Formation of the Interflavanyl Bond in Oligomeric Proanthocyanidins

The aptitude to utilize this important class of biomolecules to the maximum benefit of mankind depends on the accurate knowledge of their structures. Owing to the adverse effects of conformational mobility 

The factors that control the feasibility and the stereochemical course of the coupling process, as well as the methods to establish the configuration at C(4) of the condensation products and the mode of interflavanyl linkage were sufficiently reviewed (4, 27-29). Acid-catalyzed reactions to produce flavan-4-carbocations or A-ring quinone methides that may react with the A-rings of flavan-3-ols to produce oligo- and polymeric proanthocyanidins have been so successfully employed that they were called biomimetic syntheses (39, 40). The most recent variations of this theme are now briefly discussed. The nomenclature delineated in ref. (1) will be consistently employed. 

Synthesis of the First Profisetinidins with Epiflsetinidol Constituent Units 

The elegance of this simple biomimetic approach to the synthesis of proanthocyanidin oligomers was demonstrated during synthesis of the mixed profisetinidin trimers (22) and (23), i.e. analogues possessing different ABC and GHI chain extender units. Triflavanoid (22) with its fisetinidol ABC and epifisetinidol GHI units was formed by acid-catalyzed reaction of fisetinidol-(4a- 8)-catechin (24) (57) and epifise- 

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