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 1 H and 13 C 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 IC 50 values of 0.57, 3.11 and 5.83 μM, respectively.