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.