13C NMR-spectroscopy of polymethoxyxanthones

PRENYLATED XANTHONES FROM Rheedia acuminata

“…The localization of the chromene group at C-2 and C-3 and C-1 hydroxyl was established by com parison of their Ή and l3 C NMR data (Table 1) with reported chemical shifts for the closely related compound (SEN et ai, 1980. The signal of the OCH 3 group in the ,3 C NMR at à 61.8 indicates that it is bonded to C-5, between two ortho groups (CHAUDHURI et al, 1978). The localization of the ã,ã-dimethyallyl at C-7 was confirmed by the long range (J 3 ) coupling, and NOE experiments.…”

.ALPHA.-Glucosidase Inhibitors Ellagic Acid Derivatives with Immunoinhibitory Properties from Terminalia superba

“…Additionally, 13 C-NMR spectrum showed chemically equivalent signals at 108.9 ppm (C-2ٞ/6ٞ) and 145.5 ppm (C-3ٞ/5ٞ) and justified why the 13 C-NMR spectra displayed 26 carbon signals instead of 28 as shown by the molecular formula. The 13 C-NMR spectra (Table 1) displayed signals of two hindered methoxyl groups 8) at d 61.1 and 61.8, thus allowing placement of the aromatic methoxyl group at C-3 and C-3Ј. This indicates that the galloyl and xylose units are linked to ellagic acid via 4-O-and 4Ј-O-linkages respectively.…”

Computer supported structure elucidation of polymethoxy‐ and polyacetoxyxanthones. V—13C NMR spectroscopy of substituted xanthones

“…(The bridges at C atoms 4a and 8b are not counted as substituents for statistical reasons, in contrast to Ref. 4.) Carbon atoms 4a and 8b have a steric parameter only if both positions 3 and 4 or both 1 and 2, respectively, are substituted.…”

“…The chemical shift deviations of two substituted and neighbouring C atoms, C-1-C-4, from pure additivity have been observed to be up to 12ppm according to their position in the xanthone ring. 4 The ortho high field effects of a methoxy substituent on a substituted neighbouring C atom are decreased by this amount, and are named by us as positive steric effects. They are expressed as steric parameters (STP), which are required in addition to the four position parameters in the multiple linear regression analyses.…”