Metabolic Pool and Condensation Aptitudes of Tannin Precursors

Composition of the Metabolic Pool and Condensation Aptitudes of Tannin Precursors 

With the fore-mentioned chemical principles in mind, examination of the oligomeric composition of several extracts which contain tannins is now possible in terms of stereochemistry, functionality and condensation aptitudes of their precursors or intermediates. From this it is evident that condensations follow the path of least resistance, being regulated by the stability of the carbocation arising from the flavan-3,4-diol, by the nucleophilicity of the substrate, by steric factors contributed by both electrophile and nucleophile, by the 2,3-trans or 2,3-m stereochemistry of the electrophile, and by the relative concentrations of reactants. 

Finally it is notable that where the flavan-3,4-diols (-)-teracacidin (5) and (—)-melacacidin (6) predominate in the woods of many Acacia spp. [e. g. in the section Plurinerves (25)], evidence of natural tannin formation is largely absent. This phenomenon is in line with the reduced nucleophilicity of its A-ring due to both vicinal functionalization (7,8-disubstitution) and the previously cited inductive effect of the 4-hydroxyl. These factors further limit the prospect of self-condensation, in the complete absence of suitable nucleophilic substrates [cf. 24) — 26)]. 

Condensed tannins of the prodelphinidin, procyanidin and propelargo-nidin series should, however, be amenable to the same direct synthetic approach as outlined in the foregoing [cf. biflavanoid procyanidin formation (72)] although 5-hydroxylation probably introduces an additional steric factor into condensation reactions, thereby reducing yields. 

Geissman, T. a., and N. N. Yoshimura Synthetic proanthocyanidin. Tetrahedron Letters 1966, 2669. 

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