these operations yields tri- and tetradepsides.


Preparation of Didepsides

A simple application of these syntheses is offered by _p_-hydroxybenzoic
acid. When the chloride of its carbomethoxy derivative is allowed to
interact with _p_-hydroxybenzoic acid in aqueous alkaline solution, in
the cold, the alkali salt of carbomethoxy-_p_-hydroxybenzoic acid is
formed:--[Footnote 1: _Ber._, 1909, 42, 216.]

CH_3.CO_2.O.C_6H_4.CO.Cl + NaO.C_6H_4.COONa
= CH_3.CO_2.O.C_6H_4.CO_2.C_6H_4.CO_2.Na + NaCl.

Being sparingly soluble, the salt in this case is readily deposited as
crystals, but is readily converted into the free acid by hydrochloric
acid. In most other cases, however, the alkali salts are easily soluble
and the aqueous solution is then directly acidified with a mineral
acid. The chlorides, being for the most part solids, the mode of
procedure is as follows:--the hydroxybenzoic acid required for coupling
is dissolved in normal or double-normal alkali (the volume calculated
per molecule acid), a little acetone added, and the mixture well cooled;
a further molecule of 2N caustic soda and the chloride (I molecule)
dissolved in dry acetone are added in small portions, whilst stirring,
to the mixture. In spite of the low temperature the coupling proceeds
quickly and the sparingly soluble product can in most cases be
precipitated from the solution by acidifying and diluting with water. In
case of more easily soluble coupling products the acetone is driven off
under reduced pressure or the liquid acidified and diluted, and the
substance extracted with ether. Instead of alkali, dimethylaniline may