Conformational analysis of oligomeric profisetinidins

English: The profisetinidins are an important class of condensed tannins, or proanthocyanidins. Historically, studies towards the structure and conformation of proanthocyanidins were done on their peracetate and permethyl acetate derivatives. A current upsurge in industrial and biological applications of proanthocyanidins has prompted the present efforts at detailed analysis of the conformational behaviour of the naturally occurring free phenolic oligomeric profisetinidins. Studies towards the structure and conformational analysis of a small number of free phenolic dimeric procyanidins that are 4→8 coupled and only one free phenolic dimeric profisetinidin, fisetinidol-(4α→8)-catechin, have hitherto been reported. This study centres on the use of 1H, 13C, gradient COSY, COSY 45, COSY 90W, NOESY PH and HMQC NMR experiments in different solvents and at different temperatures to assign the hydrogen and some carbon resonances of the free phenolic profisetinidins that are found in commercially important southern hemisphere trees, namely Black Wattle (Acacia mearnsii) and Quebracho (Schinopsis balansae). These results, together with data obtained from CD spectra in methanol, were then used to study the conformations of these compounds. Dimers with 2,3-trans-3,4-trans (2,4-cis) configuration, namely fisetinidol-(4α→8)-catechin, fisetinidol-(4α→6)-catechin, ent-fisetinidol-(4β→8)-catechin and ent-fisetinidol-(4β→6)- catechin all displayed sets of duplicate resonances on 1H NMR spectra, indicating the presence of rotamers on an NMR time-scale at ambient temperatures. The proton resonances of the rotamers of the 4→6 linked dimers displayed insignificant chemical shift differences due to the similar magnetic environments and linear shape of both rotamers. The proton resonances of the rotamers of the 4→8 linked dimers displayed significant chemical shift differences due to the presence of compact and extended rotamers resulting in large changes in magnetic environment due to anisotropic effects. The type of solvent, temperature as well as the relative presence of water or cadmium nitrate had a strong influence on the relative concentrations of the rotamers, the conformations of the heterocyclic C- and F-rings as well as the visibility of hydroxy groups. The F-rings all displayed A/E- conformational exchange with line shapes indicating possible skewed boat conformations in some instances. The C-ring conformations ranged from rings with A/E conformational exchange to preferred E-conformers. The dimers with 2,3-trans-3,4-cis (2,4-trans) configuration, namely fisetinidol-(4β→8)- catechin, fisetinidol-(4β→6)-catechin and ent-fisetinidol-(4α→8)-catechin displayed only one set of resonances on 1H NMR spectra at ambient temperatures. The presence of intramolecular hydrogen bonding and limited conformational exchange was confirmed by the following observations: a) Selective broadening of proton resonances in both the heterocyclic and aromatic regions of 1D NMR spectra. b) Sharpening of resonances in 1D spectra at elevated temperatures. c) The presence of abundant coupling between heterocyclic and aromatic ring protons as observed on 2D spectra. d) Coupling between 2-HC and 4-HC on 2D NMR spectra. The C-rings had preferred A-conformations, with the F-rings displaying A/E conformational exchange with line shapes indicating possible skewed boat conformations in some instances. 2D NMR experiments afforded estimations, in some cases, of the angles between the plane of the B-ring and the 2-CC→2-HC bond, the plane of the D-ring and the 4-CF→4-HF bond as well as the plane of the D-ring and the 4-CC→4-HC. The resonances of 4-CC of the 2,3-trans-3,4-trans dimers displayed significant chemical shift differences (± 41 ppm) compared to 4-CC of the 2,3-trans-3,4-cis dimers (± 31 ppm).This could serve as a possible indicator of the relative configurations of the C-rings of 2,3-trans profisetinidins dimers. CD studies of al seven abovementioned dimers, as well as four trimers from Acacia mearnsii displayed complex curves with a number of strong Cotton effects. Although some trends were observed, it was abundantly evident that the chiroptical characteristics of this class of compounds are too complex to be interpreted in terms of the empirical quadrant rule.