Hexahydroxydiphenoyl esters

Hexahydroxydiphenic acid in a bound form (3) is readily detected in a plant extract by an old but very distinctive color test - the Procter-Paessler reaction. Esters of type 3 react with nitrous acid to give a carmine or rose red, changing to brown-green, then purple, and finally indigo blue. The chemical basis of this reaction has not been discussed but it has been employed by Bate-Smith (6, 7) for the quantitative determination of 3 in plant extracts, and it forms the basis of a very useful spray reagent for the detection of 3 by paper chromatography (6, 7, 43). 

Okuda (89) has recently correlated (H-)-hexamethoxydiphenic acid (32) from corilagin (30) with the same acid derived from the dibenzocycloctane lignan, schizandrin (31), whose chirality had been shown to be R by comparison with the X-ray-derived molecular structure of the lignan, gomisin D (54) (Fig. 7.2.8). 

It is interesting to note that the same conclusions regarding the chirality of hexahydroxydiphenoyl groups bound to D-glucopyranose follow from theoretical considerations. Two assumptions are necessary  

1) that the ester carbonyl groups of the hexahydroxydiphenoyl residue have, like other ester groups (19), a preference for the eclipsed conformation of the ester carbonyl oxygen and the adjacent hydrogen atom on the glucose ring (33) and 

2) that in the hexahydroxydiphenoyl group there is a preference for the anti or opposed arrangement of the two carbonyl groups with the C-O dipoles aligned antiparallel (34). 

Change in Chemical Chemical shift Shift relative to (d6 acetone, 5 ppm TMS) p-penta-o-galloyl D-glucose (A5) 

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Tanaka, T., Nonaka, G., Ishimatsu, M., Nishioka, I., and Kouno, I., 2001, Revised structure of cercidinin A, a novel ellagitannin having (R)-hexahydroxydiphenoyl esters at the 3,4-positions of glucopyranose, Chem. Pharm. Bull. 49 486-487. 

Ellagitannins are characterized by the presence of hexahydroxydiphenoyl esters (HHDP 1.94) with a polyol such as glucose. Acid hydrolysis of ellagitannins will result in the release of HHDP units, which will spontaneously form ellagic acid (1.96), as described in Chapter 1, section 3.13.3. Hence, the quantification of ellagic acid reflects the content of ellagitannins present in the sample. 

The chirality of the other hexahydroxydiphenoyl esters may be determined by measurements of circular dichroism (CD) and comparison with The CD spectra of hexahydroxy-... 

Biogenetically one important and perhaps unexpected variation of the hexahydroxydiphenoyl ester group found in a limited number of metabolites is the dehydro hexahydroxydiphenoyl ester (45) first identified by Schmidt °> and later noted in other metabolites of the families Gerania-ceae, Euphorbiaceae and Aceraceae. Isomerization occurs in solution and leads to an equilibrium mixture of internal hemi-acetal forms (45a, 45b) he attainment of... 

Hexahydroxydiphenoyl esters (2B, 2C). A very widely distributed metabolic fingerprint is that in which 3-penta-Q-galloyl-Erglucose (40) - it is assumed - is further transformed by oxidative coupling of pairs of adjacent galloyl ester groups (2, 3 and 4, 6) on the 3-D-glucopyra-nose core (Fig. 15, 2B). Some details of this pattern of metabolism were hinted at in earlier work by Hillis and Siekel, by Jurd, by Wilkins and Bohm, and Schmidt. 

In chemical terms a perhaps unexpected but biosynthetically interesting variant of the hexahydroxydiphenoyl ester group first noted by Schmidt is its dehydro-derivative (22). It was identified by Schmidt as a structural component of brevilagin 1 and 2, polyphenols from the fruit pods of Caesalpinia brevifolia (133, 134). Subsequent work has shown the ester to be present in the structure of other plant polyphenols, notably geraniin (51) in plants of the Geraniaceae (Thble 7.2.9). 

Table 7.2.6. Hexahydroxydiphenoyl ester derivatives, Group 2B The structure of some dimeric and trimeric C-O oxidatively coupled metabolites...

Table 7.2.8. Hexahydroxydiphenoyl ester metabolites Group 2B, occurrence in plant families...

Fig. 8. C Chemical shift of the D-glucose carbon atoms in some hexahydroxydiphenoyl esters of D-glucose relative to p-penta-o-galloyl-D-glucose, (26,19)...

Scheme 9. The metabolism of hexahydroxydiphenoyl esters in higher plants — class B...

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