Patent Application
20050222184
The present invention relates to a novel process for preparing intermediates that may be used for preparing compounds with antiviral activity, and in particular HIV protease inhibitors having the formula given below:
in which
The preparation of such compounds is described, for example, in U.S. Pat. No. 5,914,332, which is incorporated herein by reference. Among these, the compound of main relevance is the compound known as Lopinavir, the structural formula of which is given below.
The intermediate of interest is (S)-tetrahydro-a-(1-methylethyl)-2-oxo-1(2H)-pyrimidine-acetic acid, shown below,
the preparation of which is also described in the abovementioned US patent.
In particular, in U.S. Pat. No. 5,914,332, (S)-tetrahydro-a-(1-methylethyl)-2-oxo-1(2H)-pyrimidine-acetic acid is obtained by reacting valine with acrylonitrile and methyl chloroformate, and then hydrogenating on Raney-nickel the product thus obtained, as shown by the reaction scheme below.
(S)-tetrahydro-α-(1-methylethyl)-2-oxo-1(2H)-pyrimidineacetic acid.
According to the process discussed above, the (S)-tetrahydro-α-(1-methylethyl)-2-oxo-1(2H)-pyrimidineacetic acid is obtained in an overall yield of 25%. In addition, apart from the rather modest yield, the process under consideration has a second non-negligible drawback, namely the use of a catalyst based on Raney-nickel. Specifically, as is known, nickel is a metal that is not disposed of easily; secondly, Raney-nickel may cause allergies and give rise to sensitization phenomena. In addition, Raney-nickel is classified as an agent that can cause irreversible effects and is thus considered potentially carcinogenic.
A process for preparing (S)-tetrahydro-α-(1-methylethyl)-2-oxo-1(2H)-pyrimidineacetic acid has now been found, characterized not only by yields that are surprisingly superior to those of the process described in U.S. Pat. No. 5,914,332, but also, in its preferred embodiment, by the use of a catalyst that is less toxic and easier to dispose of than nickel, with obvious advantages in terms of the environment and health at work.
The process according to the present invention is characterized in that it comprises the following steps:
In the optimum embodiment of the invention, step (a) is performed in water at a temperature of 0-25° C., and preferably at 0-5° C. In particular, the L-valine is reacted with approximately equimolar amounts of acrylonitrile; the reaction is preferably performed with 1-5 M concentrations of the two compounds.
For the purposes of the present invention, the expression “the N-(2-cyanoethyl)-L-valine thus obtained is isolated” means that the product obtained in step (a) is isolated from the reaction mixture in amorphous or crystalline form, in a purity at least greater than or equal to 95% and preferably 97%. The isolation under consideration may be performed by the usual methods that will be obvious to a person skilled in the art; the product will preferably be precipitated, filtered and dried under vacuum.
Step (b) is preferably performed in water, normally working at a pH of between 8.0 and 12.0 (preferably between 9.0 and 10.5) and at a temperature of between 0 and 40° C. and preferably between 20 and 25° C. In this case also, the N-(2-cyanoethyl)-L-valine is reacted with an excess of an alkyl chloroformate, preferably methyl chloroformate; the reaction is preferably performed with 0.5-3 M concentrations of the two compounds.
As mentioned previously, the hydrogenation catalyst referred to in step (c) is preferably rhodium and even more preferably rhodium supported on charcoal. The hydrogenation is performed at a pressure of 4-7 bar and preferably 6-7 bar and at a temperature of 35-65° C. and preferably 40-60° C., preferably working in basic medium in the presence of ammonia gas, ammonium hydroxide or sodium methoxide, preferably ammonia gas; the solvent used is usually an alkyl alcohol, preferably methanol or aqueous-alcoholic mixtures.
Finally, as regards the cyclization, this is preferably performed in water at the reflux temperature of the solvent, i.e. at about 100° C. This reaction is conveniently performed by basic catalysis; this cyclization is promoted by working at a pH of between 12 and 13; the pH is preferably regulated using NaOH.
To allow the process according to the present invention to be understood more clearly, it is given schematically below.
The (S)-tetrahydro-α-(1-methylethyl)-2-oxo-1(2H)pyrimidineacetic acid is then isolated according to standard methods that will be obvious to those skilled In the art; it is preferably extracted with methylene chloride after acidification of the reaction mixture, and then dried under vacuum.
As may be seen from the examples that follow, via the process of the present invention, the (S)-tetrahydro-α-(1-methylethyl)-2-oxo-1(2H)-pyrimidineacetic acid is obtained in an overall yield of 38%, i.e. in a yield 52% higher than that obtained by working according to the process described in U.S. Pat. No. 5,914,332. In addition, besides the advantages in terms of the environment and health at work discussed previously, the present process involves a hydrogenation at temperatures that are distinctly lower and safer (50° C.) than those used in U.S. Pat. No. 5,914,332 (100° C.) without, however, adversely affecting, but rather improving, the overall yield.
The examples that follow are given purely for the purpose of illustration and should not be understood as limiting the invention.
L-valine (100 g) was suspended in water (100 ml) and an 85% solution of potassium hydroxide (56 g) in water (100 ml) was added at 20° C. The reaction mixture was stirred at this temperature until the valine was fully dissolved.
The solution was cooled to 0-5° C. and acrylonitrile (45 g) was added slowly over about 30 minutes at 0-5° C. The reaction mixture was stirred at 0-5° C. for 4-5 hours. Water (250 ml) was added and the solution was acidified to pH 5 with concentrated hydrochloric acid (about 70 ml). The suspension was then stirred at 0-5° C. for 1 hour and the solid was filtered off and washed with water (25 ml). The solid was dried at 60° C. under vacuum to give 137 g of N-(2-cyanoethyl)-L-valine (91% yield). m.p. 245-250° C.; 13C-NMR (50 MHz, D2O) δ: 171.76, 117.27, 68.34, 42.59, 28.86, 17.79, 16.91, 14.48; 1H-NMR (200 MHz, D2O) δ: 3.43 (d, 1H), 3.30 (t, 2H). 2.89 (t, 2H), 2.18-2.09 (m, 1H), 0.93 (d, 3H), 0.89 (d, 3H); IR (KBr) cm−1: 3467, 2260, 1577; MS (El): 171 [M+1], 130, 125, 84, 81.
N-(2-cyanoethyl)-L-valine (120 g) was dissolved in a solution of sodium hydroxide pearls (22.6 g) in water (360 ml). The pH was adjusted to 9.5-10.5 with 30% sodium hydroxide (about 12 ml) and the reaction mixture was stirred until the N-(2-cyanoethyl)-L-valine was fully dissolved. Methyl chloroformate (100 g) was slowly added dropwise at 20-25° C., with simultaneous addition of 30% sodium hydroxide solution (about 144 ml), while maintaining the pH between 9.0 and 10.5. The reaction mixture was stirred at 20-25° C. for 20-30 minutes.
Further methyl chloroformate (33 g) and 30% sodium hydroxide (about 86 ml) were then simultaneously added dropwise at 20-25° C. while maintaining the pH between 9.0 and 10.5. The reaction mixture was stirred at 20-25° C. for 20-30 minutes.
Methylene chloride (240 ml) was added and the reaction mixture was acidified slowly at 20-25° C. with concentrated hydrochloric acid (about 168 ml) to pH 1.5. The phases were separated and the aqueous phase was extracted with methylene chloride (240 ml). The combined organic phases were evaporated under vacuum and the crude N-(2-cyanoethyl)-N-(methoxycarbonyl)-L-valine thus obtained was used directly for the following reaction without further purification.
The crude N-(2-cyanoethyl)-N-(methoxycarbonyl)-L-valine obtained in Example 2 was dissolved in methanol (240 ml) and a solution of ammonia gas (72 g) in methanol (360 ml) was added, followed by addition of wet 5% rhodium-on-charcoal (2.4 g solids). The reaction mixture was then hydrogenated at 6-7 bar and 50° C. At the end of the reaction, the catalyst was filtered off and washed with methanol (50 ml). The methanolic solution was then evaporated under vacuum to give the N-(3-aminopropyl)-N-(methoxycarbonyl)-L-valine, which was used directly for the following reaction without further purification.
The crude N-(3-aminopropyl)-N-(methoxycarbonyl)-L-valine obtained in Example 3 was dissolved in water (580 ml) and aqueous 30% sodium hydroxide solution (130 ml). The reaction mixture was refluxed until the cyclization was complete. The reaction mixture was then cooled to 15-20° C. and sodium chloride (82 g) and methylene chloride (500 ml) were added. The aqueous phase was acidified with concentrated hydrochloric acid (about 120 ml) at pH 1 and the phases were separated. The aqueous phase was extracted with methylene chloride (2×500 ml) and the combined organic phases were evaporated under vacuum. The residue was treated with hot ethyl acetate (400 ml), cooled to 0-5° C. and filtered, the solid being washed with ethyl acetate (about 48 ml) to give after drying 75 g of crude product.
The crude product was dissolved in hot isopropanol (175 ml) and hot ethyl acetate (690 ml) was then added. The suspension was cooled slowly to 0-5° C. and the solid was filtered off and washed with ethyl acetate (about 50 ml) to give after drying at 50-60° C. under vacuum 59 g of (S)-tetrahydro-α-(1-methylethyl)-2-oxo-1(2H)-pyrimidineacetic acid (42% yield over three steps). m.p. 176-177° C.; 13C-NMR (50 MHz, DMSO) δ: 173.47, 156.26, 62.63, 42.53, 27.44, 22.64, 20.60, 19.82; 1H-NMR (200 MHz, DMSO) δ: 12.56 (s, 1H), 6.38 (s, 1H), 4.25 (d, 1H), 3.35-3.06 (m, 4H), 2.03-2.15 (1H), 1.83-1.71 (m, 1H), 0.92 (d, 3H), 0.81 (d, 3H); IR (KBr) cm−1: 3307, 1695, 1613; MS (El): 202 [M+2], 200, 157, 155, 141, 113.
| Number | Date | Country | Kind |
|---|---|---|---|
| MI2002A 001168 | May 2002 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB03/02262 | 5/28/2003 | WO | 11/23/2004 |
