Factors affecting product formation in the photolysis of α-spirocyclopropyl ketones

Abstract

Photolysis of spiro[2.5]oct-7-en-4-ones results in exclusive β-cleavage in methanol to give 2-ethenylcyclohex-2-en-1-ones and in hydrocarbon solvents to give (E)- and (Z)-2-ethylidenecyclohex-3-en-1-ones. The solvent effect is ascribed to the role of methanol in stabilizing trienol intermediates and engendering their intermolecular ketonization. Photolysis of 8-methylenespiro[2.5]octan-4-ones in methanol gives 2-ethenylcyclohex-2-en-1-ones and bicyclo[4.3.0]non-1(6)-en-2-ones; irradiation in methanol-O-d indicates that the former type of product is formed mainly via a trienol intermediate. Photolysis of 8-methylenespiro[2.5]octan-4-ones in benzene gives 2-ethenylcyclohex-2-en-1-ones together with (E)- and (Z)-2-ethylidene-3-methylenecyclohexan-1-ones; the latter undergo photoconversion to the former during the irradiation. 6,6-Dimethylspiro[2.4]-heptan-4-one on photolysis in methanol undergoes ct-cleavage to give 4-methoxy-7,7-dimethyl-5-oxaspiro[2.5]octane as the major product, whereas under the same conditions 6,6-dimethylspiro[2.5]octan-4-one is reduced to the corresponding alcohol and 6,6,8-trimethylspiro[2.5]octan-4-one undergoes β-cleavage to give 2-ethyl-3,5,5-trimethylcyclohexan-1-one as the major primary product; this trichotomy is interpreted in terms of ring-size and steric factors.