A PROCESS FOR THE PREPARATION OF 4- (DES-DIME ILAMINO) -TETRACICLINES The present invention relates to a new process for the preparation of 4- (des-dimethylamino) -tetracyclines using reagents that are more environmentally friendly thereby providing less Emanation problems thus allowing a simpler purification. These compounds have a wide therapeutic application. The 4- (des-dimethylamino) -tetracyclines do not exhibit the antibacteriological activity that is typical of tetracyclines but have a remarkable therapeutic activity as described in the following patent applications: treatment of diabetes and other dysfunctions (US 5,532,227, EP 599,397); treatment of inflammatory diseases (US 5,552,297, WO 9,808,480); treatment of arthritis, ulcers and muscular diseases (EP 435,362, 586,020); prophylactic treatment for diseases of the teeth and gums (AU 9,338,210, WO 9,213,515). These compounds can be administered alone or in combination with other medicaments. The processes for the preparation of 4- (des-dimethylamino) -tetracyclines described in the literature employ a mixture of zinc and acetic acid. An excess of the reducing agent, ie zinc, is used to obtain the elimination of the amino group C-4, starting with the triamylammonium salts of the tetracycline to be reduced (J. Amer. Chem. Soc. 80, 1654 (1958 )). The acid addition salts can also be reduced by zinc and acetic acid, producing the 4- (de-dimethylamino) -tetracyclines, but in this particular case there will be a tendency for the product to have the 6-hydroxy group also reduced ( J. A er. Chem. Soc. 76, 3568 (1954)). This method is problematic with respect to the level of waste discharge, which is potentially contaminating and to the emanations produced during the industrial preparation of the 4- (des-dimethylamino) -tetracyclines that result from the presence of metallic zinc without consuming and from the elimination of zinc salts. The presence of zinc can also be problematic with respect to the purification of the product manufactured by this process due to the high capacity of the tetracycline derivatives to form stable chelates with the metals. Thus, in the prior art, significant amounts of zinc can remain in the final 4- (des-dimethylamino) -tetracycline. It should be noted that one of the most significant indications for the use of the 4- (des-dimethylamino) -tetracyclines is as inhibitors of metalloprotose (MP) (EP 435,362A). The mode of action of these compounds involves composing with zinc in the enzyme so that the presence of zinc in the 4- (des-dimethylamino) -tetracyclins can have a detrimental effect on the activity. The purpose of the present invention is to find a clean process that allows the elimination of the C-4 substituent, without using zinc, thus overcoming both environmental and purification problems. In the prior art process related to the synthesis of the 4- (des-dimethylamino) -tetracyclines, the tetracycline to be reduced is methylilized in a first step with methyl iodide and in a second step the trimethylamine salt is reduced with a mixture containing zinc and acetic acid. The method for the synthesis of the 4- (des-dimethylamino) -tetracyclines described in the present invention allows the elimination of the dimethylamino group C-4, its advantage over the method of the prior art is that it does not use zinc. The reduction of the trimethylamine salts of the tetracycline, using this method, is carried out electrochemically in an aqueous solution. In this way, the problems associated with the use of zinc and acetic acid are eliminated. The final products that do not have amphoteric properties such as the known tetracyclines can be isolated in this way by extraction by organic solvents providing compounds in good yield and with a purity of over 97%. The present invention relates to a process for the preparation of 4- (des-dimethylamino) -tetracyclines. The tetracycline to be reduced is treated with an alkylating agent, preferably methyl iodide in an organic solvent, preferably acetone, the resulting trialkylammonium salt is thereby electrolytically reduced in an acid solution of aqueous pH. The pH of the solution is maintained between 0.5 and 5.0, preferably between 1.0 and 3.0. The tetracycline reduced in C-4 is obtained in this manner, with a purity typically above 97% by high performance liquid chromatography (HPLC). In a first stage and according to the methods described hereafter, the tetrac-cline to be reduced is suspended in an organic solvent, preferably acetone and the suspension is maintained between 25 and 40 ° C, preferably between 30 and 33 ° C , although other temperatures outside this range can be used. The alkylating agent, preferably methyl iodide, is subsequently added. The reaction mixture is stirred in the above temperature ranges for the period of time necessary to obtain the alkylation of the nitrogen atom at C-4. Typically, the reaction is complete between 48 and 72 hours at 30-33 ° C. The course of the reaction can be monitored by HPLC or by any other suitable method. The excess of the ethylating agent is removed by distillation under atmospheric pressure. The solvent is reduced to half the volume or less, by distillation. The iodide salt of the tetracycline alkylated to C-4 is isolated by direct precipitation of the solution in an ether, such as an isopropyl ether or alternatively by dissolving the residue in a lower molecular weight alcohol, followed by the addition of a no. solvent such as, for example, ethyl or isopropyl ether. The trimethylammonium salt of tetracycline is dissolved in water and an acid is added to maintain the solution at an acidic pH. The acid used for this purpose is preferably acetic acid in a concentration of 30 to 70%, preferably 50 to 60%. The solution is then purified with an inert gas, such as nitrogen or argon and kept in an electrochemical cell under an inert atmosphere to which an electrolyte used to allow the passage of the electric current is added. The referred electrolyte may be a potassium or sodium salt, such as chloride, bromide, iodide or acetate, preferably potassium chloride in a concentration of between 0.01 and 1 mole, with the preferred range being 0.1 to 0.5 mole. The construction materials of the electrodes used in the electrochemical cell for applying the electric current to the solution can be chosen from platinum, mercury, stainless steel or carbon, the preferred electrodes being platinum and mercury. After placing the tetracycline salt solution in a temperature range between 1Q and 40 ° C, preferably between 20 and 25 ° C, a direct current between 0.5 and 1.4 volts is applied between the two electrodes of the cell, preferably between 1 and 1.15 volts. The potential is maintained for a period of time sufficient to allow the reduction of the trimethylammonium salt of the tetracycline. The course of the reaction can be controlled by? PLC. The end point of the reaction is conveniently reached after 2 to 6 hours, although slower or faster reaction times may be possible at lower or higher temperatures respectively. After the reaction is complete, an aqueous solution containing hydrochloric acid (0.5 to 2N) is added, as well as an organic solvent where the 4- (des-dimethylamino) -tetracyclines, such as, among others, chloroform, are soluble, dichloromethane or ethyl acetate, preferably dichloromethane, to allow extraction of the reduced tetracycline from the aqueous phase. The organic phase is washed with dilute hydrochloric acid and subsequently with water. After the solution is dried and concentrated under reduced pressure, the residual acetic acid can be azeotropically distilled with, for example, cyclohexane. The 4- (des-dimethylamino) -tetracycline is obtained after drying the mixture at 25-40 ° C, as a yellow solid the purity of which is typically more than 97% by HPLC. The 4- (des-dimethylamino) -tetracycline can also be isolated by crystallization with acetone after removal of the acetic acid. The following examples serve only to illustrate the different aspects of the invention and are not considered in any way a limitation of the specification and claims thereof. EXAMPLE 1
The preparation of 1, 4, 4a, 5, 5a, 6, 11, 12a-octahydro-3, 10, 12, 12a-tetrahydroxy-1,1-dioxonaphthacene-2-carboxamide. 10 g of 4-dimethylamino-1,4,4, 5, 5a, 6, 11, 12a-octahydro-3, 10, 12, 12a-tetrahydroxy-1,1-dioxonaphthacene-2-carboxamide were suspended in 100 ml of acetone. The reaction mixture was heated to 30-33 ° C followed by the addition of 13.7 ml of methyl iodide. After stirring for 72 hours at room temperature, the product was completely dissolved and the reaction was completed after an additional 24 hours. The solvent was distilled under atmospheric pressure and then the residue was dissolved in 110 ml of acetone. The solution was evaporated to half its volume and then added for one hour to 700 ml of ethyl ether, after which it was precipitated. The iodide salt of the tetracycline, trimethylated on the 4-nitrogen, was then dried at 35-40 ° C, yielding the desired compound with a purity of about 97% by HPLC. 10 mg of 4-trimethylammonium-1,4,4, 5a, 5, 5a, 6, 11, 12a-octahydro-3, 10, 12, 12a-tetrahydroxy-1,1,1-dioxonaphthacene-2-carboxamide iodide were dissolved and mg of potassium chloride in 5 ml of an aqueous solution of acetic acid (50%). This solution was placed in the compartment of an electrochemical cell-equipped with a drip mercury electrode and a secondary electrode covered with platinum. A reference electrode, (calomel) immersed in a solution containing 0.1 mol of potassium chloride in 50% aqueous acetic acid was placed in the other compartment of the referred cell. The two compartments of the cell were separated by a 'porous glass membrane. The solution was purified with nitrogen for approximately 20 minutes. A direct current of -0.85 volts in relation to the calomel electrode was then applied for about 2 hours at room temperature. The solution was then acidified with 0.5 ml of 2N hydrochloric acid and extracted three times with 5 ml of dichloromethane. The combined organic phases were subsequently extracted with 0.5N hydrochloric acid and dried with anhydrous sodium sulfate. The titled compound was isolated after distillation of the solvent under reduced pressure and dried at 35-40 ° C in the form of a yellow solid with a purity greater than 97% by HPLC. EXAMPLE 2 An alternative method for the preparation of
1, 4, 4a, 5, 5a, 6, 11, 12a-octahydro-3, 10, 12, 12a-tetrahydroxy-1,1,1-dioxonaphthacene-2-carboxamide. 500 mg of 4-trimethylamine- 1, 4, 4a, 5, 5a, 6, 11, 12a-octahydro-3, 10, 12, 12a-tetrahydroxy-1, 11-dioxonaphthano-2-carboxamide were dissolved in a mixture that It contains water / acetic acid (120 ml: 180 ml) to which 7.5 g of potassium chloride were added. The reaction mixture was degassed and kept under a nitrogen atmosphere during the course of the reaction. After 30 minutes, a potential difference of 1.15 volts was applied-between the two stainless steel electrodes and the potential remained constant for approximately 6 hours. After the reaction was completed, 5 ml of 1N hydrochloric acid and 100 ml of dichloromethane were added. The phases were separated and the aqueous phase was extracted twice with 50 ml of dichloromethane. The organic phases were combined and extracted three times with 20 ml of 0.5N aqueous hydrochloric acid. The organic phase was dried with anhydrous sodium sulfate and evaporated under reduced pressure. The residual acetic acid was removed by co-distillation with cyclohexane. The yellow solid obtained from this dish was dried under vacuum between 30 and 35 ° C, producing the titled compound with a purity greater than 97% by HPLC. EXAMPLE 3 The preparation of 1, 4, 4a, 5, 5a, 6, 11, 12a-octahydro-3, 5, 10, 12, 12a-pentahydroxy-6a-methyl-1, 11-dioxonaphthacene-2-carboxamide. 10 g of doxycycline were dispersed in 100 ml of acetone. The reaction mixture was placed in a thermostatic bath at 30-33 ° C and 13 ml of methyl iodide was added. No change in color or temperature of the suspension was observed. After stirring for 48 hours at room temperature, total dissolution was observed and the reaction was completed after an additional 24 hours (conversion >)96% by HPLC). The solvent was distilled under atmospheric pressure and the residue was dissolved in methanol (35 ml) and precipitated with ethyl ether (700 ml). The yellow solid was filtered, washed with ethyl ether and dried at 35-40 ° C, yielding the trimethylammonium salt with a purity greater than 96% by HPLC. 200 mg of 4-trimethylammonium-1,4,4,5,5,6,11,16-octahydro-3, 5, 10, 12, 12a-pentahydroxy-6a-methyl-1, 11-dioxonaphthacene-2 was dissolved. -carboxamide iodide in a water / acetic acid mixture (48ml: 72ml) to which 3 g of potassium chloride were added. The reaction mixture was degassed and kept under a nitrogen atmosphere during the course of the reaction. After 30 minutes, a potential difference of 1.15 volts was applied between the two stainless steel electrodes and the power remained constant for approximately 6 hours. After the reaction was complete, 5 ml of 1N hydrochloric acid and 60 ml of dichloromethane were added. The phases were separated and the aqueous phase was extracted one times each with 60 ml of dichloromethane. The organic phases were combined and extracted three times with hydrochloric acid. diluted aqueous. The organic phase was dried with anhydrous sodium sulfate and evaporated under reduced pressure. The residual acetic acid was extracted with cyclohexane. The yellow solid obtained in this way was dried at room temperature under vacuum, yielding the titled compound with a purity greater than 98% by HPLC.