Control by a Rigid Skew in the Bridge

The efficacy of a rigid skew in the bridge between the diene and the dienophile in directing the diastereomeric outcome of the intramolecular Diels-Alder reaction was first demonstrated by Kametani [36]. It was previously known 

There are early reports [40,41] of intramolecular Diels-Alder reactions of diastereomeric mixtures, one component of which reacted more readily than the other. These can be rationalized by assuming that in the transition state leading to 6/6 cyclization, the bridging atoms will be staggered in their relationship one to another, and thus will assume a chair-like conformation. 

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