13C CP/MAS NMR studies of flavonoids

C CP/MAS NMR spectra were recorded to characterize the solid-state conformation of flavonoids: galangin, kaempferol, quercetin, myricetin, chrysin, apigenin, luteolin, acacetin, hesperetin and naringenin. Correct assignment of the signals in solution-state spectra (using APT, HSQC and HMBC) and solid-state spectra (using dipolar dephased and short contact sequences) for the above flavonoids is reported. The locked conformation of OH and OCH3 groups in solids results in an increased shielding of carbon adjacent to C—OH or C—OCH3 hydrogens, enabling conclusions as to the orientations of these groups to be drawn. The C-5—OH hydroxyl (and C-3—OH) points towards the carbonyl group oxygen, forming intramolecular hydrogen bonds. Considerations of C2′ and C6′ carbon shielding suggest that ring B is not coplanar with the benzopyran fragment in kaempferol, acacetin and myricetin. Copyright © 2001 John Wiley & Sons, Ltd.