aromatic acids are acted upon by phosphorus chloride, and another group,
which can subsequently be easily removed, must therefore be introduced
to cover the disturbing influence referred to. For this purpose, Fischer
[Footnote: _Ber_., 1908, 41, 2860.] chose the carbomethoxy group, and
this investigator succeeded, by the action of chlorocarbonic alkyl
ester and alkali upon hydroxybenzoic acid in cold aqueous solution, in
obtaining substances with the properties required. [Footnote: _Ber._,
1908, 41, 2875.] In such substances (_e.g._, salicylic acid) where the
hydroxyl occupies the ortho-position to the carboxyl, complete
carbomethoxylation does not take place, whereas the _m_- or _p_-
positions offer no hindrance. In the case of the _o_-position, however,
the action of chlorocarbonic alkyl ester is successfully assisted by the
presence of dimethylaniline in an inert solvent, _e.g._,
benzene.[Footnote: U.S. Pat, 1,639,174, 12, xii., 1899.] The difficulty
encountered by the _o_-position is eliminated when the carboxyl is not
directly linked to the benzene nucleus, _e.g._, _o_-cumaric acid. Many
hydroxybenzoic acids require an excess of chlorocarbonic methyl ester,
which then also, to some extent, attacks the carboxyl group; but on
dissolving the product in acetone and treating it with bicarbonate the
carboxyl group as such is again restored without splitting off the
carbomethoxy group.[Footnote: _Ber._, 1913, 46, 2400.] In this way all
hydroxybenzoic acids may be carbomethoxylated. [Footnote: _Ibid._,
1908, 41, 2877, 2881, 2882; 1909, 42, 226, 218, 223, 225; Liebig's
_Ann._, 1912, 391, 357, 366; _Ber._, 1913, 46, 1145, 2390, 2400.] The
carbomethoxy group is easily removed by excess of aqueous alkali in the
cold, and is also partially removed when insufficient alkali is present;
the latter fact is of importance in the synthesis of didepsides.


Chlorides of Carbomethoxyhydroxybenzoic Acids