Leptosins O-S, cytotoxic metabolites of a strain of Leptosphaeria sp isolated from a marine alga

Leptosins O (1), P (2), Q (3), R (4) and S (5) have been isolated from a strain of Leptosphaeria sp. originally separated from the marine alga Sargassum tortile. Their absolute stereostructures have been elucidated on the basis of spectroscopic analyses of their acetate derivatives (6-10) using various 1D and 2D NMR techniques and some chemical transformations. The NMR and NOE spectral analyses of 6-10 revealed that they exist in a single conformer of B type in CDCl3. Among these metabolites, leptosins O (1) and P (2) exhibited significant cytotoxicity against cultured P388 cells