Einfache Umwandlung von(–)‐( R )‐3‐Hydroxybuttersäure in das (+)‐( S )‐Enantiomere und dessen Lacton(‒)‐( S )‐4‐Methyloxetan‐2‐on

Simple Conversion of ( R )‐3‐Hydroxybutanoic Acid to the ( S )‐Enantiomer and its Lactone (–)‐( S )‐4‐Methylixetan‐2‐one Condensation of ( R )‐3‐hydroxybutanoic acid ( 1 ) with ethyl orthoacetate gives a 2‐ethoxy‐substituted (1,3)dioxanone 2 which is thermally labile: at ca. 100°, two competing processes commence, one leading to ethyl ( R )‐3‐acetoxybutanoate ( 3 ), the other one ‐ with complete inversion of configuration ‐ to the ( S )‐4‐methylixetan‐2‐one ( 4 ) and ethyl acetate. These can be readily separated by fractional distillation. Thus, enantiomerically pure ( S )‐3‐hydroxybutanoic acid ( ent‐ 1 ) and l‐ 2‐alkyl‐3‐hydroxybutanoic‐acid derivatives (such as 6 and 8 ) become available from the biopolymer PHB, the precursor to the acid 1 .