摘要:
The Hanessian method to prepare the N-Cbz-L-vinylglycine methyl ester has been improved to obtain reproducibility an alternately protected version of this useful synthon in optically pure, crystalline form. A nitrone cycloaddition route has been developed to synthesize erythro- and threo-L-beta-hydroxyornithine having stereoisomeric purities of > 99%. A parallel route described recently, proceeding from Hanessian's methyl vinlglycinate, gives these oxidatively modified alpha-amino acids in inferior enantiomeric excess owing to partial racemization that occurs in the oxidative decarboxylation to the protected vinylglycines themselves. The intermediate isoxazolidine generated in the present synthesis was separately converted to proclavaminic acid, a key intermediate in the biosynthesis of the beta-lactamase inhibitor clavulanic acid, and its absolute configuration was established as L-threo by unambiguous correlation to L-glutamic acid.