Regioselective synthesis and biological evaluation of some novel thiophene-containing heterocyclic scaffolds as potential chemotherapeutic agents

Abstract

In the reaction of 2-amino-3-carbethoxythiophene derivative 2b with hydrazine hydrate the respective aminocarbohydrazide 3 was isolated. Treatment of the latter product with carbon disulfide in alkaline medium caused a heterocyclization to give 1,3,4-oxadiazole-2-thione 5 rather than 3-amino-2-thioxothieno[2,3-d]pyrimidin-4-one 4, which could be obtained on treatment of carbohydrazide 3 with thiourea. The structure of products 4 and 5 was proved by spectroscopic methods and chemical transformations. Derivatization led to two novel series of condensed and uncondensed thiophenes, which were of significant interest for biological study. Since compound 4 contains two adjacent reactive functional groups, it reacted readily with different electrophilic reagents to provide a series of thieno[2,3-d]pyrimidin-4-one derivatives with annelated bridgehead nitrogen heterocycles 8-11, whereas a second series of 3-heteroaryl-substituted thiophenes 13-17 was obtained by thioamide functionalization in 1,3,4-oxadiazole derivative 5 using various chemical reagents. The new thiophene-based derivatives were evaluated for their preliminary antimicrobial activity against a representative panel of Gram-positive and Gram-negative bacteria as well as fungi strains. The compounds tested displayed different levels of inhibitory effects, with the assays carried out on two pathogenic bacteria and two pathogenic fungi. Of these compounds, the monocyclic aminothiophene derivative 15 showed the highest effect on pathogenic bacteria, while the tricyclic condensed thiophene derivative 8 was observed to have the same inhibitory effect against pathogenic mould (Aspergillus flavus) as the reference drug Amphotericin B. For those derivatives belonging to first series of condensed thiophenes with annelated bridgehead nitrogen heterocycles, it has been observed that the antibacterial effect was in general found to be significantly higher than the corresponding uncondensed analogues. Although most of the condensed and uncondensed thiophenes under investigation showed generally remarkable in vitro antibacterial activity, unfortunately, no significant antifungal activity was observed with any of the compounds tested except for 8. Surprisingly, compound 8 displayed an excellent effect on fungus (A. flavus) on the one hand, whereas the lowest effect on bacteria on the other. The attachment of a triazolothiadiazine moiety to a thiophene ring could be considered as a promising strategy for the development of new therapeutic antibacterial agents related to the aminothiophene system.