be employed, with the exclusion of water as a solvent for the purpose of
coupling.

Another suitable method of obtaining _o_-didepsides is that of treating
_o_-hydroxybenzoic acids with phosphorus trichloride and dimethylaniline
(_e.g_., synthesis of disalicylic acid, Boehringer & Sons).[Footnote:
Ger. Pat., 211,403.]

The carbomethoxy derivatives of the depsides are as a rule crystalline
substances of distinct acidic character, and decompose alkaline
carbonates.

The elimination of the carbomethoxy group may be brought about by dilute
alkaline solutions in the cold, or by aqueous ammonia. If the depside
formed is so stable as to resist the action of alkali for several hours,
the use of the latter is very convenient for the purpose required. The
substance is dissolved directly in sufficient normal alkali to
neutralise the carboxyl group and a further 2 molecules of caustic soda
for each carbomethoxy group to be eliminated are added. The temperature
should be about 20 C., when the reaction as a rule is completed after
one-half to three-quarters of an hour. It is usual, however, to use an
aqueous ammonia solution in considerable excess, whereby the temperature
should again be about 20 C., and the solution of ammonia normal or half
normal.

The didepsides so far investigated are crystalline bodies, sparingly
soluble in cold water; they--as a rule--decompose when fused, possess
acid reaction, and are dissolved by bicarbonates. On account of the
presence of a free phenolic group they give a coloration with ferric
chloride; if the phenolic group occupies the _o_-position to carboxyl,