Gymnodimine C, an isomer of gymnodimine B, from Karenia selliformis

Gymnodimine C (1), an oxidized analogue of the spiro-imine algal toxin gymnodimine (3), was isolated from extracts of the cells of Karenia selliformis. The structure of gymnodimine C, determined by one-and two-dimensional H-1 NMR spectroscopy and mass spectrometry, was found to be isomeric with gymnodimine B (2) at C-18.