Robinson Annulations of Sterically Hindered α-Carbomethoxycyclohexanones

Abstract

In conjunction with our interest in the synthesis of quassinoids such as bruceantin¹ (1), we required a synthesis of a tricyclic intermediate bearing a functionalized angular methyl group at C-8. One approach which we considered involved the Robinson annulation² of a bicyclic β-ketoester 2 and methyl vinyl ketone to obtain the tricyclic enone 3. The major obstacle in such an approach involved the Aldol condensation step in the annulation process in which an enolate at C-11 must condense with a highly substituted carbonyl group at C-9. There are conflicting reports in the literature on the success of similar condensations,^3,4 and we elected to test the feasibility of this approach.