Olivanic acid analogues. Part 2. Total synthesis of some C(6)-substituted 7-oxo-1 -azabicyclo[3.2.0] hept-2-ene-2-carboxylates

A number of 6-substituted 7-oxo-1 -azabicyclo[3.2.0] hept-2-ene-2-carboxylates related to the olivanic acids were prepared from the phosphorane (32). Generation of the anion α to the azetidin-2-one carbonyl group, followed by reaction with electrophiles and intramolecular cyclisation using the Wittig procedure gave the bicyclic products; in all cases the thermodynamically favoured trans-stereochemistry about the azetidinone ring predominated. In contrast, some less readily available cis-substituted analogues were obtained from the cyclohexa-1,4-diene derived phosphorane (61). The synthetic utility of a masked acetonyl ester group for preparing the free acids of these azabicycloheptene ring systems is described.