Synthetic Tannins | Page 6

C_98 H_82 N_14 O_96) and proposed the formula, C_58
H_40 O_33, the constitution of which would be--
R_1 R_1 | | }C--O--O--C{ | | | R_2 | R_2 O R_1 | R_1 | |
| }C--O--O--C{ | | R_2 R_2
where R1= CO C_6 H_2 (OH)_3 and R2= C_6 H_2 (OH)_2
Nierenstein [Footnote: _Ber._, 1905, 38, 3841; 1907, 40, 917; 1908, 41,
77 and 3015; 1909, 42, 1122 and 3552; _Chem. Ztg._, 1907, 31, 72;
1909, 34, 15.] considers tannin to be a mixture of digallic acid and
leucotannin, the latter possessing the formula--
^-------CH.OH--O----^ OH | | | | HO V OH HOOC V OH OH
The optical activity of tannin is expressed in this formula and its
probability is corroborated by Nierenstein, who was able to resolve the
acetylated tannin by fractional precipitation into pentacetyl tannin (M.P.
203°-208° C.) and pentacetyl leucotannin (M.P. 166° C.). By oxidation,
the former is converted into ellagic acid, and on hydrolysis with dilute
sulphuric acid readily yielded gallic acid. Hydrolysis of the pentacetyl
leucotannin, however, yielded gallic aldehyde, and oxidation yielded
purpurotannin (a naphthalene derivative) in addition to ellagic acid.
Nierenstein [Footnote: _Ber._, 1910, 43, 628.] also succeeded in
converting tannin into carboethoxytannin, the latter on saponification
yielding crystalline, inactive digallic acid. On acetylating pentacetyl
leucotannin with acetyl chloride a hexacetyl derivative (M.P. 159° C.)

is obtained, the strychnine salt of which is resolved into both of the
active components. This proves the presence of digallic acid and
leucotannin in tannin lev. pur. Schering investigated by Nierenstein.
The latter author [Footnote: Liebig's _Ann._, 1912, 386, 318; 388, 223.]
later considered tannin to be polydigalloylleucodigallic acid anhydride
and the simplest tannin to be a digalloylleucodigallic acid anhydride.
This view, however, would not stand subsequent criticisms, being in
disagreement with the earlier observations of molecular weight and
acidic properties of tannin. Manning [Footnote: _Ibid._, 1912, 34, 918.]
believed to have isolated a pentethylester of the pentagalloyl glucoside
from tannin, but this was shown to be the ethyl ester of gallic acid.
Feist [Footnote: _Ber._, 1912, 45, 1493.] had arrived at the conclusion
that tannin was a glucose compound, and maintained that tannin from
Turkish galls was a compound of glucogallic acid combined as an ester
with 2 molecules gallic acid. But Fischer and Strauss [Footnote: _Ibid._,
1912, 45, 3773.] synthetically prepared a glucoside of gallic acid
exhibiting differences from Feist's preparation which were so great that
the latter no longer could be considered a single glucoside of gallic
acid.
Fischer and Freudenberg [Footnote: _Ibid._, 1912, 45, 2717; 1913, 46,
1127.] subsequently elaborated a method of purifying tannin, and on
investigating the purified substance, arrived at the conclusion that no
other hydroxybenzoic acid than gallic acid was present in tannin. On
repeating Strecker's hydrolysis they obtained 7-8 per cent, sugar, and
hence concluded that 1 molecule of glucose was combined with about
10 molecules of gallic acid. Owing to the difficulty of isolating the
intermediary hydrolysis products, and the subsequent impossibility of
drawing any conclusions as to the constitution of tannin, the latter
investigators decided to adopt the methods offered by synthesis. Their
basic idea was the absence of carboxylic groups in tannin, and that
hence the total gallic acid must be present in ester form. These
conditions are fulfilled if one views tannin as being an ester compound
of 1 molecule of glucose and 5 molecules of digallic acid, of similar
construction as, for example, pentacetyl glucose. Fischer and
Freudenberg succeeded in preparing the former by shaking a mixture of

finely powdered glucose, chloroform, and quinoline with an excess of
tricarbomethoxygalloyl chloride for twenty-four hours and precipitating
the resulting product with methyl alcohol; suitably purified, a light
amorphous colourless substance was obtained which proved to be
penta-(tricarbomethoxygalloyl) glucose. Careful saponification with
excess alkali in acetone-aqueous solution at room temperature yielded a
tannin very closely resembling tannin, identified as pentagalloyl
glucose. It is doubtful, however, whether this substance is
homogeneous, and it is probably a mixture of two stereoisomers.
Fischer and Freudenberg, therefore, further concluded that tannin is
mainly an ester compound of glucose and 5 molecules _m_-digallic
acid. Elucidation on this point offered itself advantageously in
Herzwig's methylotannin, [Footnote: _Ber._, 1905, 38, 989.] which is
obtained by the interaction of diazomethane and tannin. The first step
was then to prepare pentamethyl-_m_-digallic acid
CH_3.O_______ ______COOH CH_3.O{_______}--CO.O--{______}
CH_3.O CH_3.O O.CH_3
from trimethylgalloyl chloride and the _m-p_-dimethyl ether of gallic
acid; the chloride of this substance, coupled with [Greek: a]- and
[Greek: b]-glucose, yields--
_CH.OR | | | CH.OR H_______O.CH_3 | | R=CO{_______}O.CH_3
O{ CH.OR H O | | H_____O.CH_3 | CH CO{_____}O.CH_3 | | H
O.CH_3 |_CH.OR
CH_2.OR
[Illustration: Penta-(pentamethyl-_m_-digalloyl)-glucose.]
The [Greek: a]- and [Greek: b]-derivatives thus obtained differ in their
behaviour towards polarised light, and are, again, probably mixtures of
two stereoisomers, _i.e._, mixtures of derivatives of