Distribution of Flavan-3-ols

More than 40 flavan-3-ols are now known natural products (Thble 7.6.5). The most commonly reported flavan-3-ols, catechin and epicatechin, are broadly dis- 

Although a number of flavan-3-ols of the IS configuration are known, their distribution is quite restricted in comparison with the above compounds. Both eAZ/-epiafzelechin and eA epicatechin are found in several species of the Palmae (73). Afzelia xylocarpa and Colophospermum mopane contain eA2/-fisetinidol, and e epifisetinidol is also present in the latter species (66, 82, 323). The flavan-3-ols readily undergo epimerization through inversion of configuration at C-2, making precautions necessary in their isolation. The above compounds clearly are not artifacts, however. 

Natural methylated flavan-3-ols are extremely rare and it has only been in the past few years that most examples have been found. The first of these compounds to be reported was 4 -0-methylepigallocatechin, a compound with interesting muscle-relaxant properties (74, 182, 375). Since then, 3 -0-methylepicatechin from Symplocus (356), a 4,7,3 - (or 4 )-tri-0-methylepicatechin and 5,7-di-O-methyl-3, 4 -methylenedioxyepicatechin from Cinnamomum cassia (256) have been reported. The most recent example is the 5,7,4 -tri-0-methylcatechin, obtained from Viguiera quinqueradiata (71). Prenylated flavan-3-ols are also rare with broussinol, l,3 -prenyl-3,7,4 -trihydroxyflavan, only recently having been reported from Broussonetia papyrifera in Fusarium solaniAnkcted bark (348). This compound is a true phytoalexin, for, this and related compounds do not occur in healthy tissues. 

Catechins are particularly good nucleophiles, so it is not surprising to find these compounds substituted at the A-ring with other natural products. The bark of Quercus stenophylla contains ellagitannins that are linked by carbon carbon bonds to the C-6 or C-8 of catechin (277). Kopsirachin (25), from Kopsia dasyrachis, is a most interesting example of this type of catechin derivative in which both the C-6 and C-8 positions are substituted by the alkaloid skytanthine (160). Other examples of this type of compound are the C-6, C-8, and C-6 -l- C-8 cinnamate derivatives of catechin isolated from Cinchona succirubra (273, 279). It can be predicted that many more examples of this type of catechin derivative will be found in the future. 

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Table 57.2 Distribution of flavan-3-ol monomers in food products... 

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