Azoles. Part 9. Synthesis of derivatives of thieno[2,3-d]thiazole, 4H-pyrrolo-[2,3-d]thiazole, 2H-pyrazolo[3,4-d]thiazole and isoxazolo[3,4-d]thiazole from thiazolidine-2,4-dione

The aldehyde group of 2,4-dichlorothiazole-5-carbaldehyde was protected and the chlorine atom at position 2 was replaced by hydrogen and a methylthio group, via Cl → Li exchange, to give 4-chloro-thiazole-5-carbaldehyde and its 2-methylthio derivative, respectively. Ethyl 2-mercaptoacetate reacted with 4-chlorothiazole-5-carbaldehydes to give thieno[2,3-d]thiazoles. Saponification of ethyl thieno[2,3-d]thiazole-5-carboxylate and attempted decarboxylation of the resulting acid failed to yield enough of the parent thieno[2,3-d]thiazole to allow full characterisation owing to its high volatility.4-Azidothiazole-5-carbaldehydes were prepared by nucleophilic displacement of the chlorine atom in the corresponding 4-chlorothiazole-5-carbaldehyde and converted into alkenes and Schiff's bases by standard procedures. On being heated in toluene these afforded the corresponding 4H-pyrrolo-[2,3-d]thiazole or 2H-pyrazolo[3,4-d]thiazole. lsoxazolo[3,4-d]thiazole and its 2-methylthio derivative, both unstable, were obtained by heating the corresponding 4-azidothiazole-5-carbaldehyde in bromobenzene.