5-Deoxy-proanthocyanidins

Flavan-3-ols are derived from flavanones, via 3-hydroxy-flavanones (or dihydroflavonols). Naiingenin (31) gives rise to 5,7-dihydroxyproanthocyanidins, whereas liquiritigenin (32) (Fig. 12.11) appears to be the precursor for 5-deoxy-proanthocyanidins, both via the intermediacy of flavan-3-ols and the same basic pathway as in Fig. 11.16 (Heller and Forkmann, 1988 Lewis and Yamamoto, 1989 Stafford, 1989). 

The occurrence of propelargonidins (1, 4) in woody families is more widespread (see Table 7.7.4). These are normally accompanied by procyanidins and sometimes prodelphinidins. A possible tendency is for them to occur in families in the Rosidae group, the 5-deoxy proanthocyanidins occurring exclusively here. Propelargonidins, on current evidence, are absent from the Magnoliidae (the most primitive group), Caryophyllidae, and Hamamelidae. 

Cronje, A. et al.. Oligomeric flavanoids. Part 16. Novel prorobinetinidins and the first A-type proanthocyanidin with a 5-deoxy A- and a 3,4-cw-C-ring from the maiden investigation of commercial wattle bark extract, J. Chem. Soc., Perkin Trans. 1, 2467, 1993. 

Steynberg, P.J. et al., Cleavage of the interflavanyl bond in 5-deoxy (A-ring) proanthocyanidins,... 

With reference to Table 1, butiniflavan (11) is named from three 3-deoxy (C-ring) proanthocyanidin dimers based on a 4-substituted (25)-7,3, 4 -trihydroxyflavan moiety [a (2,S)-flavan unsubstituted at C-3 possesses the same orientation as a C-2 substituent in (2if)-flavan-3-ols] obtained from Cassia petersiana The name is derived from the structural relationship between flavan (11) and the (2,S)-7,3, 4 -trihydroxyflavanone, butin... 

The potential of this development for the structural elucidation of the proanthocyanidin condensed tannins, especially the 5-deoxy analogues, from important commercial sources is clear. In addition, the method facilitates the ready definition of the absolute configuration of the chainterminating flavan-3-ol moiety in 5-deoxyoligoflavanoids, especially in... 

The proanthocyanidins containing 5-deoxy upper units are known to be far more stable to acid-catalyzed cleavage than the 5,7-dihydroxy procyanidins. Therefore, provided that reaction conditions are sufficiently mild (i.e. 0.1 M HCl, 2 to 4 hours at 25 °C) - such as those employed by Botha et al. (26-29) - an assumption of kinetic control of the regioselectivity of condensation of these compounds seems appropriate. Botha et al. (26-29) have done a great deal of work to define relationships of the structures of flavan-4-carbocations or quinone methides and flavan-3-ols on the regioselectivity of their condensations in the synthesis of proanthocyanidins (Thble 7.6.6). 

The significance of flavan-3,4-diols in plants rests primarily on their probable role as precursors of the polymeric proanthocyanidins. Co-occurrence of the 5-deoxy compounds - i.e., quibourtacacidins, mollisacacidins, and robinetinidins - with the related proanthocyanidins in Acacia species and the ready synthesis of naturally occurring proanthocyanidins from reactions of these flavan-3,4-diols with catechin under mild acidic conditions constitutes heavy but not definitive evidence for this thesis (31, 315-317). 

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