Asymmetric synthesis of beta-amino acids by addition of chiral enolates tonitrones via N-acyloxyimininm ions

N-Acyloxyiminium ions, generated by the reaction of nitrones with acyl hali des, are highly reactive species and undergo facile reaction with a wide ra nge of nucleophiles, such as ketene silyl acetals, titanium(IV) and boron e nolates, hydrido- and allyltin(IV) reagents, and alkynyltitanium(IV) reagen ts, to give alpha -substituted amine derivatives. Optically active beta -am ino acids can be prepared by the reaction of N-acyloxyiminium ions with bot h boron and titanium(IV) enolates bearing chiral auxiliaries. Reversal of d iastereoselectivity was observed by the reactions of the boron and titanium (IV) enolates. Using these reactions, all of the four stereoisomers of alph a -methyl-beta -phenylalanines, for example, can be prepared highly diaster eoselectively. Cyclic N-acyloxyiminium ions are useful for the asymmetric s ynthesis of pyrrolidine and piperidine alkaloids; (5R,8R,8aS)-5-cyano-8-met hylindolizidine which is a common key intermediate for synthesis of 5-subst ituted 8-methylindolizidines, was prepared selectively.