Namalides B and C and Spumigins K–N from the Cultured Freshwater Cyanobacterium Sphaerospermopsis torques-reginae

Chemical investigations of the terrestrial cyanobacterium Sphaerospermopsis torques-reginae ITEP-024 from northern Brazil afforded namalides B (1) and C (2), the first analogues of this anabaenopeptide-like metabolite to be described. Four other related peptides (3–6), termed spumigins K–N, were also identified. Planar structures and absolute configurations for 1, 2, and 3a–6a were deduced by a combination of 2D NMR, HRMS analysis, and Marfey’s methodology. Spumigins K–N (3–6) are the first examples of spumigins containing a 2-hydroxy-4-(4-hydroxyphenyl)butanoic acid (Hhpba) in the N-terminal position. Compounds 1 and 2 inhibited carboxypeptidase A with IC50 values of 0.75 and 2.0 μM, respectively.