Synthesis and characterization of N-glucosylated dithiadiazepine derivatives through carbon-sulfur bond formation

Introduction

Heterocyclic ring systems that contain nitrogen and sulfur atoms are important scaffolds in medicinal chemistry [1–7]. Diazepines are seven-membered heterocyclic compounds with two nitrogen atoms. Such molecules possess a wide range of medicinal properties [8]. Substitution of the diazepine segment in the bioactive molecule often leads to improvement of pharmacological activities [9]. Some of 1,2-diazepine derivatives are used in the treatment of epilepsy, malignant gliomas, and amyotrophic lateral sclerosis [10–12]. Synthesis, ESR, and X-ray diffraction studies of 1,4,5,7-dithiadiazepine and 1,6,2,4-dithiadiazepine derivatives have been reported [13–15]. Although extensive synthetic studies have been published for various thiadiazepines [16–18], to our best knowledge, not much work has been reported for the synthesis of 1,3,5,6-dithiadiazepines.

Carbohydrates play a vital role in numerous biological processes including modification of proteins, molecular recognition, and immune response [19]. Carbohydrates are an important moiety in many natural antibiotics including bleomycin, neocarzinostatin, and other DNA-targeting drugs [20]. Introduction of carbohydrates into synthetic drugs leads to new hybrid molecules of considerable interest. For example, high levels of glucosylation imparts molecular changes that accompany malignant transformations, which is characteristic of cancer cells [21]. The carbohydrate moiety can be expected to act as drug carrier and improve the selectivity of compounds for cancerous cell lines.

In view of the biological importance of diazepine derivatives [22–24] and in continuation of our work on the synthesis of N-glucosylated heterocyclic compounds [25–28], N-glucosylated dithiadiazepine derivatives have been synthesized through the formation of carbon-sulfur bonds. The current study aims at the synthesis of new N-glucosylated dithiadiazepine derivatives, which are uncommon 7-membered heterocycles.

Results and discussion

1,6-Diaryl-2,5-dithio-bis-ureas 2a–e have been synthesized by reacting two equivalents of aryl isothiocyanates 1a–e with one equivalent of hydrazine hydrate at ambient temperature [29, 30]. The IR spectrum of 2c shows an absorption band at 3210 cm^-1, which indicates the presence of the NH functional group. The absence of an absorption band at 2800 cm^-1 and presence of an absorption band at 628 cm^-1 for C=S indicated that the symmetric dithio-bis-ureas 2a–e exist in the thione tautomeric form. Further support for this structural feature comes from analysis of the ¹H NMR spectrum of 2c, which does not show a peak attributable to the SH group [31] but shows a peak in the low field region at δ 9.60 for four NH protons [32]. Three signals at δ 7.37, 7.20, and 6.96 belong to eight aromatic protons. The signal in the upfield region at δ 2.33 is attributed to the methyl protons.

The symmetrical dithio-bis-ureas 2a–e were then allowed to react with N-tetra-O-acetyl-β-d-glucopyranosyl isocyanodichloride (4) in a mixture of CHCl₃-DMF (1:1) under reflux conditions to yield hydrochloride salts of 4,7-bis(arylamino)-2-tetra-O-acetyl-β-d-glucopyranosylimino-1,3,5,6-dithiadiazepines 5a–e, as depicted in Scheme 1. During the intermolecular cyclocondensation reaction, evolution of the hydrogen chloride gas was clearly noticed by testing with moist blue litmus paper. The final compounds 5a–e were found to be in the salt forms, which, upon treatment with saturated solution of sodium bicarbonate, were transformed into free bases 6a–e. Formation of these products may be rationalized by the mechanism shown in Scheme 2.

Scheme 1

Synthetic route for the synthesis of compounds 6a–e from symmetrical dithio-bis-ureas (2a–e).

Scheme 2

Plausible mechanism for the formation of compound 6 through C-S bond formation.

For optimization of the cyclocondensation reaction, the preparation of 6a was carried out in different solvents including CHCl₃, DCM, a mixture of CHCl₃-DMF, and DMF under reflux condition. The time needed for completion of the reaction varied from 5 to 10 h. The effect of solvent on the reaction time and product yield was observed. The yield of 6a was found to increase significantly when the reaction was carried out in the 1:1 CHCl₃-DMF mixture. The reaction conducted in other solvents gave the desired product 6a in a very low yield. The optimized solvent was used for the conversion of 2b–e to 6b–e.

Detailed analyses of IR, ¹H NMR, ¹³C NMR, and mass spectra of compounds 6a–e were fully consistent with the given structures. The IR spectra of compounds 6a–e show two strong bands in the ranges of 3300–3100 cm^-1 and 1742–1753 cm^-1 due to the N-H and ester carbonyl stretching, respectively. The ¹H NMR spectra show a multiplet due to the glucosyl ring protons in the range of δ 5.35–3.83 ppm and a characteristic triplet or doublet for anomeric proton (H₁) of the glucose moiety within the region of δ 5.30–5.40. Coupling constant (J) of H1/H2 within the range of 12.3–9.3 Hz indicates that the glucosyl moiety exists as the β anomer [33]. The acetyl protons of the glucosyl moiety resonate as a multiplet within δ 2.06–1.83. In addition, the ¹H NMR spectra of compounds 6a–e exhibit a broad singlet centered at δ 9.70 for the NH-aryl functionality. The aromatic protons also appear in the expected region as are other protons of the substituents. The ¹³C NMR spectra of compounds 6a–e show numbers of signals that are fully consistent with the number of carbon atoms in the molecules. The signal for β anomeric carbon is observed within the range of δ 83.50–84.06. The mass spectra of compounds 6a–e show ion peaks [M+1]⁺ and the characteristic fragment ion peaks at 169, 331, and 413 for the glucosyl moiety.

Conclusions

We achieved the synthesis of 1,3,5,6-dithiadiazepine derivatives 6a–e having glucosyl residue through a C-S bond formation in an efficient manner. The overall conversion was carried out in three steps in good yields.

Experimental

Melting points were measured using an electro-thermal apparatus and are not corrected. FT-IR spectra were recorded using KBr disks on a Perkin Elmer FT-IR spectrophotometer. ¹H NMR (400 MHz) and ¹³C NMR (400 MHz) spectra were recorded on a Brucker Avance II 400 NMR spectrometer. Electron-impact mass spectra were obtained at an ionizing potential of 70eV. Optical rotations were measured using an Equip-Tronics Digital Polarimeter EQ-801. Purity of compounds was checked by thin layer chromatography, which was performed on aluminum sheet Silica Gel 60 F₂₅₄ (Merck). The spots were visualized by exposure to UV light and iodine vapor.

1,6-Diaryl-2,5-dithio-bis-ureas 2a–e were synthesized by the previously described method and exhibited virtually identical physical properties as reported in the literature [29–31]. N-Tetra-O-acetyl-β-d-glucopyranosyl isocyanodichloride (4) was synthesized by excessive chlorination of N-tetra-O-acetyl-β-d-glucopyranosyl isothiocyanate (3) using the previously reported method [25, 26].