RKS-1778, a new mammalian cell-cycle inhibitor and a key intermediate of the [11]cytochalasin group.

In the course of screening for the mammalian cell-cycle inhibitors, we have isolated a new [11]cytochalasin, RKS-1778 (1), and epoxycytochalasin H (2) from a fungus, Phoma sp. SNF-1778. The structure of 1 was determined to be 21-acetoxy-18-hydroxy-10-phenyl-5,6,16,18-tetramethyl[11]cytochalasa-6,13,19-trien-1-one, on the basis of spectroscopic methods, including 1 H- and 13 C-2D NMR techniques. RKS-1778 (1) may be a precursor of 2 and the key direct product of a proposed biosynthetic intramolecular Diels-Alder reaction. Both 1 and 2 completely arrested the cell-cycle progression of tsFT210 cells in the M phase at concentrations of 2.1 and 2.0 μM, respectively.