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The course and rate of direct biomimetic condensations of flavan-3,4-diois, as potential electrophiles, with progressively complex nucleophilic flavanoids to form oligomeric tannins is gene— rally regulated by the functionality and hence stability of the parent carbocation generated under mild acid conditions; by the ste— reochemistry of the electrophile; by the relative nucleophilicity of competing centres on the complex substrates; by steric effects contributed by both the functionality and the stereochemistry of the nucleophile; by contributory hyperconjugative effects in both electro-and nucleophiles; and by stoichiometric considerations. Analogies available from analysis of natural sources indicate that the same precepts are obeyed during tannin biogenesis. Syntheses based on parent flavanoids of known absolute configuration, coupled with ultra high resolution 1H n.m.r. spectroscopy for determining relative configurations and condensation sequence of f lavanoid units in the resultant tannins, supported by circular dichroism, enable definition of their structures and absolute configurations. Condensed tannins are probably the most ubiquitous of all plant phenolics, and their exceptional concentrations in the barks and heartwoods of a variety of tree species, e.g. Acacia mearnsii (black wattle), Schinopsis spp. (quebracho) and Rhizophora spp. (mangrove) have resulted in their commercial extraction with the initial objective of applying the extracts in leather manufacture. In most instances this has led to the exploitation of indigenous forests (quebracho, mangrove), but the wattle bark industry in South Africa and also elsewhere, is uniquely based on sustained af forestation. Due to a slow but progressive world—wide decline in the use of vegetable extracts as tanning materials, the range of industrial applications of wattle ('Mimosa') extract has been extended through sustained local development to flocculants for water treatment; to depressants in ore flotation; to a variety of cold and thermosetting adhesives for manufacturing composite wood products; and to adhesives designed for the packaging industry.1 These established industrial uses are based on the 70% phenolic ('tannin') content of the natural extract, representing a molecular gradation over the mass range 300-3000 and with a number average mass approximating to 1250.2 Knowledge of tannin structure and extract composition has contributed significantly to some of these developments. The chemistry of those condensed tannins which are of commercial significance, and also of others of more academic 'interest, has until of late represented an unattractive and therefore much neglected area of research. Inhibiting factors have been the extreme complexity of tannin extract composition; the high chirality of the constituent oligomers; their relative sensitivity towards stereochemical inversion; and susceptibility to autoxidation. Our dissatisfaction with the obvious limitations of chemical and spectrometric analysis prompted an exploration into methods of direct synthesis.30 The approach was based on the premise that flavan—3,4—diols as potential electro— philes, and nucleophilic flavan—3—ols were involved in initiating con4ensa— tion, while a combination of flavan-3,4-diols and the immediate condensation products presumably 'propagates' the condensation sequence. Generalized principles which have accrued from this study will constitute the subject of our review.

Structure and function in the biomimetic synthesis of linear, angular and branched condensed tannins