Expedient syntheses of (+)-cis-(2R,3S)-3-hydroxyproline and (–)-(1S,5S)-2-oxa-6-azabicyclo[3.3.0]octan-3-one (the Geissman–Waiss lactone): formal enantioselective syntheses of (–)-retronecine and related pyrrolizidine alkaloids

Abstract

Yeast reduction of the keto-proline (5) affords the hydroxyproline derivative (6)(diastereoisomeric excess > 99%cis; enantiomeric excess, e.e., 80%); subsequent hydrolysis and crystallisation gives (+)-cis-(2R,3S)-3-hydroxyproline (7)(93% e.e.) which has been homologated to the bicyclic lactones (10) and (11), precursors of (–)-retronecine, (+)-platynecine, (–)-croalbinecine and related pyrrolizidines.