The "C n.m.r. spectra of 36 natordy occurring xanthones are reported and all chemical shifts assigned. The shifts in substitoted xanthones can be predicted from sobstitoent effects evaloated for simple derivatives. The agreement between calcolated and observed s h i decreases as the extent of sobstitotion in the xanthone increases.Xanthones appear widely in nature in both higher plants and micro-organisms.' These substances possess the carbon skeleton 1 and differ from each other principally in the extent and pattern of hydroxylation. In contrast to detailed 'H n.m.r. results' for this class p J J J / / of compounds there have been few 13C n.m.r. studies. We wish to report now the 13C chemical shifts for 36 naturally occurring xanthones.It has been shown in several series of compounds that shifts in polysubstituted molecules can be predicted from substituent effects evaluated for more simple derivative^.^.^ We have determined hydrox methoxy-, and acetoxy-substituent effects from the ' C n.m.r. spectra of xanthone and several monosubstituted derivatives. The agreement between experimental shifts and values calculated from these substituent effects is examined for several polysubstituted xanthones of natural origin.This study was undertaken in conjunction with a C-biosynthetic study' of the fungal metabolite, ravenelin (1,4,8-trihydroxy-3-methylxanthone), to confirm assignments in the spectrum of that compound.
Y-,
13
EXPERIMENTALquently, chemical shift values are accurate to within *l.O Hz, i.e. ztO.06 ppm. A pulse width of 9 ps (45"), and a pulse interval of 4.0 s, were used to obtain each spectrum. Chemical shifts were measured from the centre peak of the solvent signal and corrected using the appropriate expression: ~T M S = S+, , ,
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