Palladium(0)-catalyzed isomerization reactions of aziridines bearing an alpha,beta-unsaturated ester group: A thermodynamic preference for chiral alkyl (2E)-4,5-cis-4,5-epimino-N-(alkyl- or arylsulfonyl) 2-enoates over the other three stereoisomers

Palladium(0)-catalyzed reactions of five sets of four stereoisomeric 4,5-epimino-N-(methanesulfonyl) or -N-(arylsulfonyl) 2-enoates reveal that 4,5-cis-(2E)-isomers are thermodynamically more stable than other isomers, in accord with calculations. A highly stereoselective synthesis of (E)-alkene dipeptide isosteres having the desired stereochemistries from unwanted stereoisomeric 4,5-epimino-N-(arylsulfonyl) 2-enoates is also presented.