REACTION OF ETHYL DIAZOACETATE WITH 1,3-THIAZOLE-5(4H)-THIONES

Reaction of ethyl diazoacetate (2a) and 1,3-thiazole-5(4H)-thiones 1a, b in Et(2)O at room temperature leads to a complex mixture of the products 5-9 (Scheme 2). Without solvent, 1a and 2a react to give 10a in addition to 5a-9a. In Et(2)O in the presence of aniline, reaction of 1a, b with 2a affords the ethyl 1,3,4-thiadiazole-2-carboxylate 10a and 10b, respectively, as major products. The structures of the unexpected products 6a, 7a, and 10a have been established by X-ray crystallography. Ethyl 4H-1,3-thiazine-carboxylate 8b was transformed into ethyl 7H-thieno[2,3-e][1,3]thiazine-carboxylate 11 (Scheme 3) by treatment with aqueous NaOH or during chromatography. The structure of the latter has also been established by X-ray crystallography. In the presence of thiols and alcohols, the reaction of la and 2a yields mainly adducts of type 12 (Scheme 4), compounds 5a, 7a, and 9a being by-products (Table I). Reaction mechanisms for the formation of the isolated products are delineated in Schemes 4-7: the primary cycloadduct 3 of the diazo compound and the C=S bond of 1 undergoes a base-catalyzed ring opening of the 1,3-thiazole-ring to give 10. In the absence of a base, elimination of N-2 yields the thiocarbonyl ylide A', which is trapped by nucleophiles to give 12. Trapping of A' by H2O yields 1,3-thiazole-5(4H)-one 9 and ethyl mercaptoacetate, which is also a trapping agent for A', yielding the diester 7. The formation of products 6 and 8 can be explained again via trapping of thiocarbonyl ylide A' either by thiirane C (Scheme 6) or by 2a (Scheme 7). The latter adduct F yields 8 via a Demjanoff-Tiffeneau-type ring expansion of a 1,3-thiazole to give the 1,3-thiazine.