The structure of cyanoginosin-LA, a cyclic heptapeptide toxin from the cyanobacterium Microcystis aeruginosa

The structure of cyanoginosin-LA (previously referred to by us as toxin BE-4) is cyclo(-D-Ala-L-Leu-erythro-β-methyl-D-isoAsp-L-Ala-Adda-D-Glu-N- methyldehydroAla) (17a), where Adda refers to the novel β-amino acid residue of 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (11). the connectivity of Adda is deduced from 1H- and 13C-n,m.r. data, but the stereochemistry at carbons 2,3,8, and 9 remains unknown. The residue sequence is derived from the mass spectra of cyclic and linear derivatives of the toxin. A new nomenclature for the related toxins of Microcystis aeruginosa is discussed.