Applications of proton magnetic resonance spectroscopy in the structural investigation of xanthones

Abstract

Computed chemical shifts and coupling constants for xanthone in deuteriochloroform and trifluoroacetic acid are in agreement with experimental data at 60,100, and 220 MHz. The increased dispersion of the signals a t 220 M H z in deuteriochloroform shows four distinct aromatic proton signals which can be analysed directly by first-order methods. Fundamental parameters for xanthone in carbon tetrachloride, pentadeuteriopyridine, hexadeuterioacetone, and hexadeuteriodimethyl sulphoxide have also been measured. Additive diamagnetic shifts for urtho-, meta-, and para-hydroxy-, alkoxy-, and alkyl substituents have been determined empirically and experimental and predicted chemical-shifts for natural and synthetic xanthones are in agreement, providing steric hindrance of vicinal groups is considered. The influence of been correlated.

Naturally Occurring Xanthones: Chemistry and Biology

“…1 H NMR gives information about the substitution pattern on each ring. Acetylated derivatives have been utilised in the structure determination of glycosides [81]. The number and relative position of acetyl and methoxy groups can be determined by observing the shift for the position of absorption for the aromatic protons which occurs upon replacing methoxy group by an acetyl group.…”

Structure Elucidation of Xanthone Derivatives: Studies of Nuclear Magnetic Resonance Spectroscopy

“…In 1 H-13 C COSY, the 13 C NMR spectrum appears along one axis and the 1 H NMR spectrum along the other; H-C shift correlation is evidenced by spots in the 2D display. 1 H-1 H COSY simply identifies which protons are coupled with each other, and 1 H- 13 C COSY identifies all proton and carbon atoms which are connected by a 13 C-1 H coupling over one bond. The latter is also of great use in assigning proton resonances when the carbon resonances for the same sites are known and vice versa.…”

“…The early 1 H and 13 C NMR studies of xanthone derivatives have been well documented in the 1970s [1][2][3][4][5][6][7]. These 1 H NMR studies were mainly devoted to the structure elucidation of natural derivatives and were based on chemical shifts, which were estimated from electronic and anisotropic effects, and also on spin coupling data for aromatic protons.…”

“…The proton and carbon resonances assignments were also based on the empirical rules of the additivity of substituent effects evaluated for simple xanthone derivatives. 1 H and 13 C MNR spectra remain the primary tools used by chemists to analyse xanthone derivatives on a routine basis. The first is of systematic use, while the second is not always reported.…”

Naturally occurring pentaoxygenated, hexaoxygenated and dimeric xanthones: a literature survey

“…A closer scrutiny of chemical shifts of the aromatic protons allows prediction of the oxygenation pattern 21 . There are detailed NMR results for this class of compounds 159 1 H NMR and 13 C NMR are the most useful tools in structure elucidation of xanthones 37 .…”