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 β-pinene, the euglobal IIb skeleton, which possesses a β-phellandrene moiety, is formed by the cycloaddition via skeletal rearrangement of β-pinene.