Stereoselective total synthesis of the tricyclic sesquiterpene (±)-kelsoene by an intramolecular Cu(I)-catalyzed [2+2]-photocycloaddition reaction

The sesquiterpene kelsoene (1) has been stereoselectively prepared starting from the known 1,2,3-trisubstituted cyclopentane 7 in 14 synthetic operations. The intramolecular Cu(I)-catalyzed photocycloaddition of the precursor 10 which proceeded in 89% yield established the relative configuration of the two stereogenic centers (C-1, C-8) within the cyclobutane ring. The stereochemistry at C-6 was adjusted by the stereoselective hydrogenation of the unsaturated enone 15.