Cytotoxic Benzo[j]fluoranthene Metabolites from Hypoxylon truncatum IFB-18, an Endophyte of Artemisia annua

Two new benzo[j]fluoranthene-based secondary metabolites named daldinone C (1) and daldinone D (2), along with two known metabolites, altechromone A and (4S)-5,8-dihydroxy-4-methoxy-α-tetralone, were isolated from the CHCl3/MeOH (1:1) extract of a solid culture of the endophyte Hypoxylon truncatum IFB-18 harbored inside the symptomless stem tissue of Artemisia annua. The structures of the new compounds were elucidated by MS and 1D and 2D NMR spectra and by X-ray diffraction analysis. Their absolute configurations were determined unambiguously by a combination of their CD data and the established exciton chirality rule. Compounds 1 and 2 were substantially cytotoxic against SW1116 cells, with IC50 values of 49.5 and 41.0 μM, respectively, comparable to that (37.0 μM) of 5-fluorouracil. The biosynthetic pathway for 1 and 2 was postulated with the natural occurrence of benzo[j]fluoranthene analogues discussed in brief.