Stereochemistry of valine and isoleucine biosynthesis: IV. Synthesis,configuration, and enzymatic specificity of α-acetolactate and α-aceto-α-hydroxybutyrate

A new synthetic route, involving acetylenic intermediates, has been developed for the preparation of the valine and isoleucine biosynthetic intermediates α-acetolactic acid (III) and α-aceto-α-hydroxybutyric acid (IV) including the optically active form of these labile acids. The absolute configuration of acetolactate methyl ester XV was confirmed as (R)-(?), and the configuration of XVI was also established as (R)-(?). Two trideuterio analogs of acetolactate were prepared by this route. The acetolactate anion was found to undergo a rapid room-temperature degenerate rearrangement resulting in racemization and methyl interchange. The isomeroreductase of Salmonella typhimurium was found to be specific for the (S) enantiomers of III and IV, allowing conclusions about the conformation of IV during the ethyl migration step in isoleucine biosynthesis. Acetolactate decarboxylase of Acidobacterium aerogenes was found to decarboxylate specifically the (S) enantiomers of III and IV, forming (?)-acetoin from III with inversion of configuration.