Photochemical oxa-di-π-methane rearrangement approach to [3.3.3]propellanes. Total synthesis of sesquiterpene hydrocarbon (±)-modhephene

Abstract

A photochemical approach to the construction of the [3.3.3] propellane and tricyclo[5.3.1.04,11]undecane ring systems through a common strategy employing oxa-di-π-methane rearrangement as the key step is delineated. Starting from the readily available bicyclo[4.3.0]nona-1(6),2-diene 6, the tricyclo[5.2.2.01,5]undecenones 5 and 12 were prepared through a Diels–Alder strategy employing α-chloroacrylonitrile as the ketene equivalent. Sensitised irradiation of enone 5 furnished the tetracyclo[4.3.2.02,6.02,9]undecanone 4, which on regioselective, reductive C–C bond cleavage provided access to [3.3.3]propellanone 14. The tricyclic ketone 12 through a similar protocol furnished the tricyclo[5.3.1.04,11] undecanone 16. The tetracyclic ketone 4 has been further elaborated in a straight-forward manner to afford the sesquiterpene hydrocarbon (±)-modhephene.