BIOMIMETIC RADICAL POLYCYCLIZATIONS OF ISOPRENOID POLYALKENES INITIATED BY PHOTOINDUCED ELECTRON-TRANSFER

Isoprenoid polyalkene radicals, formed by anti-Markovnikov addition of a nucleophile to their parent radical cations, which are readily acce ssible via photoinduced electron transfer, undergo cascade cyclization s. The regioselectivity is efficiently controlled by the substitution pattern, i.e., the generally observed 6-endo-trig mode is replaced by 5-exo-trig, if electron-deficient double bonds (e.g. I,1-dicyanovinyl groups) are involved. Moreover, remarkably high asymmetric inductions have been achieved by the use of chiral spirocyclic dioxinones, derive d from the chiral auxiliary (-)-menthone, notably remotely located fro m the initiation site of the cyclizations. These asymmetric photoinduc ed cyclizations constitute strong evidence of spontaneous coiling/fold ing of the terpenoid polyalkene chain and give ready access to the ena ntiomerically pure tricyclic terpenoids of complementary chiralities b y means of the single chiral auxiliary (-)-menthone.