Stereoselective synthesis of beta-hydroxy-alpha-amino acids beta-substituted with non-aromatic heterocycles

We have stereoselectively synthesised b-hydroxy-a-amino acids b-substituted with non-aromatic heterocycles by means of a condensation reaction between enantiomerically pure heterocyclic aldehydes and the (R)-(+)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine (Schollkopf's reagent) as a chiral auxiliary. The stereocontrolled addition gave mixtures of diastereoisomers whose steric configurations were assigned on the basis of spectroscopic data and X-ray analysis. Upon controlled hydrolysis, the adducts were transformed into the corresponding methyl esters of b-hydroxy-b-heterocyclic substituted a-amino acids.