FIELD OF THE INVENTION
The present invention relates to an improved process for preparation of for preparation of (2S,3R-)-2-;amino-3-(3,4-dibenzyloxy)phenyl)-3-hydroxypropanoic acid compound of formula-II, which is an useful intermediate for preparation of Droxidopa. The present invention also relates to novel intermediate Nickel complex compound of Formula VI useful in the preparation of (2S, 3R)-2-amino-3-(3,4-dibenzyloxy)phenyl)-3-hydro'xypropanoic acid compound of formula-II.
BACKGROUND OF THE INVENTION:
Droxidopa is chemically known as (2S, 3R)-2-amino-3-(3J4-dihydroxyphenyl)-3-hydroxypropanoic acid. It is also known as L-threo-(2S, 3R)-3-(3, 4-dihydroxyphenyl)serine or (-)-threo-3-(3,4-dihydroxyphenyl)-L-serine and it is structurally represented by the following formula (I)

Droxidopa has been approved by USFDA and marketed by Lundbeck under the trade name Northern® in the form of capsules with dosages of 100 mg, 200 mg and 300 mg for oral administration. Northera® is indicated for the treatment of orthostatic dizziness, lightheadedness, or the "feeling that you are about to black out" in adult patients with symptomatic neurogenic orthostatic hypotension caused by primary aulonomic failure, dopamine beta-hydroxylase deficiency, and non-diabetic autonomic neuropathy.
Droxidopa is a synthetic amino acid precursor which acts as a prodrug to the neurotransmitter norepinephrine (noradrenaline), which is extensively distributed throughout the body.
It has been observed that the L-threo enantiomer is the desired isomer having desired activity. Administration of the active L-threo enantiomer of the compound (I), substantially free of its other isomers, would essentially enable a reduction in the dose of drug. Due to the importance of the L-threo enantiomer of the compound (I) as an oral, there exists a need to develop an economical and efficient synthetic process.for its production.

Droxidopa and its pharmaceutically acceptable salts were first disclosed in US patent No. 3,920,728 (hereinafter referred as US728). US '728 disclose a process for preparation of Droxidopa, which is as shown below:

US Patent No. US 4,480,109 A discloses another process for preparation of Droxidopa, which involves the resolution of D,L-threo isomers using optical active amine. The process is shown as follows:

US Patent No. 4,319,040 discloses a process for preparation of Droxidopa which is shown as • follows:


wherein R is methyl, isopropyl or isobutyl.
US Patent No. 4,562,263 (hereinafter referred as US '263) discloses a process for preparation of Droxidopa, which is shown as follows:

US patent publication No. 2013/253061 disclosed a process for preparation of Droxidopa, which is shown as follows:

Droxidopa
In view of the above, all the prior art processes for the preparation of Droxidopa involves a
resolution method, which involves the use of resolving agent. The use of resolving agent
makes the process expensive. Though the recyclization of the resolving agent is possible; this
. _ _ recyclization pr-OCjessJavplves. additional processing which makes the process more

expensive. The yield of compound obtained after resolution is 50% due to the loss half of the racemate. The undesired enantiomer cannot be recycled and is discarded. Further the prior art process involves use of hydrazine, which is genotoxic reagents. Therefore, the prior art processes are not suitable for commercial manufacturing.
Thus, there is a need to develop an alternative asymmetric synthesis which would provide the desired L-threo isomer with high yield and to avoid genotoxic material.
J. Am. Chem. Soc. 107, 4252-59 (1985) disclosed a general method of Diastereo- and Enahtioselective Synthesis of beta-Hydroxy-alpha-amino acids by condensation of aldehydes and ketones with Nickel complex of glycine schiff s base which involves formation of metal complex as:
n;R'=H;X=0;andM=Ni . The J. Am. Chem. Soc. 107, 4252-59 (1985) further disclosed hydrolysis of the metal complex to give desired compound.

*"*7 M fill ■+* (S) - QP f R) - D«lr*d forapound
However, though this process involves recovery of starting material, the isolation of desired compound carried out through column chromatography.
Journal of fluorine chemistry 75, 93-101, (1995) discloses the synthesis of 2- and 6-fluoro analogues of threo-3-(3,4-dihydroxyphenyl)serine:



However all the attempts to enatio, diatereo selective approaches to these analogues either results no product or produced predominantly erythro diastereomer.
WO 2005085178 Al discloses a process for preparation of Droxidopa, which is shown as follow:

The present inventors during development when repeated the same process, which results in the formation product with low yield.
US Patent Publication No. 2008/108830 discloses a process for preparation of Nickel complex using optically active 2-[(N-Benzylprolyl)amino]-5-chlorobenzophenone as follows:


Nickel complex
Further, this patent publication neither generically nor specifically discloses the condensation of Nickel complex with an aldehyde compound to prepare compounds useful in the synthesis of Droxidopa.
Thus, there is a need to develop a process for preparation of Droxidopa, which avoids the synthetic process involving chiral resolution, genotoxic materials to obtain desired L-threo isomer, thereby making the process of the present invention simple, efficient, cost-effective and industrially feasible to provide Droxidopa with high yield and purity.
OBJECTIVES
The objective of the present invention is to provide Droxidopa in high yield and high stereo selectivity.
Another object of the present invention is to provide a stereoselective process for the preparation of Droxidopa using asymmetric induction, thereby avoiding synthetic process involving chiral resolution.
Another object of the present invention is to provide a process for the preparation of Droxidopa involving novel intermediates of formula VI.
Another object of the present invention is to provide a process for the preparation of Droxidopa, which avoids genotoxic materials/reagents.
Another object of the present invention is to provide a process for the preparation of Droxidopa which is simple, efficient, cost-effective and industrially feasible process.

SUMMARY OF THE INVENTION
The process for preparation of Droxidopa compound of formula I,

which comprises of:
a) Condensation of (R)-N-(2-benzyl-4-chlorophenyl)-l -benzylpyrrolidine-2-carboxamide
hydrochloride compound of formula-Ill
with glycine and Nickel salt in presence of a base in organic solvent to provide compound of formula-IV;

b) optionally, isolating the compound of formula-IV;
c) condensation of the compound of formula-IV with 3,4-dibenzyloxybenzaldehyde compound of formula-V



d) optionally, isolating the compound of formula-VI;
e) hydrolysis of the Nickel complex compound of formula-VI with an acid in organic solvent, followed by treating with a base to provide (2S,3R)-2-amino-3-(3,4-dibenzyloxy) phenyl)-3-hydroxypropanoic acid compound of formula-N; and
V
f) converting the compound of formula-II to Droxidopa compound of formula-I.
A process for the preparation of (2S,3R)-2-amino-3-(334-dibenzyloxyphenyl)-3-hydroxypropanoic acid compound of formula-II,

which comprises of:
a) condensation of compound of formuia-IV

with 3,4-dibenzyloxybenzaldehyde compound of formula-V


in presence of a base in organic solvent to provide Nickel complex compound of formula-VI;

b) optionally, isolating the compound of formula-VI;
c) hydrolysis of the Nickel complex compound of formula-VI with an acid in organic
solvent, followed by treating with a base to provide to provide compound (2S,3R)-2-amino-3-
(334-dibenzyloxypheny])-3-hydroxypropanoic acid compound of formu!a-II
A novel intermediate Nickel complex of compound of formula-VI

A recovery process of (R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide-hydrochloride compound of formula-Ill, which comprises of:
a) taking the filtrate after separating the solid (2S,3R)-2-amino-3-(3,4-dibenzyloxy) phenyl)-3-hydroxypropanoic acid compound of formula-II,
b) optionally, distilling off the solvent from the filtrate and adding another suitable solvent to the obtained residue, and

1' '
c) aclding aqueous hydrochloric acid to a solution of step-a) or step-b) to provide (R)-N-(2-benzyi-4-chlorophenyl)-1 -benzylpyrrolidine-2-carboxamide hydrochloride compound of formula-Ill.
DETAILED DESCRIPTION OF THE INVENTION:
In an embodiment of the present invention provides a process for preparation of Droxidopa compound of formula-I, which comprises of condensation of (R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrroiidine-2-carboxamide hydrochloride compound of formula-Ill with glycine in presence of Nickel salt and a base in organic solvent provides compound of formula-IV. Further condensation of obtained compound of formula-IV with 3,4-dibenzyloxybenzaldehyde compound of formula-V in presence of a base in organic solvent to provide Nickel complex compound of formula-VI. Hydrolysis of the Nickel complex co'mpound of formula-VI with acid in organic solvent provides acid addition salt of (2S,3R)-2-amin6L3-(354-dibenzyloxy)phenyl)-3-hydroxy propanoic acid compound of formula-II. Further treating the obtained salt with a base to provide (2Ss3R)-2-amino-3-(3,4-dibenzyloxy)phenyl)-3-hydroxypropanoic acid compound of formula-II. Further compound of formula II is converted to Droxidopa.
In one embodiment, the present invention relates to a process for the preparation of (2S,3R)-2-amino-3-(3,4-dibenzyloxyphenyl)-3-hydroxypropanoic acid compound of formula-II, which comprises of condensation of compound of formula-IV with 3,4-dibenzyloxybenzaldehyde compound of formula-V in presence of base in organic solvent provides Nickel complex compound of formula-VI. Hydrolysis of the Nickel complex compound of formula-VI with an acid in organic solvent to provide an acid addition salt of (2S,3R)-2-amino-3-(3,4-dibenzyloxyphenyl)-3-hydroxypropanoic acid. Further treating the obtained salt with a base to provide (2S,3R)-2-amino-3-(3,4-dibenzyloxyphenyl)-3-hydroxypropanoic acid compound of formula-II.
In another embodiment, in the process of the present invention the term "hydrolysis" used in reference to any step of reaction corresponds to the decomposition of metal complex compound of formula-VI.
In another embodiment, in the Drocess of the oresent invention the intermediate comDound of " "f6rrnula"-lV,'Nfcktel^compiex compound of formula-VI and the acid addition salt of compound

of, formula-II are isolated during the reaction, or in-situ converted to the compound of
forniula-I I.
■i.i -i
In 'another embodiment throughout the invention, the "solvent" is selected from the halogenated solvents such as dichloromethane (MDC), ethylene dichioride (EDC), chloroform and the like; alcoholic solvents such as methanol, ethanol, isopropanol, butanol and the like; ketone solvents such as acetone, methyl isobutyl ketone, dimethylformamide and the like; ether solvents such as tetrahydrofuran (THF), 2-methyltetrahydrofuran (MeTHF), diethyl ether, isopropyl ether, methyl isobutylether, methyl t-Bu ether (MTBE), dioxane and the like; ester solvents such as ethyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, tert-butyl acetate and the like; hydrocarbon solvents such as toluene, xylene and the like; nitriles such as acetonitrile, propionitrile, butyronitrile, acrylonitrile, and the like; Polar aprotic solvents are selected from the group comprising of dimethylformamide (DMF), dimethylacetamide (DMA), acetonitrile, dimethylsulfoxide (DMSO) and the like; or mixtures thereof.
In another embodiment, the "base" is selected from inorganic base selected from alkali metal hydroxide such as sodium hydroxide, potassium hydroxide and the like; alkali metal alkoxide such as sodium methoxide, sodium ethoxide, sodium isoporpoxide, sodium tert-butoxide, potassium tert-butoxide and the like; alkali metal carbonates such as sodium carbonate, potassium carbonate and the like, alkali metal bicarbonate such as sodium bicarbonate, potassium bicarbonate and teh like, ammonia, or organic base selected from NR1R2R3, wherein each of Ri, R2 and R3 are same or different, is selected from C1-C7 alkyl; NHR4R5 wherein each of R4 and Rs, which can be the same or different, is Ci -C7 alkyl; NH2R6 where R6 is C1-C7 alkyl; and pyridine or mixtures thereof.
In-another embodiment throughout the invention, the "acid" is inorganic acid or organic acid selected from the group consisting of hydrochloric acid, hydrobromic acid, hydrofluoric acid, nitric acid, sulfuric acid and phosphoric acid, acetic acid and trifluro acetic acid or mixtures thereof.
In another embodiment throughout the invention, Nickel salts selected from nitrate salts such as NifNOVb etc., a halogenate salts such as Ni.CJ, Ni,Br„etc„ an acejate.^alt suqh&s Ni(OAc)2. etc., and a sulfate such as NiSC>4 and hydrates thereof, preferably Ni(N03)2.6H20.

In. another embodiment, the condensation of compound of formula-Ill with glycine in presence of Nickel salt is carried out at a temperature ranging from about 25°C to about i'00o.C, preferably from 40°C to 70°C, more preferably at 50-55°. While, the reaction time is not particularly limited, it is generally 10 minutes to 50 hours, preferably 30 minutes to 3 hours.
In another embodiment, the condensation of compound of formula-IV and compound of formula-V is carried out at temperature ranging from 10 to 50°C, preferably at 10-20°C, for a period of 1- 10 hours, preferably 2-3 hours.
In another embodiment, the hydrolysis of Nickel complex compound of formula-VI with an acid is carried out at a temperature ranging from about 10°C to about 100°C, preferably at 40°C-45°C for a period of 1-10 hours, preferably for 4 hours.
In another embodiment, the compound of formula-II prepared according to present invention can be utilized in the preparation of Droxidopa by known methods.
In another embodiment, the (R)-N-(2-benzyl-4-chiorophenyl)-1 -benzylpyrrolidine-2-carboxamide hydrochloride compound of formuia-III used as a starting material in the present invention can be prepared by any of the known methods in the art, for example treating D-proline with benzyl chloride in presence of base such as potassium hydroxide in isopropanol to provide N-benzyl-D-proIine, which is further reacted with of (R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride compound of formula-VII in presence of thionyl chloride in dimethylformamide and acetonitrile. Further the process represented schematically as follows:


In another embodiment of the present invention provides novel Nickel complex represented by compound of formula-VI, which is useful as a intermediate in the synthesis of Droxidopa.
In another embodiment of the present invention, after separating the solid compound of formula-II, the filtrate is taken for further recovery of starting material. The recovery process carried out either by adding acid directly to filtrate obtained (or) by distilling off the solvent from the filtrate and again taking the residue into another suitable solvent and followed by addition of an acid to the obtained solution provides (R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride compound of formula-Ill.
In the following section embodiments are described by way of examples to illustrate the process of invention. However, these do not limit the scope of the present invention. Variants of these examples would be evident to persons ordinarily skilled in the art.
Examples:
ExampJe-1: Preparation of Benzyl-D-proIine
d-^rpline (300 gm) and potassium hydroxide (439 gms) were charged in isopropanol (3000 ml) and the reaction mixture was stirred for 15-30 minutes at 40-45°C. Benzyl chloride (358 gms) was added to the reaction mixture at 40-45°C and stirred for 6 hours. The reaction mixture was cooled to 0-5°C and hydrochloric acid was added to adjust pH to 6.0. Filtered the reaction mixture and distilled off the solvent from the filtrate to get residue. Acetone (900 ml) was added to the obtained residue at 25-30°C and stirred for 4-5 hours. Filtered the solid and washed with'acetone and then dried to get Benzyl-D-proline. Yield: 430 gms.
ExampIe-2: Preparation of (R)-N-(2-benziyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride (Formula-Ill)
Benzyl-D-proline (100 gms) and Acetonitrile (500 ml) were charged in Dimethylformamide (35,-gms) at 25-30° C and the reaction mixture was cooled to -20 to -15°C. Thionyl chloride (75 gms) was added to the reaction mixture at -20 to -15°C and stirred for 2 hours. (2-amino-5-chlorophenyl)(phenyl)methanone (102 gm) was added lot-wise to the reaction mixture at -20 to -15°C and raised the temperature of the reaction mixture to25-3.0°C and stirred for 16-18 hours. Cooled the reaction mixture to U-^C filtered the precipitated solid and washed

with water followed by acetone and then dried to get (R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride. Yiefdf: 150 gms.
ExampIe-3: Process for preparation of (2S,3R)-2-amino-3-(3,4-dibenzyloxyphenyl)-3-hydroxypropanoic acid (Formula-II)
(R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride
compound of formula-Ill (100 gms), glycine (24 gms) and Ni(N03)2. 6H2O (71 gms) were charged in methanol (500 ml) at 25-30°C. The reaction mixture was heated to 50-55°C and stirred for 30 minutes. Cooled the reaction mixture to 15-20°C and sodium methoxide powder (64 gms) was added to it. The temperature of the reaction mixture was raised to 55-60°C and stirred for 2 hours. Cooled the reaction mixture to 10-15°C. added 3,4-dibenzyloxybenzaldehyde (80 gms) followed by sodium methoxide solution (250 ml) to the reaction mixture at 10-15°C and stirred for 2 hours. Sulfuric acid solution (100 ml) was added slowly to the reaction mixture at a temperature below 20°C. Heated the reaction mixture to 40-45°C and stirred for 4 hours. Distilled off the solvent completely to get (2S,3R)-2-amino-3-(3,4-dibenzyloxyphenyl)-3-hydroxypropanoic acid - sulfate salt. Water (500 ml) followed by ethyl acetate (800 mi) were added to the obtained salt and neutralized the reaction mixture with aqueous ammonia. Filtered the precipitated solid and dried to get (2S,3R)-2-amino-3-(3,4-dibenzyloxyphenyl)-3-hydroxypropanoic acid. Yield: 65 gm.
Example-4: Recovery process of (R)-N-(2-benzyl-4-chIorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride (Formula-Ill)
The ethyl acetate filtrate obtained in example-3 was taken and distilled off the solvent at 35-40°C to get residue. Acetone (650 ml) was added to the obtained residue at 25-30°C, Cone. HC1 (30 ml) was added slowly to it and stirred for 2 hours at 25-30°C. Filtered the solid and washed- with acetone and then dried to get (R)-N-(2-benzyl-4-chlorophenyl)-l-benzy.lpyrrolidine-2-carboxamide hydrochloride. Yield: 70 gms.
Example-5: Recovery process of (R)-N-(2-benzyl-4-chlorophenyl)-l-benzylpyrrolidine-2-carboxamide hydrochloride (Formula-Ill)
I E fiP1? etloytacetate;filtrate ©bminedLia.example^r^wasjtakeji ;and.€o.nc. HCi (30]mjj) was added slowly to it and stirred for 2 hours at 25-30°C. Filtered the solid, washed with acetone and v

then .dried to get (R)-N-(2-benzyl-4-ch!orophenyl)-l-benzylpyrrolidine-2-carboxamide
hydrochloride.
Yield: 70 gms.
Example-6: Preparation of compound of formula-TV
(R)j^-(2-benzyl-4-chiorophenyl)-l-benzylpyrrolidine-2-carboxaniide hydrochloride (100 gms), Glycine (28 gms) and and Ni(N03)2. 6H2O (84 gms) were charged in methanol (500 ml) at 25-30°C. The reaction mixture was heated to 50-55°C and stirred for 30 minutes. Cooled the reaction mixture to 15-20°C and sodium methoxide powder (74 gms) lot-wise added to it. Heated the temperature of the reaction mixture to 55-60°C and stirred for 2 hours. Pre heated water was added to reaction mixture at 50-55°C and stirred for 1 hour. Cooled the reaction mixture to 25-30°C and stirred for 10-12 hours. Filtered the precipitated solid, washed with water and dried to get compound of formula-IV. Methyl tertiary butyl ether (300 ml) was added to the obtained solid at 25-30°C and stirred for 1 hour 30 minutes. Filtered the solid, washed with methyl tertiary butyl ether and dried to get compound of formula-IV. Yield; 100 gms
<*■ Example-7: Preparation of 3,4-dibenzyloxybenzaldehyde (Formula-V)
3,4-dihydroxybenzaldehyde (100 gms), Benzyl chloride (220 gms) and potassium carbonate
(500 gms) were charged in dimethylformaide (500 ml) at 25 - 30°C. The reaction mixture
was heated to 110 - 120°C and stirred for 10-12 hours at 110-120°C. Cooled the reaction
mixture to 25-30°C, water was added to it and stirred for 1 hour at 25-30°C. Filtered the
precipitated solid and washed with water. Water was added to the obtained solid and stirred
for 1 hour 30 minutes at 25-30°C. Filtered the solid, methanol was added to it. The reaction
mixture was heated to 60-65°C and stirred for 1 hour. Cooled the reaction mixture to 25-
30°C. Filtered the precipitated solid, washed with methanol and dried to get 3,4-
• dibenzyloxybenzaldehyde.
Yield: 200 gms.
'■ I Examplc-8: Preparation of Droxidopa (Formula-I)
(?S, 3R)-2-amino-3-(3,4-dibenzyloxy)phenyl)-3-hydroxypropanoic acid (100 gms),
methanol (1000 ml), 10 % Pd/C (4 gms) and Conc.HCl solution (prepared from 30 ml
.T p tCqnc.HOJ and i30£mlf^atef)Mwsr^^ at-j.2£-30?Cg.Cooled the
reaction mixture to 15-20°C and stirred for 3-4 hours at 15-20°. Filtered the reaction

mixture, pH of reaction mixture was adjusted to 3.2-3.8 with triethylamine (34 gms). . Filteredt.the precipitated solid and washed with methanol and water and dried to get ' Droxidopa. , Yield: 42-44 gms
Example-9: Preparation of Droxidopa Hydrochloride salt
Droxidopa (100 gms) was charged in water (250 ml) at 25-30°C and stirred for 15 minutes. The reaction mixture was cooled to 20-25°C. Cone. HC1 (60 ml) was slowly added to the reaction mixture at 20-25°C and stirred for 1 hour 30 minutes to 2 hours. Isopropanol (600 ml) was slowly added to the reaction mixture at 20-25°C and stirred for 1 hour 30 minutes to 2 hours. Filtered the solid and washed with isopropanol to get Droxidopa Hydrochloride.
Yield: 105 gms
t
Example-10: Preparation of Pure Droxidopa (Formula-I)
Droxidopa Hydrochloride (100 gms) was charge in water (1000 ml) at 25-30°C and stirred fpr 10-1*5 minutes. Cooled the reaction mixture to 20 - 25°C. Cone. HC1 (30 ml) was slowly added to the reaction mixture at 20-25°C and stirred for 10-15 minutes. Activated carbon was ' added and stirred for 20-30 minutes at 20-25°C. Filtered the reaction mixture, pH of the filtrate was adjusted to 3.5 to 4.0 with a solution of sodium carbonate (30 gms) and water (300 ml) at 25-30°C and stirred for 30 minutes to 1 hour. Filtered the solid, water was added to it and stirred for 1 - 1 14 hour. Filtered the solid, washed with isopropanol and dried to get Pure Droxidopa.