In the HMBC spectrum (Fig. 1),
the less shielded isolated aromatic proton (dH 7.42) showed
correlations to the carbonyl carbon, and it was therefore
attached to C-8. Proton H-8 showed cross-peaks with a
monosubstituted olefinic carbon (dC 122.1) which had to be
C-16. Proton H-16 correlated with C-8 and an oxygenated
aromatic carbon (dC 146.6, C-6) whilst its cis-olefinic partner
proton (H-17) gave a cross-peak to a substituted aromatic
carbon (dC 119.3, C-7). The correlations revealed that the 2,2-
dimethylpyran ring was linear to the xanthone B ring at C-6 and
C-7 with C-6 being oxygenated (dC 146.6),