Diastereoselective Diels−Alder Reactions of N-Sulfonyl-1-aza-1,3-butadienes with Optically Active Enol Ethers:  An Asymmetric Variant of the 1-Azadiene Diels−Alder Reaction

The first detailed study of a room-temperature asymmetric Diels−Alder reaction of N-sulfonyl-1-aza-1,3-butadienes is reported enlisting a series of 19 enol ethers bearing chiral auxiliaries, with many providing highly diastereoselective (endo and facial diastereoselection) reactions, largely the result of an exquisitely organized [4+2] cycloaddition transition state. Three new, readily accessible, and previously unexplored auxiliaries rationally emerged from the studies and provide remarkable selectivities (two of these give 49:1 endo:exo and 48:1 facial selectivity) that promise to be useful in systems beyond those detailed.