P. D. Collier, T. Bryce, R. Mallows
1973
Citations
1
Influential Citations
91
Citations
Quality indicators
Journal
Tetrahedron
Abstract
Abstract The structures, relative configurations and precursors of seven pigments isolated from the theaflavin fraction of black tea have been established by synthesis, and by NMR and mass spectrometry. The four principal pigments are the bis-flavan-substituted 1′,2′-dihydroxy-3,4-benzotropolones theaflavin, the corresponding isomeric 3- and 3′-monogallates, and the 3,3′-digallate. Collapsed spin-spin couplings and nuclear Overhauser effects in these and a range of mono-flavan-substituted model compounds are interpreted in terms of steric hindrance to rotation of flavan groups with respect to the benzotropolone ring. As a result of these steric effects, the 3-gallate and 3,3′-digallate exist in rotameric forms which are clearly distinguished in the temperature-dependent CD spectra.