METHOD FOR THE PREPARATION OF ALKYL ESTERS OF 3,4-EPOXYBUTYRIC ACID

Abstract: Method for the preparation of an alkyl ester of 3,4-­ epoxybutyric acid, of formula: R being a linear or branched alkyl group, containing from 4 to 10 C atoms, wherein the corresponding vinylacetate is cata­ lytically oxidized in a biphasic liquid system comprising: a) an aqueous phase, containing H₂O₂ as oxidizing agent; b) an organic phase, containing said vinylacetate and a catalyst selected from peroxidic compounds of formula: (R₁ R₂ R₃ R₄ N)₃ PW₄O₂₄ as well as, optionally, a solvent immiscible with the aqueous phase.

Description: The present invention relates to a method for the preparation of alkyl esters of 3,4-epoxybutyric acid of formula: wherein R is a linear or branched alkyl group containing from 4 to 10 C atoms, starting from the corresponding vinyl-acetates of formula: werein R has the meaning given above. Alkyl esters of 3,4-epoxybutyric acid (I) are an interesting class of compounds, endowed with peculiar features, which can be used as monomers and co-monomers in several kinds of polymerizations and co-polymerizations; furthermore they are important intermediates for sophisticated products endowed with biological and/or pharmaceutical activity. Starting from alkyl esters of 3,4-epoxybutyric acid there may be obtained functional adenine derivatives, which are very useful in affinity chromatography or as macromolecular co-enzymes, not diffusable and water-soluble, these derivatives being obtained by allowing esters of epoxybutyric acid to react with adenine or derivatives thereof. Starting from the al kyl esters of 3,4-epoxybutyric acid it is possible to obtain the chloride of (R,S)-carnitine (and other analogous derivatives of carnitine). The importance of the R- (-) form thereof in the transfer of fatty acids through membranes is well known; carnitine catalyzes the oxidation of long-chain fatty acids and takes part in the transfer, by means of enzymes, of fatty acids activated by cytoplasm towards the β-oxidation sites within mitochondrias. It is furthermore known to use the alkyl esters of 3,4-epoxy-butyric acid for the synthesis of derivatives of 2-oxo-1-pyrrolidine-acetamide, as described in EP-A-154 490; said derivatives are valuable psychotropic drugs which can restore the activity of cognitive processes impaired by different pathologic conditions. The preparation of alkyl esters of 3,4-epoxybutyric acids, however,was hitherto been carried out with not very satisfactory results, with clearly negative influences at the industrial-economic level; these influences partially or totally make void the advantageous features of alkyl esters of 3,4-epoxybutyric acid. It is known to prepare esters of 3,4-epoxybutyric acid by oxidation of the corresponding vinylacetetes with peracids (like, for instance, peracetic or m-chloroperbenzoic acid in organic solvents; however, yields and long reaction times (4-10 days) ` as well as the cost of the oxidizing agents, which have to be used in at least stoichiometric quantity with respect of the substrate make this process of limited practical interest. There also is reported a synthesis of the methyl ester of 3,4-epoxybutyric acid starting from epichlorohydrin, by carbomethoxylation with carbon mono-oxide and methanol (in the presence of cobalt cabonyl), formation of 4-chloro-3-hydroxybutyric acid methyl ester and subsequent epoxide closure with silver oxide; however, this process is of no practical interest at the industrial level, because of drastic conditions, low yields and the high cost of some reactants. EP-A-109273 describes peroxide compositions of formula QXW 4 O 24-2n . Wherein Q represents a cation of an onium salt, X is P or As and n is 0, 1 or 2. Said compositions are reported to be suitable particularly as Catalysts for the epoxidation of olefinic compounds. The compounds actually oxidized in the presence of said peroxide compositions, however, are e.g. olefins, allylchloride and α, β-unsaturated carboxylic acids. There has now been found a simple and cheap method for the synthesis of alkyl esters of 3,4-epoxybutyric acid, free of the above mentioned drawbacks; this method provides products which, because of their purity level and low price (also resulting from the very high yields of the synthesis),are fairly good intermediates for the preparation of many chemical compounds endowed with biological and/or pharmaceutical activity. In its broadest form the present invention relates to a method for the preparation of alkyl esters of 3,4-epoxybutyric acid of formula (I), wherein the corresponding vinylacetate of formula-(II) is catalytically oxidized, under phase transfer conditions, in a-biphasic liquid system comprising: a) an aqueous phase, containing hydrogen peroxide; b) an organic phase, containing said vinylacetate and a catalyst selected from the peroxidic compounds of formula: (R₁ R₂ R₃ R₄ N)₃ PW₄O₂₄ as well as, optionally, a solvent immiscible with the aqueous phase, R₁, R₂, R₃ and R₄, the same or different from each other, being hydrocarbon groups, the total number of the carbon atoms of R₁, R₂, R₃ and R₄ ranging from 20 to 70 and preferably from 25 to 40. The oxidation reaction can be represented as follows: The epoxide is obtained within comparatively short periods of time (1 - 3 hrs), with high yields based on H₂O₂ (80-85%) and with a fairly good selectivity with respect to the vinylacetate (about 90%), with clear positive influences on the possible industrial application. When the alkyl group R contains 4 to 10 carbon atoms the esters are obtained with better yields, R contains from 1 to 3 C atoms. The best results were obtained when using a particular catalyst of formula: Catalysts (III) and (IV) consist of peroxidic compounds containing tungsten, P and a quaternary ammonium cation showing sufficient affinity for lipids, obtained according to usual techniques. These catalysts, for instance, can be prepared by reacting a tungstic acid (or an alkaline tungstate), phosphoric acid (or an alkaline phosphate) and hydrogen peroxide in an acidic aqueous phase with a quaternary salt selected from methyltrioctylammonium chloride (known on the market as ALIQUAT® 336) and dimethyl [dioctadecyl (75%) + dihexadecyl (25%)] ammonium chloride (known on the market as ARQUAD® 2HT), contained in an organic phase immiscible with the aqueous phase. The reaction of the inorganic reactants can be carried out between 20° and 80°C; thereafter, the quaternary salt (dis solved in a solvent, for instance 1,2-dichloroethane, is added, preferably at room temperature, while continuing the stirring of the biphasic mixture for 15-30 minutes. The acidic phase had a pH lower than 2; in order to obtain said range of values, the pH is adjusted, if necessary, with a mineral acid (for instance H₂SO₄ or HCl). The molar ratios of the reactants must denerally be as follows: for 1 mole of P, 4 moles of W and up to 2 moles of quaternary salt are used; as to H₂O₂, from 2.5 to 6 moles of H₂O₂ are sufficient for 1 mole of W. After having separated the phases, by evaporating the organic phase, the compound (III) or (IV) is obtained in syrupy or solid form respectively. The epoxidation reaction is carried out according to the double phase technique; the organic phase contains the vinylacetate, the catalyst and, optionally, a solvent immiscible with the aqueous phase. If a solvent is not used, a suitable excess of vinylacetate can be used. Chlorinated hydrocarbons (like, for instance, dichloroethanes, trichloroethane and tetrachloroethylene) or aromatic, optionally substituted, hydrocarbons (e.g. benzene, toluene or xylenes) can be used as immiscible solvents. Generally, it is possible to work under strong stirring, at a temperature between 50° and 90°C, preferably between 65° and 75°C, under atmospheric pressure, and the reaction time (depending on the used catalyst and its amount, the working temperature and the concentration of the vinylacetate in the organic phase) generally between 1 and 3 hours; the catalyst is preferably used in a H₂O₂: catalyst molar ratio between 200 and 100. Finally, it is advisable to work with a H₂O₂: substrate molar ratio between 1:1 and 1:2 and preferably from 1:1.5 to 1:1.6 when a solvent is used; the concentration of substrate (II) in the organic phase generally is between 30 and 90%, and preferably between 50 and 80% by weight. It is advisable to use, in the aqueous phase,a H₂O₂ concentration between 1 and 70%, and preferably between 10 and 30% by weight; thereby H₂O₂ conversions of 98-99% may be achieved. At the end of the reaction, after having separated the phases, the product (present in the organic phase), may be separated from the catalyst (together with unreacted vinylacetate (II) and with the solvent) by a quick pre-distillation under vacuum; the distillate then is fractionated under vacuum, thus obtaining product (I) in pure form; obviously, these are usual methods, allowing possible alternatives. The method according to the invention can be performed employing usual apparatus and expedients. The catalyst is sufficiently stable and, therefore, it can be prepared and stored until it is used. The invention will be further described in the following examples, given to merely illustrate the present invention, without limiting the scope thereof. The concentrations of hydrogen peroxide and phosphoric acid are expressed in the examples as grams per 100ml of water. EXAMPLE 1 a) Preparation of the catalyst (C₃₇H₇₈N)₃PW₄O₂₄ Into a 100ml, four necked flask provided with blade stirrer and dropping funnel, there were introduced, at room temperature, 3.30 g of Na₂WO₄.2H₂O, dissolved in 20ml of water, 1.5ml of 40% H₃PO₄ and 3ml of 30% by weight H₂SO₄. Thereafter, 2ml of 40% H₂O₂ were added. 40ml of 1,2-dichloroethane containing 3.10 g of ARQUAD® 2HT were added, under stirring, over 2-3 minutes; ARQUAD® 2HT contains 75% of active substance. Stirring was continued for 20 minutes at room temperature and the phases were then decanted and separated. The organic (lower) phase was evaporated, 3.6 g of a white solid being thus obtained; the analytical data were in agreement with the indicated formula, as pointed out hereinafter: ACTIVE OXYGEN: found = 4.60% (determined by adding a known excess of Na₂SO₃ in basic medium and by back-titrating iodometrically in acidic medium) theoretical = 4.63% (on the basis of 8 active oxygen atoms). The aqueous phase containing the excess of reactants was recycled into the flask and used again for another preparation, upon the necessary additions (about 1.80 g of Na₂WO₄. 2 H₂O dissolved in a small amount of water and neutralized with about 1.8ml of 30% b.w. H₂SO₄, 0.35ml of 40% H₃PO₄ and about 1ml of 40% H₂O₂). b) Preparation of 3,4-epoxybutyric acid isobutyl ester. Into a,250ml, four-necked flask provided with blade stirrer, reflux cooler and thermometer, there were introduced, at room temperature, 17ml H₂O, 17.04ml of 40% H₂O₂ (about 200 mmols), 2.8 g of the catalyst prepared according to example 1a) (about 1 mmol), dissolved in 30ml of dichloroethane and 42.6 g (about 300 mmols) of isobutyl vinylacetate. Under vigorous stirring, the biphasic mixture was heated to 70°C and kept at this temperature for 2.5 hours; a conversion yield based on H₂O₄)>99% was obtained (determination by iodometric titration of the aqueous phase). The organic (lower) phase was separated, filtered and, after having evaporated the solvent, quickly pre-distilled under vacuum (6.65-26.6 hPa 5-20 mmHg), what left a residue (4.7 g) containing the catalyst and the heavy products (about 1.9 g). The distilled product was analyzed gas-chromatographically, using a glass column (1.5 m x 2 mmID) packed with 5% Carbowax® 20M on Chromosorb® W "HP" (60-80 mesh) at 75-200°C (10°C/1' ; internal standard: n-hexanol) and was found to contain 26.8 g of epoxide (169.6 mmols) and 16 g of unreacted vinylacetate (112.7 mmols), the epoxide yield (calculated on the charged H₂O₂) being thus 84.8% and the epoxide selectivity (calculated on the consumed vinylacetate) being 90.5%. Thereafter the epoxide was separated from vinylacetate by fractional distillation under vacuum (boiling point = 116-118°C at 53hPa (40 mmHg). IR and ¹H -NMR spectra of the product were in agreement with the formula, (I) of the epoxide, wherein R was: EXAMPLE 2 a), Preparation of the catalyst (C₂₅H₅₄N)₃PW₄O₂₄ Example 1 a)was repeated, replacing ARQUAD® 2HT by 1.6 g of methyltrioctylammonium chloride (ALIQUAT® 336), thereby obtaining 2.82 g of a syrup; the analytical data were in agreement with the indicated formula, as shown hereinafter: ACTIVE OXYGEN: found = 5.67% (determined by adding a known excess of Na₂SO₃ in basic medium and by back-titrating iodometrically in acidic medium) theoretical = 5.68% (on the basis of 8 active oxygen atoms). b) Preparation of 3,4-epoxybutyric acid isobutyl ester Example 1 b)was repeated, using the catalyst prepared according to example 2 a) (2.26 g, about 1 mmol), thereby obtaining analogous results [26.86 g (170 mmols) of epoxide; 85% yield (on the charged H₂O₂)]. EXAMPLE 3 (Comparative) 10ml of H₂O, 6.56ml of 40% H₂O₂ (77 millimoles), 1.3 g (0.57 millimoles) of the catalyst prepared according to example 2 a), dissolved in 20ml of benzene, and 20 g (200 millimoles) of the methyl ester of vinylacetate were placed in a 100ml flask provided with blade stirrer, reflux cooler and thermometer. The mixture was heated to 70°C under vigo rous stirring and kept at said temperature for 2.5 hours ,the organic phase being then treated as in example 1. Obtained were 4.52 g of epoxide (39 millimoles), corresponding to an epoxide yield (calculated on the charged H₂O₂) of 50.3%.

Claims: Method for the preparation of alkyl esters of 3,4-epoxy-butyric acid of formula: wherein R is a linear or branchend alkyl group containing from 4 to 10 C atoms, wherein the corresponding vinylacetate of formula: CH₂=CH-CH₂-COOR is catalytically oxidized under phase transfer conditions in a biphasic liquid system comprising: a) an aqueous phase, containing hydrogen peroxide; b) an organic phase, containing said vinylacetate and a catalyst selected from the peroxidic compounds of formula: (R₁R₂R₃R₄N)₃PW₄O₂₄ as well as, optionally, a solvent immiscible with the aqueous phase, R₁, R₂, R₃ and R₄ are equal to each other or different, being hydrocarbon groups altogether having a number of carbon atoms ranging from 20 to 70 and preferably from 25 to 40. Method according to claim 1, wherein the catalyst is selected from the peroxidic compounds of the formulae: (C₂₅H₅₄N)₃PW₄O₂₄ and (C₃₇H₇₈N)₃PW₄O₂₄. Method according to one or both of the claims 1-2, wherein said ester is the isobutyl ester of 3,4-epoxybutyric acid. Method according to one or more of the claims 1-3, wherein the oxidation temperature is from 50 to 90°C, the H₂O₂: vinylacetate molar ratio is from 1:1 to 1:2 and the H₂O₂: catalyst molar ratio is between 200 and 100. Method according to one or more of the claims 1-4, wherein the H₂O₂ concentration in the aqueous phase is between 1 % and 70 % and preferably between 10 % and 30 % by weight. Method according to one or more of the claims 1-5, wherein the concentration of the vinylacetate in the organic phase, when the operation is carried out in the presence of a solvent immiscible with water, is from 30 % to 90 % and preferably from 50 to 80 % by weight. Method according to one or more of the claims 1-6, wherein said solvent is selected from the group consisting of chlorinated aliphatic hydrocarbons and aromatic, optionally substituted, hydrocarbons. Method according to one or more of the claims 1-5, wherein the vinylacetate to be oxidized is also used as the solvent for the reaction.

Inventors: COASSOLO, ALFREDO, D'ALOISIO, RINO, VENTURELLO, CARLO, DR.
Applicants: TECNOPART S.R.L.
Application year: 1986
Main classification: C07D303
IPC codes: C07D301, B01J31, C07D303, C07B61