Isolation and Structures of Nostopeptolides A1, A2 and A3 from the Cyanobacterium Nostoc sp. GSV224

The isolation and total structure determinations of nostopeptolides A1 (1), A2 (2) and A3 (3) are described. These cyclic depsipeptides, which are devoid of cytotoxic, antifungal and inhibition of protease activities, are characteristic constituents of the cryptophycin-producing cyanobacterium Nostoc sp. GSV224. Structure elucidation by NMR analysis was made more challenging by the existence of each nostopeptolide in three conformations. One-dimensional TOCSY experiments proved to be very useful in isolating and identifying the nine amino acid residues and the butyryl group in each compound. The absolute stereochemistries of 1–3 were determined by comparing the amino acids in the acid hydrolyzates directly with authentic standards.