Additions and cycloadditions of ketenes to 1,3-thiazoles and its alkyl derivatives

1,3-Thiazole and its 4-methyl and 5-methyl derivatives react with tert-butylcyanoketene (TBCK) and dichloroketene (DCK), affording Michael-type addition products at C2, viz., 2-acylthiazoles, and 2:1 cycloadducts (with TBCK only) which proved by X-ray analysis to be bicyclic systems constituted by a thiazoline and a piperidine-1,3-dione ring condensed across the C-N bond. 2-Ethyl- and 2-isopropyl-1,3-thiazole undergo acylation by DCK at the Cαof the alkyl chain. The latter thiazole gives also a 2:1 cycloadduct, which X-ray analysis showed to be a bicyclic system constituted by a thiazoline and an oxazinone ring condensed across the C-N bond. A mechanism is envisaged involving the quatemization of the thiazole nitrogen by the ketene to give an N-thiazolium enolate system which owing to proton exchange between the C2of the ring or the Cαof the 2-alkyl chain and its enolate portion is in equilibrium with an N-acylthiazolium ylide or zwitterion, respectively. Subsequent reactions of these ...