Molecular structure and tautomerization of the 1:1 complex of luteoskyrin and rugulosin

A novel 1:1 natural product complex (1) containing luteoskyrin (2) and rugulosin (3) was isolated from the acetone extract of culture broth of Myrothecium sp. In DMSO solution, the 1H and 13C NMR spectra showed only half proton and carbon signals indicating that both compounds exist as a symmetric enol–enol form. In contrast, in the solid state, X-ray analysis revealed that 2 bound 3 with high specificity through intermolecular hydrogen bonds and π–π interactions, and both 2 and 3 tautomerized to a non-symmetric enol–ketone form due to the strong linear hydrogen bonding between the ketone group and the alcoholic hydroxyl group. In addition, complex 1 showed potent cytotoxic activity against cell lines KB (human epidermoid carcinoma cell), HT-29 (human colon cancer cell) and 3T3 (mouse embryonic fibroblast cell) with IC50 values of 0.57, 3.11 and 5.83 μM, respectively.