Ring Enlargement of Polyhydroxylated Pyrrolidines to Piperidines by Mitsunobu Reaction: A Fortuitous Synthesis of 1-Deoxy-<scp>l</scp>-allonojirimycin
corresponding R-protected pyrrolidines and 2-deoxy-piperidines in different ratios depending on the stereochemistry of the starting pyrrolidine and the nature of the acid R-OH. A mechanistic scheme is proposed involving the formation of an aziridinium ion as an intermediate. A piperidine derivative obtained in 74% yield was converted in four steps into the title allonojirimycin.