Cucurbitane triterpenoids from Leucopaxillus gentianeus

In addition to the known bioactive triterpene cucurbitacin B (1), two new cucurbitane triterpenoids, namely, leucopaxillones A (3) and B (4), exhibiting a new oxygenation pattern among cucurbitacins, have been isolated from the mushroom Leucopaxillus gentianeus (syn. L. amarus). Cucurbitacin B (1) imparts a bitter taste to the flesh of the fungus; however, it occurs in the fruiting bodies mainly esterified as tasteless fatty acid esters 2a-c. In vitro growth inhibitory effects of compounds 1-4 on proliferation of four different human tumor cell lines (A549, CAKI-1, HepG2, MCF-7) were evaluated by using a 1-day MTT assay. Only cucurbitacin B was highly active on all lines. Free cucurbitacin B is presumed to be formed in vivo by an enzyme-mediated scission of esters 2a-c, thus constituting a chemical weapon that protects the mushrooms against parasites and predators. Compounds 1-4 are structurally different from the other few cucurbitacins isolated from Basidiomycetes, being, instead, more similar to those occurring in plants. In particular, cucurbitacin B (1) seems to represent an interesting example of secondary metabolite convergence between distant taxa such as fungi and vascular plants, where they likely exert a similar role of protection. The structures of the compounds were established by means of spectroscopic methods and X-ray diffraction on a single crystal. The absolute configuration of leucopaxillone A has been assigned on the basis of CD chirality rules.