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 β-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.