^-------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,