Chemical-shift and coupling-constant data
for protons in 68 flavan derivatives are reported.
Coupling-constant data for interactions
involving the 2-, 3-, and 4-protons have been used to define the configuration
of the 2-, 3-, and 4-substituents and the conformation of the heterocyclic
ring. It is shown that chemical-shift data for protons of the heterocyclic ring
and of 3- and 4-acetoxyl groups are of little value in stereochemical studies.
Analysis of the absorptions of the
aromatic protons shows that N.M.R. is useful for determining the oxygenation
pattern in rings A and B.
Nuclear magnetic resonance
spectra of 80 compounds belonging to several classes of flavan
derivative have been analysed, and chemical shifts and coupling constants of
heterocyclic ring protons and aromatic protons are tabulated and discussed. Compounds
examined include flav-2-enes, flav-3-enes,
3-substituted flavanones and flavans, 2- and
4-substituted isoflavans, 3,4-disubstituted flavans,
and flavan-4-ols. The geometrical configurations of
substituents in the heterocyclic ring follow unequivocally
from the magnitudes of the vicinal coupling constants.
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