Fischer [Footnote: Ber., 1908, 41, 2875.] obtained a digallic acid (M.P. 275°-280° C) by coupling tricarbomethoxygalloyl chloride with dicarbomethoxygallic acid.
Nierenstein [Footnote: Ibid., 1910, 43, 628.] obtained, from the carbethoxy compound of tannin, a crystalline, optically active digallic acid, M.P. 268°-270° C. The pentacetate of this substance, obtained by reduction and acetylisation, yielded hexacetylleucotannin. A pentamethyldigallic acid methyl ester of the composition
((O.CH_3)_3)C_6H_2——COO——-C_6H_2((OCH_3)_2)COO.CH_3
was obtained by Mauthner [Footnote: Jour. f. prakt. Chem., 1911, 84, 140.] from the chloride of trimethylgallic acid and the methyl ester of the acid from the glucoside of syringin; on saponification with caustic potash the former compound yielded trimethylgallic acid and syringic acid.
Fischer [Footnote: Ber., 1913, 46, 1116.] synthesised the so-called m-digallic acid by coupling tricarbomethoxygalloyl chloride with carbonylgallic acid and subsequent splitting off of CO_2. The m-digallic acid appears as rather thick, colourless, microscopic needles containing about 16 per cent. water of crystallisation, M.P. 271° C. They are slightly soluble in cold, soluble in hot water, and very soluble in methyl and ethyl alcohols. Their aqueous solution gives dark blue coloration with ferric chloride, and precipitates gelatine and quinine.
Fischer and his students [Footnote 5: Ibid., 1912, 45, 915, 2709; 1913, 46, 1116.] prepared quite a number of digallic acid derivatives, amongst which are the following:—
Pentamethyl-m-digallic acid methyl ester, C_20H_22O_9.
Pentacetyl-m-digallic acid, C_24H_20O_14.
Pentamethyl-m-digallic acid, C_19H_20O_9.
Pentamethyl-m-digalloyl chloride, C_19H_19O_8Cl.
Pentamethyl-p-digallic acid, C_19H_20O_9.
Pentamethyl-p-digallic acid methyl ester, C_20H_22O_9.
Hydrolysis of digallic acid yields gallic acid; oxidation, on the other hand, ellagic acid and luteic acid (Luteo Säure), which can be separated by shaking with pyridine. The reduction of digallic acid yields, by different methods, the same reduction compound, [Footnote: Nierenstein, Abderhalden's "Handb. d. biochem. Arbeitsm.," vi. 154.] viz., the racemic leucodigallic acid, which differs from digallic acid by being devoid of any tannoid properties; the latter distinction may be ascribed to the transformation of the tannophor group—CO.O—, to the tannoid-inactive group CH(OH)—O—.
The successful resolving of racemic leucodigallic acid into both of its optically active components can only be brought about through the d- or l-hexacarbethoxyleucodigallic acid on introducing the latter into a 1 per cent. pyridine solution and heating to 45°-50° C., whereby the d- or l-acid is formed accompanied by a strong evolution of carbon dioxide.
Hydrolysis of leucogallic acid yields gallic acid and gallic aldehyde; oxidation by means of hydrogen peroxide yields ellagic acid and luteic acid, and oxidation with potassium persulphate and sulphuric acid, in acetic acid solution, yields purpurotannin (see below) [Footnote: Liebig's Ann., 1912, 386, 318.].