Interflavanyl Bond in Oligomeric Proanthocyanidins

The acid catalyzed cleavage of the interflavanyl bond in proanthocyanidins exhibiting C(5)-oxygenation of the A-ring of their chain-extender units with sulfur (79, 80) and oxygen (81) capturing 

Similar conditions also effected cleavage of the interflavanyl bond in the fisetinidol-(4a-+8)-catechin permethylaryl ether (68) to afford tetra-0-methylcatechin (75) (21%), the l,3-diarylpropan-2-ol (78) (12%), and tri-O-methylfisetinidol (81) (12%). Such a rupture of the interflavanyl bond in the permethylaryl ether (68) introduced an important dimension to these cleavages in relation to the chemistry of the 5-deoxyoligo-flavanoids where the additional chromatographic steps involved with derivatization are often prerequisites for sample purity. The liberation of the chain-terminating flavan-3-ol unit (5) or (75), irrespective of 

The protonated species (73/74) presumably also served as precursor to the 4P-deuteriotri-0-methylfisetinidol (82) by delivery of hydride ion from the P-face in a predominant Sn2 mode. Compound (82) persistently formed also when fisetinidol-(4a- 8)- and (4p- 8)-catechin hepta-O-methyl ethers (68) and (70) were treated with Na(CN)BD3 in TEA. This observation prompted an investigation of the structural features of the substrates that direct the stereochemistry of the delivery of hydride ion at C(4) in intermediates of type (73/74). Whereas treatment of the epifisetinidol-(4p- 8)-catechin hepta-O-methyl ether (85) with Na(CN)BD3 afforded the 4P-deuteriotri-0-methylepifisetini-dol (87) (18.5%), tetra-O-methylcatechin (75) (32%) and the (25)-l,3-dideuterio-l,3-diarylpropan-2-ol [6%, enantiomer of compound (80)], the nEfisetinidol-(4p- 8)-catechin hepta-O-methyl ether (86) gave 4a-deuteriotri-O-methyl- nf-fisetinidol [13%, the enantiomer of compound (82)], tetra-O-methylcatechin (75) (24%) and the (25)-l,3-dideuterio-l,3-diarylpropan-2-ol [12%, enantiomer of (80)]. 

the formation of the 4P-deuteriofisetinidol- and epifisetinidol derivatives (82) and (87) from the reduction of the profisetinidin permethylaryl ethers (68), (70), and (85) with Na(CN)BD3 in TEA, and of the enantiomer of compound (82) during reduction of the fisetinidol-(4p- 8)-catechin derivative (86), indicated that the deuterium ion is consistently delivered to C(4) of a protonated species of type (73 / 74) from the side opposite to the 2-aryl group of the C-ring. This 

The potential of this development for the structural elucidation of the proanthocyanidin condensed tannins, especially the 5-deoxy analogues, from important commercial sources is clear. In addition, the method facilitates the ready definition of the absolute configuration of the chainterminating flavan-3-ol moiety in 5-deoxyoligoflavanoids, especially in 

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