Synthesis of (R)‐ and (S)‐2‐hydroxy‐3‐enoic acid esters

The synthesis of (R)‐ and (S)‐2‐hydroxy‐3‐enoic acid esters [(R)‐1a‐d and (S)‐1a‐c] is described. The (R) enantiomers were prepared by a Pinner synthesis from the corresponding (R)‐cyanohydrins [(R)‐2a‐d], which in turn were obtained by R‐oxynitrilase‐ (E.C. 4.1.2.10)‐catalyzed addition of HCN to the α,β‐unsaturated aldehydes 3a‐d. For the preparation of the (S) enantiomers an inversion of the configuration had to be implemented. A critical evaluation of the two possible sequences: inversion of the configuration of the cyanohydrins followed by solvolysis of the nitrile function, and solvolysis of the cyanohydrins followed by inversion of the configuration of the resulting α‐hydroxy esters, came out in favor of the latter pathway.