Umsetzung von Diazoessigsäure‐ethylester mit 1,3‐Thiazol‐5(4 H )‐thionen

Reaction of Ethyl Diazoacetate with 1,3‐Thiazole‐5(4 H )‐thiones Reaction of ethyl diazoacetate ( 2a ) and 1,3‐thiazole‐5(4 H )‐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 4 H ‐1,3‐thiazine‐carboxylate 8b was transformed into ethyl 7 H ‐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 1a and 2a yields mainly adducts of type 12 ( Scheme 4 ), compounds 5a,7a , and 9a being by‐products ( Table 1 ). 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 CS 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 H 2 O yields 1,3‐thiazole‐5(4 H )‐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.