Pyrrolizidine alkaloid analogues. Preparation of semisynthetic esters of retronecine

Abstract

Methods have been devised for the selective esterification of retronecine (I) to give analogues of the naturally occurring hepatotoxic pyrrolizidine alkaloids. Selective esterification at C-9 of retronecine (I) with simple acids was achieved by using NN′-dicyclohexylcarbodi-imide. Selective esterification and esterification with αβ-unsatur-ated and α-hydroxy-αα-dialkyl acids was achieved through the intermediacy of N-acylimidazoles.