Conformational analysis of oligomeric profisetinidins

[摘要] English: The profisetinidins are an important class of condensed tannins, or proanthocyanidins.Historically, studies towards the structure and conformation of proanthocyanidins were doneon their peracetate and permethyl acetate derivatives.A current upsurge in industrial and biological applications of proanthocyanidins has promptedthe present efforts at detailed analysis of the conformational behaviour of the naturallyoccurring free phenolic oligomeric profisetinidins. Studies towards the structure andconformational analysis of a small number of free phenolic dimeric procyanidins that are4�? coupled and only one free phenolic dimeric profisetinidin, fisetinidol-(4α�?)-catechin,have hitherto been reported.This study centres on the use of 1H, 13C, gradient COSY, COSY 45, COSY 90W, NOESYPH and HMQC NMR experiments in different solvents and at different temperatures toassign the hydrogen and some carbon resonances of the free phenolic profisetinidins thatare found in commercially important southern hemisphere trees, namely Black Wattle(Acacia mearnsii) and Quebracho (Schinopsis balansae). These results, together with dataobtained from CD spectra in methanol, were then used to study the conformations of thesecompounds.Dimers with 2,3-trans-3,4-trans (2,4-cis) configuration, namely fisetinidol-(4α�?)-catechin,fisetinidol-(4α�?)-catechin, ent-fisetinidol-(4β�?)-catechin and ent-fisetinidol-(4β�?)-catechin all displayed sets of duplicate resonances on 1H NMR spectra, indicating thepresence of rotamers on an NMR time-scale at ambient temperatures. The protonresonances of the rotamers of the 4�? linked dimers displayed insignificant chemical shiftdifferences due to the similar magnetic environments and linear shape of both rotamers.The proton resonances of the rotamers of the 4�? linked dimers displayed significantchemical shift differences due to the presence of compact and extended rotamers resulting inlarge changes in magnetic environment due to anisotropic effects.The type of solvent, temperature as well as the relative presence of water or cadmium nitratehad a strong influence on the relative concentrations of the rotamers, the conformations ofthe 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 boatconformations in some instances. The C-ring conformations ranged from rings with A/Econformational exchange to preferred E-conformers.The dimers with 2,3-trans-3,4-cis (2,4-trans) configuration, namely fisetinidol-(4β�?)-catechin, fisetinidol-(4β�?)-catechin and ent-fisetinidol-(4α�?)-catechin displayed only oneset of resonances on 1H NMR spectra at ambient temperatures. The presence ofintramolecular hydrogen bonding and limited conformational exchange was confirmed by thefollowing observations:a) Selective broadening of proton resonances in both the heterocyclic and aromaticregions 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 protonsas 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 conformationalexchange with line shapes indicating possible skewed boat conformations in some instances.2D NMR experiments afforded estimations, in some cases, of the angles between the planeof the B-ring and the 2-CC�?-HC bond, the plane of the D-ring and the 4-CF�?-HF bond aswell as the plane of the D-ring and the 4-CC�?-HC.The resonances of 4-CC of the 2,3-trans-3,4-trans dimers displayed significant chemical shiftdifferences (± 41 ppm) compared to 4-CC of the 2,3-trans-3,4-cis dimers (± 31 ppm).Thiscould serve as a possible indicator of the relative configurations of the C-rings of 2,3-transprofisetinidins dimers.CD studies of al seven abovementioned dimers, as well as four trimers from Acacia mearnsiidisplayed complex curves with a number of strong Cotton effects. Although some trendswere observed, it was abundantly evident that the chiroptical characteristics of this class ofcompounds are too complex to be interpreted in terms of the empirical quadrant rule.

[发布机构] University of the Free State