CASCADE CYCLIZATIONS OF TERPENOID POLYALKENES TRIGGERED BY PHOTOELECTRON TRANSFER - BIOMIMETICS WITH PHOTONS

Light-induced cyclizations of suitably functionalized polyalkene terpe noids, such as geranyl, all-trans-farnesyl, and all-trans-geranylgeran yl derivatives, via formation of radical cations are proven to be a po werful method for the single-step synthesis of mono- and mostly all-tr ans-fused polycyclic compounds from readily available precursors. Wher eas some of these highly stereo- and chemoselective transformations re quired the use of micellar media, they can now be conveniently perform ed in homogeneous solutions upon suitable choice of the electron accep ters and of the functionality pattern of the polyalkene substrates. Mo reover, the mode of cyclization, i.e., 6- vs. 5-membered ring formatio n and termination of the cyclization cascades, are steered efficiently by the substituents of the polyalkenes (polyalkenyl acetate vs. alpha ,beta-unsaturated ethyl polyalkenoate and polyalkene-1,1-dicarbonitril e). At the same time, the protic solvents used add highly stereoselect ively to the omega-alkene sites of the polyalkens in anti-Markovnikov sense which strongly suggests that radical cations are intercepted. In terestingly, the transformations achieved here upon photoelectron tran sfer parallel the biosynthetic paths of non-oxidative terpene cyclizat ions which are thought to occur purely by protonation of the isoprenoi d polyalkenes.