Stereoselective electrophile-induced mono- and bis-cyclisation–fragmentation reactions of alkenyl oxime O-allyl and O-benzyl ethers. Synthesis of dihydropinidine

Phenylseleny bromide-induced cyclisation of γ- and δ-unsaturated aldoxime and ketoxime O-allyl and O-benzyl ethers is followed by a slow fragmentation of the resultant oxyiminium ions furnishing cyclic iminium salts which are readily reduced to pyrrolidines, piperidines or tetrahydroisoquinolines by sodium borohydride; dialkenyl oximes yield indolizidines and quinolizidines by an analogous sequence terminating in a mercury(II)-induced cyclisation.