Process for the preparation of (N-phenylacetyl-N-2,6-xylil)methyl alaninate

Abstract: The present invention relates to a process for the preparation of (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate.

Description: The present invention relates to a process for the preparation of (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate. In particular, the so prepared compound is used in fungicidal compositions comprising said compound corresponding to (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate, in which more than 50% of said compound consists of the laevorotatory enantiomorph, and one or more known fungicides, said fungicidal compositions being used in the agricultural field for controlling fungine disease which seriously damage agricultural crops. Many of the compounds which are used for controlling phytopathogen fungi in agrarian practice have at least one asymmetrical centre. In particular, when these compounds have only one asymmetrical centre, or chiral centre, the two enantiomorphs can have a different fungicidal activity. More specifically, when only one of the enantiomorphs has a significant biological activity, it is possible for the same fungicidal effect to be obtained using the more efficient enantiomorph at a dosage which is half that of the raceme. Obviously when both enantiomorphs have a comparable biological activity, the reductions in the expected applicative dosages should in any case be less than 50% with respect to those of the raceme. Most fungicidal compounds containing at least one chiral centre are normally sold as a racemic mixture as it is difficult to compensate the higher cost relating to the production of the enantiomorph in its pure form, with additional commercial advantages. The necessity of improving the environmental impact has recently led to a re-evaluation of the use of single enantiomorphs to obtain at least partial reductions in the dosages used, thus diminishing the quantity of xenogene substances dispersed in the environment and improving the environmental impact of phytoiatric treatment. In addition, it is particularly advantageous if products with a fungicidal activity can be easily degraded by the vegetable host at the end of the period for which the fungicidal activity is required, so as to guarantee the minimum quantity of residual active principle. The reduction in residues of main principle in vegetables when they are picked, is in fact linked to potential risk for possible consumers of the agricultural product: the smaller the quantity of residual active principle, the lower the risk will be for the consumer. The compound (N-phenylacetyl-N-2,6-xylyl)-DL-methyl alaninate, also known under the trade-name of Benalaxyl, is particularly efficient in the control of diseases caused by Oomycetes. Oomycetes are responsible for many diseases of economically important crops such as, for example, grapes, potatoes, tomatoes and tabacco. This fungicide is described in U.S. patents 4.291.049 and 4.425.357. Benalaxyl has one asymmetrical centre and consists of an equimolecular mixture of the two enantiomorphs. When prepared according to the methods described in the two U.S. patents cited above, Benalaxyl is obtained as a racemic mixture in which the enantiomorphs are present in equimolecular quantities. "Pesticide Science" (1985), Vol. 16, pages 277-286, on the other hand also describes the preparation of the laevorotatory enantiomorph, corresponding to (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate and shows its greater activity with respect to both the dextrorotatory enantiomorph corresponding to (N-phenylacetyl-N-2,6-xylyl)-L-methyl alaninate and the raceme, corresponding to (N-phenylacetyl-N-2,6-xylyl)-DL-methyl alaninate. This greater activity has been observed by both experimenting the laevorotatory enantiomorph in tests in vitro, and by applying it on infected plants or on plants infested after its appication, and also by applying it to the earth or plant seed, to control the pathogenes present in the soil. Racemic Benalaxyl is degraded with a different rate depending on the type of vegetable and portion to which it is applied. For example, the degradation rate of racemic Benalaxyl in hops makes the use of this product unsuitable as an antifungal agent, to the detriment of its distinct fungicidal efficiency: in fact, after two weeks following treatment, the residual levels of active principle are still high. Together with its good preventive activity, Benalaxyl also has considerable curative efficiency: it is in fact capable of blocking infections already in development consequently allowing intervention after infection. In agrarian practice it is customary to intervene within 72 hours of a possible infection such as, for example, rainfall or abundant night-dew. Literature of the known art specifies that the MIC (minimum concentration of active principle necessary for inhibiting the development of disease) of the laevorotatory enantiomorph of Benalaxyl, when applied on the leaves of vines cultivated in a vase and infected 24 hours earlier with spores of Plasmopara viticola, is 5 mg/l, the MIC of the dextrorotatory enantiomorph is 100 mg/l and the MIC of the raceme is 10 mg/l. The laevorotatory enantiomorph of Benalaxyl, corresponding to (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate applied to leaves combined with one or more fungicides normally used for controlling phytopathogenic Oomycetes of economically important crops, has a higher synergetic activity than that possbily obtained with the racemic product used in a double dosage, consequently containing the same quantity of laevorotatory enantiomorph. This allows the production of more effective fungicidal compositions, having an improved environmental impact with respect to those obtained using racemic Benalaxyl. The above fungicidal compositions can be used in both curative and preventive applications, i.e. at regular intervals. In treatment at regular intervals, its curative property is still important to fight infections which have possibly arisen during the last period of the interval of treatment. A known process for preparing a compound corresponding to (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate having formula (I): wherein more than 50% of said compound having formula (I) consists of the laevorotatory enantiomorph, comprises the following steps: (a) reacting methyl ester having general formula (III): having an S-type asymmetrical carbon and wherein X represents a halogen atom such as chlorine, fluorine, bromine or iodine; or X represents an activated ester such as a paratoluenesulfonate, a mesilate or triflate; with an xylidine having formula (IV): in the presence of or without an inert organic solvent or a mixture of inert organic solvents, at a temperature ranging from 60°C to the boiling point of the solvent system selected, in the presence of or without an organic or inorganic base, obtaining N-xylyl-D-methyl alaninate having formula (V): (b) acylating the N-xylyl-D-methyl alaninate having formula (V) obtained in step (a), with a derivative of phenylacetic acid such as, for example, acyl chloride; or with a mixed anhydride obtained by reaction of the salt of phenylacetic acid with an acyl chloride such as, for example, pivaloyl chloride; or with an alkyl chloroformate such as, for example, isobutyl chloroformate; in the presence of an inert organic solvent or a mixture of inert organic solvents, at a temperature ranging from -30°C to +120°C, in the presence of or without an organic or inorganic base, obtaining the compound having formula (I). Examples of inert organic solvents which can be used in step (a) of the above process are: aromatic solvents such as, for example, toluene, xylene; protic solvents such as, for example, ethanol, propanol, butanol, octanol; dipolar aprotic solvents such as, for example, N,N-dimethyl-formamide, N-methylpyrrolidone, dimethylsulfoxide; or their mixtures. Examples of organic bases which can be used in step (a) of the above process are tertiary amines such as, for example, triethylamine. Examples of inorganic bases which can be used in step (a) of the above process are alkaline carbonates or hydrogencarbonates such as, for example, sodium hydrogencarbonate and potassium carbonate. The use of the above bases must, however, be carefully effected in order to avoid racemizations of the asymmetrical carbon. Examples of inert organic solvents which can be used in step (b) of the above process are: esters such as, for example, ethyl acetate; chlorinated solvents such as, for example, methylene chloride, dichloroethane; aromatic solvents such as, for example, toluene, xylene; hydrocarbons such as, for example, hexane, petroleum ether; or their mixtures. Examples of organic bases which can be used in step (b) of the above process are tertiary amines such as, for example, triethylamine, N-methylmorpholine; or heterocyclic amines such as, for example, pyridine. Examples of inorganic bases which can be used in step (b) of the above process are alkaline carbonates such as, for example, sodium carbonate. The Applicant has however surprisingly found, and this is therefore the object of the present invention, that step (b) of the above process is conveniently carried out in the presence of an aromatic solvent (for example, toluene, etc.), or a halogenated solvent (for example, dichloromethane, dichloroethane, etc.) or an ester solvent (for example, ethyl acetate), at a temperature ranging from -20°C to +40°C, preferably between -5°C and +25°C, in the presence of an inorganic base (for example, sodium bicarbonate, etc.) or an organic base (for example, triethylamine, pyridine, etc.). Operating under these conditions, products are obtained with a higher ratio D/S isomers than that obtained operating according to the known method described in "Pesticide Science (1985)", Vol. 16, pages 277-286, which consists in reacting N-xylyl-D-methyl alaninate having formula (V) with the chloride of phenylacetic acid in the presence of toluene, at a temperature of 80°C, without bases. In fact operating according to this method, products are obtained with a satisfactory ratio D/S isomers only after repeated crystallizations which cause considerable reductions in the yield. The methyl ester having general formula (III), can be conveniently prepared starting from aminoacid alanine, by diazotization of the amine group in the presence of a halide ion as described, for example, in "Methoden der Organischen Chemie - Band V/4 - Halogen Verbindungen" (1960), page 458, obtaining the corresponding halogenated acid having general formula (VI): wherein X has the same meaning described above, which is subsequently esterified to obtain methyl ester having general formula (III) according to any of the usual known methods in organic practice. When X, in the methyl ester having general formula (III), represents an activated ester, said methyl ester having general formula (III), is conveniently obtained from methyl lactate, a compound which is commercially available at a low cost, by reaction with a suitable derivative of methanesulfonic, paratoluenesulfonic, trifluoromethanesulfonic acids such as, for example, a chloride or an anhydride, in the presence of or without an organic base such as, for example, triethylamine, N-methylmorpholine, pyridine, or an inorganic base such as, for example, sodium bicarbonate. The xylidine having formual (IV) is a compound which is commercially available. Alternatively, the above process can be modified by carrying out the condensation of xylidine having formula (IV), described above, with an acid such as, for example, S-bromo propionic acid having formula (VII): to obtain N-aryl-D-aminoacid having formula (VIII): which is subsequently esterified with methanol (CH 3 OH) in an acid environment by the addition, for example, of hydrochloric acid or sulfuric acid, operating at a temperature ranging from 20°C to the boiling point of the solution, obtaining N-xylyl-D-methyl alaninate having formula (V) described above. Or, the N-arylaminoacid having formula (VIII) can be acylated with the chloride of phenylacetic acid, operating under the same conditions described in step (b) of the process described above for the acylation of N-xylyl-D-methyl alaninate having formula (V), or operating in an aqueous environment made basic with inorganic bases such as, for example, sodium bicarbonate or sodium hydroxide, possibly in the presence of an organic cosolvent such as, for example, methylene chloride, ethyl acetate, or tetrahydrofuran, operating at a temperature ranging from 0°C to 20°C, obtaining the acid having formula (IX): which is subsequently transformed into (N-phenylacetyl-N-2,6-xylyl)-methyl alaninate having formula (I), by esterification with methanol, in the presence of acids such as, for example, hydrochloric acid or sulfuric acid, operating at a temperature ranging from 20°C to the boiling point of the solution. S-bromo propionic acid having formula (VII) is a product which is commercially available. EXAMPLE 1 (A 1 ) preparation of N-xylyl-D-methyl alaninate. 13.5 g of 2,6-xylidine (Aldrich) are added to 16.7 g of methyl S-bromopropionate obtained by esterification of the corresponding Aldrich acid esterified with methanol under acid conditions, and the mixture is heated for 3 hours, at a temperature of 110°C. The raw material obtained is directly purified on silica gel, using a mixture of hexane and ethyl acetate in a ratio of 7/3 v/v, as eluant. 16.3 g of N-xylyl-D-methyl alaninate are obtained (yield 79%). (A 2 ) Preparation of N-xylyl-D-methyl alaninate. N-xylyl-D-methyl alaninate is also obtained by operating with an analogous process to that described above in (A 1 ) using methyl S-2-tosyloxypropionate or methyl S-mesyloxypropionate in substitution of methyl S-bromopropionate, obtaining 82% and 85% yield, respectively. (B 1 ) Preparation of (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate having formula (I) (Comparative example): A solution of 98 g of N-xylyl-D-methyl alaninate, obtained as described above and 91.4 g of phenylacetic acid chloride in 1200 cm 3 of toluene, is heated to reflux-temperature, for 2 hours, in the presence of 2.5 cm 3 of dimethylformamide. After cooling, the solution obtained is washed with aqueous bicarbonate and the organic phase is evaporated at reduced pressure after drying with sodium sulfate. The raw material thus obtained (158 g) is crystallized with abundant hexane obtaining 114 g of a white crystalline solid corresponding to (N-phenylacetyl-N-2,6-xylyl)methyl alaninate with an enantiomeric D/S ratio = 80/20. This white solid is recrystallized three times with a mixture of hexane/ethyl acetate in a ratio 95/5 obtaining decreasing quantities of (N-phenylacetyl-N-2, 6-xylyl) methyl alaninate progressively enriched with the desired D isomer: 1st crystallization: 90.0 g (yield of 60%), D/S ratio = 88/12; 2nd crystallization : 61.4 g (yield of 40%), D/S ratio = 94/6; 3rd crystallization: 25.8 g (yield of 16.8%), D/S ratio = 98.5/1.5. The above enantiomeric ratios are determined with a CHIRALCEL OD chiral column (10 µm - 4.6 x 250 mm), eluating with a mixture of hexane/isopropyl alcohol in a ratio 7/3, at a flow-rate of 0.5 ml/minute. (B 2 ) Preparation of (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate having formula ( I). 42 g of sodium bicarbonate are added to a solution of 100 g of N-xylyl-D-methyl alaninate in 500 cm 3 of toluene, cooled to a temperature ranging from 5°C to 10°C, and subsequently 75 g of phenylacetic acid chloride are slowly added dropwise. After 4 hours at room temperature, the above solution is washed with water and then evaporated at reduced pressure after drying with sodium sulfate. The raw material thus obtained is crystallized with abundant hexane obtaining 106 g of a white crystalline solid corresponding to (N-phenylacetyl-N-2,6-xylyl)-methyl alaninate with an enantiomeric D/S ratio = 98/2 (yield 92.5%). (B 3 ) Preparation of (N-phenylacetyl-N-2,6-xylyl-D-methyl alaninate having formula (I) (Comparative example): 15.5 g of phenylacetic acid chloride are added to a solution of 20.7 g of N-xylyl-D-methyl alaninate, obtained as described above, in 100 cm 3 of toluene. After 4 hours at reflux temperature, the reaction is cooled, evaporated at reduced pressure and the raw material obtained is purified on silica gel, using a mixture of hexane and ethyl acetate in a ratio 7/3 v/v, as eluant. 28.9 g of (N-phenylacetyl-N-2,6-xylyl-D-methyl alaninate having formula ( I) are obtained, with an ([α] D rotatory optical power of -27.3° (c = 1, acetone) (yield 89%). EXAMPLE 2 Analogously to what is described in Example 1, but starting from methyl R-bromopropionate, (N-phenylacetyl-N-2,6-xylyl-D-methyl alaninate having formula ( I), is obtained, in two passages, with an ([α] D rotatory optical power of + 27.4° (c = 1, acetone) (yield 89%).

Claims: A process for preparing a compound corresponding to (N-phenylacetyl-N-2,6-xylyl)-D-methyl alaninate having formula (I): wherein more than 50% of said compound having formula (I) consists of the laevorotatory enantiomorph; said process comprising the following steps: (a) reacting methyl ester having general formula (III): having an S-type asymmetrical carbon and wherein X represents a halogen atom; or X represents an activated ester such as a paratoluenesulfonate, a mesilate or triflate; with an xylidine having formula (IV): in the presence of or without an inert organic solvent or a mixture of inert organic solvents, at a temperature ranging from 60°C to the boiling point of the solvent system selected, in the presence of or without an organic or inorganic base, obtaining N-xylyl-D-methyl alaninate having formula (V): (b) acylating the N-xylyl-D-methyl alaninate having formula (V) obtained in step (a), with a derivative of phenylacetic acid; or with a mixed anhydride obtained by reaction of the salt of phenylacetic acid with an acyl chloride; or with an alkyl chlorofcrmate; in the presence of an aromatic solvent or a halogenated solvent, or an ester solvent, at a temperature ranging from -20°C to + 40°C, in the presence of an organic or inorganic base. The process according to claim 1, wherein in step (b) the aromatic solvent is toluene. The process according to claim 1, wherein in step (b) the halogenated solvent is dichloromethane or dichloroethane. The process according to claim 1 , wherein in step (b) the ester solvent is ethyl acetate. The process according to claim 1 , wherein in step (b) the inorganic base is sodium bicarbonate. The process according to claim 1 , wherein in step (b) the organic base is triethylamine or pyridine. The process according to any of the previous claims from 1 to 6 wherein in step (b) the temperature is between -5°C and + 25°C.

Inventors: COLOMBO LAURA, FILIPPINI LUCIO, MIRENNA LUIGI, PALLA OTTORINO, ZANARDI GIAMPAOLO, ZINI GUIDO, COLOMBO, LAURA, FILIPPINI, LUCIO, MIRENNA, LUIGI, PALLA, OTTORINO, ZANARDI, GIAMPAOLO, ZINI, GUIDO
Applicants: ISAGRO SPA, ISAGRO S.P.A.
Application year: 1997
Main classification: A01N37
IPC codes: C07C227, C07C233, C07C231, C07C229