| Abstract: | 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 . |
