Synthetic Tannins | Page 7

[Greek: a]- and
[Greek: b]-glucose. Compared to methylotannin, these preparations
exhibit very close resemblance to the former, from which it may be
concluded that they are closely related to this substance, and probably

possess the same or a very similar structure; the result of the above
experiments has, therefore, brought us at least in close proximity to the
structure of tannin. It must, however, be borne in mind that the analysis
and hydrolysis of tannin does not afford an explanation of the question
as to whether tannin is a compound of glucose and 10, 9, or 11
molecules of gallic acid; it is also possible, though not probable, that
tannin would contain a polysaccharide instead of glucose itself.
Similarly to sugar, the true glucosides can be coupled with
hydroxybenzoic acids, which is proved by the preparation of
tetra-galloyl-[Greek: a]-methyl glucoside; this substance, also, exhibits
tannoid character.
2. DIGALLIC ACID
Whereas, until recently, tannin had been considered to be gallic acid
anhydride, or digallic acid, closer investigations have revealed that
neither is tannin digallic acid nor is the synthetically prepared digallic
acid identical with tannin. Schiff [Footnote: _Ber._, 1871, 231 and 967.]
prepared digallic acid by the interaction of phosphorus oxychloride and
gallic acid, and believed the product obtained to be identical with
tannin; to this latter he first ascribed an ether formula (I.), later an ester
formula (II.)--
(OH)_2 (OH)_2 ¦¦ ¦¦ C_6H_2---0---C_6H_2 ¦ ¦ COOH COOH (I.)
(OH)_2 ¦¦ C_6H_2(OH)_3--C--O.C_6H_2 ¦¦ ¦ O COOH (II.)
Froda [Footnote: _Gasz. chim._, 1878, 9.] held that Schiff's
condensation product contained phosphorus or arsenic acid and
ascribed its tanning properties to the latter; according to this
investigator, digallic acid, when completely freed from arsenic acid,
does not react with gelatine or quinine. Biginelli [Footnote: _Ibid._,
1909, 39, ii. 268 and 283.] did not consider the action of arsenic acid
that of a catalyst, but held that it entered into reaction; according to his
investigations products containing arsenic (C_7H_7O_8As and
C_14H_11O_12As) are obtained when gallic acid is heated with
arsenic acid.

In his preparation of digallic acid, Iljin [Footnote: _Jour. f. prakt.
Chem._, 1911, 82, 451.] could only obtain gallic acid, and the ethyl
ether of gallic acid showing no characteristics of the tannins; when,
however, he heated gallic acid with arsenic pentoxide, he obtained
bodies exhibiting the reactions given by tannins.
Bottinger [Foonote: _Ber._, 1884, 1503.] made the first attempt at
synthesising tannin; he heated gallic acid or its ethyl ester with
glyoxylic acid or pyroracemic acid, and obtained a substance of the
composition C_14H_10O_9.2H_2O, which certainly showed some of
the characteristics exhibited by tannin, but which by no means was
identical with the latter. Bottinger's preparation is probably identical
with [Greek: b]-digallic acid, one of two dibasic isomers having the
composition--
C_6H_2(OH)_2COOH | C_6H(OH)_3COOH
the other possible isomer having the composition
C_6H(OH)_3COOH CO | C_6H_2(OH)_3
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