Reaction of thiocarbonyl S-methylides with acetylenic dipolarophiles and an unexpected rearrangement of the cycloadducts

The 1,3-dipolar cycloaddition of 2,2,4,4-tetramethyl-3-thioxocyclobutanone S-methylide (2a), generated in situ by thermal extrusion of N2 from the corresponding 2,5-dihydro-1,3,4-thiadiazole 1a, with electron-deficient acetylenic compounds yields spirocyclic 2,5-dihydrothiophene derivatives of type 4 (Scheme 2). Mixtures of diastereoisomers are obtained in the case of propiolates. The strained cyclooctyne also undergoes smooth cycloadditions with thioketone S-methylides (Scheme 3). Under acidic conditions, the spirocyclic products of type 4 and 6a isomerize, via opening of the cyclobutanone ring and aromatization of the five-membered ring, to thiophene derivatives of type 7 (Scheme 4)