PROCESS FOR THE PREPARATION OF (S)-2-(3-TERT- BUTYLUREIDO)-3,3-DIMETHYLBUTANOIC ACID

CROSS-REFERENCES TO RELATED APPLICATIONS [0001] This application claims the benefit of US Provisional Application, Serial No. 60/974,012, filed September 20, 2007, the content of which is incorporated herein by reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0002] NOT APPLICABLE

REFERENCE TO A "SEQUENCE LISTING," A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK. [0003] NOT APPLICABLE

BACKGROUND OF THE INVENTION

[0004] Compounds having a (<S)-2-(3-tert-Butylureido)-3,3-dimethylbutanoyl group have been identified as useful for the treatment of hepatitis C and related disorders, hi particular, (Ii?, 2S, 55)-3-Azabicyclo[3,l,0]hexane-2-carboxamide, N-[3-amino-l-(cyclobutylmethyl)- 2,3-dioxopropyl]-3-[(25)-2-[[[l,l-dimethylethyl]amino]carbon ylamino]-3,3-dimethyl-l- oxobutyl]-6,6-dimethyl is disclosed in U.S. Ser. No. 09/908,955 which was filed JuI. 19, 2001, and Ser. No. 10/052,386 which was filed Jan. 18, 2002, which are each incorporated herein by reference. The compound of formula I is identified as a hepatitis C virus ("HCV") protease inhibitor, useful for treating hepatitis C and related disorders. Specifically, the compound of formula I is identified as an inhibitor of the HCV NS3/NS4a serine protease.

(I)

In view of the importance of hepatitis C virus ("HCV") protease inhibitors, new, novel methods and intermediates for preparing such inhibitors are of interest.

BRIEF SUMMARY OF THE INVENTION

[0005] In one aspect, the present invention provides a process for the preparation of (S)-2- (3-ter?-Butylureido)-3,3-dimethylbutanoic acid, comprising:

(a) contacting (5)-2-amino-3,3-dimethylbutanoic acid with a silyl ester forming reagent under conditions sufficient to form an in situ silyl ester of (S^-amino-S, 3- dimethylbutanoic acid;

(b) contacting said in situ silyl ester of (5)-2-amino-3,3-dimethylbutanoic acid with tert-butylisocyanate or a synthetic equivalent thereof; and

(c) working up the reaction of step (b) to provide (<S)-2-(3-tert-Butylureido)-3,3- dimethylbutanoic acid as a solid.

[0006] Typically, the reaction workup in step (c) involves

(cl) contacting the mixture produced in step (b) with H ₂ O to quench unreacted tert- butylisocyanate;

(c2) acidifying the mixture produced in step (cl) to a pH of from 1 to 4; and in some embodiments,

(c3) filtering and drying the product.

[0007] The process of the present invention can be, and is preferably, run in a single reaction vessel. Moreover, the process can be run on large scale, typically a kilogram or multi-kilogram scale.

[0008] In a related aspect, the present invention provides a method for the preparation of a therapeutic agent, said method comprising preparing a (>S)-2-(3-tert-Butylureido)-3,3-

dimethylbutanoic acid intermediate by the process described herein and coupling the intermediate with a suitable proline derivative to produce said therapeutic agent or a therapeutic agent precursor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 provides a reaction scheme for the preparation of (S)-2-(3-tert- Butylureido)-3,3-dimethylbutanoic acid using tert-butylisocyanate.

[0010] Figure 2 provides a reaction scheme for the preparation of (S)-2-(3-tert- Butylureido)-3,3-dimethylbutanoic acid using a fert-butylisocyanate equivalent.

[0011] Figure 3 illustrates the coupling of (iS)-2-(3-tert-Butylureido)-3, 3 -dimethylbutanoic acid to a proline derivative to produce a therapeutic agent precursor, which on oxidation is converted to a therapeutic agent.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention provides a process for the preparation of a key intermediate in the production of HCV protease inhibitors. Prior to the present invention, the compound (S)- 2-(3-tert-Butylureido)-3,3-dimethylbutanoic acid had been prepared via a multistep process involving isolation and, in some cases, purification of intermediates. The present invention provides a process that is conveniently scalable and can be run in a single reaction vessel without isolation of intermediates.

[0013] Accordingly, in one aspect, the present invention provides a for the preparation of (5)-2-(3-tert-Butylureido)-3, 3 -dimethylbutanoic acid, comprising:

(a) contacting (5)-2-amino-3,3-dimethylbutanoic acid with a silyl ester forming reagent under conditions sufficient to form an in situ silyl ester of (5)-2-amino- 3, 3 -dimethylbutanoic acid;

(b) contacting said in situ silyl ester of (5)-2-amino-3, 3 -dimethylbutanoic acid with tert-butylisocyanate or a synthetic equivalent thereof; and

(c) working up the reaction of step (b) to provide (S)-2-(3-førMButylureido)-3,3- dimethylbutanoic acid as a solid.

[0014] The starting material, (5)-2-amino-3,3-dimethylbutanoic acid (also referred to as tert-butyl-L-glycine or L-tert- Leucine) can be obtained from commercial sources such as Aldrich (Milwaukee, WI).

[0015] As an initial step, a silyl ester is formed in situ by contacting the amino acid with a silyl ester- forming reagent. Examples of suitable silyl ester- forming reagents include trimethylsilylchloride, triethylsilylchloride, triisopropylsilylchloride, tert- butyldimethylsilylchloride, isopropyldimethylsilylchloride, hexamethyldisilazane, phenyldimethylsilylchloride, diisopropylmethylsilylchloride and combinations thereof. The amount of silyl ester- forming reagent can vary, typically from an excess on a mole % basis (e.g., 110 mole %, 200 mole %, 300 mole %, or more) to a catalytic amount on a mole % basis (e.g., 0.5 mole %, 1.0 mole %, 2.0 mole %, 3-10 mole %). In one group of embodiments an amount of more than 100 mole % of silyl ester-forming reagent is used. In another group of embodiments, hexamethyldisilaze is used in combination with an amount of trimethylsilyl chloride that is less than 10 mole %. A preferred silyl ester- forming reagent is trimethylsilyl chloride (or chlorotrimethylsilane). One of skill in the art will appreciate that other silyl ester forming reagents can be used so long as the ester can be conveniently removed during workup. Generally, the contacting step (a) is performed in the presence of a solvent such as methylene chloride, chloroform, dichloroethane, or tetrahydrofuran; with an amine base such as triethylamine, N,N-diisopropylethylamine, N-methylmorpholine, N- ethylpiperidine, N-methylpiperidine, N-ethylmorpholine, pyridine, 2,6-dimethylpyridine and N-methylpyrrolidine. Preferred reagents include trimethylsilylchloride or triethylsilylchloride, in combination with triethylamine or N-methylmorpholine. Preferred solvents are the chlorinated hydrocarbons such as methylene chloride and chloroform.

[0016] The reaction conditions for the formation of the silyl ester will vary, depending on the solvent used, the base used, the silylating reagent used, and other factors such as the concentration of all reactants. Generally, the reaction is run at temperatures of from O ⁰ C to the refluxing temperature of the solvent. For those instances in which an elevated temperature is used (relative to room temperature of about 25-28°C), the mixture can be held at the elevated temperature for about two to ten hours, generally about two to four hours.

[0017] Following formation of the in situ silyl ester of (5)-2-amino-3,3-dimethylbutanoic acid, the ester is contacted with tert-butylisocyanate or a synthetic equivalent thereof. The contacting is conveniently performed by addition of tert-butylisocyanate to the reaction mixture containing the in situ silyl ester, so that only a single reaction vessel need be employed. For those embodiments in which elevated temperatures are used for the in situ silyl ester formation, the mixture will typically be cooled to room temperature or lower before the addition of tert-butylisocyanate. In one embodiment, the temperature of the reaction mixture is adjusted to 20-25°C prior to isocyanate addition. The tert-butylisocyanate can be added neat (without solvent), or in a solvent or mixture of solvents. Generally, a single equivalent is used, or a small excess. In some embodiments, a synthetic equivalent of fert-butylisocyanate is used. A synthetic equivalent of tert-butylisocyanate can be derived (generally in situ) from, for example, tert-butylamine and an aryl chloroformate (e.g., phenyl chloroformate or 4-nitrophenyl chloroformate). Procedures for the in situ formation of an isocyanate equivalent are known to the skilled artisan and can be found in, for example, REAGENTS FOR ORGANIC SYNTHESIS, M. Fieser, ed., John Wiley & Sons, New York, NY (1967-2000). Additional guidance for the preparation of ureas from a carbamate (a stable isocyanate equivalent) can be found in Thavonekham, B. Synthesis 1997, 1189-1194.

[0018] The reaction can be monitored by thin layer chromatography. Upon complete reaction, as evidenced by disappearance of starting material (tert-butyl-L-glycine), the reaction mixture is worked up in a conventional manner.

[0019] The reaction workup in step (c) can be accomplished using a variety of techniques known to those of skill in the art. Generally, the workup involves:

(cl) contacting the mixture produced in step (b) with H ₂ O to quench unreacted tert- butylisocyanate; and (c2) acidifying the mixture produced in step (cl) to a pH of from 1 to 4; and in some embodiments, (c3) filtering and drying the product.

[0020] For those embodiments in which water is added to the resulting mixture (from step (b)), the mixture will generally be cooled, water will be added and the aqueous mixture will be acidified to a pH of from about 1 to 4. Generally, the mixture will be cooled to about 0- 20°C, more preferably about 10-20°C, before acid is added to remove the silyl ester and provide (5)-2-(3-tert-Butylureido)-3,3-dimethylbutanoic acid as a solid. Additionally, a

number of different acids can be used in the present invention, although HCl, HBr, H ₃ PO ₄ , KHSO ₄ and H ₂ SO ₄ are preferred. In the most preferred embodiments, HCl is used to acidify the reaction mixture.

[0021] Once the mixture has been acidified to a pH of about 1 to 4, the product is often obtained as a solid and can be isolated by, for example, filtration, and then dried to removed traces of solvent and/or moisture.

[0022] In a related aspect, the present invention provides methods for the preparation of a therapeutic agent, the method comprising preparing a (5)-2-(3-terMButylureido)-3,3- dimethylbutanoic acid according to the processes as described above, and coupling the acid with a suitable proline derivative to produce said therapeutic agent or a therapeutic agent precursor. The coupling process can use any of the known methods for amide formation.

[0023] The target therapeutic agents are generally selected from:

B

D.

[0024] Routes for the conversion of (5)-2-(3-tert-Butylureido)-3,3-dimethylbutanoic acid to the agents above can be found in, for example, PCT/US2006/25996, PCT/US2008/50033, and PCT/US2004/018914. Additionally, Figure 3 provides an illustration of the coupling of (S)-2-(3-tert-Butylureido)-3,3-dimethylbutanoic acid to a 4-(aryloxy)- or A- (heteroaryloxy)pyrrolidine-2-carboxamide derivative which produces a therapeutic agent precursor. Oxidation of the hydroxy group produces target therapeutic agents of the type provided above as B, C and D. Similarly, a proline derivative having a fused gem- dimethylcyclopropane at the 3- and 4-positions of the proline, can be coupled with (<S)-2-(3- /erM3utylureido)-3,3-dimethylbutanoic acid, and optionally oxidized to produce A. One of skill in the art will appreciate that coupling of (5)-2-(3-tert-Butylureido)-3,3- dimethylbutanoic acid to other pyrrolidine-2-carboxamides followed by elaboration of the 2-carboxamide portion is also an aspect of the present invention.

EXAMPLES

Experimental Section

(S)-2-(3-te^Butylureido)-3,3-dimethyIbutanoic acid (3) - Method A

[0025] A 20-L glass flask was charged with dichloromethane (7.00 L) under a nitrogen atmosphere at 25-35 ⁰ C followed by tert-butyl-L-glycine (1.00 kg; 7.62 mol) with stirring. To the suspension was added triethylamine (0.94 kg; 9.29 mol; 122 mole %) at 25-35 ⁰ C. The reaction mixture was then treated with chlorotrimethylsilane (1.15 L; 9.00 mol; 118 mole %) with stirring at 25-35 ⁰ C. The reaction mixture was then heated to reflux temperature (39- 41 ⁰ C) and maintained under reflux for 3 h. The reaction mixture was cooled to 20-25 ⁰ C and tert-butyl isocyanate (0.76 kg; 7.67 mol; 101 mole %) was added to the reaction mass at 20- 25 ⁰ C The reaction temperature was adjusted to 25-30 ⁰ C and the resulting mixture stirred for 14-15 hours at 25-30 ⁰ C. The reaction mixture was monitored by thin layer chromatography (TLC) for the complete disappearance of tert-butyl-L-glycine. Once TLC confirmed the absence of the starting material, the reaction mixture was treated with water (4 L) at 25-30 °C. The reaction was cooled to 15-20 ⁰ C and the reaction mixture was acidified to a pH of 1.0 to 2.0 by the addition of 6N aqueous hydrochloric acid (1.0 L; 6.0 mol) at 15-20 ⁰ C. Precipitation of the product was observed after the addition of the acid was complete. The mixture was stirred for 20-30 minutes and the slurry was filtered. The filter cake was washed with water (1.00 L) and allowed to dry under vacuum on the filter until no additional solvent

was collected in the receiver. The solid was transferred to polypropylene trays and dried at 60-70 ⁰ C until loss on drying was below 0.50%. Yields of compound 3 (see Figure 1) were 1.30-1.56 kg (73.9-88.6%) for three runs. HPLC Purity of these lots ranged from 99.00 to 99.78%.

(S)-2-(3-te^-ButyIureido)-3,3-dimethyIbutanoic acid (3) - Method B

Synthetic Scheme - Use of Hexamethyldisilazane and Catalytic Chlorotrimethylsilane

H H 4) (-Bu-N=C=O, 20-25 ⁰ C ^ XNV N^CO ₂ H

5) H ₂ O O

6) 5N aq HCI

7) Filter and dry (50-60 ⁰ C) 3

[0026] tert-Butyl-L-leucine (100 g, 0.76 mol) and dichloromethane (DCM; 1000 mL) were charged to a clean, dry round-bottom flask at ambient temperature. Hexamethyldisilazane (HMDS; 319.6 mL, 1.52 mol; 200 mole%) was added slowly to the flask with stirring at room temperature over 10 to 15 minutes followed by a catalytic amount of chlorotrimethylsilane (2.4 mL; 0.018 mol; 2 mole%). The contents were reflux ed for 3 h, and then cooled to RT. tert-Butylisocyanate (86.76 mL; 0.76 mol; 100 mole%) was added at ambient temperature over 30 min. and then stirred at ambient temperature for about 16 h, monitoring the progress of the reaction by TLC (elution solvent: 20% methanol in DCM; visualization by dipping the dry plate in a ninhydrin solution, then heating until the spots appear). Once the TLC assay indicated that the reaction was complete, water (1.0 L) was added and the reaction mass was acidified to pH = 2 with 5 N aqueous hydrochloric acid. The reaction mass was stirred for 1 h at ambient temperature then the DCM was distilled under vacuum. The reaction mass was cooled to 10-15 ⁰ C and stirred for 30 minutes. The precipitated solids were filtered, washed with hexanes and dried at 50-60 ⁰ C under vacuum for 4-5 h to provide 120 g (68%) of (5)-2-(3-tert-butylureido)-3,3-dimethylbutanoic acid. HPLC: 98.8%; Chiral HPLC: 97.8%; ¹ H NMR (DMSO-D ₆ , 200 MHz): δ 12.3 (br s, IH), 5.9- 5.95 (d, J= 10 Hz, 2NH), 3.85-3.9 (d, J= 10 Hz, IH), 1.2 (s, 9H), 0.84 (s, 9H).

[0027] The foregoing invention has been described in some detail by way of illustration and example, for purposes of clarity and understanding. It will be obvious to one of skill in the art that changes and modifications may be practiced within the scope of the appended claims. Therefore, it is to be understood that the above description is intended to be illustrative and not restrictive. The scope of the invention should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the following appended claims, along with the full scope of equivalents to which such claims are entitled.