Proton magnetic resonance spectra of catechin and bromocatechin derivatives: C₆- vs. C₈-substitution

Abstract

The 1Hmr spectra of 20 catechin derivatives substituted at C-6/C-8 by bromine and/or hydrogen and at oxygen by methyl, acetyl, and/or hydrogen have been analyzed in deuterated acetone, acetonitrile, and chloroform. Because of its dependence on the nature of the solvent and of the oxygen substituent, the difference between H-6 and H-8 chemical shifts has been found to be an unreliable criterion for the distinction between 8-bromo and 6-bromo isomers. In methylated catechins, double irradiation of H-8 and H-6 enhances one (MeO-7) and two (MeO-5 and MeO-7) methoxy signals, respectively, via the nuclear Overhauser effect. This permits unambiguous assignment of chemical shifts to all ring A protons. The H-6 and H-8 resonance frequencies of catechin have been determined by decoupling of the OH-5 and OH-7 protons.

Iodination and deuteration of catechin derivatives

“…The two regioisomers are readily distinguished by the characteristically different chemical shifts of their residual ring A aromatic protons (see Table l), and by the nOe difference spectra of their tetramethyl ether acetates: saturation of H-8 of 5e enhances only one methoxy signal while saturation of H-6 of 5 f enhances two. Additional verification of our structure assignments is provided by the excellent correlation of the nmr chemical shifts for all ten monoiodocatechin derivatives (Tables 1 and 2) with those for the corresponding bromo compounds ( 1,9).…”

“…ring A (C-8 rather than C-6), are based on a comparison of their proton and carbon chemical shifts (Tables 1 and 2) with those of the 6-iodo isomers (3e and Se, vide infra) and the corresponding bromocatechins (1,9), and on the nuclear Overhauser enhancement (nOe) of two methoxy signals when the aromatic ring A proton of Sf is saturated. Most of our 'H nrnr and mass spectrometry data for Sf (see Experimental) agree with those reported by Roux and co-workers (7) who obtained the compound in less than 1 % yield from catechin by successive iodination, methylation, chromatographic purification of the intermediate tetramethyl ester, and acetylation.…”

Identification of Two Novel Proanthocyanidins in Green Tea

“…In the case of 1 after the addition of Cd(NO 3 ) 2 to the substrate solution (Kiehlmann and Tracey, 1986), the previously broad hydroxyl group protons appeared as sharp signals. Signal assignments were straightforwardly afforded by COSY and HMQC spectra.…”

Oligomeric catechins: An enabling synthetic strategy by orthogonal activation and C(8) protection

“…Its steric and electronic effects would hopefully reduce the C(8) nucleophilicity, preventing the self-reactions. Regio- selective installation of a bromine atom was easily achieved by treatment of 3 with N-bromosuccinimide in CH 2 Cl 2 (22), giving the requisite C(8)-bromide 12 in 95% yield (Scheme 6). Moreover, the corresponding phenylthio derivative 13 was obtained in a high ␤-selectivity (␣͞␤ ϭ 5:95) by treatment of 12 with PhSH in the presence of BF 3 ⅐OEt 2 in 91% yield (15).…”