The reaction of methyl 3,3,3-trifluoropyruvate (1) with 2,5-dihydro-1,3,4-thiadiazoles 4a, b in benzene at 45O yielded the corresponding methyl 5-(trifluoromethy1)-1,3-oxathiolane-5-carboxylates 5a, b (Scheme I ) via a regioselective 1,3-dipolar cycloaddition of an intermediate 'thiocarbonyl ylide' of type 3. With methyl pyruvate, 4a reacted similarly to give 6 in good yield. Methyl 2-diazo-3,3,3-trifluoropropanoate (2) and thiobenzophenone (7a) in toluene underwent a reaction at 50'; the only product detected in the reaction mixture was thiirane 8a (Scheme 2). With the less reactive thiocarbonyl compounds 9H-xanthene-9-thione (7b) and 9H-thioxanthene-9-thione (7c) as well as with 1,3-thiazole-5(4H)-thione 12, diazo compound 2 reacted only in the presence of catalytic amounts of Rh2(OAc),. In the cases of 7a and 7b, thiiranes 8b and Sc, respectively, were the sole products (Scheme 3 ) . The crystal structure of Sc has been established by X-ray crystallography (Fig.). In the reaction with 12, desulfurization of the primarily formed thiirane 14 gave the methyl 3,3,3-trifluoro-2-(4,5-dihydro-l,3-thiazol-5-y1idene)propanoates (E)and (2)-15 (Scheme 4). A mechanism of the Rh-catalyzed reaction via a carbene addition to the thiocarbonyl S-atom is proposed in Scheme 5. ') Vorgetragen von H. H. am 'Symposium on Fluoroorganic Chemistry', Warschau, 17. Mai 1996.