Enantiomerically pure vinylketene acetals in the Diels-Alder reaction. Catalysis and facial selectivity.

Abstract

A series of enantiomerically pure vinylketene acetals which function as diastereoselective dienes in the Diels-Alder reaction have been developed. Most selective are (4 R, 5 R)-4,5-diphenyl-2-(2-methyl-2-propenidene)-1,3-dioxolane ( 8) and (4 R, 5 R)-4,5-diphenyl-2-(( E)-2-butenidene)-1,3-dioxolane ( 9), which utilize phenyl rings as asymmetric directing groups. It was shown through NOE analysis that the major product obtained from the reaction of diene 9 and N-methylmaleimide is due to Re exo approach of the dienophile to the diene, with the two endo transition states also being manifest in the product mixture in small amounts. The relative order of the transition state energies were determined to be Re exo < Re endo < Si endo < Si exo. Both molecular mechanics and π-charge density calculations were used to rationalize the high level of diastereoselectivity observed in the reactions of diene 8 and N-methylmaleimide (48:1).