Bonding Positions at Nucleophilic Centres

Our starting-point was the required definition of the bonding positions at either C-6 or C-8 of the presumed terminal (-h)-catechin [(2/ 3S)-2,3-rm .s-flavan-3,3, 4, 5,7-pentaol] unit which serves as initial nucleophile in most tannins under consideration. The solvent shift method for methoxyl function developed by Pelter et al (3), although applicable to the methyl ethers of biflavonoids (i. e. compounds with 3- or 4-carbonyl function), was less reliable when applied to the methyl ether acetates of biflavanoids, often leading to ambiguous or erroneous conclusions.  

Conditions for Interflavanoid Bonding. Stereochemical Course of the Reaction. 

With the knowledge that flavan-3,4-diols are freely associated with flavan-3-ols [e. g. (H-)-catechin, (—)-epicatechin, ( —)-fisetinidol, ( + )-epifisetinidol] in tannin mixtures comprised of these molecular entities, experimental conditions were sought which would permit formation of C-4 —C-6 or 8 interflavanoid links. The conditions selected had to be sufficiently mild to avoid self-condensation of the flavan-3,4-diol, to avoid anthocyanidin formation as a side-reaction, to permit stereochemical conclusions (i. e. the reaction should be under kinetic rather than under thermodynamic control), and to allow assessments regarding the re-gioselectivity of the condensation. 

Apart from defining the ideal conditions for biomimetic condensations under kinetic control, the derivatives of these optically pure 4-arylflavan-3-ols also offered the first opportunity for fomulating a chiroptical rule which defines the absolute stereochemistry at C-4 of flavanoid units of this type and hence in biflavanoids and higher oligomers. Contrary to earlier 

Thus the sign of the Cotton-effect in the fore-mentioned instances correlates with the position of the 4-aryl (or 4-flavanyl) function above 

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