Stereoselective synthesis of 4-amino-3-hydroxy- 2-methylpentanoic acids: stereochemistry of the amino acid occurring in the marine toxin janolusimide

Four diastereomers of 4-amino-3-hydroxy-2-methylpentanoic acid, an amino acid constituent of the hexapeptide portion of the antitumor antibiotic bleomycin A2, have been stereoselectively synthesized by crotylboration of N-Boc-L-alaninal. The synthesis allowed the assignment of the stereochemistry as 2R,3S,4S to the 4-amino-3- hydroxy-2-methylpentanoic acid occurring in the tripeptide marine toxin janolusimide.