RING EXPANSION OF AZIRIDINES TO PIPERIDINES USING THE AZA-WITTIG REARRANGEMENT

A one-pot, two-step synthesis of unsaturated piperidines from 2-keto a ziridines is reported. Treatment of a range of 2-keto aziridines with two equivalents of a phosphonium ylide generates intermediate vinyl az iridines which rearrange by a [2,3]-sigmatropic shift to create a new carbon-carbon bond with concomitant ring opening of the aziridines to give the unsaturated piperidines. This three- to six-membered ring exp ansion allows the synthesis of N-unsubstituted piperidines and pipecol ic acid derivatives.