# Patent

## Title
Synthesis of piperazine ring

## Abstract

A novel process for preparing a compound of the formula I: 




wherein

R1 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy or arylalkoxy; R2 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy, arylalkoxy, tosyl, formyl, acetyl or amine; and R3 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy or arylalkoxy is disclosed. These compounds are useful in the synthesis of the antidepressant mirtazapine and other tetracyclic compounds.

## Background
RELATED APPLICATION
This application is a continuation of application Ser. No. 09/545,011, filed Apr. 7, 2000, now U.S. Pat. No. 6,339,156, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/130,048, filed Apr. 19, 1999.


FIELD OF THE INVENTION
The present invention relates to methods for the synthesis of piperazine rings, particularly for the preparation of heterocyclic compounds useful as intermediates in the synthesis of piperazinoazepines such as the antidepressant mirtazapine.
BACKGROUND OF THE INVENTION
Mirtazapine, also known as 1,2,3,4,10,14b-hexahydro-2-methylpyrazine [2,1-a]pyrido[2,3-c]benzazepine, is an antidepressant suitable for oral administration. It has a tetracyclic chemical structure unrelated to other classes of antidepressants such as selective seratonin reuptake inhibitors (SSRIs), tricyclics or monoamine oxidase inhibitors. Mirtazapine belongs to the piperazinoazepine group of compounds, and has the following structural formula 




Known methods for the preparation of piperazine ring derivatives have low yields (9-30%), expensive reagents, and many reaction steps (Roderick, W. R. et al., J. Med. Chem 9, 1966, 181-185). It is desirable to have methods for preparing piperazine ring derivatives with fewer steps, high yields and inexpensive raw materials.
SUMMARY OF THE INVENTION
The present invention relates to a novel process for preparing a compound of the formula 




wherein R1 denotes substituted or unsubstituted alkyl, aryl, arylalkoxy, tosyl, formyl, benzoyl, acetyl or amine; R2 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy or arylalkoxy; land R3 denotes substituted or unsubstituted alkyl, alkoxy, aryl, aryloxy or arylalkoxy; by reacting a compound of the formula 




wherein R2 and R3 are as defined above and R4 and R5 are independently selected from the group consisting of fluoro, chloro, bromo and iodo,

    
    
        with a compound of the formula H2N—R1, wherein R1 is as defined above. Preferably the reaction is performed in the presence of a solvent. Polar aprotic solvents such as, dimethyl formamide, dimethylacetamide and dimethylsulfoxide are preferred.
    
    




DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a new process for preparing piperazine rings suitable for use in the synthesis of the antidepressant mirtazapine and other tetracyclic compounds such as those disclosed in U.S. Pat. No. 4,062,848 to van der Burg, the contents of which are incorporated herein by reference.
The process of the present invention comprises the steps of reacting a compound of formula II: 




wherein

R4 and R5 are independently any of the of radicals selected from the group that consists of fluoro, chloro, bromo and iodo; and R2 and R3 are as defined above; with a compound of the formula H2N—R1, wherein R1 is as defined above. Preferably, R1 denotes aryl, acetyl, formyl, benzoyl, amine, or tosyl. Most preferably, R1 is tosyl. In order to remove any doubt, the tosyl radical is defined as the group of formula VI: 




wherein Me represents a methyl group. Preferably R2 denotes methyl, R3 denotes phenyl, R4 denotes chloro, and R5 denotes chloro.

Preferably, the compounds of formulae I and II are compounds of formulae IV and V accordingly: 




In a preferred embodiment, the present invention relates to a process for preparing a compound of formula XI: 




wherein Ph represents a phenyl group, which comprises reacting a compound of formula XII: 




with a compound of formula XIII: 




Preferably this reaction takes place in the presence of a strong base such as sodium hydroxide (NaOH), sodium hydride (NaH), potassium hydroxide (KOH), potassium hydride (KH), sodium methoxide (NaOMe) and sodium amide (NaNH2). Sodium hydroxide and sodium hydride are preferred.

Preferred solvents for the above reaction are any one or more of the solvents selected from the group that consisting of dimethyl formamide (DMF), dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO), xylene, benzene, ethylbenzene, acetonitrile, toluene and ethers with high boiling points such as ethyleneglycol dimethyl ether, diethyleneglycol dimethyl ether, and propyleneglycol dimethyl ether.
The compound of formula XI may be further hydrolized to give the compound of formula XIV: 




The compound of formula XIV is known as 4-methyl-2-phenylpiperazine. Compounds of formula XI may be hydrolyzed by reacting a compound of the formula XI with acid to give compounds of the formula XIV. Preferred acids for the reaction are strong acids such as sulfuric acid (H2SO4), hydrochloric acid (HCl), phosphoric acid (H3PO4) and p-toluene sulfonic acid. A more preferred acid is sulfuric acid with a concentration of about 98%. Preferably the reaction is carried out in aqueous solution.

The compound of formula XIV may be used in the preparation of the mirtazapine (1,2,3,4,10,14b-hexahydro-2-methyl-pyrazino[2, 1-a]pyrido[2,3-c][2] benzazepine), as shown in Schemes 1 and 2 below. 




As shown in Scheme 1, the compound 3-cyano-2(4-methyl-2-phenyl-1-piperazynyl) pyridine may be prepared starting from 2-chloro-3-cyano pyridine and 4-methyl-2-phenyl-piperazine.
Starting from 3-cyano-2-(4-methyl-2-phenyl-1-piperazynyl)pyridine, mirtazapine can be prepared by two routes, which are further presented in Scheme 2: 




In accordance with the present invention, mirtazapine produced by the process of the present invention may be prepared as pharmaceutical compositions that are particularly useful for the treatment of depression. Such compositions comprise mirtazapine with pharmaceutically acceptable carriers and/or excipients known to one of skill in the art.
EXAMPLES
The following examples are given for the purpose of illustrating the present invention and shall not be construed as being limitations on the scope or spirit of the invention.
Example 1





A suspension of sodium hydride (60%, 1.1 g) in DMF (20 mL) was prepared. p-Toluenesulfonamide (2.2 g) was dissolved in DMF (10 mL) and added continuously to the sodium hydride suspension. After mixing at room temperature the suspension was heated to 60-70° C.
After that, the solution of beta-chloro-N-methyl-N-chloroethyl phenylethylamine (3 g) in DMF (10 mL) was added dropwise and mixed overnight. The reaction mixture was poured into a mixture of water (21 g) and ice (40 g).
After 4 hours the precipitate was filtered, washed with water (2×30 mL) and dried in an oven to give 3.3 g of the product.
Example 2
Preparation f 4-methyl-2-phenyl piperazine





Tosyl piperazine (1.2 g) was dissolved in water (Ig) and H2SO4 (98%, 3 mL) while heating to 110° C. After 20 minutes at 110-120° C. the reaction mixture was poured into water (10 mL) and ice (20 g).
The solution was alkalized to pH 13 with NaOH (47%) and the product was extracted into ether. After phase separation the organic phase was evaporated to dryness to give the 4-methyl-2-phenyl piperazine in 75% yield.
Although certain presently preferred embodiments of the invention have been described herein, it will be apparent to those skilled in the art to which the invention pertains that variations and modifications of the described embodiment may be made without departing from the spirit and scope of the invention. Accordingly, it is intended that the invention be limited only to the extent required by the appended claims and the applicable rules of law.

## Claims

1. A method for preparing a compound of the formula 




wherein R1 denotes phenylalkoxy, tosyl, benzoyl, or formyl; R2 denotes alkyl; and R3 denotes phenyl,
comprising the step of reacting a compound of the formula 





wherein R2 and R3 are as defined above and R4 and R5 are independently selected from the group consisting of fluoro, chloro, bromo and iodo,
with a compound of the formula H2N—R1, wherein R1 is as defined above.

2. The method of claim 1, wherein R1 is selected from the group consisting of formyl, benzoyl, and tosyl.

3. The method of claim 1, wherein R1 is tosyl.

4. The method of claim 1, wherein R2 is methyl.

5. The method of claim 1, wherein R4 is chloro.

6. The method of claim 1, wherein R5 is chloro.

7. The method of claim 1, wherein the reaction is performed in a solvent selected from the group consisting of DMF, DMAC, ethers, ethyleneglycol dimethyl ether, diethyleneglycol dimethyl ether, propyleneglycol dimethyl ether, DMSO, xylene, benzene, ethylbenzene, acetonitrile and toluene.

8. The method of claim 7, wherein said solvent is DMF.

9. The method of claim 1, further comprising the step of adding a strong base.

10. The method of claim 9, wherein said strong base is selected from the group consisting of sodium hydroxide, sodium hydride, potassium hydroxide, potassium hydride, sodium methoxide and sodium amide.

11. The method of claim 10, wherein the base is sodium hydroxide.

12. The method of claim 10, wherein the base is sodium hydride.

13. The method of claim 1, further comprising the step of removing R1 by hydrolysis.

14. The method of claim 13, wherein R1 is removed by hydrolysis using a strong acid.

15. The method of claim 14, wherein the acid is selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid and p-toluene sulfonic acid.

16. The method of claim 15, wherein the acid is sulfuric acid.

17. The method of claim 16 wherein the sulfuric acid has a concentration of about 98%.

18. A compound of the formula: 



wherein R1 denotes tosyl, formyl, or benzoyl; R2 denotes methyl; and R3 denotes phenyl.

19. A method for preparing 4-methyl-2-phenylpiperazine comprising hydrolyzing a compound of claim 18 using an acid.

20. The method of claim 19, wherein the acid is sulfuric acid.

21. A method for preparing 3-cyano-2-(4-methyl-2-phenyl-1-piperazinyl) pyridine comprising:
hydrolyzing a compound of claim 18 to form 4-methyl-2-phenylpiperazine; and reacting 4-methyl-2-phenylpiperazine with a 3-cyano-pyridine to form 3-cyano-2-(4-methyl-2-phenyl-1-piperazinyl) pyridine.

22. A method for preparing mirtazapine comprising the steps of:
hydrolyzing a compound of claim 18 to form 4-methyl-2-phenylpiperazine; 
reacting 4-methyl-2-phenylpiperazine with a 3-cyano-pyridine to form 3-cyano-2-(4-methyl-2-phenyl-1-piperazinyl) pyridine; 
converting 3-cyano-2-(4-methyl-2-phenyl-1-piperazinyl) pyridine to 3-carboxy-2-(4-methyl-2-phenyl-1-piperazinyl) pyridine; and 
converting 3-carboxy-2-(4-methyl-2-phenyl-1-piperazinyl) pyridine to mirtazapine.

23. A compound of the formula: 




wherein R1 denotes phenylalkoxy, tosyl, benzoyl, or formyl; R2 denotes alkyl; and R3 denotes phenyl.

24. The compound of claim 23, wherein R1 is formyl, benzoyl, or tosyl.

25. The compound of claim 23, wherein R2 is methyl.

26. The compound of claim 23, wherein R1 is tosyl.