Synthesis of the Benzotricyclooctane Ring System. Intramolecular Cycloaddition of Indene Derivatives

Article abstract of DOI:10.1021/jo00141a018

The photosensitized triplet reactions of several 1-allyl-substituted indenes have been studied.The triplet-sensitized irradiations gave benzotricyclo<3.3.0.0^2,7>octanes in good yield by means of a novel intramolecular <2 + 2> cycloaddition.The effect of substituents on the regioselectivity of the sensitized rearrangement was studied in some detail.With the simple 1-allyl-substituted isomer, 1,5-cyclization of the excited state is the preferred path.This mode of cyclization is favored on the basis of strain, radical stability, and entropy factors.We have found, however, that the normal closure predicted by the rule of five does not occur in the photosensitized irradiation of the 1-prenyl-substituted isomer.With this system, intramolecular <2 + 2> cycloaddition gives rise to the benzotricyclo<3.2.1.0>octane system.The diradical produced from the sensitized 1,4-cyclization path is long lived enough to allow internal disproportionation to complete with radical coupling.The facility with which the intramolecular <2 + 2> indene photocycloadditions occur makes this type of approach particularly attractive for the synthesis of some unusual polycyclic ring compounds.