Ring enlargement of polyhydroxylated pyrrolidines to piperidines by Mitsunobu reaction: A fortuitous synthesis of 1-deoxy-L-allonojirimycin

Chiral 3,4-dibenzyloxy-5-hydroxymethyl-2-thiazolylpyrrolidines under Mitsunobu conditions (R-OH, Ph3P, DIAD in THF, 80 °C) afforded the 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