PROCESS FOR PREPARING VINCAMINE AND RELATED ALKALOIDS

Abstract: (-)-Vincadifformine and related compounds are con­ verted to (+)-vincamine, 16-epivincamine, apovincamine and related alkaloids by ozonization in an acid medium, followed by treatment with base.

Description: PROCESS FOR PREPARING VINCAMINE AND RELATED ALKALOIDS The invention relates to the conversion, with high yields and by a simple route, of (-)vincadifformine and related compounds into (+)vincamine, 16-epivincamine, apovincamine and alkaloids of related structure. Processes for the conversion of (-)vincadifformine (a product obtainable by reducing tabersonine) into vincamine and correlated alkaloids are already known. For example, Belgian Patent Specification No. 761.628 of Omnium Chimique describes the conversion of tabersonine into vincamine by treating the reduction product of tabersonine, i.e. (-)vincadifformine, with per-acids as oxidizing agents, with subsequent treatment with reducing agents and acids. A subsequent patent achieves the same conversion using oxygen in the presence of chromium, cobalt and copper salts. Other processes (Parcor, French Patent Specification No. 2358412, Buskine, Netherlands Patent Application No. 7509118) employ oxygen as oxidizing agent acting on the anion of the vincadifformine. A further process (Boehringer, Belgian Patent No. 870978) carries out the same conversion by utilizing hydrogen peroxide in the presence of soluble salts of the hydrated oxides of vanadium, chromium, molybdenum and tungsten. The results which can be achieved with all these processes are, in one way or another, unsatisfactory. In some cases, the yields obtained in practice are distinctly lower than those reported in the Examples of the Patent Specifications, or the products are difficult to purify. Also, some processes involve environmental disadvantages due to the presence of metallic salts (copper, chromium, etc.) in the effluents; and finally, reagents of very high cost sometimes need to be employed. The present invention provides a very simple and economic process which enables vincamine (and alkaloids of related structure) to be obtained in high yields and in a state of remarkable purity. According to the present invention, there is provided a process for converting (-)vincadifformine or a compound of related structure to (+)rincamine, 16-epivincamine, apovincamine or an alkaloid of related structure characterised in that a solution of (-)-vincadifformine or of a compound of related structure in an acid reaction medium is contacted with ozone, the resulting product is treated with base and the alkaloid(s) so-obtained are isolated. The term "products of the related structure to (-)-vincadifformine" as used herein refers to compounds containing the same nucleus as (-)vincadifformine, i.e. a nucleus of the structure: EMI2.1 The nucleus may be substituted by one or more groups which do not interfere with the ozonization reaction e.g. CH3 0 groups; and the -CO- group may form part of an ester group other than a methyl ester, or a free -COOH group. Similarly, various isomeric forms may be used, e.g. (+)vincadifformine and also salts of the free alkaloid. Thus, for example, the starting material may have the formula EMI2.2 in which R represents hydrogen or methoxy, whereupon ozonization of the reaction mixture, and the subsequent alkalization, leads to products of the following structural formulae: EMI3.1 in which R has the above-indicated significance. The compounds obtained can be separated from one another by means of chromatography or by equivalent methods. If the process is performed starting from (-)-vincadifformine (I, with R = H), there will be obtained in practice a mixture of vincamine, 16-epivincamine and apovincamine (II, III and IV, respectively, with R = H), with a distinct preponderance of vincamine. Where R in the starting material is CH3 0 as in ll-methoxy-(-)-vincadifformine (V) EMI3.2 the major product of the process of the invention will be 11methoxy -(+)-vincamine (II, with R= CH30-) otherwise known as 3 vincine. The process of the invention may be carried out with or without isolation of intermediate products formed as a result of the ozonization. Thus for example 16-hydroxyvincadifformine (or products of related structure) may be isolated as an inter mediate product, particularly where the ozonization is carried out at relatively low temperatures, e.g. around OOC. This intermediate product may then be converted to the desired final product by treatment with an acid, particularly by heating in the presence of an acid. In general, the contacting with ozone can be carried out by dissolving the (-)vincadifformine (or related compounds) in dilute mineral acids, or in carboxylic acids compatible with the ozonization process, or in alcohols such as methanol, and causing the mixture to absorb the amount of ozone necessary for completion of the conversion. The ozonization may take place at temperatures preferably between OOC and +70 0C when the operation is performed in aqueous medium, or between r70 and +70 0C when the operation is performed in organic medium. The reaction times depend on the temperature and on the rate of introduction and of absorption of O3. After complete conversion of the (-)-vincadifformine (or related compounds), the reaction mixture is brought to, or maintained at, a temperature between 300C and 700C for, for example, between 10 minutes and 1 hour. Following this, treatment of the resulting solution with a base and subsequent extraction with solvents immiscible with water, such as for example, ethers, alcohols or chlorinated hydrocarbons, a product mixture is obtained, from which the individual components may be obtained in a pure state by, for example, chromatographic methods or fractional crystallization. By using the process of the invention and operating under appropriate conditions, for example in a methanol solution and at temperatures between -30 and 1OOC, direct crystallization of pure (+)-vincamine, in very high yields, is obtained, after alkalization and concentration, by simple cooling. Compared to the prior art, this constitutes a significant technical advance. A further advantage of the process of the invention is that it enables, in practice, the solutions of (-)-vincadifformine which are obtained by hydrogenation of tabersonine to be subjected directly to the treatment with ozone without isolation of intermediates. The following Examples serve to illustrate the invention. EXAMPLE 1 2 grams (5.91 mmoles) of (-)-vincadifformine are dissolved in 143 ml of a 0.65N solution of sulphuric acid and 0 treated with 6.5 mmoles of ozone, at 65-70 C, for a period of 20 minutes. The red solution is then maintained at this temperature for another 20 minutes. The reaction mixture is cooled to room temperature-and made alkaline with ammonia to pH 8.5. The partially precipitated alkaloids are extracted three times with 50 ml of methylene chloride. The combined organic extracts are cdncentrated to dryness and the residue is taken up in 35 ml of methanol. The solution is left over night in a refrigerator and 885 mg of (+)vincamine are obtained (yield 42.1%). The mother liquors of crystallization are concentrated to dryness and chromatographed on silica gel, the alkaloids being eluted with a mixture of methylene chloride and methanol with a varying gradient of concentrations. The elution is initiated with pure methylene chloride. The first fraction contains 50 mg of apovincamine (yield 2.5%) and a subsequent fraction contain (+)-vincamine (150 mg) (yield 7.2%). There are also obtained, in a subsequent fraction, 430 mg of 16-epivincamine (yield 20J67.). The UV, IR, NMR and MS spectra of the products are identical to those of authentic samples and the same is true for the normal physicochemical tests. EXAMPLE 2 1.4 grams of (-)-vincadifformine are dissolved in 100 ml of 1N acetic acid and 1N sodium acetate (pH 3.75) and 0 are ozonized at a temperature of 25 C until the starting alkaloid had completely disappeared. The reaction mixture is heated for 1 hour at 700C, subsequently treated with ammonia to bring the pH to 8.5 and extracted with methylene chloride. 1.4 g of a mixture of vincamine and 16-epivincamine are obtained and by crystallization of the residue, 0.72 g of pure vincamine is obtained. By a chromatographic procedure, 0.02 g of vincamineand 0.3 g of 16-epivincamine are obtained. EXAMPLE 3 1.7 grams of (-)-vincadifformine are dissolved in 100 ml of iN acetic acid and 1N sodium acetate at pH 3.75 and are ozonized at a temperature of OOC until the starting alkaloid had completely disappeared. At the end of the reaction, the reaction mixture is made alkaline with NaOH to pH 8.5 and extracted three times using 50 ml of methylene chloride each time. On evaporation to dryness under vacuum, the organic phase yields an amorphous product which, after chromatography on silica gel and elution with benzene and triethylamine in a ratio of 85:5, yields 1.07 g of 16-hydroxyvincadifformine. The structure of the compound is confirmed by the IR, NMR, MS and W data. EXAMPLE 4 1.05 grams of 16-hydroxyvincadifformine are dissolved in 50 ml of methanol and 1.5 ml of 967. sulphuric acid are added; the solution is heated for 20 minutes with weak reflux. On disappearance of the 16-hydroxyvincadifformine-and after neutralization of the solution with ammonia, the reaction mixture is diluted with 50 ml of H20 and extracted three times with 50 ml of methylene chloride at a time. After evaporation of the solvent to dryness, the residue is crystallized from methanol and after crystallization and chromatography of the mother liquors, 0.720 g of vincamine and 0.3 g of 16-epivincamine are obtained. EXAMPLE 5 20 grams (59.1 mmoles) of (-)-vincadifformine are dissolved in 100 ml of methanol to which 7.092 g (118.2 mmoles) of glacial acetic acid are added. At a temperature maintained at -200C, the solution is caused to absorb an equimolar amount of 03 in the course of about 3 hours. The solution is then brought to room temperature and after about half an hour it is brought to pH 7 with concentrated NH40H.. 40 ml of methanol are eliminated by distillation and 0 then the residue is cooled to O C. Following filtration using a pump, 15.17 g (71%) of pure (+)-vincamine are directly obtained. Smaller amounts of (+)-vincamine and, moreover, apovincamine and 16-epivincamine are obtained by treatment of the mother liquors in the manner set forth in the preceding Examples. Similar results as regards yield and purity of the product are obtained by replacing the acetic acid by a corresponding amount of sulphuric acid, or by operating directly on methanol solutions of (-)-vincadifformine obtained by hydrogenation of tabersonine. It will be appreciated that in the above Examples compounds of related structure may be used instead of (-)-vincadifformine as starting material to produce corresponding alkaloids of related structure to (+)-vincamine, 16-epivincamine or apovincamine. Such alkaloids will in general contain a nucleus of the structure EMI7.1

Claims: CLAIMS: 1. A process for converting (-)-vincadifformine or a compound of related structure to -(+)-vincamine, 16-epivincamine, apovincamine or an alkaloid of related structure characterised in that a solution of (-)-vincadifformine or of a compound of related structure in an acid reaction medium is contacted with ozone, the resulting product is treated with base and the alkaloid(s) soobtained are isolated. 2. A process according to Claim 1 wherein the starting material is (-)-vincadifformine and the products include one or more of (+)-vincamine, 16-epivincamine and apovincamine. 3. A process according to Claim 1 wherein the starting material is 15-methoxy-(-)-vincadifformine and the products include ll-methoxy-(+)-vincamine (vincine). 4. A process according to any preceding claim wherein the oxonization is effected in an organic solvent at between about -70 and +700C. 5 A process according to Claim 4 in which the organic solvent comprises one or more carboxylic acid compatible with ozone. 6. A process according to Claim 4 in which the organic solvent comprises methanol. 7. A process according to any one of Claims. 1 to 3 in which the ozonization is effected in an aqueous medium at between about 0 and +700C. 8. A process according to any preceding claim in which 16-hydroYincadifformine or a compound of related structure is isolated as an intermediate product and subsequently converted to to the dcsired final product by heating in the presence of an acid. 9 A process-according to any preceding claim wherein the starting material comprises an acidified solution of (-)-vincadifformine obtained by hydrogenation of tabersonine and sai solution is subjected directly to ozonization.

Inventors: BOMBARDELLI, EZIO, GABETTA, BRUNO
Applicants: INVERNI DELLA BEFFA S P A
Application year: 1981
Main classification: C07D461
IPC codes: C07D471, C07D461