摘要:
                                Formic acid-mediated cyclization reactions of N-(3-alkenyl)-N-(methoxycarbonyl)-acetoxyglycine esters are described.  The major reaction products are 4-formyloxypipecolic acid derivatives, formed with low stereoselectivity at C-4.  The several subtle features of the cyclization process are satisfactorily explained by a mechanism involving (1) a rapid cationic aza-Cope rearrangement of the incipient iminium ion and (2) participation of the ester moiety through formation of a relatively stable bicyclic dioxycarbenium cation as pivotal intermediate.