Direct irradiation of B,y-unsaturated ketones with accessible allylic yhydrogens has been reported to yield intramolecular y-hydrogen abstraction,' intramolecular cycloaddition'e and 1,3-acyl shift and a-cleavage processes la,b,f
. As part of a study of the effect of stereochemistry on the relative photoreactivity of $,y-unsaturated ketones toward these processes, we describe the solution photochemistry of 2-aceto-2-methylmethylenecyclohexane, 1, in which the acetyl group can be equatorial or axial, and trans-2-aceto-2-methyl-4t-butylmethylenecyslohexane, 2, in which the acetyl group is held equatorial. In a related study, Lewis and Johnson have recently noted that a-methylcyclohexyl phenyl ketone undergoes predominantly intramolecular transannular hydrogen abstraction when the bensoyl group is axial and a-cleavage when the benzoyl group is equatorial.' Irradiation' of a solution of 1 in benzene (0.6M) for seven hours results in greater than 95% loss of starting material and a 55% yield of two products, 3 and 4 in a ratio of 3.2 to 1.0.' Products 3 and ,S are purified by preparative glpc' and assigned the structures shown in equation 1 based on analogy and the following data: 3, an oil, ir (Ccl@) 3650, 3500, and 1660 cm"; nmr (Ccl*, 6OMH2, 6) 0.8-3.1 (m, 15H, with singlets at 1.13, 1.24 and 1.5, last concentration dependent),5.28 (m, 1H); mass spectrum (70 eV), m/e (rel intensity) 152 (M+, 5); 4, a solid, mp 74-5', ir (Ccl&) 3600, 3450, 1645 and 900 cm-'; nmr (CCls,