New synthetic approach to enantiopure bicyclic sulfonium salts: swainsonine analogues.

The enantioselective synthesis of bicyclic sulfonium salts 8 or 9, thioanalogues of swainsonine derivatives, is described. The synthetic strategy is based on a stereo- and regiospecific transannular cyclization reaction of nine-membered cyclic sulfides, mediated by Me 3 SiI or carried out under acidic catalysis.