Lobocyclamide B from Lyngbya confervoides. Configuration and Asymmetric Synthesis of β-Hydroxy-α-amino Acids by (-)-Sparteine-Mediated Aldol Addition

(Matrix Presented) Lobocyclamide B, a cyclododecapeptide containing five β-hydroxy-α-amino acid residues, was isolated from Lyngbya confervoides. This is the first reported occurrence of γ-hydroxythreonine in a natural peptide. Optically active β-hydroxy-α-amino acids required for configurational analysis of the title compound were prepared using a novel (-)-sparteine-mediated asymmetric aldol addition of N-(diphenylmethylene)glycine tert-butyl ester to aldehydes. The method is general for aliphatic and aryl aldehydes and notable for operational simplicity.