Methods for the structural investigation of xanthones: Part II. Location of hydroxyl groups by ultraviolet and visible spectroscopy1

Ultraviolet spectral shifts in presence of certain additives may be used to determine the hydroxylation pattern of xanthones. Thus, 3-hydroxyxanthones are characterized by an intense band around 355 nm which appears in presence of sodium hydroxide, o- and p-Dihydroxyxanthones decompose in presence of this reagent, the reaction rate depending on the position of the substituents. While the spectra of 1-and 2-hydroxyxanthones do not suffer modification in presence of sodium acetate, 3- and 4-hydroxy groups produce typical shifts which are dependent on the presence of additional oxygen functions. The spectra of 1-hydroxy-, as well as of o-dihydroxyxanthones, are affected by the addition of aluminum chloride. 1- and 4-Hydroxy groups with unsubstituted p-positions may be detected and distinguished through the visible spectrum (350–750 nm) of the reaction product of the xanthone with 2,6-dichlorobenzoquinone chloroimide (Gibbs test). Relatively acidic hydroxyls give rise to two maxima in this region. Two maxima, even if of different relative intensity, are also obtained with o- and p-dihydroxy-, as well as with o- and p-hydroxyallylxanthones.