Methods for the preparation of benzophenone derivatives

Abstract: The present invention relates to a process for the manufacture of benzophenone derivatives and is based on the separation of compounds of formula (I), from corresponding isomeric compounds of formula (II), by selective crystallization of corresponding salts of compounds of formula (I).

Description: The present invention relates to a process for the manufacture of benzophenone derivatives. The process is based on the separation of a compound of formula (I), wherein R 1 is C 1 -C 8 -alkyl and R 2 is hydrogen, C 1 -C 8 -alkyl or halogen, from the respective, corresponding isomeric compound of formula (II), wherein R 1 and R 2 are as defined above. Compounds of formula (I) are important intermediates for the manufacture of pharmaceutically active compounds, for example for the manufacture of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone. 3,4-Dihydroxy-4'-methyl-5-nitrobenzophenone is known per se and is described, for example, in European Patent Application No. 237,929. The substance is a catechol-O-methyl-transferase (COMT) inhibitor and is especially suitable for the therapy of Parkinson's disease and for the treatment of depressions and similar disease states. The synthesis is mentioned in the aforementioned European Patent Application and is also described by Borgulya et al. (Drugs of the Future (1991) 16 , 719-721). It is based essentially on the reaction of 4-bromotoluene with 4-(benzyloxy)-3-methoxybenzaldehyde in the presence of butyllithium to give 4-(benzyloxy)-3-methoxy-4'-methylbenzhydrol. By oxidation to 4-(benzyloxy)-3-methoxy-4'-methylbenzophenone and subsequent debenzylation there is obtained 4-hydroxy-3-methoxy-4'-methylbenzophenone. After regioselective nitration to 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone and subsequent hydrolysis of the methoxy group there is obtained the aforementioned inhibitor 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone. The synthesis of other analogous catechol-O-methyltransferase inhibitors is described by Borgulya et al. (Helvetica Chimica Acta (1989) 72 , 952-968). The known synthetic route has, however, considerable disadvantages, which are primarily associated with the high number of steps of the synthesis and the time/apparatus expenditure associated therewith. Moreover, relatively expensive starting materials, such as vanillin and 4-bromotoluene, are required for this synthetic route. Surprisingly, it has now been found that these disadvantages can be largely avoided when a process set forth in the claims is used for the manufacture of the corresponding starting material or in carrying out the corresponding overall synthesis. Accordingly, in the manufacture of benzophenone derivatives of formula (I), wherein R 1 is C 1 -C 8 -alkyl and R 2 is hydrogen, C 1 -C 8 -alkyl or halogen, salts of the compounds of formula (I) are separated from the salts of the corresponding isomeric compounds of formula (II), wherein R 1 and R 2 are as defined above, by selective crystallization. Surprisingly, the corresponding salts of compounds of formula (II) have a far better solubility compared with the salts of the corresponding isomeric compounds of formula (I). The substituent R 2 can be situated in the ortho-, meta- and para-position. In a preferred embodiment it is situated in the para-position. Especially preferred processes are those in which compounds in which R 1 and R 2 represent methyl groups are used. The crystallization of compounds of general formula (I) and the thereby as selective as possible separation of compounds of general formula (II) can be effected by crystallization of a salt, e.g. the sodium or lithium salt, preferably the sodium salt, of a compound of formula (I). Water or water-containing solvent mixtures can be used as the solvent for the crystallization. Aqueous salt solutions are preferred solvents for the selective crystallization. For example, the isomeric compounds of formulae (I) and (II) can be extracted from an organic phase with an aqueous potassium hydroxide solution. To the aqueous phase there can be added e.g. a saturated aqueous sodium chloride solution. Thereby, the sodium salt of the corresponding compound of formula (I) crystallizes out, while the corresponding sodium or potassium salt of the isomer of formula (II) remains in solution. The precipitated sodium salt can be separated and subsequently washed e.g. with a dilute, aqueous sodium chloride solution. The thus-purified compound of formula (I) can then be transferred into an organic phase by acidification, e.g. with dilute hydrochloric acid, and extraction with an organic solvent, e.g. ethyl acetate. Where desired, the compound of formula (I) can subsequently be purified further according to known methods. The term "C 1 -C 8 -alkyl" used for the definition of the general formulae relates to straight-chain or branched alkyl groups which contain a maximum of eight, preferably a maximum of four, carbon atoms. The term embraces, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec.butyl, tert.butyl, pentyl, hexyl and the like. The term "halogen" stands for fluorine, chlorine, bromine and iodine. The process in accordance with the invention can be used, for example, in the manufacture of benzophenone derivatives. Thus, benzophenone derivatives of formula (I) can be obtained by reacting a compound of formula (III), wherein R 1 is C 1 -C 8 -alkyl, with an activated acid derivative, e.g. with a reactive acid derivative of formula (IV) wherein R 2 is as defined above and R 3 is halogen, preferably chlorine, under Friedel-Crafts acylation conditions. The Friedel-Crafts acylation is a standard reaction which will be known to a person skilled in the art. In the reaction an aromatic ketone is formed by the reaction of the aromatic compound with an acylating agent under the influence of a Friedel-Crafts catalyst. The reaction of a compound of formula (III) with a compound of formula (IV) is described in detail in Example 1. One of the two C 1 -C 8 alkyl ether bonds R 1 -O- is subsequently cleaved from the resulting compound of formula (VII), wherein R 1 and R 2 are as defined above. When the Friedel-Crafts acylation is carried out with aluminium trichloride, the cleavage is conveniently effected e.g. by the addition of aluminium trichloride. The isomers of formulae (I) and (II) which result from the cleavage can then be separated by the process in accordance with the invention. Accordingly, the present invention likewise embraces a process for the manufacture of benzophenone derivatives characterized by subjecting a compound of formula (III) and a reactive acid derivative of formula (IV) to a Friedel-Crafts acylation, cleaving off one of the two substituents R 1 and separating the resulting compound of formula (I) from the isorneric compound of formula (II) according to one of the processes described above. The process in accordance with the invention is especially suitable for the manufacture of 4-hydroxy-3-methoxy-4'-methylbenzophenone, an intermediate for the manufacture of pharmaceutically active substances. Thus, 3,4-dimethoxy-4'-methylbenzophenone is obtainable by reacting veratrol and p-toluoyl chloride under Friedel-Crafts conditions. After cleavage of one methyl group, 4-hydroxy-3-methoxy-4'-methylbenzophenone can be separated from 3-hydroxy-4-methoxy-4'-methylbenzophenone by selective crystallization of the corresponding sodium salt or lithium salt. Compounds of formula (I) can be further processed to pharmaceutically active compounds with the aid of the process in accordance with the invention. Thus, compounds of formula (I), wherein R 1 and R 2 are as defined above, obtained by means of the process described above, can be converted by nitration into compounds of formula (V), wherein R 1 and R 2 are as defined above. After demethylation, there are obtained compounds of formula (VI) The selective nitration in accordance with the process step described above is in principle known and can be carried out, for example, using 65% aqueous nitric acid, as described in Example 2. For the demethylation, the nitration product of general formula (V) can be treated with a solution of aluminium trichloride in triethylamine and methylene chloride. After the reaction, the product, which can be liberated e.g. by acidic hydrolysis, can, for example, be extracted with methylene chloride and, if desired, purified further by recrystallization. As set forth above, the compounds obtained in this manner are inhibitors of catechol-O-methyltransferase (COMT) and are suitable, for example, for the treatment of Parkinson's disease. This process is especially suitable for the manufacture of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone. Thus, 4-hydroxy-3-methoxy-4'-methylbenzophenone can be obtained according to one of the processes described above and subsequently can be converted by selective nitration into 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone. This compound is subsequently demethylated to 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone. The present invention relates to all aspects described above and especially to the use of the process for the manufacture of the compounds named above as well as to all compounds which are manufactured according to one of the said processes. The following Examples serve exclusively to illustrate the invention and have no limiting character. Examples Example 1: Manufacture of 4-hydroxy-3-methoxv-4'-methvlbenzophenone Veratrol (pyrocatechol dimethyl ether, 1,2-dimethoxybenzene; 1.01 eq.) is dosed at 15-25°C and subsequently p-methylbenzoyl chloride (1.00 eq.) is dosed at 25-30°C into a suspension of aluminium trichloride (1.04 eq.) in methylene chloride, with 3,4-dimethoxy-4'-methylbenzophenone forming in a Friedel-Crafts acylation. After completion of the evolution of hydrogen chloride gas, aluminium trichloride is added in two portions (1.04 eq., 0.18 eq.) at intervals of 3 hours to the reaction mixture at 25°-28°C. In order to complete the methyl ether cleavage, the reaction mixture is stirred at 32-35°C for a further 3-4 hours. The reaction mixture is cooled to 20°C and stirred in a three phase mixture of dilute hydrochloric acid, methylene chloride and ice. The phases are separated. The aqueous phase is extracted with methylene chloride. The combined extracts are purified in succession with dilute hydrochloric acid and with water and finally washed with aqueous sodium bicarbonate solution. The washed neutral methylene chloride phase is extracted with an about 1N aqueous potassium hydroxide solution (1.02 eq.), whereby the two isomeric hydroxymethoxybenzophenones are dissolved as potassium salts in the aqueous phase. The methylene chloride phase is separated. A saturated, aqueous sodium chloride solution is dosed into the aqueous phase. Thereby, the sodium salt of 4-oxy-3-methoxy-4'-methylbenzophenone crystallizes out, while the sodium or potassium salt of the isomeric 3-oxy-4-methoxy-4'-methylbenzophenone remains in solution. The sodium salt of 4-oxy-3-methoxy-4'-methylbenzophenone is isolated, washed with dilute sodium chloride solution and subsequently dissolved in a two phase mixture of ethyl acetate and dilute hydrochloric acid. The aqueous phase is separated, the organic phase is washed with water, dried by azeotropic distillation, concentrated and filtered while hot. The filtered solution is cooled and treated with n-hexane, with the product crystallizing upon cooling. In order to complete the crystallization, the resulting suspension is stirred at -15°C for 2 hours. The crystals are isolated and washed with a mixture of ethyl acetate/n-hexane and subsequently dried at 45°-55°C in a vacuum. Yield of 4-hydroxy-3-methoxy-4'-methylbenzophenone: 50-64% of theory based on p-methylbenzoyl chloride used. Example 2: Manufacture of 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone 65% aqueous nitric acid (1.23 eq.) is dosed into a solution of 4-hydroxy-3-methoxy-4'-methylbenzophenone in glacial acetic acid at 20°-25°C. After stirring for 2 h. the resulting 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone is precipitated by the addition of water. The crystals are isolated, washed with dilute acetic acid and subsequently with water. The product can be stored in water-moist form or in pre-dried form until it is used in the subsequent step (Example 3). Yield of 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone: 80-87% of theory [based on] 4-hydroxy-3-methoxy-4'-methylbenzophenone used (dry weight basis). Example 3: Manufacture of 3.4-dihydroxy-4'-methyl-5-nitrobenzophenone a) Demethylation of 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone and isolation of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone as the crude product Water-moist 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone is dried by azeotropic distillation with methylene chloride. A solution of aluminium trichloride (1.36 eq.) in triethylamine (5.30 eq.) and methylene chloride is dosed into the dried methylene chloride solution of 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone at an internal temperature of 5°-15°C. Subsequently, the reaction mixture is boiled under reflux for at least 2.5 hours. As soon as the reaction has finished, the reaction mixture is cooled to 20°-25°C and stirred into a two phase system consisting of methylene chloride and dilute hydrochloric acid of 0°C. The product liberated by the acidic hydrolysis is extracted with methylene chloride. The cooled methylene chloride extracts are washed with dilute hydrochloric acid and subsequently with water, filtered and concentrated in a vacuum. The distillation residue is dissolved in methylene chloride, filtered and subsequently concentrated. The concentrate is cooled, with the product crystallizing out. In order to complete the crystallization, the suspension is stirred at -15°C for at least 3 hours. The crystals are isolated, washed with cold methylene chloride and stored in a methylene chloride-moist form until processed in the subsequent purification step. b) Purification of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone (crystallization from ethanol/water 1:1 (v/v). The methylene chloride-moist crude crystallizate is dissolved in abs. ethanol at 55°-65°C and filtered while hot. The filtrate is concentrated, with the product beginning to crystallize. Water is dosed into the 50% (w/v) concentrate of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone/ethanol at 45-55°C such that a solvent mixture ethanol/water of 1:1 (v/v) results. In order to complete the crystallization, the suspension is cooled stepwise to -10°C and is stirred at this temperature for at least 3 hours. The crystals are isolated, dried at 40°C in a vacuum and subsequently micronized. Yield of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone: 72-85% of theory based on 4-hydroxy-3-methoxy-4'-methyl-5-nitrobenzophenone used.

Claims: A process for the manufacture of benzophenone derivatives of formula wherein R 2 is hydrogen, C 1 -C 8 -alkyl or halogen, which process comprises a) obtaining the compound of formula wherein R 1 is C 1 -C 8 -alkyl and R 2 is as defined above, by a selective crystallization from a mixture of isomeric potassium salts of formula I and by a. adding a sodium or lithium salt to said mixture of potassium salts to crystallize the compound of formula (I); and b. isolating the compound of formula (I) in crystalline form as a sodium or lithium salt, b) subsequently producing compounds of formula (V) by nitration of a compound of formula (I) wherein R 1 and R 2 are as defined above, and c) after subsequent dealkylation, obtaining compounds of formula (VI). A process according to claim 1, wherein R 2 is situated in the para-position. A process according to claims 1 and 2, wherein R 1 and R 2 are methyl. A process according to claims 1-3, wherein the selective crystallization of a compound of formula (I) is effected in an aqueous solvent. A process for the manufacture of 3,4-dihydroxy-4'-methyl-5-nitrobenzophenone according to claims 1 - 4, which process comprises a) obtaining 4-hydroxy-3-methoxy-4'-methylbenzophenone, b) producing 4-hydroxy-3-methexy-4'methyl-5-nitro-benzophenone by nitration and c) obtaining 3,4-dihydroxy-4'methyl-5-nitrobenzophenone by demethylation. A process for the manufacture of benzophenone derivatives of formula (I) wherein R 1 is C 1 -C 8 -alkyl and R 2 is hydrogen, C 1 -C 8 -alkyl or halogen, which process comprises selectively crystallizing a mixture of isomeric potassium salts of formula I and by a. adding a sodium or lithium salt to said mixture of potassium salts to crystallize the compound of formula (I); and b. isolating the compound of formula (I) in crystalline form as a sodium or lithium salt.

Inventors: BRUHIN JUERG, BRUHIN, JUERG, Bruhin, Jürg
Applicants: HOFFMANN LA ROCHE, F. HOFFMANN-LA ROCHE AG
Application year: 1998
Main classification: C07C45
IPC codes: C07C205, C07C201, C07C45