Reactions of Singlet Oxygen with Enol Esters

摘要

Singlet oxygenation of (l-adamantylidene)ethyl acetate (4) and 6,6-dimethylcyclohex-l-enyl acetate (7) produces only ene reaction products. Photooxygenation of Delta 1,6-2-oxabicyclo(4.4.0)decen-3-one (9), in contrast, yields ene, acyl-shifted, and (2+2)cycloaddition products. The product distribution resulting from oxidation of 9 indicates that attack of singlet oxygen (1O2) occurs exclusively on the same side of the double bond as the ester functional group. The bimolecular rate constant for reaction of 9 with 1O2 is found to be ca. 50 times larger than those of 4 and 7. These results are explained most economically by invoking the initial formation of perepoxide intermediate. In the case of 9, stabilization of the transition state leading to the perepoxide by interaction of the incoming 1O2 molecule with the ester functionality produces the observed rate enhancement and stereospecificity. (Author)