Straightforward Synthesis of Aromatic Polycyclic Terpenoids through Biomimetic Cascade Cyclizations Triggered by Photochemical Electron Transfer

Photoinduced biomimetic cascade cyclizations of suitably functionalized terpenoid arylpolyalkenes via radical cationic intermediates represent a powerful route to polycyclic compounds in a single operational step. In all cases examined, the radical cation, created selectively and exclusively at the ψ‐polyalkene position, is trapped in anti ‐Markovnikov fashion by water, liberating a free radical site, which ultimately initiates cascade‐style bond formation to afford the all‐chair polycyclic conformation containing one aromatic moiety. The products, some of which are podocarpic‐type structures 17 − 19 , are beneficially 3β‐hydroxylated. Mechanistically, the reaction is probably initiated by a radical cation trapped by water prior to the cyclization event, which is driven by a pure radical‐type process. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)