CS261262B1 - Process for the preparation of hydrochlorides of lower alkyl esters of α-L-aspartyl-L-phenylalanine - Google Patents

Abstract

The solution relates to a method for producing hydrochlorides of lower alkyl esters of alpha-L-aspartyl-L-phenylalanine, in which R represents a CH3 or C2H5 group. The said compounds are distinguished by an intensive sweet taste and can be used, especially in the form of methyl ester, as advantageous non-caloric sweeteners. The said hydrochlorides are prepared in high yields and purity from the corresponding alkyl esters of N-butyloxycarbonyl-alpha-L-aspartyl-L-phenylalanine by the action of an aqueous solution of hydrogen chloride in the medium of a lower carboxylic acid with 1 to 3 carbon atoms or in ethyl acetate, which generates acetic acid in the reaction medium, at a temperature of 5 to 40 °C, while the desired lower alkyl ester is isolated in the form of an addition salt with hydrochloric acid. Compared to known processes, the process is faster, more economical and more advantageous.

Description

The present invention relates to a process for the preparation of the lower alkyl esters of alpha-L-aspartyl-L-phenylalanine of the general formula I, h ₂ n

CH-CO-NH-CH-COOR ch ₂ -coohch ₂

HCl (I) wherein R is CH 3 or CH 3. These compounds excel in an intense sweet taste [Mazur R.H. Schlattér J. Μ., Goldkamp A.H .: J. Amer. Chem. Soc. 91, 2684 (1969)] and can. in particular the methyl ester of formula I, wherein R is CH3, as preferred non-caloric sweeteners.

In the years following the discovery of the sweetening properties of said dipeptide methyl ester, a number of methods for its preparation have been developed and published to enable the production of this dipeptide methyl ester on an industrial scale. The methods differ from each other in the choice of protecting groups for the amino or beta-carboxylic function in the L-aspartic acid component and the methods by which the two amino acid components are combined into a protected dipeptide from which the protecting group is finally cleaved under appropriate conditions.

An important requirement for the choice of the amino-protecting group of L-aspartic acid is its ease of introduction and the possibility of simple, gentle, rapid and complete removal from the synthesized N-protected alkyl esters of L-aspartyl-L-phenylalanine. The rate of deprotection of the amine function of these dipeptide alkyl esters is of considerable importance in preventing subsequent reactions such as transestirification and hydrolysis processes, increased formation of L-aspartyl-L-phenylalanyl-dioxopiperazine, etc., which otherwise lead to undesirable contamination of the final product.

One of the most preferred protecting groups is the tert-butyloxycarbonyl group, referred to in the literature as the abbreviation BOC, whose advantages for the temporary protection of amino functions in amino acids and peptides are generally well known. It was used by Yasuda N., Yamatani T., Ariyoshi Y. and Sato N. for the synthesis of the esters of N-BOC-alpha-L-aspartyl-L-phenylalanine and N-BOC-beta-L-aspartyl-L-phenylalanine. U.S. Patent 1,339,101 (1973)].

Numerous methods have been developed to remove the tert-butyloxycarbonyl group from N-BOC-amino acids and N-BOC-peptides and derivatives thereof. Anhydrous trifluoroacetic acid is commonly used in peptide chemistry [Schwyzer R. et al., Angew.

Chem. 72, 915 (1960)], solutions of dry hydrogen bromide or hydrogen chloride in glacial acetic acid [McKay, R. C., Albertson, N. F., J. Amer. Chem. Soc. 79, 4,686 (1957)], in diethyl ether [Guttman S., Boissonnas R. A., Helv. Chim. Acta 41, 1852 (1958)], in 1,4-dioxane [Guttmann S .: Helv. Chim. Acta 44, 721 (1961)] or in ethyl acetate [Schwyzer R. et al., Helv. Chim. Acta 42,1702 (1959)]. In special cases, p-toluenesulfonic acid was also used in diethyl ether [Goodracre J., Ponsdorf R. J., Stirling J .: Tetrahedron Lett. 1975-3609J or other acidolytic agents.

We have now found that the stated anhydrity of the reaction medium and reagents is not always a necessary condition for the reaction to proceed successfully and that the N-BOC group can be cleaved from the protected alpha-L-aspartyl-L-phenylalanine lower esters with an aqueous solution hydrochloric acid. At the same time, the process according to the invention makes it possible to take advantage of the advantage that the final hydrochlorides of these lower esters, in particular the methyl ester, readily crystallize in the form of hydrates from water-containing solvents.

The process according to the invention is characterized in that the lower alkyl esters of N-BOC-alpha-L-aspartyl-L-phenylalanine of the formula II,

BOC-NH-CH-CO -NH-CH-COOR ₂ -cooh-ch ₂ -Ό- "in which R is CH 2 or C 2 H 5, convert to the lower alkyl esters of alpha-L-aspartyl-L-phenylalanine of the general formula Wherein R is as defined above by treating with a concentrated aqueous solution of hydrogen chloride in a suitable solvent such as a monovalent carboxylic acid having 1 to 3 carbon atoms, preferably acetic acid, or ethyl acetate, which generates acetic acid in the reaction medium. In this environment, the BOC protecting group is completely cleaved from the amine function. By-products of the cleavage are, as in other analogous cases, carbon dioxide and 2-methylpropene.

In the process according to the invention to a stirred suspension of N-BOC-.alpha.-L-aspartyl-L-phenylalanine of the formula II in which R has the abovementioned meaning, in a lower monobasic carboxylic acid of a C ₃ to C _3, or in ethyl acetate was added at 5 to 40 ° C, preferably at 20 to 25 ° C, an aqueous solution of 30 to 40% by weight of hydrochloric acid. The reaction mixture is stirred until gas evolution subsides, diluted with ethyl acetate, and the precipitated alpha-L-aspartyl-L-phenylalanine ester hydrochloride crystals of formula I wherein R is as defined above are aspirated.

The method of the invention is fast, simple and productive. An important advantage thereof is that it can also be used for the treatment of technical mixtures of the esters of N-BOC-alpha-L-aspartyl-1-phenylalanine of the general formula II in which R is as defined above with isomeric esters of N-BOC- beta-L-aspartyl-L-phenylalanine of formula III, boc-nh-ch-ch ₂ -co-nh-ch-ch ₂

III

COOH COOR wherein R is CH CH or CjH ^, since the alpha-isomer of formula (I) wherein R is as hereinbefore defined preferably crystallizes from the reaction medium after completion of the reaction of the invention, in high yield and high purity, and optionally undesired beta the N-BOC protecting group isomer remains in solution. The process according to the invention thus makes it possible to eliminate subsequent separation and cleaning operations.

Further details of the invention will be apparent from the following examples which illustrate but not limit the generality of the process of the invention.

He did

To a stirred suspension of 15.6 g (0.04 mol) of N-tert-butyloxycarbonyl-alpha-L-aspartyl-L-phenylalanine methyl ester, mp 153-154 ° C; (α / Ιθ ^Ο = ^{+ 5} ' ⁶ ° < ^cx · ⁰ ' · ^CH 3 ° C ^H ) ^{in 45 ml} of acetic acid, add 7,8 ml of concentrated hydrochloric acid (34%) in one go. After stirring for 20 to 30 minutes, a clear, homogeneous solution is formed which is diluted with 35 ml of ethyl acetate and filtered rapidly. An additional 200 mL of ethyl acetate was added to the filtrate with stirring. The crystalline suspension is cooled to 5 to 3 ° C and kept at that temperature with stirring for 10 hours. The product is filtered off with suction, washed with 2 * 20 ml of ethyl acetate and dried at 50 DEG C. to constant weight.

13.6 g (93.8%) of alpha-L-aspartyl-L-phenylalanine methyl ester hydrochloride dihydrate are obtained in a chromatographic quality having a content of 99 to 100%.

Example 2

Analogously to Example 1, the tert-butyloxycarbonyl group was also eliminated from the ethyl ester of N-tert-butyloxycarbonyl-alpha-L-aspartyl-L-phenylalanine [mp 131-133 ° C; [α] D = +2.5 (c 1.0, CH 3 COOH)]. The hydroxyde of alpha-L-aspartyl-L-phenylalanine ethyl ester was obtained in the non-hydrate form, mp 164-166 ^U C.

Example 3

To a stirred suspension of 20 g (0.050 7 mol) of finely divided N-tert-butyloxycarbonyl-L-aspartyl-L-phenylalanine methyl ester (a mixture of alpha and beta isomers) in 58 ml of glacial acetic acid was added at 20-25 ° C. 10 ml conc. hydrochloric acid (34%).

The reaction mixture is stirred at this temperature until gas evolution (CO 2; 2-methylpropene) has subsided and a clear solution is formed. The solution is stirred with 0.1 g of activated carbon and the mixture is passed through a # 4 sinter funnel. The filter charcoal is washed with 2 ml of glacial acetic acid.

A total of 300 mL of ethyl acetate was added portionwise to the combined filtrates with stirring.

The crystalline suspension is cooled with stirring to 3-5 ° C and the product is filtered off with suction after 3-6 hours of crystallization, washed with 2x30 ml of cold (5 ° C) ethyl acetate and dried thoroughly at 50 ° C until constant weight.

16 g (86%) of .alpha.-L-aspartyl-L-phenylalanine hydrochloride methyl ester dihydrate are obtained in a chromatographic quality of 99 to 100%.

He attaches

To a stirred suspension of 20 g of finely divided N-tert-butyloxycarbonyl-L-aspartyl-L-phenylalanine methyl ester (a mixture of alpha and beta isomers) in 60 ml of ethyl acetate is added 20 ml of concentrated hydrochloric acid (20-25 ° C). 34%). The mixture was stirred at room temperature until gas evolution ceased (45-60 minutes), then 240 ml of ethyl acetate was added to the crystalline suspension, and the mixture was cooled to 3-5 ° C. After cooling for 3 h, the product is filtered off with suction, washed with 2x30 ml of cold ethyl acetate (5 ° C) and dried at 50 ° C to constant weight.

Yield 15.8 g (84.85%) of alpha-L-aspartyl-L-phenylalanine methyl ester hydrochloride dihydrate containing 98-100%.

EXAMPLE 5 g (0.050 7 mol) of finely divided N-tert-butyloxycarbonyl-L-aspartyl-L-phenylalanine methyl ester (a mixture of alpha and beta isomers) are suspended in 58 ml of propionic acid (98-99%). 10 ml of concentrated hydrochloric acid (34%) are added to the suspension while stirring at room temperature. The reaction mixture is stirred at room temperature until gas evolution ceases. Further procedure, including event. filtration of the activated carbon solution and adding 300 ml of ethyl acetate is the same as in Example 3.

15.8 g (84.85%) of alpha-L-aspartyl-L-phenylalanine methyl ester hydrochloride dihydrate are obtained with a content of 98 to 100%.

Claims (3)

SUBJECT OF THE INVENTION A process for the preparation of the lower alkyl ester hydrochlorides of the general formula I, alpha-L-aspartyl-L-phenylalanine h ₂ n-CH-CO-NH-CH-COOR CH-COOH _CH2 -O • HCl (i) wherein R is CH ₃ or C ₂ H ₁ , by reaction of the lower alkyl esters of N-butyloxycarbonyl-alpha-L-aspartyl-L-phenylalanine of formula II, BOC-NH-CH-CO-NH-CH-COOR AND CH ₂ -COOH-CH ₂ (II) wherein R represents a CH 2 group or by reaction of a mixture of these esters with lower alkyl esters N-butyloxycarbonyl-beta-L-aspartyl-L-phenylalanine of formula III, BOC-NH-CH-CH ₂ -CO-NH-CH-CH ₂ - | ^í (III) COOH COOR wherein R is CH ₃ or C ₂ H ₅ , with an excess of 30 to 40% by weight aqueous hydrochloric acid, characterized in that the elimination of the butyloxycarbonyl group is carried out in a C 1 -C 3 monohydric carboxylic acid environment, or their alkyl esters with an alkyl of up to 5 to 40 ° C, whereupon the desired lower alkyl alpha-L-aspartyl-L-phenylalanine is isolated in the form of a hydrochloride acid addition salt and the optionally present beta-isomer remains entirely in the mother liquors. 2. Process according to claim 1, characterized in that the reaction is carried out at a temperature of 20-25 ° C. 3. Process according to claim 1 or 2, characterized in that acetic acid or ethyl acetate is used as the solvent.