Alkyl 5-amino-2-mercaptothiazole-4-carboxylates (5) have been prepared in a one-pot synthesis by the action of hydrogen sulfide on a mixture of alkyl 2-oximino-2-cyanoacetate and trialkyl orthoformate in pyridine; reduction of the oximino group to imino followed by its trapping with trialkyl orthoformate has been instrumental. S-Alkylation of 5 with (ethenyl/ethynyl)methyl bromides has been performed under phase-transfer catalysed conditions avoiding any possible ring-transformation reaction. Thermal 'sulphur to nitrogen' rearrangement of ethyl 5-amino-2-allylmercaptothiazole-4-carboxylate (12b) has furnished 5-amino-3-allyl-4-ethoxycarbonylthiazole-2(3H)-thione (15); addition of AIBN has lowered the reaction-temperature and improved the yield. Similar rearrangement of ethyl 5-amino-2-propargylmercaptothiazole-4-carboxylate (14b) has furnished 5-methylpyrrolo[3,2-d]thiazole-2(2H,4H,6aH)-thione (17) and 3a-ethoxycarbonyl-5- methylpyrrolo[3,2-d]thiazole-2(2H,3H,3aH)-thione (16); addition of AIBN has improved the yield of 17.