A pair of new isocoumarin epimers from soil fungus Hypoxylon sp

In our research on novel secondary metabolites from micro-organisms, two new (1-2) and four known dihydroisocoumarins (3-6) were derived from soil fungus Hypoxylon sp. Their structures were determined with extensive NMR data analysis and ECD calculation comparing with those of experimental CD spectra. Interestingly, compounds 1 and 2 possessed the same planar structure and very similar NMR data, suggesting 1 and 2 were a pair of epimers at either C-3 or at C-4, confirmed by the totally opposite cotton effect around 250 nm in the CD spectra of 1 and 2. Moreover, for the first time, we revealed that the CD absorption peak at 250 nm was dominated by C-3 orientation, rather than the orientation of C-3 substituents, by intensive ECD investigations.