Diels-Alder reaction between vinylcyclohexene and benzoquinones. A shortcoming of the FMO approach

Abstract

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

Origins of the Regioselectivities in the Diels−Alder Reactions of Vinylindenes with 1,4-Quinone Monoketal and Acrolein Dienophiles

“…The uncatalyzed reactions of 1-methyl-2-vinyl-4-hydroxycyclohexene with cyclohexenones by Stork et al, vinylcyclohexene with propiolic acid by Nazarov and co-workers, and vinylcyclohexene with 1-formylcyclohexene by Bergmann et al all provide products analogous to product A as the major product, with the unsubstituted terminus of the diene attacking the most electophilic terminus of the dienophile. Pitea and co-workers obtained similar results with the reactions of vinylcyclohexene with benzoquinones …”

Metathesis Reactions of Carbohydrates: Recent Highlights in Alkyne Metathesis

“…Regarding the cycloaddition of 1,3‐dienes 72 and 78 with 1,4‐benzoquinone and acrolein, the two cycloadducts arise from an endo approach from the same face as the α substituents. Concerning the regioselectivity of the reactions, in the case of acrolein as dienophile, the observed results are in agreement with previous observations in related systems 66,67…”