Diastereoselective synthesis of 2-amino-4-phosphonobutanoic acids by conjugate addition of lithiated Schöllkopf's bislactim ethers to vinylphosphonates.

Conjugate additions of lithiated bislactim ethers derived from cyclo-[Gly-Val] and cyclo-[Ala-Val] to alpha-, beta-, or alpha,beta-substituted vinylphosphonates allow direct and stereoselective access to a variety of 3- or 4-monosubstituted and 2,3-, 2,4-, or 3,4-disubstituted 2-amino-4-phosphonobutanoic acids (AP4 derivatives) in enantiomerically pure form. The relative stereochemistry was assigned by X-ray diffraction analysis or NMR study of 1,2-oxaphosphorinane derivatives. Competitive eight-membered "compact" and "relaxed" transition-state structures are invoked to rationalize the stereochemical outcome of the conjugate additions.