Synthesis of D-alloisoleucine from L-isoleucine and from (S)-2-methylbutan-1-ol. Synthesis of isostatine
Three different synthetic routes to the uncommon α-amino acid D-alloisoleucine were studied. The first was based on the stereospecific inversion of configuration of the C-2 stereogenic carbon of L-isoleucine. The second involved acetylation of L-isoleucine with epimerisation at the C-2 carbon, giving a mixture of L-isoleucine and D-alloisoleucine, which was resolved enzymically with hog kidney acylase. In a new approach, an epimeric mixture of LL-isoleucine and D-alloisoleucine was synthesized from (S)-2-methylbutan-1-ol and was again resolved enzymically. The D-alloisoleucine produced in these syntheses was subsequently transformed into the γ-amino acid isostatine.