Total Synthesis and Stereochemical Assignment of Nostosin B

Nostosins A and B were isolated from a hydrophilic extract of Nostoc sp. strain from Iran, which exhibits excellent trypsin inhibitory activity. Nostosin A was the most potent natural tripeptide aldehyde as trypsin inhibitor up to now. Both r- and s-2-hydroxy-4-(4-hydroxy-phenyl)butanoic acid (Hhpba) were prepared and incorporated into the total synthesis of nostosin B, respectively. Careful comparison of the NMR spectra and optical rotation data of synthetic nostosin B (1a and 1b) with the natural product led to the unambiguous identification of the r-configuration of the Hhpba fragment, which was further confirmed by co-injection with the authentic sample on HPLC using both reversed phase column and the chiral AD-RH column.