Synthetic Tannins | Page 9

to
ellagic acid; the former consists of faintly yellow needles, M.P.
329°-336°C. It is soluble in water, precipitates gelatine, and is adsorbed
by hide powder. It occurs with gallic acid, tannin, and ellagic acid in
dividivi, myrabolams, algarobilla, and chestnut wood extracts.
Other bodies of this class include:--
METELLAGIC ACID, Cl_4H_6O_5, derived from methoxybenzoic
acid, and recrystallised from acetic acid, forms small crystalline needles,
M.P. 273°-276° C., and yields fluorene on distillation with zinc dust.
----CO.O---- ^ ---------- ^ | | | | V ---O.CO--- V OH
FLAVELLAGIC ACID, C_14H_6O_9, is obtained by the oxidation of
gallic acid with concentrated sulphuric acid and potassium persulphate.
It crystallises from pyridine in prismatic needles melting above 360° C.
Distillation with zinc dust yields fluorene (see above)--
----CO.O---- ^ ---------- ^ OH | | | | HO V ---O.CO--- V OH OH OH

By heating ellagic acid for three-quarters of an hour at 185° C. with
concentrated sulphuric acid, ceruleo-ellagic acid (dioxyellagic acid),
C_14H_6O_10, is formed as yellowish needles, M.P. 360° C., which
are but little soluble in the usual solvents. The acid is slightly soluble in
strong caustic soda solution, the colour of the solution, on diluting,
changing to green and blue.
LUTEIC ACID (Luteo Saure, pentoxybiphenylmethylolide carboxylic
acid),C_14H_8O_9, occurs, in addition to ellagic acid, in myrabolams--
[Footnote: Ber., 1909, 42, 353.]
----CO.O---- ^ ---------- ^ OH | | | | HO V OH HOOC V OH OH OH
It is obtained by extracting myrabolams for one hour and a half, under
reflux condenser, with pyridine, filtering and adding twice the volume
of water to the filtrate and boiling till complete solution is obtained.
After about thirty hours a reddish powder deposits, from which ellagic
acid may be extracted with pyridine; the mother-liquor on being
concentrated yields luteic acid. It is also obtained by oxidising tannin
with hydrogen peroxide, the other oxidation product being ellagic acid,
and the two may then be separated as indicated above. Luteic acid
forms reddish needles which are decomposed, with evolution of gas, at
338°-341° C. Heated with 10 per cent. caustic soda solution it yields
ellagic acid. In pyridine solution the carboxyl group maybe eliminated
by hydrogen iodide, whereby pentoxybiphenylmethylolide is formed as
long silky needles, which do not melt below 300° C. The same
substance may also be obtained when ellagic acid is boiled with
concentrated caustic potash solution. When luteic acid is treated with
diazomethane, it yields the methyl ester of
pentamethoxybiphenylmethylolidcarboxylic acid.
4. DEPSIDES
The most common decomposition products of the natural tannoids are
hydroxybenzoic acids, notably gallic and proto-catechuic acids;
furthermore, other aromatic and aliphatic hydroxy compounds
frequently occur. So far, however, attempts at explaining the
constitution of the complex decomposition products obtained by

hydrolysing high molecular tannoids have not been successful. On the
other hand, the constitution of the simpler natural tannoids is known to
a greater or less extent; of these, lecanoric acid (Lecanorsäure) is the
best known, being an ester anhydride of orsellic acid (a
dihydroxytoluylic acid). It combines with erythrite, forming another
tannoid, erythrine. The fact that hydroxybenzoic acids are constantly
encountered together with the products obtained on hydrolysis of the
tannins, seems to point toward the conclusion that anhydrides of
hydroxybenzoic acids are frequent constituents of the natural tannoid
molecules.
The assumption that, for instance, in tannin at least part of the gallic
acid radicals are combined with one another is highly probable, and is
supported by the formation of tri- and dimethylgallic acid from
methylotannin, [Footnote: Herzig, _Monatshefte f. Chemie_, 1909, 30,
343.] and by the formation of ellagic acid when tannin is oxidised.
[Footnote: Nierenstein, Ber., 1908, 41, 3015.] Further proof is brought
forward by the existence of the pentacetyl-tannin, [Footnote: Schiff,
_Ann. d. Chem_., 1873, 170, 73.] and by the results of hydrolysis
which has yielded up to 104 per cent. anhydrous gallic acid fiom tannin
[Footnote: Sisley, _Bull. Soc. Chim_. 1909, 5, 727.]
Of the three classes of isomeric anhydrides which can be formed from
hydroxybenzoic acids, the chemistry of the natural tannins is only
concerned with the class comprising the ester anhydrides. If the
carboxyl of the first molecule combines with a hydroxyl of the second
molecule (ester formation), then a substance possessing character
similar to that of a hydroxybenzoic acid is formed, which is capable of
combining up with a further molecule in the same way. It is natural to
assume that this ester form is much more prevalent in Nature than a
combination of two carboxyls by the elimination of water. From the
point of view of the chemistry of the tannins, therefore, the
starting-point would naturally be that of synthesising the ester
anhydrides of hydroxybenzoic acids. Amongst the small number of
synthetically prepared ester anhydrides of hydroxybenzoic acids, a few
occur exhibiting the properties of the natural tannoids.

In order to simplify