Abstract
Proanthocyanidins (PACs) are natural plant-derived polymers consisting of flavan-3-ol monomers. Quebracho (Schinopsis lorentzii and balansae) heartwood and mimosa (Acacia mearnsii) bark extracts are the major industrial sources of PACs. These commercial extracts are often sulfited to reduce their viscosity and increase their solubility in water. The chemical process of sulfitation is still poorly understood regarding stereochemical influences during the reaction and during the cleavage of the interflavanyl bond of oligomers. To gain a better understanding of sulfitation, two diastereomeric flavan-3-ol monomers were sulfited under industrial conditions, and procyanidin B-3 (catechin-4α→8-catechin) were sulfited to investigate interflavanyl bond cleavage with sulfitation at C-4. Treatment of diastereomeric flavan-3-ols 2R,3S-catechin and 2R,3R-epicatechin with NaHSO3 at 100 °C in aqueous medium afforded the enantiomeric (1R,2S)- and (1S,2R)-1-(3,4-dihydroxyphenyl)-2-hydroxy-3-(2,4,6-trihydroxyphenyl)propane-1-sulfonic acid, respectively. Utilizing computational NMR PD4 calculations it was determined that the direction of stereoselective nucleophilic attack is controlled by the C-3 configuration of the flavan-3-ols catechin and epicatechin. Sulfitation of the catechin-4α→8-catechin dimer 7 (procyanidin B-3) under the same conditions led to the cleavage of the interflavanyl bond yielding the C-4 sulfonic acid substituted catechin momomer. From the heterocyclic ring coupling constants it was determined that nucleophilic attack occurs from the β-face of the dimer leading to the 2,3-trans-3,4-cis isomer as product.
Highlights
The tanning of animal skins or hides into durable leather has been practiced since antiquity, and the original method used was vegetable pit tanning [1]
Tannins are secondary metabolites found in higher plants and mainly consist of hydrolysable tannins and proanthocyanidins (PACs), known as condensed tannins (Figure 1)
Sulfitation of the catechin-4α→8-catechin dimer 7 under the same conditions led to the cleavage of the interflavanyl bond yielding the C-4 sulfonic acid substituted monomer 8 and
Summary
The tanning of animal skins or hides into durable leather has been practiced since antiquity, and the original method used was vegetable pit tanning [1]. Vegetable material containing tannins reacts irreversibly with the protein constituents of hides to yield soft leather, which is resistant to microbial degradation, water, heat and abrasion [2]. Tannins are secondary metabolites found in higher plants and mainly consist of hydrolysable tannins (polyesters of gallic or hexahydroxydiphenic acid and D-glucose) and proanthocyanidins (PACs), known as condensed tannins (complex oligomers of flavan-3-ol monomers) (Figure 1). The four major trees from which tanning material is extracted are mimosa (Acacia mearnsii bark) and quebracho (Schinopsis lorenzii and S. balansae, heartwood) for condensed tannins [3,4] and chestnut (Castanea sativa and C. dentate, wood) [5] and tara (Caesalpinia spinosa, pods) for hydrolysable tannins [6]. Hydrogen bond formation between the hydroxy groups of the polyphenolic PACs and proteins forms the basis for many of the tannin extract applications.
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