Four tetrahydroxanthone dimers (1-4) and four biogenetically related monomers (5-8), including the new derivatives 4-6, were isolated from the endophyte Phomopsis longicolla. The absolute configurations of 2-4 were established for the first time by TDDFT electronic circular dichroism calculations, and that of phomoxanthone A (1) was revised by X-ray crystallography. Phomoxanthone A (1) showed the strongest pro-apoptotic activity when tested against a panel of human cancer cell lines, including cisplatin-resistant cells, whereas it was up to 100-fold less active against healthy blood cells. It was also the most potent activator of murine T lymphocytes, NK cells, and macrophages, suggesting an activation of the immune system in parallel to its pro-apoptotic activity. This dual effect in combating cancer cells could help in fighting resistance during chemotherapy. Preliminary structure-activity studies of isolated compounds and derivatives obtained by semisynthesis (9a-11) hinted at the location of the biaryl axis and the presence of acetyl groups as important structural elements for the biological activity of the studied tetrahydroxanthones.
Chromatographic analysis of an ethyl acetate extract of Pestalotiopsis virgatula rice cultures yielded four new -pyrone derivatives, pestalotiopyrones I-L, and the new (6S,1'S,2'S)-hydroxypestalotin, in addition to three known compounds. The planar structures of the new compounds were elucidated on the basis of extensive NMR spectroscopic and mass spectrometric analyses. The absolute configurations of the new compounds were determined on the basis of biosynthetic considerations, coupling constants and for pestalotiopyrones L by TDDFT ECD calculations of solution conformers.
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