B-Type Proanthocyanidins

X-ray and CD analysis. The structure of procyanidin B-1 was unequivocally confirmed by x-ray analysis of its deca-(9-acetyl derivative by Weinges, one of the pioneers in the field of proanthocyanidin chemistry. One of the most powerful methods to establish the absolute configuration at C-4 of the T-unit in dimeric A- and B-type proanthocyanidins remains the chiroptical method via application of the aromatic quadrant rule. This has been repeatedly demonstrated by the author s own work and several other contributions listed in Refs. 7-12. 

The flavan-3-ols (catechins), flavan-4-ols/flavan-3,4-diols (leucoanthocyani-dins), A-type proanthocyanidins, B-type proanthocyanidins including the... 

Proanthocyanidins with one A-type linkage have two less hydrogen than those of the B-type proanthocyanidins. A procyanidin trimer gave rise to [M-H] m/z 865, whereas a procyanidin trimer with one-type linkage yielded [M H] m/z 863. A-type interflavan bond differs from B-type bound in that they do not undergo QM cleavage. Thus,... 

In B-type proanthocyanidins (Fig. 9B.2), the flavanol constitutive units are linked by C4-C8 and/or C4-C6 bonds, opening the possibility for branched structures. 

Only B-type proanthocyanidins have been formally identified in grapes, with small amounts of dimers and trimers containing 4-6 linkages occuring along with the... 

Jannie P. J. Marais studied at the University of the Free State, Bloemfontein, South Africa where he obtained his Ph.D. in organic chemistry in 2002. His research focused on characterization of the free phenolic profile of South African red wine, and the stereoselective synthesis of flavonoids, for example, flavan-3,4-diols and flavanones. He joined the National Center for Natural Products Research at the University of Mississippi as a postdoctoral associate in July of 2002, where he worked with Dr. Ferreira on the stereoselective synthesis of flavonoid precursors, the semi-synthesis of proanthocyanidin oligomers, characterization of proanthocyanidin profiles of selected transgenic plants, and the synthesis of radioactive antimalarial 8-aminoquinolines. He was promoted to associate research scientist in 2005, where his main area of research still remains the synthesis of A- and B-type proanthocyanidins, starting at the monomeric level and continuing through the tri- and tetra-meric, to the deca-mer level. 

The B-type procyanidins include a mixture of oligomers and polymers composed of flavan-3-ol units linked mainly through C4 C8 and/or C4 C6 bonds, and represent the dominant class of natural proanthocyanidins. Among the dimers, procyanidins Bl, B2, B3 and B4 (Fig. 2a) are the most frequently occurring in plant tissues. Procyanidin B5 (EC-(4j6 6)-EC), B6 (catechin-(4o 6)-catechin), B7 (EC-(4/3 6)-catechin) and B8 (catechin-(4q 6)-EC) are also widespread (Eig. 2b) [17-19]. 

On the other hand, the flavan-3-ol units can also be doubly linked by an additional ether bond between C2 07 (A-type). Structural variations occurring in proanthocyanidin oligomers may also occur with the formation of a second interflavanoid bond by C-0 oxidative coupling to form A-type oligomers (Fig. 3) [17,20]. Due to the complexity of this conversion, A-type proanthocyanidins are not as frequently encountered in nature compared to the B-type oligomers. 

The A-type proanthocyanidins are characterized by a second ether linkage between an A-ring hydroxyl group of the lower unit and C-2 of the upper unit. Since they are less frequently isolated from plants than the B-types, they have been considered unusual structures [18,19]. The first identified A-type proanthocyanidin was procyanidin A2 isolated from the shells of fruit of Aes-culus hippocastanum. Since then, many more A-type proanthocyanidins have been found in plants, including dimers, trimers, tetramers, pentamers and ethers [18,21]. 

Proanthocyanidins of the B-type are characterized by singly linked flavanyl units, usually between C-4 of the chain-extension unit and C-6 or C-8 of the chain-terminating moiety. They are classified according to the hydroxylation pattern of the chain-extension units (see T able 11.1). 

Foo, L.Y. Lu, Y. Howell, A.B. Vorsa, N. 2000. A-type proanthocyanidin trimers from cranberry that inhibit adherence of uropathogenic P-fimbriated Escherichia coli. J. Nat. Prod. 63 1225-1228. 

Monomeric units may be linked, to form oligomers and polymers, by C4-C6 and/or C4-C8 bonds (B-type) or doubly linked, with an addition C7-0-C2 ether linkage (A-type). Besides, flavan-3-ol units may be encountered as 3-0-esters, in particular with gallic acid, or as glycosides (25). Finally, the degree of polymerization (DP) may vary greatly as proanthocyanidins have been described up to 20,000 in molecular weight (26). 

Indirect evidence for the intermediacy of a /)-quinone methide of type (213) in the oxidative conversion of B- into A-type proanthocyanidins came from the oxidation of epigallocatechin (216) with the homogenate of banana fruit flesh polyphenol oxidase. " Besides racemization at C-2, the oxidative conversion also gave retro-ct-hydroxydihydrochalcone (219) (Scheme 24), presumably via initial oxidation of (216) to the B-ring quinone methide (217). Hydration gave the unstable hemiacetal (218) that would equilibrate with the 1,3-diarylketone (219). It was also shown that laccase (EC 1.10.3.2) catalyzed the conversion of procyanidin B-2 (5) into procyanidin A-2 (215). 

In contrast to the aforementioned proanthocyanidins of the B-type where the constituent flavanyl units are linked by only one bond, analogues of the A-class possess an unusual second ether linkage to C(2) of the T-unit. Studies focussing on the oxidative formation of this bond are surprisingly limited and are hitherto restricted to the use of hydrogen peroxide (73) and dioxygen (74-76) as oxidants. 

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