Cyclisations of homoallylic sulfonamides proceed smoothly via carbenium ion generation using trifluoromethanesulfonic (triflic) acid, the ease of cyclisation being directly related to the ion stability to give good to excellent yields of the corresponding pyrrolidines. Both toluene-and nitrophenyl-sulfonyl groups are suitable for all substrates tested whereas the corresponding carbamates are only useful in cases of tertiary and highly stabilised ...
