General procedure for the synthesis of (4-hydroxyphenyl)(2,3,4-trihydroxyphenyl)methanone from o-hydroxybenzenetriol and p-hydroxybenzoic acid: 0.11 moles of p-hydroxybenzoic acid and 0.1 moles of pyrophosphoric acid as an acylating agent as well as 0.015 moles of concentrated sulfuric acid were added in a reactor and mixed well. An inert gas was passed for 10 minutes to remove air. After increasing the system temperature to 70-80°C, the reactor was closed, the temperature was adjusted to 110-130°C, the pressure was maintained at 0.1-5 MPa and the reaction was carried out for 3 hours. Upon completion of the reaction, the reaction mixture was cooled to 50-60 °C and held at this temperature for 1.5 hours. Subsequently, 200 ml of 20% ethanol solution was added and crystallized at 0-5°C. The crystals were collected by filtration and dried to give the target product (4-hydroxyphenyl)(2,3,4-trihydroxyphenyl)methanone. The molar yield of this step was 78.16% and the purity was ≥98.5% by HPLC.
