Cucurbitane Triterpenoids from Leucopaxillus gentianeus

In addition to the known bioactive triterpene cucurbitacin B (<BO>1</BO>), two new cucurbitane triterpenoids, namely, leucopaxillones A (<BO>3</BO>) and B (<BO>4</BO>), exhibiting a new oxygenation pattern among cucurbitacins, have been isolated from the mushroom Leucopaxillus gentianeus (syn. L. amarus). Cucurbitacin B (<BO>1</BO>) imparts a bitter taste to the flesh of the fungus; however, it occurs in the fruiting bodies mainly esterified as tasteless fatty acid esters <BO>2a</BO>−<BO>c</BO>. In vitro growth inhibitory effects of compounds <BO>1</BO>−<BO>4</BO> 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 <BO>2a</BO>−<BO>c</BO>, thus constituting a chemical weapon that protects the mushrooms against parasites and predators. Compounds <BO>1</BO>−<BO>4</BO> are structurally different from the other few cucurbitacins isolated from Basidiomycetes, being, instead, more similar to those occurring in plants. In particular, cucurbitacin B (<BO>1</BO>) 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.