13C NMR spectral assignment of the A-ring of polyoxygenated flavones

Abstract

The 13C NMR spectra of polyhydroxyflavones were systematically examined by using about 70 examples which have a 4-methoxyphenyl B-ring, such as 5,6,7-, 5,7,8 and 5,6,8-trioxygenated and 5,6,7,8-tetraoxygenated flavones and flavonols, and the following results were obtained. In the 13C NMR spectra of flavones, the signals at the 2-, 3- and 4-positions in the C-ring were hardly affected by the substituents on the A-ring except for those at the 5-position and shifted regularly by introduction of a methoxy or hydroxy group at the 3-position. The signals at the 5- to 10-positions in the A-ring were greatly affected by the substituents and oxygenated patterns on the A-ring and exhibited the respective characteristic pattern reflecting the A-ring substituents without the effects of the substituents on the B- and C-rings. The substituent effects on the A-ring were estimated from the spectral data. Consequently, it is found that the structures of polyhydroxyflavones can be correctly defined by their 13C NMR spectral assignment. Additionally, we examined the structures of some natural flavones previously reported.