, J = 8.4 Hz), 7.30 (d, 2H, Ar–H, J = 8.0 Hz). ¹³C NMR (100.63 MHz): δ 29.81, 36.75, 40.88, 53.93 (Adamantane-C), 98.46 (Thiazole C-5), 122.24, 128.42, 129.40, 130.33, 131.61, 137.53 (Ar–C), 148.47 (Thiazole C-4), 159.07 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-chlorophenyl)-4-(p-tolyl)thiazol-2(3H)-imine 5p. Yield: 90%, m.p.: 188–190 °C (EtOH), Mol. Formula (Mol. Wt.): C₂₆H₂₇ClN₂S (435.03). ¹H NMR (500.16 MHz): δ 1.65–1.67 (m, 6H, Adamantane-H), 1.85–1.86 (m, 6H, Adamantane-H), 2.07 (s, 3H, Adamantane-H), 2.28 (s, 3H, CH₃), 5.82 (s, 1H, Thiazole-H), 6.92 (d, 2H, Ar–H, J = 8.0 Hz), 6.98 (d, 2H, Ar–H, J = 8.0 Hz), 7.04–7.06 (m, 2H, Ar–H), 7.15–7.17 (m, 2H, Ar–H). ¹³C NMR (125.77 MHz): δ 21.33 (CH₃), 29.87, 36.83, 40.91, 53.80 (Adamantane-C), 97.03 (Thiazole C-5), 127.90, 128.18, 129.09, 129.50, 130.47, 131.25, 138.02, 138.69 (Ar–C), 149.0 (Thiazole C-4), 158.89 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-chlorophenyl)-4-(4-methoxyphenyl)thiazol-2(3H)-imine 5q. Yield: 82%, m.p.: 160–162 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₆H₂₇ClN₂OS (451.03). ¹H NMR (500.16 MHz): δ 1.65–1.67 (m, 6H, Adamantane-H), 1.85–1.86 (m, 6H, Adamantane-H), 2.07 (s, 3H, Adamantane-H), 3.75 (s, 3H, OCH₃), 5.78 (s, 1H, Thiazole-H), 6.70 (d, 2H, Ar–H, J = 9.0 Hz), 6.95 (d, 2H, Ar–H, J = 9.0 Hz), 7.04 (d, 2H, Ar–H, J = 9.0 Hz), 7.16 (d, 2H, Ar–H, J = 8.5hz). ¹³C NMR (125.77 MHz): δ 29.85, 36.80, 40.86, 53.84 (Adamantane-C), 55.30 (OCH₃), 96.47 (Thiazole C-5), 113.76, 124.84, 128.23, 129.39, 130.53, 131.71, 137.78, 138.44 (Ar–C), 149.43 (Thiazole C-4), 159.36 (Thiazole C-2).

(Z)-N-(Adamantan-1-yl)-3-(4-chlorophenyl)-4-(naphthalen-2-yl)thiazol-2(3H)-imine 5r. Yield: 89%, m.p.: 177–179 °C (EtOH/CHCl₃), Mol. Formula (Mol. Wt.): C₂₉H₂₇ClN₂S (471.06). ¹H NMR (500.16 MHz): δ 1.68 (s, 6H, Adamantane-H), 1.88–1.89 (m, 6H, Adamantane-H), 2.09 (s, 3H, Adamantane-H), 5.99 (s, 1H, Thiazole-H), 6.99–7.02 (m, 1H, Ar–H), 7.10–7.15 (m, 4H, Ar–H), 7.44–7.48 (m, 2H, Ar–H), 7.58 (d, 1H, Ar–H, J = 9.0 Hz), 7.69 (s, 1H, Ar–H), 7.71–7.75 (m, 2H, Ar–H). ¹³C NMR (125.77 MHz): δ 29.87, 36.83, 40.93, 53.90 (Adamantane-C), 98.34 (Thiazole C-5), 125.49, 126.59, 127.11, 127.76, 127.89, 128.13, 128.28, 130.05, 130.37, 131.72, 132.77, 133.12, 137.79, 138.71 (Ar–C), 148.94 (Thiazole C-4), 158.96 (Thiazole C-2).

Preparation of the single crystal of compounds 5d and 5f

Analytically-pure samples of compounds 5d and 5f. (50 mg) were dissolved in 15mL cyclohexane/CHCl₃ and EtOH/CHCl₃ (1:1, v/v), respectively. Slow evaporation of the solutions at room temperature yielded the single crystals as colorless prisms.

Molecular Modelling and molecular docking analysis

The structures of 5e and 5k were drawn using ChemSketch Freeware (www.acdlabs.com) and these structures were subsequently optimized with the B3LYP/6-31G level of theory using the Gaussian 09 program⁶⁵. The optimized structures were used for molecular docking simulations. The crystal structure of SIRT1 catalytic domain bound to NAD and an EX527 analog was retrieved from the protein data bank (PDB ID: 4I5I). The molecular docking simulation against SIRT1 was performed using the AutoDock Vina program⁶⁶ available in PyRX-0.8 virtual tools⁶⁷. The chain A of 4I5I and the ligands 5e and 5k were prepared and used for docking calculation. The active site grid was constructed based on the position of NAD ligand. To check the predictive power of the AutoDock Vina program, we also performed docking for the cocrystallized ligand NAD. The experimental and predicted poses were very similar. The top ranked binding poses were considered for protein–ligand interaction analysis using PLIP web server⁶⁸.

Author contributions

A.A.El-E. and M.B.El-A. designed the study and supervised the chemical synthesis. E.T.W. synthesized characterized the compounds. M.S.M.A. and S.G.-G. performed the X-ray data collection and the resolution of the crystal structure. E.E.H. conceived the antimicrobial activity studies. S.T. conducted the molecular docking analysis. All authors contributed in the preparation of the manuscript, discussed the contents and approved the submission.

Funding

Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability

The crystallographic data for the structures of compounds 5d (CCDC #: 2,155,510) and 5f. (CCDC #: 2,155,511) could be obtained free of charge from the Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk/data_request/cif).

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-022-25390-0.