The Diels‐Alder cycloadditions of facially dissymmetric maleic anhydride 1 with facially nonequivalent exocyclic 1,3‐butadienes(dimethylidenebicyclo[2.2.2]octene 3 and 2,3,5,6‐tetramethylidenebicyclo[2.2.2]‐octene (4)) were investigated. In each cycloaddition, the reaction occurred via the course in which 1 added exclusively by its syn‐face (same face as the etheno‐bridge) onto either π‐face of the exocyclic 1,3‐butadiene systems to produce only two of the four possible stereoisomeric monocycloadducts (8a/8b and 9a/9b). In the Diels‐Alder cycloaddition of 1 with bis‐exocyclic butadiene 4, however, both monocycloadducts 9a and 9b underwent subsequent cycloaddition with distinctive facial selectivity to produce the Cs‐symmetric bis‐cyclohexanobarrelene 10a as only bis‐cycloadduct.

There are four conceivable products of the title addition, however only two of them have been prepared. MNDO, AMI, and PM3 quantum-chemical computations have been used in order to rationalize the observation. Activated complexes have been computed for the four possible additions and it was found that one C-C bond rather than two is created on the path from the reactants to the activated complex (the C-C bond varies between 1.70 and 1.87 A). Only one activated complex has been localized on each reaction path. The two types of products observed in experiment are computed to be both thermodynamically more stable and kinetically more feasible, and hence the computations agree with the observations.

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