Cyclobutanes and Cyclobutenes from Photochemical Cleavage of Some Bicyclo[3.2.0]heptan-2-ones

Abstract

Irradiation of solutions of cyclopent-2-enone or 3-methylcyclopent-2-enone and allyl alcohol with light from a medium-pressure mercury lamp through a Pyrex filter with a cut-off at 295 nm furnishes excellent yields of isomeric mixtures of substituted bicyclo[3.2.0]heptan-2-ones as the primary products. Both enones give predominantly the head-to-head isomer with exo stereochemistry. The primary products turn out to be photolabile and undergo Norrish type I cleavage followed by hydrogen abstraction, but the outcome is influenced by the presence of a methyl group at C5. In the absence of such a methyl group the C1–C2 bond is cleaved, to give rise to two cyclobutenyl aldehydes in better than 90% combined yield; when a methyl group is attached at C5 both C–C bonds to the carbonyl group are broken and a mixture of cyclobutyl aldehydes and ketenes are formed in close to quantitative combined yield.