Claims:1. An improved process for preparation of substantially pure Tadalafil, said process comprising the steps of:
a) esterifying D-Tryptophan in presence of a solvent and thionyl/oxalyl chloride to obtain D-Tryptophan methyl propanoate hydrochloride;
b) isolating the D-Tryptophan methyl propanoate hydrochloride obtained in step (a) in a non-chlorinated solvent;
c) coupling the isolated D-Tryptophan methyl propanoate hydrochloride with piperonal in presence of a non-halogenated solvent to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate hydrochloride;
d) condensing (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with chloro acetyl chloride in presence of a base to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate; and
e) cyclisation and condensing of (1R,3R)-Methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with 15-20% w/w methyl amine in presence of a another solvent and water to provide substantially pure tadalafil
wherein said substantially pure tadalafil is free from trans isomer impurity.
2. The process as claimed in claim 1, wherein the solvent in step (a) is selected from methanol, dimethyl carbonate, diazomethane, dimethyl sulfate, methyl chloride and mixture of solvents.
3. The process as claimed in claim 2, wherein the solvent in preferably methanol or mixture of methanol and 2-methyl tetrahydrofuran.
4. The process as claimed in claim 1, wherein the non-chlorinated solvent in step (b) is selected from 2-Methyl tetrahydrofuran, cyclopentyl methyl ether, Di-tert-butyl ether, diglyme, dimethoxy ethane, and tetrahydropyran.
5. The process as claimed in claim 1, wherein the non-halogenated solvent in step (c) is selected from least water immiscible alkyl chain extended homologue alcohols.
6. The process as claimed in claim 5, wherein the non-halogenated solvent is isobutyl alcohol, diethylene glycol, propylene glycol and glycerol.
7. The process as claimed in claim 6, wherein the non-halogenated solvent is preferably isobutyl alcohol.
8. The process as claimed in claim 1, wherein base in step (d) is an alkaline earth metal phosphate, sulfate, carbonate or a buffer salt.
9. The process as claimed in claim 8, wherein said buffer salt is selected from magnesium carbonate, sodium phosphate dibasic, sodium dihydrogen orthophosphate, sodium acetate, potassium phosphate dibasic, potassium hydrogen phthalate, and ammonium dihydrogen orthophosphate and mixtures thereof.
10. The process as claimed in claim 9, wherein said buffer salt is preferably magnesium carbonate.
11. The process as claimed in claim 1, wherein in step (e) the other solvent is selected from methanol, isobutyl alcohol, diethylene glycol, propylene glycol, glycerol and mixtures thereof.
12. The process as claimed in claim 12, wherein the other solvent is preferably methanol.
13. The process as claimed in claim 1, wherein at least 90% of the substantially pure tadalafil is having the particle size between 41 to 95 microns.
14. The process as claimed in claim 1, wherein at least 50% of the substantially pure tadalafil is having the particle size of between 11 to 41 microns.
15. The process as claimed in claim 1, wherein at least 10% of the substantially pure tadalafil is having the particle size less than 10 microns.
, Description:FIELD OF THE INVENTION
The present invention relates to a novel and easily scalable synthesis, improved process for the preparation of pure Tadalafil. The present invention more particularly relates to a novel process for the preparation of pure Tadalafil i.e., (6R-trans)-6-(1,3-benzodioxol-5-yl)- 2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino [1', 2':1,6] pyrido[3,4-b]indole-1,4-dione having formula (I),via singlekey raw material called D-Tryptophan.

BACKGROUND OF THE INVENTION
The chemical name of Tadalafil is (6R, 12aR)-2, 3, 6, 7, 12, 12a-hexahydro-2-methyl 6-(3,4-methylenedioxyphenyl)-pyrazino[2', 1': 6,1]pyrido [3,4-b]indole-1, 4-dione and molecular formula is C22H19N3O4 . It has a molecular weight is 389.40. The current pharmaceutical product containing this drug is being sold by Icos-Lilly using trade name CIALIS, in the form of tablets.
Tadalafil of formula (I) is a tetra cyclic derivative, potent and selective inhibitor of cyclic guanosine 3,5-monophosphate-(cGMP)-specific phosphodiesterase type 5(PDE5) having utility in a variety of therapeutic area where such inhibition is thought to be beneficial. Therefore, it has utility in the treatment of various disorders, including stable, unstable and variant (Prinzmetal) angina, hypertension, pulmonary hypertension, congestive heart failure, renal failure, arteriosclerosis, conditions of reduced blood vessel potency (post-PTCA) etc. Tadalafil is potent drug useful for treatment of erectile dysfunction.

Formula(I)

U.S. Pat. No. 5,859,006 describes a process for the preparation of Tadalafil and its intermediate of formula (V) which involves reacting D-Tryptophan methyl ester with piperonal in the presence of dichloromethane and trifluoroacetic acid which provides cis and trans isomers of compound. The isomers are separated by chromatography. The cis-isomer is then reacted with chloroacetyl chloride and methylamine to give Tadalafil. The product of the reaction is purified by chromatography. However, the process has several disadvantages including being time consuming and requiring 4–5 days at 0–5° C for the completion of reaction. The overall yield obtained is low, with 37–42% cis-isomer, which is desired, and 27–47% trans isomer (undesired) in coupling stage. It uses highly corrosive and toxic material Trifluoroacetic acid. It requires either fractional crystallization or column chromatography of the reaction mixture in order to separate the isomers before epimerization to form pure cis isomer in coupling stage. Moreover, the process requires 24–72 hours for epimerization in a volatile solvent dichloromethane.
WO2004/011463 discloses a process of preparation of Tadalafil via modified Pictet-Spengler reaction in which D-Tryptophan methyl ester hydrochloride and piperonal is condensed in anhydrous Isopropyl alcohol to provide hydrochloride salt of coupling product. The product further is reacted with chloroacetyl chloride and then with methyl amine to give Tadalafil. However this process requires anhydrous IPA specifically less than 0.1% water content which is practically difficult to achieve in commercial scales.
WO2005/068464 discloses process of preparation in which D-Tryptophan methyl ester and piperonal are condensed in the presence of TFA, solvent and molecular sieves to give mixture of compound as Cis and Trans isomers which is treated with aqueous hydrochloric acid to form hydrochloride salt of Cis-isomer which in turn is reacted with chloroacetyl chloride and then methylamine to give Tadalafil of formula (I).However, in this process highly toxic and corrosive reagent trifluoroacetic acid is used which should be avoided normally for regulatory compliance purpose.
In summary, the known processes for preparation of Tadalafil suffers from the following drawbacks:
a) They are time consuming and require 4–5days at 0–5° C for the completion of reaction.
b) Overall yield obtained is low, with 35–45% Cis-isomer, which is desired, and 20–45% Trans isomer (undesired).
c) Highly corrosive and toxic material Trifluoro acetic acid is used in several patents.
d) Chlorinated solvents are used in the process which are non-ecofriendly solvents
e) The processes require either fractional crystallization or column chromatography of the reaction mixture in order to separate the isomers before epimerization to form pure Cis isomer of (V). The process also requires 24–72 hours for epimerization in a volatile chlorinated solvent like dichloromethane.
f) The process requires critical moisture check of the solvent like Isopropyl alcohol in some literature patent.
Accordingly there is a need for an improved and cost effective process which solves above mentioned problems, particularly a process that directly synthesizes desired stereoisomer of the compound.

OBJECT OF THE PRESENT INVENTION
It is an object of the present invention to provide an improved and industrially realizable, economical process for preparing Tadalafil.
It is another object of the present invention to provide an improved process for the preparation of Tadalafil which is simple and convenient.
It is yet another object of the present invention to provide an improved process for the preparation of Tadalafil comprising reaction of D-Tryptophan methyl ester hydrochloride with piperonal in presence of Isobutyl alcohol to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate hydrochloride without any further purifications it is converted to subsequent stages to get tadalafil.

It a further object of the present invention to provide a process in which the cis isomer is obtained as major product.

It is therefore an object of the present invention to provide an improved process for the preparation of Tadalafil of formula (I).

It is yet a further object of the present invention to provide an improved process for the preparation of tadalafil comprising condensation of (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with Chloroacetyl chloride in presence of base magnesium carbonate to give (1R,3R)-methyl-1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate.

It is yet another object of the present invention is to provide simple process which eliminates the use of highly corrosive and hazardous chemicals such as trifluoro acetic acid volatile solvents, which involve a regular moisture content (being below 0.1%) check.

It is still another object of the present invention to provide a process which eliminates the use of chromatographic purification at various stages.

SUMMARY OF THE INVENTION

In one aspect the present invention provides a process for preparation of substantially pure Tadalafil, said process comprising the steps of:
a) esterifying D-Tryptophan in presence of a solvent and thionyl chloride to obtain D-Tryptophan methyl propanoate hydrochloride;
b) isolating the D-Tryptophan methyl propanoate hydrochloride obtained in step (a) in a non-chlorinated solvent;
c) coupling the isolated D-Tryptophan methyl propanoate hydrochloride with piperonal in presence of a non-halogenated solvent to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate hydrochloride;
d) condensing (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with chloro acetyl chloride in presence of a base to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate; and
e) cyclisation and condensing of (1R,3R)-Methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with 15-20% w/w methyl amine in presence of a another solvent and water to provide substantially pure tadalafil
wherein said substantially pure tadalafil is free from trans isomer impurity.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention provides a process for the preparation of tadalafil, substantially free from structurally similar impurities and having a purity of above 99.5%. Particularly the process comprising the steps of:
a) esterifying D-Tryptophan in presence of a solvent and thionyl chloride to obtain D-Tryptophan methyl propanoate hydrochloride;
b) isolating the D-Tryptophan methyl propanoate hydrochloride obtained in step (a) in a non-chlorinated solvent;
c) coupling the isolated D-Tryptophan methyl propanoate hydrochloride with piperonal in presence of a “non-halogenated solvent” to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate hydrochloride;
d) condensing (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with chloro acetyl chloride in presence of a base to obtain (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate; and
e) cyclisation and condensing of (1R,3R)-Methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate with 15-20% (weight/weight) methyl amine in presence of a another solvent and water to provide substantially pure tadalafil
Wherein said substantially pure tadalafil is free from Trans isomer impurity.

The present inventors found that upon using higher homologues of aliphatic or aromatic alcohols such as isobutyl alcohol as solvent for coupling of D-Tryptophan methyl ester hydrochloride with piperonal, the resultant (1R,3R)-methyl-1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylatehydrochloride is free from critical trans isomer impurity and has higher yield.

Route of synthesis (ROS) of the present invention is depicted below:

The non-chlorinated solvent used in step (b) of the present invention can be selected from may preferably be 2-methyl tetrahydrofuran, cyclopentyl methyl ether, Di-tert-butyl ether, diglyme, dimethoxy ethane, and tetrahydropyran. It may preferably be 2-methyl tetrahydrofuran.

The solvent in step (c) of the present invention is selected from alkyl chain extended homologue alcohols such as isobutyl alcohol, diethylene glycol, propylene glycol and glycerol. It may preferably be isobutyl alcohol.

The base in step (d) of the present invention is selected from buffer salts like sodium, potassium mono or di hydrogen phosphates or sulfates such as magnesium carbonate, magnesium, sodium phosphate dibasic, magnesium, sodium dihydrogen orthophosphate, sodium acetate, potassium phosphate dibasic, potassium hydrogen phthalate, and ammonium dihydrogen orthophosphate. It may preferably be magnesium carbonate.

In an embodiment the present invention also includes recycling and reuse of isobutyl alcohol, which can be considered as ‘green solvent’. Isobutyl alcohol used in the present process is easily recoverable from moisture content by conventional dehydration techniques, viz., azeotrope distillation, atmospheric distillation, fractional column distillation / purification among others thus leading to a cost efficient process.
In an embodiment of the present invention Tadalafil made process for preparation of the present invention complies with all pharmacopeia grades in ‘green chemistry’ way.
According to the present invention substantially pure tadalafil means pharmaceutical grade tadalafil free from impurities as specified in the table below.
Enantiomeric and Diastereomeric purity method
6R,12aS diastereomera Not more than 0.1 0.03 0.01 0.05
6S,12aS enantiomerb Not more than 0.1 Not detected Not detected 0.05
6S,12aR diastereomerc Not more than 0.1 Not detected Not detected 99.09

a. (6R,12aS)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.
b. (6S,12aS)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.
c. (6S,12aR)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.

Further the present process produces drug product of desired particle size substantially pure tadalafil wherein the particle size of at least 90% is between 41 to 95 microns. Preferably the particle size of at least 50% of it is between 11 to 41 microns. More preferably, the particle size of at least 10% of it is less than 10 microns.
The process of the present invention has the following advantages:
1) It avoids corrosive and toxic reagent such as trifluoro acetic acid.
2) Avoids use of chlorinated solvents which are non-ecofriendly
3) The duration of reaction period is relatively shorter.
4) The overall yield for getting Cis-isomer is extremely high and presence Trans isomer is < / = 0.5% in the particular processing stage
5) The process is simple easy to handle and provides better yield and improved quality.
6) It provides relatively pure Tadalafil of formula without repeated crystallization or column chromatographic purification
7) Particle size of the Tadalafil achieved is d90 as >41-95 microns, d50 as 11-41 microns and d10 as <10 microns
The following examples are meant to illustrate the present invention. The examples are presented to exemplify the invention and are not to be considered as limiting the scope of the invention.
Example-1A: Preparation of Methyl-2-amino-3-(1H-indol-3-yl)propanoate hydrochloride:

Round bottom flask was charged with 500 ml of methanol and 100 g of D-Tryptophan at 25-30°C, slowly added 82 g of thionyl chloride over a period of 30-45 min, heated the reaction mixture to 60-65°C, maintained for 4-5 hours, distilled out the methanol completely under vacuum at below 50°C, cooled the reaction mixture to 30-35°C, charged 50 ml of 2-methyl tetrahydrofuran, stirred for 10-15 min, distilled out the solvent completely under vacuum at below 45°C, cooled to 30-35°C, charged 300 ml of 2-methyl tetrahydrofuran, cooled to 0-5°C, stirred for 60-90 min, filtered and washed the wet cake with chilled 100 ml of 2-methyl tetrahydrofuran, dried under tray drier at NMT 55°C to get 118 g of methyl-2-amino-3-(1H-indol-3-yl)propanoate hydrochloride; purity NLT 98.5 %; Yield 95 %.

Example-1B: Preparation of Methyl-2-amino-3-(1H-indol-3-yl)propanoate hydrochloride:

Round bottom flask was charged with 500 ml of methanol and 100 g of D-Tryptophan at 25-30°C, slowly added 82 g of thionyl chloride over a period of 30-45 min, heated the reaction mixture to 60-65°C, maintained for 4-5 hrs, distilled out the methanol completely under vacuum at below 50°C, cooled the reaction mixture to 30-35°C, charged 50 ml of 2-methyl tetrahydrofuran, stirred for 10-15 min, distilled out the solvent completely under vacuum at below 35°C, again charged 300 ml of 2-methyl tetrahydrofuran, distilled out the solvent completely under vacuum at below 35°C to get 120 g of methyl-2-amino-3-(1H-indol-3-yl)propanoate hydrochloride; purity 99 %; Yield 96 %.
Example-2: Preparation of (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate hydrochloride:

Round bottom flask was charged with 400 ml of Isobutyl alcohol, 100 g of methyl-2-amino-3-(1H-indol-3-yl)propanoate hydrochloride from example-1 at 25-30°C, slowly added 62 g of piperonal, heated to 80-90°C, maintained for 15-20 hours, cooled to 25-30°C, then to 5-10°C and 0°C, filtered and washed the wet cake with pre-cooled 100 ml of Isobutyl alcohol and further purified from isobutyl alcohol, dried under tray drier at about 60°C to get 140g (1R,3R)-methyl-1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate hydrochloride; purity >99.5%; Yield 92 %.
Example-3 Preparation of (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate
Round bottom flask was charged with 100 g of (1R,3R)-methyl 1-(benzo[d][1,3]dioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate from example-2, 200 ml of purified water at 25-30°C, cooled to 15-20°C, charged 35.5 g of magnesium carbonate, slowly cooled to 0-5°C, added slowly 75 g of chloroacetyl chloride over a period of 60-90 min, maintained the reaction mixture for 4-5 hours, added 400 ml of purified water, pH of the reaction mixture was adjusted slowly to 7.5-8.0 using approximately 300 ml of 12 % aqueous magnesium carbonate solution, raised the temperature of the reaction mixture to 30-35°C,charged 100 ml of Isobutyl alcohol at 35-40°C, raised the temperature to 40-45°C and charged 300 ml of Isobutyl alcohol, cooled the reaction mixture to 20-25°C, stirred for 2-2.5 hours, filtered and washed with 100 ml of Isobutyl alcohol to get 100g of (1R,3R)-methyl1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate; purity 98 %; Yield 91 %
Example-4 Preparation of Tadalafil:

Round bottom flask was charged with 1 liter of methanol, 100 g of (1R,3R)-methyl-1-(benzo[d][1,3]dioxol-5-yl)-2-(2-chloroacetyl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate from example-3, and 150 ml of 15-20% methanolic methyl amine at 25-30°C, heated to 45-50°C, stirred for 5-6 hours, cooled to 10-15°C, maintained for 2-2.5 hours, filtered and washed with 50 ml of methanol. The obtained wet cake was taken into round bottom flask and charged 1 liter purified water, stirred for 90-120 min, filtered and washed with 200 ml of purified water. The obtained wet cake was taken in 500 ml of methanol, stirred for 90-120 min, filtered and washed with 100 ml of methanol, product again taken in water 500-1000 ml, heated to 70 - 75°C, cooled to 0°C dried under vacuum at NMT 50°C to get 80 g Tadalafil; purity 99.5 %; Yield 88 %.
Example-5: Chemical analysis of Tadalafil:

Three consecutive three batches of Tadalafil were prepared through the process described in the present invention and their yields were determined in (Table- 1). The product was completed analyzed as per its In-House specifications (Table-2A) and one of the three batches was chemically characterized by 1H NMR (Structure -1 and Table-3), 13C NMR (Structure-2 and Table-4), Mass (Structure-3, Table-5) and Elemental analysis (Table-6).
Table: 1: Yield details for the process of the present invention
Sr.No Batch number Input Output
1 TDF-A131-013 100 g 133.5 g
2 TDF-A131-014 100 g 133 g
3 TDF-A131-015 100 g 133 g
Foot notes for table:

Input: D-Tryptophan (KSM)

Output: Tadalafil (Active Pharmaceutical ingredient)

The yield is calculated from compound I (Example 1) to Tadalafil (Example 4).

Table-2A:
Analytical summary:
S. No. Test Specification Ex no:TDF-A131-013-04 Ex no: TDF-A131-014-04 Ex no:TDF-A131-015-04
1. Description White or almost white powder. White powder White powder White powder
2. Solubility Freely soluble in dimethyl sulfoxide, slightly soluble in methylene chloride and practically insoluble in water. Complies Complies Complies
3. Identification
A) By HPLC The retention time of the major peak of the sample solution should conform to that of the identification solution in enantiomeric and diastereomeric purity method Complies Complies Complies
B) By IR The IR absorption spectrum of sample should conform to the spectrum of standard. Complies Complies Complies
4. Loss on drying (%w/w) Not more than 0.5 0.14 0.13 0.08
5. Residue on ignition (%w/w) Not more than 0.10 0.07 0.06 0.08
6. Enantiomeric and Diastereomeric purity method
6R,12aS diastereomera Not more than 0.1 0.03 0.01 0.05
6S,12aS enantiomerb Not more than 0.1 Not detected Not detected 0.05
6S,12aR diastereomerc Not more than 0.1 Not detected Not detected 99.09
7. Chromatographic Purity by HPLC (% w/w)*
Individual impurity Not more than 0.1 0.07 0.05 0.05
Total impurities Not more than 0.3 0.07 0.05 0.05
8. Assay by HPLC (% w/w)
(on dried basis) Not less than 97.5 and Not more than 102.5 98.64 98.89 99.09
*Reporting threshold-0.05%
Chemical Name of impurities:
a) (6R,12aS)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.
b) (6S,12aS)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.
c) (6S,12aR)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.
Packaging and Storage: Preserve in well-closed containers. Store at room temperature.

Table 2B:
Sr. No Reference Chemical Name Structure
1 Impurity A (6R,12aS)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.

2 Impurity B (6S,12aS)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino[1’,2’:1,6]pyrido[3,4- b]indole-1,4-dione.

3 Impurity C (6S,12aR)-6-(1,3-Benzodioxol-5-yl)-2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino [1’,2’:1,6] pyrido[3,4- b]indole-1,4-dione.

Structure 1: Tadalafil

Table-3:
1H NMR SPECTRUM ASSIGNMENTS FOR TDF-A124-051-01
PROTON CHEMICAL SHIFT(ppm) MULTIPLICITY NO OF PROTONS
12 11.028 s 1
3 7.555 – 7.536 d (J=7.6 Hz) 1
1 7.307 - 7.287 d(J=8 Hz) 1
2 & 21 7.078 – 6.979 m 2
8 6.867 s 1
9 & 10 6.783 s 2
7 6.132 s 1
11 5.923 s 2
5 4.418 – 4.378 dd(J=4.4, J=12.0 Hz) 1
6 4.200 – 4.157 d(J=17.18 Hz) 1
61 3.969 – 3.926 d(J=17.18 Hz) 1
4 3.549 – 3.498 dd(J=4.8, J=15.99 Hz) 1
41 3.008 – 2.969 dd(J=12.39 Hz, J=15.58) 1
13 2.933 s 3

s= Singlet, d=Doublet, m=Multiplet. dd=Doublet of doublet
The Proton Nuclear Magnetic Resonance experiment was done using DMSO-d6 as solvent.

Structure 2: Tadalafil

Table-4:
13C NMR SPECTRUM ASSIGNMENTS FOR TDF-A124-051-01
CARBON DEPT CHEMICAL SHIFT (ppm)
10 C 166.909
13 C 166.580
18 C 147.118
20 C 146.130
1 C 137.049
15 C 136.275
16 C 134.003
6 C 125.836
3 CH 121.300
22 CH 119.398
4 CH 118.929
5 CH 118.147
17 CH 111.363
21 CH 108.012
2 CH 107.049
7 C 104.818
19 CH2 100.949
9 CH 55.570
14 CH 55.414
12 CH2 51.487
11 CH3 32.881
8 CH2 23.224

The Carbon Nuclear Magnetic Resonance experiment was done using DMSO-d6 as solvent.

Structure 3: Tadalafil

Molecular mass: 389.40 g/mol
Exact Mass: 389.14 g/mol
Table: 5
Details Expected mass Obtained results
M+H 390.4 389.9

Elemental analysis (C, H, N,S)
Theoretical and experimental values for the elemental analysis of TDF-A124-051-01are given in the Table. Data obtained from elemental analysis is matching with the theoretical values of Molecular formula: C22H19N3O4.

Table: 6
Element Theoretical (% w/w)
Determined (% w/w)
Carbon 67.86 67.86
Hydrogen 4.92 4.92
Nitrogen 10.79 10.79