Photoinduced vinylcyclopropane-cyclopentene rearrangement: A methodology for chiral bicyclo[3.2.0]heptenes. Formai syntheses of (±)-grandisol and naturally occurring ()-Δ 9(12)-Capnellene and its antipode

Abstract

The problem of achieving reaction selectivity in cis -alkyl vinylcyclopropanes in favour of their rearrangement to cyclopentenes, obviating the competing low energy process of the [1,5] (homo) sigmatropic hydrogen shift, is addressed. It has been demonstrated that the readily available bicyclo[4.1.0]heptenes derived from (+)-Δ 3-carene. ( 1), upon photosensitized irradiation, are conveniently transformed into chiral cis -bicyclo [3.2.0]heptenes. The synthetic potential of this strategy has been demonstrated in realization of efficient formal syntheses of the important insect pheromone (±)-grandisol ( 2) and both the enantiomers of Δ 9(12)-capnellene ( 3) from readily available 1, a major component of Indian turpentine oil.