Diels–Alder reactions of 2-carbomethoxy-2-cyclohexen-1-one

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Abstract
Under stannic chloride catalysis, 2-carbomethoxy-2-cyclohexen-1-one (5) was found to undergo Diels–Alder reaction with a variety of dienes to give directly the cis-1-octalone system possessing a functionalized substituent at the angular position. With unsymmetrically substituted dienes, the adducts were those predicted on the basis of the normal rules (ortho and para) governing the orientation of Diels–Alder addition. In cases where differing secondary orbital overlap would affect the stereochemistry of the adducts, the major isomer in each case was that produced by secondary overlap of the diene with the ester carbonyl rather than the ketone carbonyl, presumably due in part to a steric effect. The degree of stereoselectivity was further shown to be dependent upon both the reaction temperature and the diene used and was generally enhanced when the reaction was carried out at lower temperatures or when a bulkier diene was involved.

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