Process for stereoselective reduction of β-ketoesters

Description

PRIORITY CLAIM

The present application claims the benefit under 35 U.S.C. § 371 of International Application No.: PCT/IN03/00166, filed Apr. 22, 2003, the entire contents of each of these applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a novel stereoselective process for preparing optically active dihydroxy ester derivatives of Formula I which are useful intermediates for the synthesis of HMG-CoA enzyme inhibitors like atorvastatin, cerivastatin, rosuvastatin, itavastatin, and fluvastatin.

BACKGROUND OF THE INVENTION

Ester derivatives of the Formula I

• wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl;
• wherein P represents any suitable protecting group; and R₂ is alkyl or aryl;
  are valuable chiral synthons for synthesizing compounds which are known anti-hyptercholesterolemic agents having an inhibitory effect on HMG-CoA reductase (See U.S. Pat. Nos. 5,003,080, 5,169,857, 5,354,772; PCT Application WO 01 85702; European Patent Application EP 0304063).

The most common approach for achieving stereoselective synthesis of compounds of Formula I is the reduction of Formula II

• wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl; wherein P represents any suitable protecting group; and R₂ is alkyl or aryl;
• using special borane reagents (See U.S. Pat. Nos. 5,273,995, 5,470,981, 5,489,691). However, reagents such as methoxydiethylborane are hazardous and expensive.

U.S. Pat. No. 6,001,615 describes an enzymatic synthetic route. This process, however, is not industrially scalable and involves large volumes.

U.S. Pat. No. 5,399,722 describes a process starting from commercially available ethyl-ω-chloroacetoacetate or its benzyloxy derivative. Disadvantages of this process are that a stereoselective reduction using a ruthenium-BINAP catalyst is employed and the desired compound of Formula I is obtained in six steps.

U.S. Pat. No. 5,481,009 describes a process starting from 4-phenyl-3-butenoic acid and achieves the desired compound in about 5 steps. The process uses hazardous steps (e.g. ozonolysis) to obtain the desired product.

Exemplary synthetic approaches for the preparation of statins using compounds of Formula I are depicted in Schemes 1-6.

The present invention has several advantages over known methods. The process of the present invention is safe and non-hazardous, cost-effective, industrially scalable, requires few steps, and is commercially viable.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a novel process for the preparation of compounds of Formula I

• wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl; wherein P represents any suitable protecting group; and R₂ is alkyl or aryl;
  the process comprising a step of reacting a compound of Formula II

• wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl; wherein P represents any suitable protecting group; and R₂ is alkyl or aryl;
  with sodium borohydride in presence of a metal halide (e.g. CeCl₃) or metal alkoxide (e.g. Ti(OiPr)₄) (See Scheme I). In one embodiment, the reagents are non-hazardous, easily available and inexpensive.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS OF THE INVENTION

The compound of Formula II is an important intermediate for the preparation of many drug molecules, especially HMG Co-A reductase inhibitors. HMG Co-A reductase inhibitors are useful as inhibitors of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG CoA reductase) and are thus useful as hypolipidemic or hypocholesterolemic agents.

In one aspect, the process of the present invention is a new, improved, industrially scalable, economical, and commercially feasible method for preparing intermediates used for the preparation of HMG CoA reductase inhibitors. An exemplary process according to the present invention is depicted in Scheme 1.

• wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl; wherein P represents any suitable protecting group; and R₂ is alkyl or aryl;

In certain embodiments, the present invention provides a novel process for the preparation of compounds of Formula I

wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl; wherein P represents any suitable protecting group; and R₂ is alkyl or aryl;

by reacting a compound of Formula II

• wherein R₁ is —CN, —OP, alkyl, aryl or heteroaryl; wherein P represents any suitable protecting group; and R₂ is alkyl or aryl; with sodium borohydride in the presence of a metal halide or a metal alkoxide. In one embodiment, the metal halide is anhydrous CeCl₃. In another embodiment, the metal halide is hydrated CeCl₃. In yet another embodiment, the metal alkoxide is Ti(OiPr)₄.
  The reagents are easily available and inexpensive.

Compounds of Formula I are important intermediates for the preparation of HMG Co-A reductase inhibitors. Exemplary synthetic approaches for the preparation of such inhibitors using compounds of Formula I can be found in schemes 2-6. (See scheme 2 (Atorvastatin), scheme 3 (Cerivastatin), scheme 4 (Itavastatin), scheme 5 (Rosuvastatin) and scheme 6 (Fluvastatin)).

The illustrated embodiments have been set forth only for the purposes of example and should not be taken as limiting the invention. Therefore, it should be understood that, within the scope of the appended claims, the invention may be practiced other than specifically described herein.

EXAMPLE 1

Preparation of tert-butyl (3R,5R)-6-cyano-3,5-dihydroxyhexanoate

A solution of tert-butyl (5R)-6-cyano-5-hydroxy-3-oxohexanoate (10 g, 0.044 mol) in THF (60 mL) was stirred under nitrogen and methanol (20 mL) was added. The reaction mixture was stirred for 15 min. and cooled to −50° C. to −55° C. Anhydrous CeCl₃ (10.8 g, 0.044 mol) was added and stirred for 30 min., maintaining the temperature between −50 and −55° C. Sodium borohydride (2.5 g, 0.066 mol) was added in 6 portions maintaining the temperature between −70 and −90° C. The resulting mixture was stirred for 1 h at the same temperature. After warming the reaction mixture to room temperature (RT), it was concentrated to a residue under vacuum at about 45° C. Methanol (60 mL) was added and the resulting mixture was concentrated. The resulting residue was cooled to RT, water (50 mL) was added and the resulting aqueous mixture was extracted with ethyl acetate (100 mL×2). The combined organic layer was washed with brine solution (50 mL), and concentrated to obtain the title compound. Yield: 9 g.

EXAMPLE 2

Preparation of tert-butyl (3R,5R)-6-cyano-3,5-dihydroxyhexanoate

A solution of tert-butyl (5R)-6-cyano-5-hydroxy-3-oxohexanoate (10 g, 0.044 mol) in THF (60 mL) was stirred under nitrogen and methanol (20 mL) was added. The reaction mixture was stirred for 15 min. and cooled to −50° C. to −55° C. CeCl₃.7H₂O (16.4 g, 0.044 mol) was added and stirred for 30 min., maintaining the temperature between −50 and −55° C. Sodium borohydride (2.5 g, 0.066 mol) was added in 6 portions maintaining the temperature between −70 and −90° C. The resulting mixture was stirred for 1 h at the same temperature. After warming the reaction mixture to RT, it was concentrated to a residue under vacuum at about 45° C. Methanol (60 mL) was added and the resulting mixture was concentrated. The resulting residue was cooled to RT, water (50 mL) was added and the resulting aqueous mixture was extracted with ethyl acetate (100 mL×2). The combined organic layer was washed with brine solution (50 mL), and concentrated to obtain title compound. Yield: 5 g.

EXAMPLE 3

Preparation of tert-butyl (3R,5R)-6-cyano-3,5-dihydroxyhexanoate

A solution of tert-butyl (5R)-6-cyano-5-hydroxy-3-oxohexanoate (10 g, 0.044 mol) in THF (60 mL) was stirred under nitrogen and methanol (20 mL) was added. The reaction mixture was stirred for 15 min. Ti(IV)isopropoxide (12.5 g, 0.044 mol) was added and stirred for 30 min. at room temperature. After cooling the reaction mixture to −50° C. to −55° C., sodium borohydride (1.67 g, 0.044 mol) was added in 4 portions maintaining the temperature between −50° C. and −55° C. The resulting mixture was stirred for 1 h at the same temperature. After warming the reaction mixture to RT, it was concentrated to a residue under vacuum at about 45° C. Methanol (60 mL) was added and the resulting mixture was concentrated. The resulting residue was cooled to RT, water (50 mL) was added and the resulting aqueous mixture was extracted with ethyl acetate (100 mL×2). The combined organic layer was washed with saturated ammonium chloride solution (2×50 mL), water (50 mL) and brine solution (50 mL), and concentrated to obtain the title compound. Yield: 7.5 g.

EXAMPLE 4

Preparation of (3R,5S)-6-(tert-butyl-diphenyl-silanyloxy)-3,5-dihydroxy-hexanoic acid tert-butyl ester

A solution of (5S)-6-(tert-butyl-diphenyl-silanyloxy)-5-hydroxy-3-oxo-hexanoic acid tert-butyl ester (20 g, 0.044 mol) in THF (60 mL) was stirred under nitrogen and methanol (20 mL) was added. The reaction mixture was stirred for 15 min. and cooled to −50° C. to −55° C. Anhydrous CeCl₃ (10.8 g, 0.044 mol) was added and stirred for 30 min., maintaining the temperature between −50 and −55° C. Sodium borohydride (2.5 g, 0.066 mol) was added in 6 portions maintaining the temperature between −70 and −90° C. The resulting mixture was stirred for 6 h at the same temperature. After warming the reaction mixture to RT, it was concentrated to a residue under vacuum at about 45° C. Methanol (60 mL) was added and the resulting mixture was concentrated. The resulting residue was cooled to RT, water (50 mL) was added and the resulting aqueous mixture was extracted with ethyl acetate (100 mL×2). The combined organic layer was washed with brine solution (50 mL), and concentrated to obtain the title compound. Yield: 17 g.

EXAMPLE 5

Preparation of (3R,5S)-3,5-dihydroxy-6-trityloxy-hexanoic acid tert-butyl ester

A solution of (5S)-5-dihydroxy-3-oxo-6-trityloxy-hexanoic acid tert-butyl ester (20 g, 0.044 mol) in THF (75 mL) was stirred under nitrogen and methanol (20 mL) was added. Ti(IV)isopropoxide (12.5 g, 0.044 mol) was added and stirred for 30 min. at room temperature. After cooling the reaction mixture to −50° C. to −55° C., sodium borohydride (1.67 g, 0.044 mol) was added in 4 portions maintaining the temperature between −50° C. and −55° C. and stirred for 5 h at the same temperature. After warming the reaction mixture to RT, it was concentrated to a residue under vacuum at about 45° C. Methanol (60 mL) was added and the resulting mixture was concentrated. The resulting residue was cooled to RT, water (50 mL) was added and the resulting aqueous mixture was extracted with ethyl acetate (100 mL×2). The combined organic layer was washed with saturated ammonium chloride solution (2×50 mL), water (50 mL) and brine solution (50 mL), and concentrated to obtain the title compound. Yield: 14.5 g.