714. Polymerisation of flavans. Part IV. The condensation of flavan-4-ols with phenols

Tannins: Major Sources, Properties and Applications

“…One is degradative leading to lower molecular weight products, and the second is condensative, as a result of the hydrolysis of heterocyclic rings ( p -hydroxybenzyl ether links) [38] . The generated p -hydroxybenzylcarbenium ions condense randomly with nucleophilic centres on other tannin units to form ' phlobaphenes ' or ' tanner ' s red ' [38,[45][46][47] . Other modes of condensation ( e.g .…”

Synthesis of 4-substituted flavans from 4α-halogenoflavans

“…These authors assumed that the bromoflavans they obtained were of 13configuration (2,4-cis). We ha\'e used their procedure to prepare from flavan-413-0Is a series of 4-halogenoflavans (3), ( 4), ( 5), (6), and (7) (Scheme 1) which, however, are shown to have an ot-configuration (2,4-trans) by n.m.r.3 (,£.12,3 13-14 Hz and I.J3•4 6.0-6.5 Hz), indicating that the reactions proceed with inversion of configuration at the 4-position. Other phosphorus halides, e.g.…”

“…Structural distinction between 4-aryloxyflavans 5 and 4arylflavans 6 is most easily made from mass spectral evidence (Table 2). The base peak of either 4ot-or 413-aryloxyflavans results from the loss of the 4-substituent from the molecular ion, the percentage of which is quite small, whereas the base peak of 4-arylflavans results from a retro-Diels-Alder fission 3,7 and the percentage of molecular ion is appreciable (see Scheme 2).…”

Synthesis and Epoxidation of Flav-3-enes as Methodology for the Biomimetic Preparation of Flavan-3,4-diols

“…The epoxidation of 3′,4′,5,7-tetramethoxyflav-3-ene (19i) was therefore repeated in the standard way, but after 1 hour at -78 to -40 °C, DMSO (5 equiv) was added and the reaction mixture stirred for another hour before addition of the phloroglucinol nucleophile (2 equiv) and the mixture allowed to slowly reach room temperature and stirred overnight (Scheme 7). Although some of the diol (22i) (9%) could be isolated from the reaction mixture by extraction, free phenolic compounds tend to be lousy substrates when subjected to chromatography over silica gel, so the rest of the reaction mixture was acetylated and, after work-up and removal of the solvent, an additional 16% the acetate of the 2,3-trans-3,4-cis-3,4-diol (38) was obtained. To our delight the diol derivative was accompanied by the C-4 arylated product (39) in 23% yield, thus indicating that C-4 substituted products like dimeric flavonoids/anthocyanidins, could indeed be prepared through Scheme 6 Proposed intermediates in the formation of flavan-3,4-diol diastereomers…”