The cycloaddition of 1-vinylcyclohexene to in situ obtained 3-and 4-carbomethoxy-l,2-benzoquinone gives monoadducts; both crystal structures have been determined by X-ray diffraction. In the former reaction, the formation of a bis adduct is also observed. It is shown that the FMO apprqximation gives erroneous predictions of the regioselectivities. Moreover, the PMO predictions of chemo-and regioselectivities are strongly dependent on the level of approximation in the use of the PMO equation as well as on the distances between the reaction centers. It is suggested that these difficulties can be overcome if the interaction energies are calculated for the geometry corresponding to the minimum of the nonbonding energy of each possible adduct. These minima are considered as good descriptions of the early stages of the reaction. The obtained theoretical predictions are in good qualitative and semiquantitative agreement with the experimental results. The dominant term leading to endo selectivity is the polar term.We recently investigated the efficacy of 2-carbomethoxy-l,4-benzoquinone (2CPBQ) in the cycloaddition with 1-vinylcyclohexene (VCH) in the building up of the hydrophenanthrene skeleton of 9,10-syrc-podocarpane diterpenoids.18

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