Reaction of Optically Active Oxiranes with Thiofenchone and 1-Methylpyrrolidine-2-thione: Formation of 1,3-Oxathiolanes and Thiiranes

✍ Scribed by Changchun Fu; Anthony Linden; Heinz Heimgartner

• Publisher: John Wiley and Sons
• Year: 2011
• Tongue: German
• Weight: 247 KB
• Volume: 94
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.201100043

✦ Synopsis

Abstract

The SnCl~4~‐catalyzed reaction of (−)‐thiofenchone (=1,3,3‐trimethylbicyclo[2.2.1]heptane‐2‐thione; 10) with (R)‐2‐phenyloxirane ((R)‐11) in anhydrous CH~2~Cl~2~ 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 S~N~2‐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).