Galloyl depsides

Figure 1. Structure of a gallotannin typical of Chinese gallotannin, found, e.g., in galls of Rhus semialata. Note the characteristic meta-depside linkages of the galloyl residues at C-2.

In enzymatic studies with cell-free extracts from higher plants, however, no evidence has been found to date for galloyl-CoA s involvement in the biosynthesis of / -glucogallin, or its higher galloylated derivatives an eventual role in the formation of the chemically different meta-depside bonds of gallotannins has not been investigated. 

Using NMR the formation of the additional m-depside linkages was shown to occur preferentially to those galloyl ester groups attached to C-2 or C-3, C-4 and C-6 of the D-glucopyranose (Fig. 15). 

Spectroscopic measurements combined with the use of the methanolysis reaction have provided evidence for the occurrence of the complex ester in other plants. The chemical shift of the aromatic protons of depsidically linked galloyl ester groups occurs downfield from those of simple aliphatic galloyl esters (de acetone or de DMSO, 8 values from TMS, depside 7.25 — 7.70, simple ester 6.9—7.20). It is possible using this measurement to estimate the average value of the number n for the ester isolated from a particular plant. Methanolysis followed by a quantitative hplc analysis of the products (11) and (12) confirms the nature of this analysis. 

Fig. 4. Chemical shift of aryl carbon atoms of depside galloyl esters...

The presence of depsidically linked galloyl ester groups in (18) is also indicated by nmr and by mass spectrometry. In the nmr... 

Galloyl esters of D-glucose Depside Metabolites ----------p-Penta-o-galloyl-D-glucose... 

---