Monohydroxybenzoic acids allow theoretically of tri-depsides of the type HO.C_6H_4COO.C_6H_4.COO.C_6H_4.COOH only; if, on the other hand, di- or trihydroxybenzoic acids are dealt with, two formulae are possible, viz.:—
HO.C_6H_4.COO } C_6H_3.COOH HO.C_6H_4.COO
Of the former type, two compounds are known, i.e., di-p-hydroxybenzoyl-p-hydroxybenzoic acid and vanilloyl-p-hydroxybenzoyl-p-hydroxybenzoic acid—
HO } C_6H_3.COO.C_6H_4.COO.C_6H_4.COOH CH_3O
The first named of these two compounds was obtained by Klepl, in addition to the didepside, by heating p-hydroxybenzoic acid. Fischer and Freudenberg obtained a beautifully crystalline form in the following way: carbethoxyhydroxy-benzoyl chloride was coupled with p-hydroxybenzoyl-p-hydroxybenzoic acid in alkaline solution, the compound dissolved in a mixture of pyridine and acetone, and ammonia added for the purpose of removing the carbethoxy group. The tridepside was then obtained as long needles by re-dissolving in acetone.
Both tridepsides melt well above 200° C., are practically insoluble in water, and are but sparingly soluble in practically all organic solvents. In alcoholic solution they give colour reaction with ferric chloride similar to those given by p-hydroxybenzoic acids.
Preparation of Tetradepsides
[Footnote: Fischer and Freudenberg, Liebig's Ann., 1910, 372, 32.]
Here, again, two forms are known, e.g., tri-p-hydroxybenzoyl-p-hydroxybenzoic acid—
HO.C_5H_4.COO.C_6H_4.COO.C_6H_4COO.C_6H_4 COOH
and vanilloyl-di-p-hydroxybenzoyl-p-hydroxybenzoic acid—