Synthesis and biological evaluation of novel Indolyl 4-thiazolidinones bearing thiadiazine nucleus

1 Introduction

Indole and its derivatives occupied a unique place in the chemistry of nitrogen heterocyclic compounds because of their varied biodynamic properties (Biradar et al., 2010; Mohit et al., 2007; Sharma et al., 2010) and thiadiazine derivatives are also reported to possess diverse biological activities (Jaishree et al., 2009; Prakash et al., 2008; Shivarama et al., 2002). In addition, the interest in 4-thiazolidinone has increased due to the high biological activity and broad-spectrum action of their derivatives (Verma and shailendra, 2008), such as bactericidal (Kouatli et al., 2010), fungicidal (Anshu et al., 2006), anti-convulsant (Dimri and Parmar,1978), anti-inflammatory (Bhawna et al., 1999), anaesthetic (Manrao and Kohli, 1986), potentiation of phenobarbital induced sleeping time (Manrao et al., 1982) and potent Anti-HIV agents (Maria et al., 2002). Based on these facts and in continuation of our research on the synthesis of bioactive Indole analogues (Biradar and Doddappa, 2007; Doddappa and Biradar, 2009) and interest in thiazolidinones (Renuka and Biradar, 2000) as a bioactive active entity, we designed to synthesize novel Indolyl 4-thiazolidinone derivatives as triheterocycles containing the bioactive heterocyclic ring with the expectation of enhanced biological activity of the molecule. Hence we have synthesized substituted Indolyl 4-thiazolidinones bearing the thiadiazine ring, and carried out antimicrobial assay against various bacterial and fungal microorganisms. Selected compounds were evaluated for analgesic and anti-inflammatory activities.

2 Experimental

3 Results and discussion

The requisite compound Ethyl 2-hydrazinyl-5,6-dihydro-5-oxo-4H-1,3,4-thiadiazine-6-carboxylate (2) was prepared from the literature method (Chande et al., 2000) and 2,5-disubstituted indole-3-corboxaldehydes (1a–d) were prepared from the Bischler method of Indole synthesis followed by Vilsmeier–Haack formylation (Hiremath et al., 1982). Synthetic pathway for Ethyl 2-{(2E)-2-[(2,5-disubstituted-1H-indol-3-yl) methyleno] hydrazine}-5-oxo-5,6-dihydro-4H-1,3,4-thiadiazine-6carboxylates (3a–g) and Ethyl2-[2-(2,5-disubstituted-1H-indol-3-yl)-4-oxothiazolid-3-ylamino]-5,6-dihydro-5-oxo-4H-1,3,4-thiadiazine-6-carboxylates (4a–g) is summarized in Scheme 1. An equimolar mixture of substituted thiadiazine (2), substituted indole-3-carboxaldehyde (1a–d) and sodium acetate were refluxed in ethanol (20 mL) for 6–7 h. The solution was then concentrated under reduced pressure up to half of the original volume and poured over crushed ice; it was then filtered, dried and recrystallized from ethanol to obtain compound 3a–g. The formation of compound 3a is confirmed by their spectral and analytical data as follows: IR spectrum of compound 3a as an example, showed the peak at 3162 cm⁻¹ due to indole NH groups, 3000 cm⁻¹ and 2855 cm⁻¹due to secondary aliphatic amine groups, carbonyls appeared at 1743 cm⁻¹ and 1631 cm⁻¹ for ester carbonyl and azine carbonyl. Other peaks at 1456 cm⁻¹ and 767 cm⁻¹ are due to C⚌N and –S–, respectively. ¹H NMR spectrum of compound 3a showed the peaks at 3.3 δ for –CH₂– and 2.4 δ –CH₃ for the ethyl group and 4.3 δ for thiadiazine S–CH single proton. A multiplet appeared between 7.1 and 8.1 δ due to deshielded aromatic protons. Two singlets are at 8.9 δ and 12.3 δ for N–NH and N–NH–CO of the thiadiazine ring protons. Singlet at 10 δ is due to the dishielded indole NH. Peak at 6.4 δ is due to the deshielded protons of CH⚌N. Mass spectrum of 3a has not shown the molecular ion peak, it may be due to the unstability of the molecular ion. Molecular ion has undergone into fragmentation displaying the peaks A₁ at m/z 267(7%) and m/z 269(2%). A₁ has undergone into further fragmentation by two routes. In one route it has lost N₂ and H₂ showed peaks A₂ at m/z 236(100%) and m/z 238(32%), in another route it has lost CN and N₂ to display peaks A₃ at m/z 214(6%) and m/z 216(1.5%). A₃ has then lost the chlorine radical and displayed peak A₄ at 178(7%). Fragment A₄ loses C₂H₂ to displayed peak A₅ at m/z 154(98%). This fragmentation pattern of the mass spectrum supports the proposed structure of compound 3a. (See Schemes 2 and 3)

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Scheme 1 Synthetic pathway of substituted indolyl 4-thiazolidinone derivatives.

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Scheme 2 Mass fragmentation of compound 3a.

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Scheme 3 Mass fragmentation of compound 4a.

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The final compounds 4a–g were synthesized by equimolar ratio of the compound 3a–g and thiaglycolic acid (mercapto acetic acid) in dimethylformamide with catalytic amount of zinc chloride (anhydrous), then the reaction mixture was refluxed for 6–8 h. The reaction mixture was cooled and poured into crushed ice, the product thus obtained was filtered, washed with cold water and recrystallized from ethanol/1, 4-diaxane to afford compound 4a–g. The formation of the title compounds confirmed by their spectral and analytical reports are as follows; IR spectrum of compound 4a as an example, showed characteristic peaks at 3141 cm^–1 and 3060 cm^–1 for indole and thiadiazine NH, respectively. The peaks at 1659 cm⁻¹, 1628 cm⁻¹ are due to two carbonyls. Appearance of two peaks at 799 cm⁻¹ and 762 cm⁻¹ are for the thio(-S-) group of thiadiazine and cyclized thiozolidinone ring, respectively. ¹H NMR spectrum of compound 4a showed peaks at 1.8 δ (t, 3H, CH₃) and 4.2 δ (q, 2H, CH₂) are for the ethyl attached to the thiadiazine ring. Singlets appearing at 3.6 δ (S–CH) and 12.6 δ (CO–NH) are the protons of the thiadiazine ring. A characteristic peak at 4.8 δ (s, 2H, thiazolidinone) is due to the two protons of the thiazolidinone methylene group. Singlet appearing at 3.4 δ is accounted for a proton of the thiazolidinone ring (–CH⚌). Multiplet in the region 7.3–7.8 δ (m, 8H, ArH) is due to the aromatic protons. Deshielded indole –NH– appeared at 9.9 δ. Structure of compound 4a is further supported by the mass spectrum.

Mass spectrum of compound 4a has shown the molecular ion peak M⁺ at m/z 527(2%) and m/z 529(0.5%), which is in agreement with the molecular weight of the compound. Molecular ion peak had undergone further fragmentation and displayed the peaks A₁ at m/z 452(2%) and m/z 454(0.5%), A₂ at m/z 368(8%) and m/z 370(3%). Another fragment A₃ is at m/z 326(10%) and m/z 328(3%). A₃ eliminates C₂H₄OS and displayed stable fragment A₄ at m/z 253(100%) and m/z 255(30%), A₄ then lost CN, Cl and C₂H₂N to generate A₅ at m/z 152 (1%). This fragmentation pattern of the mass spectrum supports the proposed structure of compound 4a. Structures of other derivatives of the series (3b–g) and (4b–g) were confirmed based on their spectral and analytical data. C,H,N elemental analysis for the title compounds 3a–g and 4a–g indicated that the calculated and observed values were within acceptable limits (±0.4%).

4 Conclusion

We have synthesized series of novel Ethyl 2-{(2E)-2-[(2,5-disubstituted-1H-indol-3-yl) methy leno] hydrazine}-5-oxo-5,6-dihydro-4H-1,3,4-thiadiazine-6carboxylates (3a–g) and Ethyl 2-[2-(2,5-disubstituted-1H-indol-3-yl)-4-oxothiazolid-3-ylamino]-5,6-dihydro-5-oxo-4H-1,3,4-thiadiazine-6-carboxylates (4a–g) derivatives and screened them for in-vitro antimicrobial activities against various microbial strains. We found evidently that compounds 3e, 3g, 4a, 4d, 4c, 4g and 3g, 4d, 3c, 3f are significantly active towards microbial organisms. Selected compounds on the basis of SAR studies have been evaluated for analgesic and anti-inflammatory activities. Among the title compounds 4c, 4d and 4f have shown analgesic and 4a, 4b, 4d, 4e and 4f have displayed significant anti-inflammatory activities. The bioactivity of these compounds was unknown and these studies may promote a further development of the Indolyl 4-thiazolidinones, which may lead to compounds with better pharmacological profile than standard drugs.