Abstract
The SnCl4-catalyzed reaction of (-)-thiofenchone (= 1,3,3-trimethyl-bicyclo[2.2.1]heptane-2-thione;10) with (R)-2-phenyloxirane ((R)-11) in anhydrous CH2Cl2 at -60° led to two spirocyclic, stereoisomeric
4-phenyl-1,3-oxathiolanes 12 and 13 via a regioselective ring enlargement, in accordance with previously reported reactions of oxiranes with thioketones (Scheme 3). The structure and configuration of the major isomer 12 were determined by X-ray crystallography. On the other hand, the reaction of 1-
methylpyrrolidine-2-thione (14a) with (R)-11 yielded stereoselectively (S)-2-phenylthiirane ((S)-15) in 56% yield and 87 –93% ee, together with 1-methylpyrrolidin-2-one (14b). This transformation occurs via an SN2-type attack of the S-atom at C(2) of the aryl-substituted oxirane and, therefore, with inversion of the configuration (Scheme 4). The analogous reaction of 14a with (R)-2-{[(triphenylmethyl)oxy]methyl}oxirane ((R)-16b) led to the corresponding (R)-configured thiirane (R)-17b (Scheme 5); its structure and configuration were also determined by X-ray crystallography. A mechanism via initial ring opening by attack at C(3) of the alkyl-substituted oxirane, with retention of the configuration, and subsequent decomposition of the formed 1,3-oxathiolane with inversion of the configuration is proposed (Scheme 5).
Fu, C