Asymmetric Syntheses of (2 R,3 S)-3-Hydroxyproline and (2 S,3 S)-3-Hydroxyproline.

Two synthetic routes have been developed for the asymmetric syntheses of (2 R,3 S)- and (2 S,3 S)-3-hydroxyproline. The key synthetic step in each of these strategies is the conversion of protected α,δ-dihydroxy-β-amino esters (either 2,3- anti- or 2,3- syn-configured) into β,δ-dihydroxy-α-amino esters (protected forms thereof), via the intermediacy of the corresponding aziridinium ions. The products of these stereospecific rearrangements were then cyclized and deprotected to afford (2 R,3 S)-3-hydroxyproline and (2 S,3 S)-3-hydroxyproline as single diastereoisomers (>99:1 dr) in >26% overall yield.