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 the terminology of these substances, Fischer
[Footnote: Liebig's _Ann_., 1910, 372, 35.] proposed the name
"Depsides" from [Greek: depheiv] = to tan. In analogy with peptides
and saccharides, the names di-, tri-, and polydepsides of hydroxybenzoic
acids would be suitable for these substances.

The principles underlying the synthesis of depsides are the
following:--If the chlorides of carbomethoxy (or carbethoxy)
hydroxybenzoic acids are coupled with the sodium salts of hydroxybenzoic
acids, esters are formed, _e.g._,

CH_3CO O.O.C_6H_4.CO.Cl + NaO.C_6H_4.COO.Na = NaCl
+ CH_3.COO.O.C_6H_4.CO.O.C_6H_4.COO.Na

On gently saponifying the esters, these are converted into the
corresponding hydroxy derivatives--

OH.C_6H_4.CO.O.C_6H_4.COOH

According to Fischer and Freudenberg, [Footnote: Liebig's _Ann._, 1909,
372, 32.] this method possesses the following advantages:--