DESC:FIELD OF THE INVENTION:
The present invention relates to an improved process for preparation of testosterone (I).
BACKGROUND OF THE INVENTION:
Testosterone (I) is a steroid hormone that belongs to androgen group and is found in all vertebrates. It plays a key role in development of reproductive tissues and secondary sexual characteristics as well as prevention of osteoporosis. Testosterone is chemically known as (8R,9S,10R,13S,14S,17S)-17-hydroxy-10,13-dimethyl-1,2,6,7,8,9,11,12,14,15,16, 17-dodeca hydrocyclopenta[a]phenanthren-3-one and represented by formula as shown below.

The process for preparation of testosterone (I) is described in earlier US patents 2294433 and 2679502 involving reaction of 17-Keto -3-enol ether (II, R=ethyl) with reducing agent to give 3-enol ether (III, R=ethyl) followed by dealkylation with inorganic acid such as hydrochloric acid or sulfuric acid as shown in synthetic scheme-I.
Scheme-I
This process for dealkylation to obtain testosterone is not preferable in view of commercial manufacture since these acid causes corrosion of the reactor, leads to impurity formation and an economically not viable due to requirement of further purification of testosterone by crystallization.
SUMMARY OF THE INVENTION:
The present invention is directed to an improved process for preparation of testosterone (I) that comprises dealkylation of 3-enol ether (III) with aqueous formic acid.
Preparation of testosterone (I) according to the present invention is depicted in the following synthetic scheme II:

Scheme-II
DETAILED DESCRIPTION OF THE INVENTION:
In an embodiment of the present invention provides a process for preparation of testosterone (I) comprising;
a) reducing 17-Keto-3-enol ether (II) with reducing agent to obtain 3-enol ether (III) and
b) dealkylation of 3-enol ether (III) with aqueous formic acid.
The 17-Keto-3-enol ether (II) is prepared by the methods available in the prior art such as reaction of androstenedione with trimethyl orthoformate or triethyl orthoformate.
In step (a), the reduction of 17-Keto-3-enol ether (II) is carried out using reducing agents such as sodium borohydride, lithium borohydride or metallic sodium in the presence of suitable solvent. When lithium aluminium hydride is used as reducing agent, it is advantageous to use ether solvent such as tetrahydrofuran, butyl ether and the like or benzene or mixtures thereof such as mixture of ether and benzene.
When sodium-boron hydride is used as reducing agent, it is advantageous to use alcohol solvent such as methanol, ethanol and the like, ether solvent such as dioxane or water mixture thereof such as aqueous methanol or dioxane-water mixture.
When metallic sodium is used as reducing agent it is advantageous to use alcohol solvent such as n-propanol.
The 3-enol ether (III) is subjected dealkylation by treatment with aqueous formic acid to obtain testosterone (I).
The molar quantity of formic acid used in step (b) is in the range of 0.3 to 15 moles per mole of 17-Keto-3-enol ether (II); preferably 8-10 moles per mole of 17-Keto-3-enol ether (II).
The formic acid used in the present invention is solution of formic acid in water which is in the range of 5-50%, preferably 10-30%.
The dealkylation reaction as described in step (b) is carried out at temperature of 0-100°C; preferably 50-60°C.
Testosterone (I) is isolated by conventional techniques such as filtration, concentration or evaporation. The present invention provides testosterone (I) having purity =99.9% (by HPLC) which doesn’t require further purification by crystallization from suitable solvent.
The effect of various inorganic and organic acids on dealkylation of enol ether (III, R=ethyl) is shown in Table-I

Table-I: HPLC purity of testosterone (I) obtained by de-ethylation of 3-enol ether (III, R=ethyl) with different acids.

S.No Experiment Acid Moles of acid HPLC Purity
Compound IV Testosterone (I)
1 Reference Example-1 2N HCl 14.8 3.32 96.68
2 Reference Example-2 Acetic acid 0.9 0.34 0.20
3 Example-1 Formic acid 1.0 0.13 98.78
4 Example-2 Formic acid 3.9 0.23 97.97
5 Example-3 Formic acid 7.9 0.09 99.91

De-ethylation of 3-enolether (III) with acid is complicated by the formation of androsta-2,14-dien-17-ol (IV). This impurity is lower when formic acid is used while the use of hydrochloric acid results in high impurity formation. Surprisingly, we found that de-ethylation reaction is very sluggish when acetic acid is used.

The present inventors also carried out a study on de-ethylation of enol ether (III, R=ethyl) with various mole equivalent of formic acid. The impurity formation is minimum with 7.9 mole of formic acid.
The advantages of the present invention are to provide testosterone (I) with high purity which doesn’t require purification by crystallization, minimizes impurity (IV) and avoids corrosion of reactor.
The present invention is described in the following examples, however it should be noted that the scope of present invention is not limited by the examples.
EXPERIMENTAL SECTION:
The HPLC (High Performance Liquid Chromatography) purity was obtained by using Shimadzu LC-2010 system with following parameters.
Chromatographic System:
Column : Ascentis express C18 (4.6 x 150 nm), 2.7µm
Wavelength : UV at 233 nm
Flow rate : 1.0 mL/min
Injection volume : 10 mL
Column oven temp : 40 °C
Auto sampler temp : 25 °C
Buffer: 0.02 M ammonium acetate in water.
Mobile phase-A: Buffer, acetonitrile and methanol (60:20:20 v/v).
Mobile phase-B: Buffer, tetrahydrofuran and acetonitrile (15:10:75 v/v).
Gradient Programme:
Time (In mins) Mobile phase-A (%) Mobile phase-B (%)
0.01 100 0
4 100 0
24 60 40
53 0 100
55 0 100

Retention time of testosterone: 15.5 min
Retention time of compound (IV): 29.8 min
Reference Example-1:
17-Keto-3-enol ether (II, R=ethyl) (90 g) was added to reaction flask containing methanol (720 ml) at 26°C. Sodium borohydride (13.5 g) was added and stirred at 4-6°C for 30 minutes then stirred at 25°C for 1 hour and 25 minutes. 2N HCl (900 ml) was added and stirred at 29°C for 2 hours and 50 minutes then filtered. The solid was washed with water (180 ml) and dried. Yield: 73 g, Purity: 96.48 % (by HPLC)
Reference Example-2:
17-Keto-3-enol ether (II, R=ethyl) (15 g) was added to reaction flask containing methanol (120 ml methanol), triethylamine (0.5ml) at 0°C. To the reaction mass sodium borohydride (2.25 g) was added then stirred for 1 hour 20 minutes at 5 to 10°C. Pre-cooled water (150 ml) was added and stirred. Acetic acid (20 ml) was added and stirred at 5-10°C for 1 hour 15 minutes followed by temperature raised to 55-60°C and stirred for 3 hours and 20 minutes. The reaction mass was cooled to 25-30°C. The solid was filtered, washed with aqueous methanol (15 ml methanol: 30 ml water) and dried. Yield: 12.2 g, Purity: 0.2% (by HPLC).
Example-1:
17-Keto-3-enol ether (II, R=ethyl) (50 g) was added to reaction flask containing methanol (250 ml) and sodium borohydride (6 g) at 5-6°C. The reaction mixture was stirred for 3 hours and cooled to -5°C. Aqueous formic acid (37.5 ml of HCOOH: 350 ml of H2O) was added and stirred at 60°C for 4 hours. The reaction mass was cooled to 28°C, stirred and filtered. The solid was washed with water (200 ml) and dried. Yield: 37 g, Purity: 98.78% (by HPLC).
Example-2:
17-Keto-3-enol ether (II, R=ethyl) (150 g) was added to reaction flask containing methanol (750 ml) and sodium borohydride (18 g) at 8-9°C. The reaction mixture was stirred for 3 hours and cooled to 0-3°C. Aqueous formic acid (150 ml of HCOOH: 1050 ml of H2O) was added and stirred at 74°C for 2 hours. The reaction mass was cooled to 28°C, stirred and filtered. The solid was washed with water (300 ml) and dried. Yield: 118 g, Purity: 97.80% (by HPLC).
Example-3:
17-Keto-3-enol ether (II, R=ethyl) (200 g) was added to reaction flask containing methanol (1000 ml) and sodium borohydride (24 g) at 7-10.5°C. The reaction mixture was stirred for 2 hours and cooled to 0°C. Aqueous formic acid (300 ml of HCOOH: 1400 ml of H2O) was added and stirred at 54-57°C for 2 hours. The reaction mass was cooled to 27°C, stirred and filtered. The solid was washed with water (800 ml) and dried. Yield: 151g, Purity: 99.91% (by HPLC).
,CLAIMS:1. A process for preparation of testosterone (I) comprising;

a) reducing 17-Keto-3-enol ether (II) with reducing agent to obtain 3-enol ether (III) and

Wherein R = alkyl
b) dealkylation of 3-enol ether (III) with aqueous formic acid.
2. The process according to claim 1, wherein the reducing agent is sodium borohydride or lithium borohydride.
3. The process according to claim 1, wherein step (a) is carried out in solvent is selected from alcohol, ether andaqueous solution thereof.
4. The process according to claim 3, wherein the solvent is selected from methanol, ethanol, dioxane and aqueous solution thereof.
5. The process according to claim 1, wherein the mole ratio of formic acid is in the range of 0.3 to 15 moles per mole of 17-Keto-3-enol ether (II).
6. The process according to claim 3, wherein the mole ratio of formic acid is in the range of 2-10 moles per mole of 17-Keto-3-enol ether (II).
7. The process according to claim 1, wherein the formic acid in water is in the range of 5-50%.
8. The process according to claim 5, wherein the formic acid in water is in the range of 10-30%.
9. The process according to claim 1, wherein step (b) is carried out in the temperature range of 0-100°C.
10. The process according to claim 1, wherein step (b) is carried out in the temperature range of 50-60°C.