Action of L-aminoacylase and L-amino acid oxidase on 2-amino-3- methylpent-4-enoic acid [Δ(4)-dehydroisoleucine and alloisoleucine] stereoisomers: An alternative route to a stereochemically pure compound and the application to the synthesis of (R)-2-methylbutan-1-ol

The action of L-aminoacylase and L-amino acid oxidase on the stereoisomers of 2-amino-3-methylpent-4-enoic acid was examined, to secure stereochemically pure compounds related to L-alloisoleucine. A scalemic mixture [(2S,3R)- (90.0%), (2S,3S)(3.2%), (2R,3S)- (5.9%), (2R,3R)- (0.9%)] of the N-trifluoroacetyl derivatives of the title compound, which was prepared by an asymmetric Claisen rearrangement, was submitted to Aspergillus L-aminoacylase-catalyzed hydrolysis. Only the (2S,3R)- and (2S,3S)isomers were hydrolyzed to give the corresponding amino acids. The subsequent treatment of these isomers with Crotalus adamanteus L-amino acid oxidase afforded the pure (2S,3R)-isomer, related to L-alloisoleucine, in more than 99% stereochemically pure form. Moreover, the product was converted to L- alloisoleucine and (R)-2-methylbutan-1-ol via a chemoenzymatic procedure.