Photoreactions of Tricyclic α-Cyclopropyl Ketones and Unsaturated Enones – Synthesis of Polyquinanes and Analogous Ring Systems

Abstract

Polycyclic systems of both angular and propellane type have been synthesized through intramolecular radical cyclization reactions, by photochemically induced electron transfer of tricyclic α-cyclopropyl ketones or by photolysis of unsaturated enones. In general, tricyclic α-cyclopropyl ketones, each bearing an alkynyl or alkenyl side chain at the position γ to the carbonyl group, were used as starting materials. The reactions resulted in regioselective cleavage of one cyclopropyl bond with formation of tri- to tetracyclic ring systems by a tandem fragmentation-radical/radical anionic reaction pathway. The regioselectivity of the cyclization (exo/endo) depends on the length of the unsaturated side chain. In cases involving α-cyclopropane derivatives with alkoxymethyl side chains, various non-cyclization processes were observed. In addition, the photoinduced cyclization of the corresponding bicyclic enone derivatives with the same unsaturated side chains afforded tetra- and tricyclic products of propellane type in good yields and with high stereoselectivity. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)