Proposal for structural revision of several disubstituted tricycloalternarenes.

Mono- and di-substituted tricycloalternarenes form a group of meroterpenes isolated from epiphytic fungi. In this work, we have made thirteen proposals to correct erroneous structures of disubstituted tricycloalternarenes, also known as guignardones. Thus, in this group of compounds, structures of guignardones K, L, M, W, tricycloalternarene B, 15-hydroxy-tricycloalternarene 5 b, guignardiaene D, magnardones F-H and coibanols A-C, have been revised. Moreover, we have also explained why there are only two types of disubstituted tricycloalternarenes in nature, one with a -CH-O- β-bridge between C-6 and C-4 (6R,4S-configuration), and the other with a -CH-O- α-bridge between C-4 and C-6 (4R,6S-configuration). Finally, the relative and absolute configurations of phyllostictone A and the absolute structure of phyllostictone D have been established by comparison with those of magnardones I and D, respectively.