Electrochemical synthesis of chroman and euglobal skeletons via cycloaddition reaction of o-quinone methides and alkenes

Euglobal skeletons were synthesized by the intermolecular cycloaddition reaction of terpenes and o-quinone methides generated in situ by electrochemical oxidation. In a two-phase reaction medium composed of hexane-lithium perchlorate/nitromethane, 2-[1-(propylsulfanyl)alkyl] phenols were selectively oxidized to give the corresponding unstable o-quinone methides. These intermediates were trapped in situ by unactivated alkenes or easily oxidizable terpenes to form varied chromans and spirochromans including euglobal skeletons. In particular, in the presence of beta-pinene, the euglobal IIb skeleton, which possesses a beta-phellandrene moiety, is formed by the cycloaddition via skeletal rearrangement of beta-pinene.