Synthesis of medium-sized bicyclic compounds by intramolecular cyclization of cyclic β-keto radicals generated from cyclopropanols using manganese(III) tris(pyridine-2-carboxylate) and its application to total synthesis of 10-isothiocyanatoguaia-6-ene

Bicyclic cyclopropanols having an olefinic side chain are oxidized with manganese(III) tris(pyridine-2-carboxylate) to generate cyclic beta-keto radicals with ring-expansion. These cyclize intramolecularly, affording bicyclic radical intermediates. The cyclized radicals are trapped with various radical-trapping reagents such as electron-rich or -deficient olefins, tributylstannane and diphenyl diselenide to give the corresponding functionalized products. Stereochemistries of the bicyclic products are well predicted by MM2 force field calculation. A stereoselective total synthesis of an isothiocyano sesquiterpene, 10-isothiocyanatoguaia-6-ene, is achieved using this reaction.