Proposal for structural revision of several monosubstituted tricycloalternarenes

Cycloalternarenes are a group of meroterpenes isolated from epiphytic fungi with a mono-, bi, trior tetracyclic skeleton. We have detected in the bibliography a series of monosubstituted tricycloalternarenes with erroneous structures. Thus, in this work we make several proposals to correct the structures of nineteen 4-hydroxy-tricycloalternarenes, TCA 6a, TCA 11a(2), (2E)-and (2Z)-TCA 12a, 2H-(2E)-TCA 12a, TCAs 9a and F-2, methyl nortricycloalternarate, TCAs K, L, S-W, X-2 and tricycloalterfurenes A-C, and four 6-hydroxy-tricycloalternarenes, TCA 12b, TCA 13b, tricycloalterfurene D and TCA F-3. Moreover, the graphic representation of TCA 14b and TCAs 15b-18b had been corrected. In addition, we have suggested that mono-hydroxylated tricycloalternarenes can only exist in nature substituted at the 4 alpha-or 6 beta-position (4R- or 6R-configuration), which could also be explained considering biogenetic reasons. We have also determined the C-4 and C-6 configuration of several monosubstituted tricycloalternarenes, whose planar structure had been previously determined. Thus, compounds of the series a such as TCAs 1a-8a, 11a and ACTG-toxin H have a 4R-configuration, whilst in the series b TCAs 3b-7b and TCAs 9b-11b possess a 6R-configuration.

Automated summary

In this study, we identified several monosubstituted tricycloalternarenes with erroneous structures and proposed correction strategies. We also determined that mono-hydroxylated tricycloalternarenes can only exist at specific positions, and further determined the structures and configurations of several compounds.