Unequivocal assignments of the uncoupled protons at H-3 and H-6/H-8 of pyranoflavones by 2D-NOESY experiments have made it possible to distinguish linear and angular isomers. This study also suggests that the linear structure (carpachromene, 1) assigned to a pyranoflavone isolated from Atalantia ceylanica should be revised to angular form (atalantoflavone, 4). 2D-NOESY is also useful for differentiating 3′ 4′-and 2′ 4′-disubstituted B-ring flavones.
A. Banerji and Luthria, D. L., “Structural Elucidation of Isomeric Pyronoflavones by 2D-NOESY”, Spect. Letts, vol. 24, p. 471, 1991.