Kabirimine, a New Cyclic Imine from an Okinawan Dinoflagellate

On our quest for new bioactive molecules from marine sources, two cyclic imines (1, 2) were isolated from a dinoflagellate extract, inhibiting the growth of the respiratory syncytial virus (RSV). Compound 1 was identified as a known molecule portimine, while 2 was elucidated to be a new cyclic imine, named kabirimine. The absolute stereochemistry of 1 was determined by crystallographic work and chiral derivatization, whereas the structure of 2 was elucidated by means of spectroscopic analysis and computational study on all the possible isomers. Compound 1 showed potent cytotoxicity (CC₅₀ < 0.097 µM) against HEp2 cells, while 2 exhibited moderate antiviral activity against RSV with IC₅₀ = 4.20 µM (95% CI 3.31-5.33).