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Cyclization of terpenoid polyal kenes via photo-induced electron transfer-versatile single-step syntheses of mono- and polycycles Klaus-D. Warzecha, Xuechao Xing and Martin Demuth* Max-Planck-Institut fur Strahlenchemie, D-454 13 Miilheim an der Ruhr, Germany Abstract: Light-induced radical cationic cyclizations of suitably functionalized polyalkenes, readily available from geraniol, all-trans-farnesol and all-trans-geranylgeraniol, establish a powerful access to mono- and polycyclic compounds. Within these highly stereo- and chemoselective transformations, the cyclization mode, optionally yielding five- and six-membered rings, is efficiently controlled by the substitution pattern of the polyalkenes (acetate vs. a$-unsaturated ester and dicyano groups). In all examples, protic solvents add highly stereoselectively to the malkene site of the isoprenoid starting materials in anti-Markovnikov sense suggesting that radical cation intermediates are intercepted. Notably, these trans-formations achieved upon light-induced electron transfer mimic in vivo processe