Biomimetic radical polycyclizations of isoprenoid polyalkenes initiated by photoinduced electron transfer

Abstract

Isoprenoid polyalkene radicals, formed by anti- Markovnikov addition of a nucleophile to their parent radical cations, which are readily accessible via photoinduced electron transfer, undergo cascade cyclizations. The regioselectivity is efficiently controlled by the substitution pattern, i.e., the generally observed 6<em>-endo-</em>trig mode is replaced by 5<em>-exo-</em>trig<em>, </em>if electron-deficient double bonds (e.g. 1, 1-dicyanovinyl groups) are involved. Moreover, remarkably high asymmetric inductions have been achieved by the use of chiral spirocyclic dioxinones, derived from the chiral auxiliary (-)-menthone, notably remotely located from the initiationsite ofthe cyclizations.These asymmetricphotoinduced cyclizations constitute strong evidence of spontaneous coiling/folding of the terpenoid polyalkene chain and give ready access to the enantiomerically pure tricyclic terpenoids of complementary chiralities by means of the single chiral auxiliary (-)-menthone.