Abstract:
A process is described for recovering 2-acetylthiomethyl-5-aminopentanoic acid from an impure acid addition salt thereof by treating the latter in a polar organic solvent with a base.

Description:
BACKGROUND OF THE INVENTION 
     (d,l)2-Mercaptomethyl-5-guanidinopentanoic acid is reported as a potent inhibitor of carboxypeptidase B [Ondetti, M. A., Condon, M. E., Reid, J., Sabo, E. F., Cheung, H. S., and Cushman, D. W., Biochemistry 18, 1427-1430 (1979)]. More recently, this compound has been shown to have excellent activity in inhibiting serum carboxypeptidase N (SCPN) [Hugli, T. E., Gerard, C., Kawahara, M., Scheetz, M. E., Barton, R., Briggs, S., Koppel, G., and Russell, S., Molecular and Cellular Biochemistry 41, 59-66 (1981)]. 
     Ondetti et al, supra, details a multi-step sequence for the preparation of (d,l)2-mercaptomethyl-5-guanidinopentanoic acid. It is to a modification of the described method of preparation that this invention is directed. In the sequence described by Ondetti et al., supra, an intermediate, 2-acetylthiomethyl-5-aminopentanoic acid, is prepared. The trifluoroacetate salt of this material, as described in the Ondetti et al. reference, obtained as a pale yellow impure oil, is converted to the above intermediate by a cumbersome chromatographic procedure employing AG50WX2, a cation exchange resin. It has been discovered that this conversion and product recovery can be accomplished by a simple and rapid method using a base and a polar solvent. 
     SUMMARY OF THE INVENTION 
     Therefore, this invention is directed to a process for recovering 2-acetylthiomethyl-5-aminopentanoic acid of increased purity from an impure acid addition salt thereof, which comprises treating the impure acid addition salt in a polar organic solvent with a base. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A schematic representation of the preparation of (d,l)2-mercaptomethyl-5-guanidinopentanoic acid from readily available starting materials is as follows: ##STR1## 
     The abbreviations used herein have the following meanings: 
     DCHA--Dicyclohexylamine 
     DMF--N,N-Dimethylformamide 
     Et--Ethyl 
     TFA--Trifluoroacetic acid 
     The portion of the foregoing sequence that illustrates the process of this invention involves the conversion of the trifluoroacetate salt of 2-acetylthiomethyl-5-aminopentanoic acid (X) to 2-acetylthiomethyl-5-aminopentanoic acid (XI) [Step 8, part 3]. In the Ondetti et al., supra, reference, this conversion and product recovery is carried out using a cumbersome chromatographic procedure. By the process of this invention, the product rapidly forms and, under the defined conditions, precipitates from the reaction mixture. 
     In treating the trifluoroacetate salt of 2-acetylthiomethyl-5-aminopentanoic acid in accordance with the process of this invention, two conditions must be met. First, a reagent must be employed that will generate the free amino acid product. This reagent, a base, will be used in an amount representing at least an equivalent of the amino acid salt starting material. Typical such reagents are alkoxides, such as sodium methoxide, sodium ethoxide, potassium ethoxide, and the like; amines, such as diethylamine, triethylamine, diisopropylamine, tri-n-propylamine, benzylamine, and the like. A preferred base is an amine, and, in particular, triethylamine. 
     Secondly, generation of the free amino acid must be carried out in a medium which affords solubility for the amino acid salt starting material with accompanying precipitation of the free amino acid product. Polar solvents are suitable for this purpose. Examples are nitriles, such as acetonitrile, and the like; alcohols and glycols, such as methanol, ethanol, ethylene glycol, and the like; nitromethane; nitrobenzene; amides, such as N,N-dimethylformamide, N,N-dimethylacetamide, and the like; and other typical polar solvents. Amides are preferred for use, and, in particular, N,N-dimethylformamide. 
     Typically, the process of this invention is carried out by dissolving the acid addition salt of 2-acetylthiomethyl-5-aminopentanoic acid, generally as the relatively impure trifluoroacetate oil, in the selected polar solvent, usually DMF. To this solution then is added the selected base, typically triethylamine. The base effects production of the free amino acid which, under the process of this invention, precipitates from the mixture and is readily recovered therefrom in pure form. 
    
    
     The following represents a detailed example of the preparation of (d,l)2-mercaptomethyl-5-quanidinopentanoic acid, including that portion of the overall preparation which illustrates the process of this invention. 
     EXAMPLE 
     A. Ethyl N-(p-methoxybenzyl)nipecotate hydrochloride (III) 
     To 7 liters of toluene were added 695 grams (4.4M) of ethyl nipecotate, 1304 grams (4.8M) of p-methoxybenzyl trichloroacetate, and 660 grams (4.8M) of potassium carbonate. The mixture was refluxed for 72 hours under a nitrogen atmosphere after which it was cooled, and the solvent was removed in vacuo. The resulting oil was dissolved in chloroform, and the solution was washed with 10% aqueous potassium carbonate followed by 10% aqueous hydrochloric acid. The solution then was dried over sodium sulfate and concentrated in vacuo. The resulting oil was triturated with ethyl ether, yielding 783 grams of the title compound as a solid. 
     B. N-(p-Methoxybenzyl)nipecotic acid (IV) 
     To a mixture of 1 liter of water and 5 liters of methanol were added 783 grams (2.5M) of the product from part A and 211 grams (5.3M) of sodium hydroxide. The mixture was stirred at room temperature for 17 hours after which the solvents were removed in vacuo. Toluene was added to the residue and removed in vacuo to obtain 1055 grams of crude title compound. 
     C. 1-(p-Methoxybenzyl)-3-methylene-2-piperidone (V) 
     A mixture of 800 grams (crude) of the product from part B, 6 liters of acetic anhydride, and 1 liter of triethylamine was refluxed for 4 hours. The solvents then were removed in vacuo. The resulting residue was dissolved in chloroform, and the chloroform solution was washed with water, dried over sodium sulfate, and concentrated. The resulting oil was chromatographed over silica gel using a 1:1 mixture of hexane and ethyl acetate as eluant to obtain 219 grams of the title compound. 
     D. 3-Methylene-2-piperidone (VI) 
     A mixture of 219 grams (0.99M) of the product from part C, 276 grams (2.6M) of anisole and 3 liters of trifluoroacetic acid was refluxed for 48 hours under a nitrogen atmosphere. The solvents then were removed in vacuo, and the residue was chromatographed over silica gel using ethyl acetate as eluant to afford 115 grams of the title compound. 
     E. 2-Methylene-5-aminopentanoic acid, hydrochloride salt (VII) 
     To 8 liters of 6N hydrochloric acid were added 177 grams (1.59M) of the product as prepared in part E. The resulting mixture was refluxed for 40 hours after which it was cooled and extracted with methylene chloride. The aqueous layer then was concentrated in vacuo, toluene was added, and the solvents were again removed in vacuo. The resulting residue was recrystallized from isopropyl alcohol to obtain 44.6 grams of the title compound as a solid. A second crop yielded 26.9 grams of product as an oil. 
     F. 2-Methylene-5-(p-methoxybenzyloxycarbonyl)aminopentanoic acid (VIII) 
     To a solution of 3.8 grams (53 mmol) of the product from part E in 100 ml. of water were added 6.4 grams (159 mmol) of magnesium oxide with stirring followed by a solution of 12.2 grams (59 mmol) of p-methoxybenzyloxycarbonyl azide in 100 ml. of p-dioxane. The resulting slurry was stirred at room temperature for two days. The mixture then was filtered through Hyflo and diluted with 200 ml. of ethyl acetate. AG50W-X2 Dowex 50 ion-exchange resin (200 ml. wet volume) was added, and the mixture was stirred at room temperature for 2 hours. The resin then was removed by filtration and washed with water. The filtrate layers were separated, and the aqueous layer was extracted twice with 200 ml. of ethyl acetate. The combined ethyl acetate solution was dried over magnesium sulfate and concentrated in vacuo to give 15.5 grams of the title compound as an oil. 
     G. 2-Acetylthiomethyl-5-(p-methoxybenzyloxycarbonyl)aminopentanoic acid, dicyclohexylamine salt (IX) 
     To 15.5 grams of the oil containing the product from part F were added 50 ml. of thioacetic acid. The solution was allowed to stand at room temperature for 48 hours. The thioacetic acid then was removed in vacuo without addition of heat, and three portions of benzene were added and removed in vacuo to remove excess thioacetic acid. The resulting viscous oil was dissolved in ethyl ether, and the cloudy solution was treated with a slight excess (10%) of dicyclohexylamine. The mixture was cooled to less than 0° C., and crystals formed. The crystals were harvested to give 18.2 grams of the title compound. 
     H. 2-Acetylthiomethyl-5-aminopentanoic acid (XI) 
     The product from part G (12.5 grams; 22.7 mmol) was dissolved in 200 ml. of chloroform. The resulting solution was washed with two 200 ml. portions of 10% potassium bisulfate solution. The chloroform solution then was dried over magnesium sulfate and concentrated in vacuo. 
     The residue was dissolved in 18 ml. of anisole, and the mixture was cooled to 0°-5° C. Trifluoroacetic acid (115 ml.) was added dropwise over a 15 minute period, and the resulting solution was stirred at 0°-5° C. for 1 hour. The excess trifluoroacetic acid then was removed in vacuo. The residue was dissolved in water, and the aqueous solution was extracted with ethyl ether. The aqueous layer then was lyophilized to a pale yellow oil (X). 
     The yellow oil was dissolved in 65 ml. of dry N,N-dimethylformamide, and 8.6 ml. of triethylamine were added with stirring. A precipitate formed immediately. The resulting mixture was filtered, and the solid was washed with dry N,N-dimethylformamide. The residue then was dried in vacuo overnight. The resulting dried clumps were dissolved in water and lyophilized to give 3.9 grams (19.0 mmol; 84% yield) of the title compound as a floculant off-white solid. 
     n.m.r.: (D 2  O) 1.66 (m, 4, β and γ methylenes), 2.49 (m, 1, &gt;CH--CO 2  H), 2.99 (m, 4, --CH 2  NH 2  and --CH 2  SCOCH 3 ). 
     Analysis, Calculated for C 8  H 15  NO 3  S (205): C, 46.81; H, 7.37; N, 6.82; S, 15.62. Found: C, 46.59; H, 7.07; N, 6.80; S, 15.77. 
     I. (dl)2-Mercaptomethyl-guanidinopentanoic acid (XIII) 
     To 100 ml. of 1.0N sodium hydroxide were added 4.9 g. (24.4 mmol.) of 1-guanyl-3,5-dimethylpyrazole nitrate. The resulting free base was extracted three times with 100 ml. of ethyl acetate. The ethyl acetate solutions were combined, dried over sodium sulfate, and concentrated in vacuo. The resulting residue was dissolved in 15 ml. of water, and 1.00 g. (4.9 mmol.) of the product from part H was added. The solution was degassed by evacuation, and argon atmosphere was introduced, and 1.4 ml. (9.8 mmol.) of triethylamine were added. The reaction apparatus was closed, and the solution was stirred at 40° C. for 3 hours. The aqueous solution was then washed four times with two-fold volumes of ethyl acetate. The aqueous solution was frozen and lyophilized to give 1.3 g. of a slightly impure, white solid, (mass spec shows on M-1, 246 ion; NMR good for desired product.) 
     The white solid (1.0 g.; 4.0 mmol.) was dissolved in 15 ml. of water, and the solution was degassed by evacuation. An argon atmosphere then was introduced. Concentrated NH 4  OH (15.0 ml.) was added dropwise over a 5 minute period, and the solution was stirred at room temperature for 1 hour after completion of the addition. The reaction mixture then was degassed in vacuo for two hours, frozen, and lyophilized to a colorless syrup. The resulting syrup was dissolved in a minimum volume of 0.05M ammonium acetate beffer, pH 4.15, and the solution was applied to a Bio-Rex 70 cation exchange column (3.5 cm.×38 cm.) equilibrated with the same buffer. Elution with the buffer was continued, and 20 ml. fractions were collected. All Sakaguchi positive fractions were pooled and lyophilized to give 0.310 g. of the title compound as a white solid, representing a total yield of 40% based upon the product from part H. 
     n.m.r.: (D 2  O) 1.60 (m, 4, β and γ methylenes), 2.42 (m, 1, &gt;CHCO 2  H), 2.63 (m, 2, --CH 2  SH), 3.22 (m, 2, --NH--CH 2  --). 
     Mass Spectroscopy: 206 (M +  +1), 205 (M + ), 174.