Regioselective aluminum chloride catalyzed reactions of unsaturated electrophiles with thiazoles

Aluminum chloride promotes reactions of α,β-unsaturated esters and nitriles with 1,3-thiazoles to give 1:1 and 2:1 adducts, the nature and distribution of which depend on the substituent R at C2 of the heterocycle and the nucleophile QN employed as a quencher of the reaction mixture. With 2-bromothiazole, ethyl propiolate, and dimethyl acetylenedicarboxylate give substituted (E)- and (Z)-N-vinylthiazolin-2-ones (QN, aqueous sodium bicarbonate) and 2-imino-N-vinylthiazolines (QN, benzylamine), while ethyl acrylate and acrylonitrile afford substituted N-propanoylthiazolin-2-ones (QN, aqueous sodium bicarbonate). With 1,3-thiazole and 2-ethylthiazole, ethyl propiolate gives (QN, aqueous sodium bicarbonate) N-formyl- and N-propanoylthiazolines, respectively, together with 2:1 open-chain adducts containing 2 mol of acetylene and 1 mol of thiazole. A plausible scheme accounting for the formation of the observed products involves a regioselective attack by of the multiple bond of the ester or nitrile coordinated with AlCl3 on the nitrogen of the thiazole ring to give a zwitterion with allenic structure which by action of the QN undergoes C2-R (R = Br) or C2-S (R = H, C2H5) bond fission and forms the observed products. © 1982, American Chemical Society. All rights reserved.