Roseic Acid and Roseolactones A and B, Furan-Cucurbitane Triterpenes from Russula aurora and R. minutula (Basidiomycota)

Three new seco-cucurbitane triterpenes, viz. roseic acid (1) and roseolactones A (2) and B (3), all bearing a furan moiety condensed to the B-ring, were isolated from Russula aurora and R. minutula. Their structures were elucidated using 1D and 2D NMR spectroscopy, and ESI HRMS. Compounds 2 and 3 were found to be epimers at C-23. A detailed ab initio theoretical investigation was carried out to assess their relative stereochemistries. The conformations and NMR spectra of the two isomers were calculated, and the results were compared to the experimental data. The new compounds show a bright pink reaction with sulfo-vanillin at room temperature, and they are therefore responsible for the systematically significant reaction performed by mycologists on R. aurora and R. minutula fruiting bodies. A mixture of compounds 2 and 3 was found to be weakly active against HepG2, WISH, and CAKI-1 human tumour cells.