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:—
1. The synthesis takes place at low temperatures, so that any intramolecular rearrangements are improbable.
2. The composition of the substances is controlled by the intermediary compounds, the carboalkyloxy derivatives.
3. The synthesis permits of more definite evidence as regards the structure of the resulting compounds.
4. The substances obtained are easily purified.
Depsides produced in this manner are by no means new, and were obtained by Klepl by simply heating p-hydroxy-benzoic acid (cf. Introduction, p. 4). This simple procedure, however, is not applicable to most other hydroxybenzoic acids which are decomposed at the high temperature necessary to induce reaction. Lowe and Schiff (loc. cit.) have obtained products similar to tannins, the latter investigator by removing the elements of water from gallic acid, protocatechuic acid, salicylic acid, m-hydroxybenzoic acid, cresotinic acid, phloretinic acid, and pyrogallolcarboxylic acid. These depsides, however, are amorphous substances, and it is hence difficult to substantiate their homogeneity.
Carbomethoxylation of Hydroxybenzoic Acids