Method of synthesis of pyrogallol

#This invention relates to a process for the preparation of pyrogallol. In a particular aspect, it relates to a process for the preparation of pyrogallol from glutaric ester and dialkoxymalonic ester.

#Pyrogallol has many important uses, e.g. in photographic developers, as a mordant for wool, and in staining of leather. It is usually prepared from gallic acid by heating as described by Rinderknecht and Niemann, J. Am. Chem. Soc. 70, 2605 (1948), although Stevens in U.S. Pat. No. 2,603,662 disclosed a process involving chlorination of p-tert. butylphenol and hydrolysis of the product. Gallic acid is made principally by saponification of natural product tannic acids which are chiefly obtained from nutgalls gathered in Turkey and from Tara powder obtained from a plant species found at high altitudes in the Peruvian Andes. A method of synthesis has been disclosed by C. D. Hurd, U.S. Pat. No. 3,560,569. Nevertheless, gallic acid and pyrogallol are high-priced commodities which undoubtedly have been kept from many markets where they would be widely used at a lower price.

#It is an object of this invention to provide a new process for the preparation of pyrogallol.

#The invention will be better understood by reference to the following example. It is understood, however, that the example is intended only for illustration and it is not intended that the invention be limited thereby.

#Dimethyl dimethoxymalonate was prepared by mixing trimethyl orthoformate, 300 ml (285 g or 2.8 mol) with 35 g of dihydroxymalonic acid (0.257 mol) in a reaction vessel equipped with a heat source, an agitator and a distillation column. The mixture was heated to the boiling point, 100.degree.-105.degree., and excess trimethyl orthoformate and by-products were separated by slowly distilling over a period of 13 hours until 250 ml had been collected. The distillation was continued under reduced pressure and the product was collected at 110.degree. C. at a pressure of 7 mm.

#A 38.4 g (0.2 mol) portion of the dimethyl dimethoxymalonate prepared as above was dissolved in 350 ml anhydrous benzene in a reaction vessel under a nitrogen atmosphere to prevent any tendency to oxidation, if any, using magnetic stirring. A reflux condenser fitted with a water separator was connected to the vessel and a 100 ml portion of the benzene was distilled to remove any residual moisture and to dry the apparatus. The mixture was cooled and sodium hydride (16.9 g of a 57% dispersion in mineral oil) and 1 ml of anhydrous methanol were added. The mixture was heated to reflux and dimethyl glutarate, 16 g (0.1 mol), dissolved in 150 ml of anhydrous benzene was gradually added over a period of 6.5 hr. The mixture was heated an additional 16 hr under reflux and was then cooled in an ice bath. Water, 250 ml, was slowly and cautiously added over an extended period of time and stirring was continued for 30 minutes at room temperature. The mixture was then filtered and the aqueous layer was separated. The aqueous layer was washed with 100 ml of ethyl ether and the pH of the aqueous layer was adjusted to 4 with acetic acid (60 ml) to precipitate the crude product. The crystals were separated by filtration, washed with water and air dried to yield dimethyl 4,6-dihydroxy-5,5-dimethoxy-3,6-cyclohexadiene-1,3-dicarboxylate, m.p. 143.degree.-144.degree.. The infra-red absorption spectrum and the proton magnetic resonance spectrum were consistent with the proposed structure, as were the carbon-hydrogen ratios.