Synthese von Trifluoromethyl‐substituierten Schwefel‐Heterocyclen unter Verwendung von 3,3,3‐Trifluorobrenztraubensäure‐Derivaten

Abstract

Synthesis of Trifluoromethyl‐Substituted Sulfur Heterocycles Using 3,3,3‐Trifluoropyruvic‐Acid Derivatives The reaction of methyl 3,3,3‐trifluoropyruvate ( 1 ) with 2,5‐dihydro‐1,3,4‐thiadiazoles 4a, b in benzene at 45° yielded the corresponding methyl 5‐(trifluoromethyl)‐1,3‐oxathiolane‐5‐carboxylates 5a, b ( Scheme 1 ) 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 9 H ‐xanthene‐9‐thione ( 7b ) and 9 H ‐thioxanthene‐9‐thione ( 7c ) as well as with 1,3‐thiazole‐5(4 H )‐thione 12 , diazo compound 2 reacted only in the presence of catalytic amounts of Rh 2 (OAc) 4 . In the cases of 7a and 7b , thiiranes 8b and 8c , respectively, were the sole products ( Scheme 3 ). The crystal struture of 8c 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‐1,3‐thiazol‐5‐ylidene)propanoates ( E )‐and ( Z )‐ 15 ( Scheme 4 ). A mechanism of the Rh‐catalyzed reaction via a carbene addition to the thiocarbonyl S‐atom is proposed in Scheme 5 .