Addition reactions of heterocyclic compounds. Part LXIII. New structures for some 2 : 1 molar adducts from dimethyl acetylenedicarboxylate with thiazoles and benzo-imidazoles, -oxazoles, and -thiazoles formed by novel rearrangement. Crystal and molecular structure determinations for tetramethyl 3,8a-dimethylpyrido[2,1-b]thiazole-5,6,7,8-tetracarboxylate and tetramethyl 5-methylpyrido[2,1-b]thiazole-6,7,8,8a-tetracarboxylate

2,4-Dimethylthiazole with dimethyl acetylenedicarboxylate gives tetramethyl 3,8a-dimethylpyrido[2,1-b]thiazole-5,6,7,8-tetracarboxylate, whereas thiazole and other alkylthiazoles give tetramethyl pyrido[2,1-b]thiazole-6,7,8,8a-tetracarboxylates, rearrangements having taken place. Similar rearrangements can occur in the benzothiazole and benzimidazole series, and earlier formulations proposed for a number of adducts from several heterocycles with the acetylenic ester have now been revised on the basis of 13C and 1H n.m.r. spectra. The structures of the adducts from 2-methyl- and 2,4-dimethyl-thiazoles have been established by X-ray diffraction studies.