Process for Synthesis of Coenzyme Q Compounds

1. Field of the Invention

The present invention relates to a process for the synthesis of 2,3-dimethoxy-5-methyl-6-substituted-1,4-benzoquinones having the formula (I): ##STR5## wherein R is ##STR6## in which n is an integer of 0 to 9, and A and B are hydrogens or A--B is a direct valence bond between the carbon atoms to which they are attached.

Compounds of formula (I) are known as coenzymes Q, and especially, 2,3-dimethoxy-5-methyl-6-decaprenyl-1,4-benzoquinone [2,3-dimethoxy-5-methyl-6-(3,7,11,15,19,23,27,31,34,39-decamethyltetracont adecaen-2,6,10,14,18,22,26,30,34,38-yl)-1,4-benzoquinone] in which A--B is a direct valence bond and n is 9, which is called "coenzyme Q.sub.10," participates in the electron transport system in living bodies and plays an important role for producing energy. This compound has activities of improving the state of the myocardium in patients suffering from ischemia, increasing the reduced heart beat number and competing with the Na-storing action of aldosterone, and is effective for treating and preventing of congestive heart failure, lung congestion, swelling of liver and angina pectoris.

2. Description of the Prior Art

As known processes for the synthesis of compounds of formula (I), there can be mentioned a method comprising reacting 2,3-dimethoxy-6-methyl-1,4-benzohydroquinone or a 1-monoacyl derivative thereof with an n- or iso-prenol or a reactive derivative thereof, in the presence of an acid catalyst such as a protonic acid, e.g., formic acid, sulfuric acid, hydrochloric acid, phosphoric acid, p-toluenesulfonic acid or the like, a Lewis acid, e.g., zinc chloride, aluminum chloride, a boron trifluoride-ether complex or the like, or a mixture of these protonic or Lewis acids, to obtain a corresponding hydroquinone (see Japanese Pat. No. 17513/64 and No. 3967/71), and reacting the hydroquinone with an oxidizing agent to convert it to the corresponding benzoquinone (see Japanese Pat. No. 17514/64). In such known method, however, because the yield of the condensation step is low, the yield of the intended quinone compound is very low and even the yield of the crude product is about 30% at highest. Further, each of the acid catalysts that are used in the condensation step is highly corrosive and has bad effects on the preparation equipment, and metal dissolved out from the acid catalyst contaminates the product. Accordingly, industrial working of this known method involves various disadvantages.

Further, since an acid catalyst such as those mentioned above is employed, neutralization, extraction and other post treatments must be conducted for separating the desired product from the reaction mixture, and because the amount of the catalyst used for the reaction is relatively large and the catalyst used must often be discarded after completion of the reaction, the manufacturing cost is increased and environmental pollution is readily caused. Thus, this known method involves various industrial difficulties.

In accordance with the present invention, there is provided a process for the synthesis of 2,3-dimethoxy-5-methyl-6-substituted-1,4-benzoquinones having the formula (I), which comprises reacting 2-methyl-4,5,6-trimethoxyphenol with an aryl boronic acid, a lower alkyl boronic acid or an acid anhydride thereof, to obtain a 2-methyl-4,5,6-trimethoxyphenol ester of boric acid (step 1), reacting the resulting boric acid ester with an n- or iso-prenol having the formulas: ##STR7## wherein R is as defined above, or a reactive derivative thereof, in the presence of a silica-alumina compound to obtain a 2-methyl-3-substituted-4,5,6-trimethoxyphenol ester of boric acid (step 2), and hydrolyzing the thus-obtained boric acid ester to transform it to 2-methyl-3-substituted-4,5,6-trimethoxyphenol having the formula (II): ##STR8## wherein R is as defined above, and treating the thus-obtained compound with an oxidizing agent to obtain a 2,3-dimethoxy-5-methyl-6-substituted-1,4-benzoquinone having the formula (I) (step 3).

Production of 2-methyl-4,5,6-trimethoxyphenyl borate in step 1 can be accomplished according to customary methods for production of boric acid esters, for example, a method disclosed in J. Am. Chem. Soc., 75, 213 (1953).

As the aryl boronic acid, there can be employed, for example, phenylboric acid, diphenylboric acid and tolylboric acid, and as the lower alkyl boronic acid, there can be employed, for example, methylboric acid, dimethylboric acid, dimethylboric anhydride, ethylboric acid and propylboric acid. The boronic acids have the formula RB(OH).sub.2 wherein R is aryl having 6 to 10 carbon atoms or alkyl having one to 4 carbon atoms. The acid anhydrides (RBO) include intramolecular anhydrides and intermolecular anhydrides of a boronic acid such as those mentioned above.

The stability of the boric acid ester obtained in step 1 to water is very low. Accordingly, it is preferred that the as-prepared ester be immediately transferred to the next step without isolation.

As the n- or iso-prenol or its reactive derivative used at step 2, there can be mentioned, for example, 3-methylbuten-2-ol-1, 3-methylbuten-1-ol-3, geraniol, linalool, nerol, nerolidol, phytol, isophytol, geranylgeraniol, geranyllinalool, geranylfarnesol, geranylnerolidol, farnesylfarnesol, farnesylnerolidol, geranylgeranylfarnesol, solanesol, decaprenol, isodecaprenol and halides derived from these alcohols.

As the silica-alumina compound present in the reaction mixture, there can be mentioned, for example, clay, active clay, kaolin, natural and synthetic zeolites, silica-alumina, silica-alumina-boria and silica-alumina-magnesia. An amount of the silica-alumina compound is from 1/4 to 2 times the weight of 2-methyl-4,5,6-trimethoxyphenol to be used as a reactant.

In practicing this reaction in the process of the present invention, it is preferred to use a solvent appropriately chosen from the group consisting of aromatic hydrocarbons such as benzene, toluene and xylene, ether type solvents such as diethyl ether, diisopropyl ether and tetrahydrofuran, aliphatic hydrocarbons such as pentane, hexane, heptane, octane, isooctane, petroleum ether and ligroin and ester type solvents such as ethyl acetate.

Hydrolysis of the boric acid ester in step 3 is readily accomplished simply by contacting the boric acid ester obtained in step 2 with water. In general, the hydrolysis is performed by washing the product obtained at step 2 with water. Oxidation of the phenol of above formula (II) is conducted according to a customary quinone-forming method using a mild oxidizing agent such as a silver oxide, lead oxide, ferric chloride, aqueous hydrogen peroxide or the like. For example, the phenol of the above formula (II) can easily be oxidized according to the method disclosed in Japanese Pat. No. 17514/64 or Japanese patent application laid-open specification No. 49733/73.

It is possible to conduct steps 1 to 3 in a continuous manner. Since the compound of formula (II) is relatively stable, it is possible to isolate and purify the compound of formula (II) and check the degree of completion of the reaction.

The process of the present invention provides the following important advantages over the above-mentioned conventional methods.

1. Improvement of Yield

In the process of the present invention, because the yield at the condensation step 2 is high, the final intended quinone of formula (I) can be obtained in a high yield. For example, it has been confirmed that pure coenzyme Q.sub.10 can be obtained at a yield of 65%, or higher, according to the process of the present invention.

2. Reduction of Number of Steps

A number of steps are required for obtaining a compound of formula (I) from a compound of formula (II) according to the conventional techniques. For example, the compound of formula (II) is converted to 2,3-dimethoxy-5-methyl-1,4-benzoquinone according to the method disclosed in Japanese Pat. No. 28503/74, and the thus-obtained quinone is reduced to 2,3-dimethoxy-5-methyl-1,4-benzohydroquinone, which is monoacylated if needed. Then, the resulting benzohydroquinone or monoacylated product thereof is condensed with an n- or iso-prenol, or a reactive derivative thereof, to obtain a 2,3-dimethoxy-5-methyl-6-substituted-1,4-benzohydroquinone or a monoacylate thereof (Japanese Pat. No. 17513/64), and then, the thus-obtained compound is oxidized to obtain the desired product of formula (I) (Japanese Pat. No. 17514/64). Thus, according to the conventional techniques, 4 to 5 steps are required for obtaining the desired compounds of formula (I) and complicated operations are included in these steps. In contrast, according to the process of the present invention, the synthesis is accomplished in three steps.

3. Prevention of Corrosion of Equipment and Occurrence of Environmental Pollution

In the condensation step 2, an acidic chemical such as zinc chloride, a boron trifluoride-ether complex or the like is used as a condensation catalyst in the conventional methods. Such acidic chemical is highly corrosive and a reaction vessel made of an anti-corrosive material must be used. Further, when a metal halide or the like is used as a condensation catalyst, metal ions are dissolved out and cause environmental pollution. In the process of the present invention, because these acidic condensation catalysts need not be used at all, none of these disadvantages are brought about.

In view of the foregoing, it will readily be understood that the process of the present invention is very advantageous in comparison with the conventional methods.

The present invention will now be described in more detail by reference to the following illustrative example.

a. Synthesis of 2-methyl-3-decaprenyl-4,5,6-trimethoxyphenol

To 300 ml of toluene were added 20 g of 2-methyl-4,5,6-trimethoxyphenol and 12 g of phenylboric acid, and the mixture was refluxed for 2 hours under agitation while removing the water formed by the reaction. Then, 15 ml of toluene was distilled from the reaction mixture, and the residue was cooled to 50.degree. C and 35 ml of n-hexane and 20 g of silica-alumina were added thereto. The mixture was heated at 60.degree. C for 30 minutes under agitation. Under the same conditions, a solution of 14 g of decaprenol (having a purity of 94.4%) in 10 ml of n-hexane was added dropwise to the reaction mixture over a period of 40 minutes. The mixture was further heated and agitated for 10 minutes to complete the reaction. After completion of the reaction, the reaction mixture was filtered, and the filtered solids were washed with diethyl ether. The washing liquor was combined with the filtrate, and the organic layer was recovered and washed with water and then with a 10% aqueous solution of sodium hydroxide. The organic solvent layer was recovered and the solvent was distilled therefrom to obtain 17 g of a light yellow oily product. A part of the product was sampled and purified by silica gel column chromatography to obtain a standard substance for identification. The purified product was white crystals melting at 43.degree. to 44.degree. C. Elementary analysis values of the purified product at C.sub.60 H.sub.94 O.sub.4 were as follows:

Calculated: C = 81.95%, H = 10.77% Found: C = 81.76%, H = 10.80%

b. Synthesis of 2,3-dimethoxy-5-methyl-6-decaprenyl-1,4-benzoquinone

In 150 ml of ethyl acetate was dissolved 17.0 g of the light yellow oily product obtained in step (2), and 45 g of ferric chloride hexahydrate was added to the solution and the mixture was agitated for 2 hours at room temperature. Then, 150 ml of water was added to the mixture and the resulting mixture was sufficiently shaken. The organic solvent layer was recovered, washed with water and dried with sodium sulfate salt. The solvent was distilled off to obtain 14.1 g of a reddish brown oily product. All of the thus-recovered product was purified by silica gel column chromatography using diethyl ether-n-hexane as an eluting solvent to obtain 10.1 g of a light yellowish orange oily product. The yield was 62.0% based on the decaprenol.

A part of the thus-obtained product was recrystallized from acetone to obtain yellowish orange crystals having a melting point of 49.degree. to 50.degree. C. Results of UV, IR, NMR and MAS spectrum measurements were in agreement with those of a standard substance of 2,3-dimethoxy-5-methyl-6-decaprenyl-1,4-benzoquinone.