Isolation and Structure Assignments of Rostratins A−D, Cytotoxic Disulfides Produced by the Marine-Derived Fungus Exserohilum rostratum^†

Four new cytotoxic disulfides, rostratins A−D (1−4), were isolated from the whole broth of the marine-derived fungus Exserohilum rostratum (Drechsler), a fungal strain found associated with a marine cyanobacterial mat. The structures of these cyclic dipeptides were established through chemical degradation and a variety of two-dimensional NMR techniques. The absolute configurations of the rostratins were determined by the modified Mosher method. In the case of the polyhydroxylated compound 1 and the mercaptol 4, regioselective acylation was achieved by modulating the reaction temperature while monitoring the progress of the reaction by ¹H NMR. Rostratins A, B, C, and D showed in vitro cytotoxicity against human colon carcinoma (HCT-116) with IC₅₀ values of 8.5, 1.9, 0.76, and 16.5 μg/mL, respectively.