Complete Assignment of the1H and13C NMR Spectra of Flavone and its A-Ring Hydroxyl Derivatives

Complete assignment of the 1H and 13C NMR spectra of flavone and three flavone derivatives is reported. Proton–proton and carbon–proton coupling constants of the flavones, including the extreme seven-bond long-range coupling between H-7 and H-3 in 6-hydroxyflavone (0.52 Hz) and flavone (0.27 Hz), are presented. Hydrogen bonding between 5-OH and the keto group (C-4) in 5,7-dihydroxyflavone gives rise to a large coupling constant between H-3 and C-3 (169.1 Hz), in addition to the expected low-field shifts of the 5-OH hydroxyl proton (12.92 ppm) and C-4 (181.86 ppm). This investigation revises the previously reported carbon assignments of three of the examined flavones and provides new NMR data for 6-hydroxyflavone.