Capnellane sesquiterpenes. Total synthesis of epiprecapnelladiene and Δ8(9)-capnellene

Abstract

The total synthesis of epiprecapnelladiene(29) containing an uncommon bicyclo[6.3.0]undecane carbon skeleton is described. The fused 5,8-ring system in (29) is elaborated in a single step, using a highly regio- and stereo-selective intramolecular photocycloaddition reaction from the enol benzoate(16a). The relative stereochemistry of precapnelladiene, from the soft coral Capnella imbricata, is established as that in formula (30). In a biogenetically patterned synthesis, treatment of epiprecapnelladiene(29) or its exo-methylene isomer(28), with boron trifluoride–diethyl ether is shown to lead to Δ⁸⁽⁹⁾-capnellene(32), accompanied by smaller amounts of the positional isomers (33) and (34).