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Nostosins A and B were isolated from a hydrophilic extract of Nostoc sp. strain from Iran, which exhibits excellent tryps 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.