An asymmetric induction principle and biomimetics with photons via electron transfer

Abstract

Isoprenoid polyalkene radicals, formed upon 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-endo-trig mode is replaced by 5-exo-trig, if electron-deficient double bonds are involved. Mechanistic studies revealed that these synthetically useful transformations, initiated by polyalkene radical cations being trapped by water, are propagated in a plain radical fashion and terminated upon either protonation of carbanions or hydrogen transfer. Cyclization products were used for a natural product total synthesis, i.e of stypoldione. Moreover, high asymmetric inductions in such transformations have been achieved by the use of chiral spirocyclic dioxinones, derived from the auxiliary (-)-menthone, remotely located from the initiation site of the cyclizations. These asymmetric photoinduced cyclizations are further examples of a more general enantiodivergent induction principle giving access to enantiomerically pure polycyclic terpenoids of complementary chiralities by means of the single chiral auxiliary (-)-menthone. Finally, the efficient application of solar radiation for photochemical purpose is demonstrated by the use of flat collectors which in contrast to concentrating technologies not only employ direct, but also diffuse radiation (the latter amounts to about 40% of the global radiation at central European latitude).