Absorption Spectra of Phytoxanthones

Abstract

The Ultraviolet Absorption Spectra of Phytoxanthones are reviewed.In recent years a large number of xanthones have been isolated from plant as well as microbial sources.Among other plants Guttiferae and Gentianaceae represent the principal sources of xan-4 thone derivatives1-".Reviews by bbberts2, Cottlieb , ~cheinmann~, and ~ostettmanl~, concerning chemotoxonomic importance and chemistry of natural xanrhanee have appeared.Ultraviolet absorption spectroscopy has played an important role in determining the oxygenation pattern on xanthone skeleton.In this article the importance of this technique is reviewed and absorption maxima for over one hundred natural xanthones in the presence of different additives, are tabulated.The ultraviolet absorption spectra of most xanthones consist of three intense bands in the region 230-340 nm, and a fourth, less intense, band at higher wavelengths which account for their yellow colour.The utility of ultraviolet spectral studies in assigning the oxygenation pattern of a xanthone is of grear value.This is because the ulfraviolet spectra of polyoxygenared xanthones vary significantly with change in the oxygenation pattern 2,16,22 The approach is largely emperical which provides information on the general oxygenarion 20 pattern of the carbon skeleton of xanthone .This permits location of free hydroxyl groups in the presence of certain additives such as sodium hydroxide, sodium acetate, aluminium chloride or 2,6-dichlorobenzoquinone chlorimide.Addition of sodium hydroxide ionises hydroxyl groups at all positions of xanthone skeleton resulting in significant alterations in the absorption spectrum.1-hydroxyxanthones show a reduction of the intensity of second principal maximum along with red shifts of this and other maxima at higher wavelengths.3-hydroxyxanthones in alkaline media give bands at 345-365 nm, where other xanthones, including relatively acidic 4-hydraxy derivatives absorb feebly.A hydroxyl group in the 3-(or 6 -) position of xanthone nucleus has enhanced acidity due to the presence of the 1-ring carbonyl group and can be readily detected by it's solubility in h U .741 G. c. &D&.751 P o t y g a t a r n a d e n i a G M y .761 P. p e n e a L. 771 Swehtia chiZata Buch-Hm.