Lobocyclamide B from Lyngbya confervoides.: Configuration and asymmetric synthesis of β-hydroxy-α-amino acids by (-)-sparteine-mediated aldol addition

[GRAPHICS] Lobocyclamide 13, a cyclododecapeptide containing five beta-hydroxy-alpha-amino acid residues, was isolated from Lyngbya confervoides. This is the first reported occurrence of gamma-hydroxythreonine in a natural peptide. Optically active beta-hydroxy-alpha-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.