Control by an Alkoxy Substituent on the Bridge

In the foregoing discussion, diastereomeric control is achieved by making one face of the diene or the dienophile sterically less accessible than the other. It follows that any substituent that effectively hinders only one face of the diene or the dienopldle should exert diastereomeric control [54, 55]. Note that in the latter example, either face of the dienophile can be made the more hindered, depending on the choice of cyclization solvent. 

If the asymmetric fragment exerting diastereomeric control were removable, and were itself optically pure, the result would be an enantioselective cyclization. Chiral acrylates have been used in the intermolecular Diels-Alder reaction, one of the more effective to date being phenylmenthyl (54). Roush has shown [56] that phenylmenthyl esters exert substantial diastereomeric control in the intramolecular Diels-Alder reaction also. 

The treatment in this and the preceeding chapter are not meant to close the book on the intramolecular Diels-Alder reaction. Rather, the intent is to summarize and systematize early findings. As exemplified by two recent contributions [60, 61], there are yet many opportunities for creative application of such cyclizations. 

It should be noted that if enolization toward the ring fusion is possible, subsequent epimerization can obscure the stereochemistry of the initial cycUzation. Gras (35) has shown that Mn02 in CHCI3 is sufficient to effect such epimerization. Note also that a report by Oppolzer (36 a) of exo cyclization has been withdrawn (36b). 

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