Tin tetrachloride-induced π-cyclizations of glycine cation equivalents to substituted pipecolic acid derivatives
作者:Peter M Esch、Ilona M Boska、Henk Hiemstra、Richard F de Boer、W.Nico Speckamp
DOI:10.1016/s0040-4020(01)86443-7
日期:1991.1
Cationic pi-cyclization reactions of N-(3-alkenyl)-N-(methoxycarbonyl)acetoxyglycine esters induced by tin tetrachloride in dichloromethane are described. Reactions started and quenched with water at -78-degrees-C mainly yield cis-4-hydroxypipecolic esters, whereas reactions quenched after warm-up to room temperature provide trans-4-chloropipecolic esters as major products. A mechanistic scheme is advanced which adequately explains these results. The essentials are a rapid cationic aza-Cope equilibrium of the incipient iminium cation, and participation of the ester moiety through formation of a relatively stable bicyclic dioxycarbenium cation as pivotal intermediate.