FORM 2
THE PATENT ACT, 1970
(39 of 1970)
&
The Patent Rules, 2006
COMPLETE SPECIFICATION
(See section 10; rule 13)
1. TITLE OF THE INVENTION: An improved process for the preparation of substantially pure N-carbamoyl-tert leucine of the formulae 1/IA.
2. Applicant

(a) NAME: Arch Pharmalabs Limited
(b) NATIONALITY: Indian
(c) ADDRESS: 541-A, Arch House, Marol-Maroshi Road, Marol,
Andheri (East), Mumbai - 400 059, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed.

TITLE: An improved process for the preparation of substantially pure N-carbamoyi-tert leucine of the formulae I/I A.

TECHNICAL FIELD OF TECHNOLOGY: The present invention provides an efficient process for the preparation of substantially pure N-carbamoyl-tert leucine of formula I which is a key intermediate for the preparation of various therapeutic agents or therapeutic agent precursors used as ("HCV") protease inhibitors. The said intermediate when coupled with a suitable proline derivative results into the formation of various therapeutic agents or therapeutic agent precursors used as ("HCV") protease inhibitors. A few therapeutic agents comprising use of formula I as one of the moiety are disclosed herein below. The present invention in particular relates to an improved process over the prior art resulting in to the elimination or minimization or prevention of certain impurities those are

generally formed as by-products during the preparation of compound of formula 1. The by-products of the reaction those become the prominent impurities later in the desired product are represented by formulae VI, VII, VIII and IX. The present invention relates to a process for the preparation of substantially pure N-carbamoyl-tert leucine comprising contacting tert-butyl isocyanate represented formula IV produced optionally in-situ by contacting tert-butyl amine of formula II with phosgene represented by formula III in biphasic system with tert-leucine of formula V maintaining the biphasic system as reaction medium while keeping the control over critical parameters like concentration or dilution of reacting components along with pH, molar ratio, temperature and the like. Use of biphasic system plays a vital role in keeping the control over the impurities in the final desired product.



The term "binary phase" used in the specification means system comprising two solvents system comprising water as one component and water

immisiible organic solvent as second component so that they will form a system having two phases.
The term "dilution" used herein means the reacting component under the consideration is not to be used as such in neat form but it is to be diluted with a solvent.
Therapeutic agents comprising the use of compound of formula I/IA are.

BACKGROUND OF THE INVENTION:
Infection with HCV is a major cause of human liver disease throughout the world. Although only 30% of acute infections are symptomatic, greater than 85% of infected individuals develop chronic, persistent infection. In the US, an estimated 4.5 million Americans are chronically infected with HCV. Treatment costs for HCV infection have been estimated at $5.46 billion in the US in 1997. Worldwide over 200 million people are estimated to be infected chronically. HCV infection is responsible for 40-60% of all chronic liver disease and 30% of all liver transplants. Chronic HCV infection accounts for 30% of all cirrhosis, end-stage liver disease, and liver cancer in the United States. Compounds having (S)-2-(3-tert-butylureido)-3,3-dimethylbutanoyl group moiety represented by the compound of formula IA as part of the therapeutic compound have been identified useful for the treatment of hepatitis C and related disorders. In view of the importance of hepatitis C virus ("HCV") protease inhibitors and compound of formula I being a key intermediate for the preparation of hepatitis C virus ("HCV") protease inhibitors, there has always been interest to develop improved, safe, economical and industrially viable process which can address to the deficiencies of the processes reported therein in the prior art. N-carbamoyl-tert-leucine, in general is produced by carbamoylizing the amino group of tert-leucine, i.e. 2-amino-3,3-dimethyl butanoic acid. The method disclosed in the prior art for the preparation of N-carbamoyl-amino acid comprises reacting amino acid with an isocyanic acid.
EP2221294 (hereinafter referred as ;294) discloses the process for preparing N-carbamoyl-tert-leucine, comprising contacting tert-leucine (neat) with an isocyanic acid compound in an amount of 0.9 to 1.1 times the amount of

tert-leucine by mole while keeping the pH value of the mixture at 8.0 to 13.5. Reaction of tert-butyl isocyanate with tert-leucine in water (pH adjusted to 11.5 by adding NaOH) gave N-tert-butylcarbamoyl -tert-leucine. '294 describes the crucial role of pH and control over the formation of dipeptide and bis urea impurities. '294 describes that if the pH during the reaction is low, the time of the reaction tends to be prolonged which may result in the formation of dimerization of peptide like compounds comprising the condensation of N-carbamoyl-tert leucine of the formula 1 with tert leucine of formula V as depicted in scheme I. '294 discloses that the peptide like impurity is represented by compound of formula VIII. If the pH during the reaction is high, isocyanic acid compound may get degraded and a urea compound of formula IX, which is condensation product of an amine compound, gets formed as an impurity as depicted in scheme II. As per '294 both the impurities can be prevented or controlled by keeping a control over pH and stoichiometric ratio of the reaction medium and reacting components respectively. IMPURITIES AS PER '294 PATENT SCHEME I: Low pH, prolonged time and higher temperature



Example 1 and 2 of '294 does not report the isolated yield. The yield is simply estimated on HPLC analysis. Example 2 does not mention the chiral purity of the product. Example 3 isolates the product by extraction procedures rather that isolating it as solid after pH adjustments. As per the inventors of the present invention, the N.Nr-di(tert-butyl) urea represented by formula IX (wherein R( is tert-butyl) has a very low solubility in toluene. However, as per the workup as disclosed in example 3 wherein ethyl acetate is used as extractant, the extractant will not only extract the product but will also extract the N,N;-di(tert-butyl) urea formed as a byproduct. Therefore, toluene used as antisolvent will cause the precipitation of the said impurity also resulting into the isolation of impure desired product. Inventors of the present invention have provided a solution for removing the said impurity N.N'-di(tert-butyl) urea represented by formula IX (wherein Ri is tert-butyl) by filtration before acidifying the reaction mass to isolate the desired product of formula I, thereby removing the said impurity at this step and is not carried forwarded.
Another drawback associated with this patent is the use of neat isocyanate i.e. as such without any dilution.
However the, said patent neither teach nor motivates a person skilled in the art about the role played by the dilution factor of reacting components in controlling the impurity formation nor about the role of biphasic system through out the reaction which enables to separate and remove impurity at

this stage itself. Moreover the said patent also does not talk about the possibility of existence of various possible positional isomers of tert leucine that contribute towards various possible impurities those are isomers of compound of formula I; wherein R1 is tert-butyl.
WO2009039361 (hereinafter referred as '361) discloses a process for the preparation of (S)-2-(3-tert-butylureido)-3,3- dimethylbutanoic acid comprising contacting (S)-2-amino-3,3-dimethyIbutanoic acid with a silyl ester forming reagent under conditions sufficient to form an in-situ silyl ester of (S)-2-amino-3,3- dimethylbutanoic acid and contacting this in-situ silyl ester of (S)-2-amino-3,3-dimethylbutanoic acid with tert-butylisocyanate or a synthetic equivalent thereof.
The drawback associated with this process is the additional step of silylation that comprises an additional unit operation that enhances the overall cost of the process owing to high cost of silylating agent making the process uneconomical.
Moreover the said patent application does not describe the impurity profile of the product. '361 discloses the use of synthetic equivalent of tert-butyl isocyanate that includes in-situ generation of tert-butyl isocyanate but none of the examples or embodiments described therein disclose the use of in-situ generated tert-butyl isocyanate. The product obtained shows the purity of 98.8% HPLC and chiral purity of 97.8% without disclosing the impurity profile.
JP2007332050 (hereinafter referred as '050) discloses crystals of the N-tert-butylcarbamoyl-L-tert-leucine. The process comprises gradual drop wise addition of neat tert-butyl isocyanate to a mixture of aq. NaOH solution of

L-tert-leucine and THF at a temperature of 30°C or below and the reaction mixture is stirred at room temperature for 2 h. The product is extracted with ethyl acetate and mixed with water followed by distillation to remove solvent under the reduced pressure, finally resulting into crystalline product. Process disclosed in '050 comprises physical handling of hazardous and low flash point ter-butyl isocyanate.
Inventors of the present invention found that dipeptide compound formed due to dimerisation of N-carbamoyl-tert leucine of formula I and tert leucine of formula V can also get formed due to condensation of N-carbamoyl-tert leucine of formula I with different positional isomers oftert-leucine represented by formulae B and A . Inventors of the present invention after in-depth studies have been successful in characterising the new impurity of formula VII (which is isomer of formula I; wherein R| is tert butyl) other than those reported in '294 patent. They also have provided a simplified process for the removal of impurity of formula IX.
PROBLEM TO BE SOLVED BY THE INVENTION:
Formation of the by-products those become prominent impurities of the product of formula I.
The various by-products formed during the process for the preparation of compound of formula I/IA are formed as follows:
I. When compound of formula I; (wherein R| is tert-butyl) reacts with unreacted tert-leucine of formula V it results into the formation of impurity of formula VIII.

II: When unreacted amine of formula II reacts with unreacted isocyanate of formula IV (wherein R| is tert-butyl), it results into formation of impurity of formula IX (wherein R| is tert-butyl). The problem associated with the said impurity is the limited or low solubility as a result of which it gets precipitated easily and thus remains trapped in the product. Moreover, if it gets carried forward throughout the reaction, isolation of final product of formula I becomes difficult.

(wherein R1 is tert-butyl) III: When isocyanate of formula IV(wherein R1 is tert-butyl), reacts with one positional isomer of tert-leucine of formula B, it results into the formation of impurity of formula VI (Isomer of compound of formula I; wherein R1 is tert-butyl).

IV: When isocyanate of formula IV (wherein R1 is tert-butyl) reacts with another positional isomer of tert-leucine represented by A, it results into the formation of impurity of formula VII (Isomer of compound of formula I; wherein R1 is tert-butyl).

wherein R1is tert-butyl
V. Another technical problem found was no proper separation for positional isomers of tert-leucine by HPLC under the normal conditions.
SOLUTIONS TO THE PROBLEM:
I. The impurities of formulae Vll, VIII are eliminated or minimized by
controlling the critical parameters like pH, molar ratio, temperature and
concentration i.e. increasing the dilution of reacting components and
introducing binary phase comprising water immiscible organic solvent and
water as reaction medium.
II. The impurity of formula IX is removed by the filtration rather than
removing it by extraction method. It is filtered off and separated before the
isolation of product of formula I.
Beside this, use of binary system for the preparation of isocyanate of formula IV comprising contacting aqueous solution of compound of formula

II with solution of compound of formula III in organic solvent characterized in such a way that unreacted amine of formula II remains in aqueous layer and is not carried forward to react further with isocyanate of formula IV thereby avoiding the formation of impurity of formula IX.
III. The said positional isomer of tert-leucine is not participating in the formation of impurity of formula VI. This is confirmed by the synthesis comprising the reaction of said positional isomer of tert-leucine (out sourced) with tert-butyl isocyanate and spiked with compound of formula I.
IV. The said positional isomer of tert-leucine represented by formula A is participating in the formation of novel impurity of formula VII rather than dipeptide impurity of formula VIII as reported by '294. This is confirmed by the synthesis of impurity of formula VII, comprising the reaction of said positional isomer of tert-leucine (out sourced) with tert-butyl isocyanate and spiked with compound of formula I. The retention time of compound of formula I is 7.43(molecular mass confirmed to be 230) while retention time of compound of formula VII is 11.47(molecula'r mass confirmed to be 230).
OBJECT OF THE INVENTION:
The main objective of the present invention is to provide an improved process for the preparation of substantially pure N-carbamoyl-tert leucine of formula I; comprising use of binary phase system for the reaction, dilution factor of the reacting components of the reaction together with controlling parameters like pH, molar ratio and temperature to eliminate or minimizes the formation of impurity of formula VIII.

The specific aspect of the present invention is to provide an improved process for the preparation of substantially pure (S)-2-(3-tert-butylureido)-3,3- dimethylbutanoic acid of formula IA; comprising use of binary phase system for the reaction, dilution factor of the reacting components of the reaction together with controlling parameters like pH, molar ratio and temperature to minimizes or eliminate the formation of impurity of formula VIII.
It has been observed by the inventors of the present invention that during the preparation of isocyanate of formula IV if dilution of tert-butyl amine is less i.e. concentration remains high and it results into enhancement of impurity of formula IX; (wherein R1 is tert-butyl) that creates difficulty in isolation of the compound of formula I if gets carried forward. Secondly; if the concentration of tert-leucine of formula V remains high it results into enhancement of impurities of formulae VIII and VII (in case L-tert-Leucine if contains positional isomer of as an impurity).
One of the general aspect of the invention is to use binary phase system for the preparation of isocyanate of formula IV comprising contacting aqueous solution of compound of formula II with solution of compound of formula III in an organic solvent characterized in such a way that unreacted amine of formula II remains in aqueous layer and is not carried forward to react with isocyanate of formula IV that remains in organic phase thereby minimizing/eliminating the formation of impurity of formula IX (wherein, Ri is a hydrogen atom, an optionally substituted alkyl group having 1-10

carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms).

A specific aspect of the invention is to use binary phase system for the preparation of isocyanate represented by formula IV (wherein R1 is tert-butyl) comprising contacting aqueous solution of compound of formula IT (wherein R1 is tert-butyl) with solution of compound of formula III in an organic solvent characterized in such a way that unreacted tert-butyl amine remains in aqueous layer and is not carried forward to react further with tert-butylisocyanate that remains in organic phase thereby minimizing/eliminating the formation of impurity represented by formula IX(wherein R1 is tert-butyl).

Another general aspect of the invention is to use binary phase system for the preparation of the compound of formula I comprising contacting dilute solution of isocyanate of formula IV in an organic solvent with aqueous solution of alkali metal salt of tert-leucine characterized in such a way that non volatile organic impurities get retained in organic solvent and are not

carried forward, therefore, do not get trapped with product of formula I which being an alkali metal salt remains in aqueous phase.

Another specific aspect of the invention is to use binary phase system for the preparation of the compound of formula I comprising contacting tert- butyl isocyanate in an organic solvent with aqueous solution of alkali metal salt of tert-leucine characterized in such a way that non volatile organic impurities get retained in the organic solvent and are not carried forward, therefore do not get trapped with product of formula IA which being sodium salt remains in aqueous phase.

One more genera! aspect of the invention is to remove the impurity of formula IX which being insoluble in aqueous medium is separated by

filtration before aqueous layer containing alkali metal salt of compound of formula 1 is taken for the acidification to isolate the product of formula I.

A specific aspect of the invention is to remove N,N'-di(tert-butyl) urea impurity represented by general formula IX by filtration before aqueous layer containing sodium salt of compound of formula IA is taken for the acidification to isolate the product of formula IA.

An additional aspect of the invention is to provide a process for the preparation of compound of formula I comprising the acidification of alkali metal salt of compound of formula I using monobasic acid like hydrochloric acid, hydrobromic acid and the like avoiding the formation of some unknown impurities thereby providing substantially pure compound of formula I.
One more specific aspect of the invention is to synthesize and characterize a new impurity of formula VII, wherein R1 is tert-butyl.


Another additional aspect of the invention is to use the new impurity of formula VII as reference standard for analytical purpose.
Another specific aspect of the present invention is to provide an efficient process for the preparation of compound of formula VII comprising:
a) contacting aqueous solution of tert-butyl amine with aqueous solution of triphosgene (n is 3 in formula III) an organic solvent thereby generating a binary phase to obtain tert-butyl isocyanate;

b) separating the organic layer containing tert-butyl isocyanate ( when R1 is tert-butyl in formula IV);

c) contacting the said organic layer containing tert-butyl isocyanate with aqueous solution of sodium salt of positional isomer of tert-leucine of formula A thereby generating a binary phase;

d) separating the aqueous layer

e) contacting aqueous layer from step d with monobasic acid to
precipitate the compound of formula VII;
f) isolating the compound of formula VII by filtration.
Organic solvent used is selected from the group comprising aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, ester, ethers that will form the binary phase when mixed with aqueous solvent. Alkali metal salt as used is selected from group comprising lithium, sodium and potassium.
An overall general aspect of the present invention is to provide an efficient process for the preparation of substantially pure compound of formula I comprising:
a) in-situ generation of isocyanate of formula IV comprising contacting aqueous solution of amine of formula II with solution of phosgene of formula III in an organic solvent thereby generating binary phase;
b) separating the organic layer containing isocyanate of formula IV;
c) contacting solution of isocyanate of formula IV from step b with aqueous solution of alkali metal salt of tert-leucine of formula V;
d) filtering the reaction mass to remove the impurity of formula IX (wherein, Rl is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1 -6 carbon atoms) and separating the aqueous layer;
e) contacting aqueous layer from step d with monobasic acid like hydrochloric acid, hydrobromic acid and the like to precipitate the compound of formula I;

f) isolating the compound of formula I by filtration. In case if the compound of the formula I is not obtained as solid it can be extracted and isolated by the standard established means known in the prior art.

Another general aspect of the present invention is to provide an efficient process for the preparation of substantially pure compound of formula I comprising:
a) contacting dilute solution of isocyanate of formula IV made in an organic solvent with aqueous solution of alkali metal salt of tert-leucine of formula V, thereby generating a binary phase;
b) filtering the reaction mass to remove the impurity of formula IX if any; (wherein Rt is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms) and separating the aqueous layer,
c) contacting aqueous layer from step b with mono basic acid like hydrochloric acid, hydrobromic acid and the like to precipitate the compound of formula I;
d) isolating the compound of formula 1 by filtration. In case if the compound of the formula I is not obtained as solid it can be extracted and isolated by the standard established means known in the prior art.


A specific aspect of the present invention is to provide an efficient process for the preparation of substantially pure compound of formula IA comprising:
a) in-situ generation of tert-butyl isocyanate comprising contacting aqueous solution of tert-butyl amine with solution of triphosgene represented by formula II; (wherein n is 3) in an organic solvent thereby generating binary phase;
b) separating the organic layer containing tert-butyl isocyanate;
c) contacting solution of tert-butyl isocyanate from step b with aqueous solution of alkali metal salt preferably sodium salt of tert-leucine;
d) filtering the reaction mass to remove N,N'-di(tert-butyl) urea represented by formula IX (wherein R1 is tert-butyl);
e) contacting aqueous layer from step d with hydrochloric acid to precipitate the compound of formula IA;
f) isolating the compound of formula IA by filtration or by standard
processes known in the prior art. In case if the compound of the formula
IA is not obtained as solid it can be extracted and isolated by the standard
established means known in the prior art.


A specific aspect of the present invention is to provide an efficient process for the preparation of substantially pure compound of formula IA comprising:
a) contacting dilute solution of tert-butyl isocyanate represented by formula IV; (wherein R1 is tert-butyl); made in an organic solvent with aqueous solution of alkali metal salt of tert-leucine of formula V, thereby generating a binary phase;
b) filtering the reaction mass to remove the impurity of formula IX if any; (wherein R1is tert-butyl) and separating the aqueous layer;

c) contacting aqueous layer from step b with hydrochloric acid to precipitate the compound of formula 1;
d) isolating the compound of formula I by filtration or by standard processes known in the prior art. In case if the compound of the formula IA is not obtained as solid it can be extracted and isolated by the standard established means known in the prior art.


SUMMARY OF THE INVENTION:
The present invention provides an efficient process for the preparation of substantially pure N-carbamoyl-tert leucine of formula 1 which is a key intermediate for the preparation of various therapeutic agents or a therapeutic agent precursor used as ("HCV") protease inhibitors. The said intermediate when coupled with a suitable proline derivative results into the formation of various therapeutic agents or a therapeutic agent precursor used as ("HCV") protease inhibitors.
The present invention relates to an improved process resulting in to the elimination or minimization or prevention of formation of certain impurities which are formed as by-products during the preparation of compound of formula I.
Prominent impurities are represented by compounds of formulae VI, VII, VIII and IX. The present invention relates to a process for the preparation of substantially pure N-carbamoyl-tert leucine comprising contacting tert-butyl isocyanate of formula IV produced optionally in-situ by contacting tert-butyl amine of formula II with phosgene represented by formula III in a biphasic solvent system with tert-leucine of formula V keeping biphasic system as reaction medium while keeping the control over critical parameters like concentration or dilution of reacting components along with pH, molar ratio, temperature and the like. Use of biphasic system plays a vital role for the preparation of substantially pure compound of formula I for eliminating or minimizing the impurities. The general schematic representation of the invention is as given below:

DETAILED DISCREPTION OF THE INVENTION
Disclosed herein is an efficient, economical and industrially viable process for the preparation of N-carbamoy!-tert leucine of formula (I). The disclosure of the present invention has been described in detail under the following heads:
DESCRIPTION OF THR FIGURES:
Figure 1 represents online HPLC chromatograph for the reaction comprising the use of neat isocyanate,
Figure II represents HPLC chromatograph for the reaction mass comprising the use of hydrochloric acid for acidification during the isolation of
Figure III represents HPLC chromatograph for the reaction mass comprising the use of sulphuric acid for acidification during the isolation of compound of formulae I/I A results into impure product.
The present invention is further explained by taking particular examples as
given herein below:

2. Use of diluted tert-butyl isocyanate:
1. In-situ generation of tert-butyl isocyanate:

The inventors of the present invention observed that the process for the preparation of compound of formula IA comprising use of tert-butyl isocyanate in next with L-tert-leucine as reported in the prior art results into the formation of impure product as depicted in chromatograph given herein below in figure I:

Figure - I
Inventors of the present invention have studied the importance of introducing the binary system through out the process right from the preparation of isocyanate of formula IV and its further use for the preparation of compound of formula I. Use of binary system for the preparation of isocyanate of formula IV comprising contacting aqueous solution of compound of formula II with solution of compound of formula III in an organic solvent and as a result unreacted amine of formula II

remains in aqueous layer and is not carried forward to react with isocyanate of formula IV thereby avoiding the formation of impurity of formula IX wherein R, is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms,.
It has been observed by the inventors of the present invention that during the preparation of isocyanate of formula IV if dilution of tert-butyl amine is less i.e. concentration remains high it results into the increased quantity of impurity represented by formula IX, (wherein R1 is tert-butyl) which when carried forward creates difficulty in isolation of the compound of formula I. Inventors of the present invention have also observed that the high concentration of tert-leucine of formula V in the reaction mixture results into the formation of higher quantity of impurities of formulae VII or VIII. However, inventors of the present invention disclose herein that when the same reaction for the preparation of compound of formula IA comprising contacting optionally in-situ generated tert-butyl isocyanate in diluted form with insitu generated sodium salt of L-tert-Ieucine is carried out in diluted form having low concentration of reactants in the reaction mixture using binary reaction solvent system results into the formation of substantially pure compound of Formula I as evident from the chromatograph (showing single peak with 99.13% purity) given herein below in figure II:

using Arch process (Crude):

Figure II
Inventors of the present invention disclose herein that acidification of sodium salt of compound of formula I or IA to obtain compound of Formula 1 or IA using sulphuric acid results in the formation of impure product as it forms some unknown impurities. The chromatograph of the product obtained by acidification with sulphuric acid to obtain compounds of Formulae I and IA is given herein below in Figure III. However, use of monobasic acid like hydrochloric acid, hydrobromic acid etc.preferably hydrochloric acid for the same purpose results in the formation of substantially pure products of formulae I and IA avoiding the formation of unknown impurities as depicted in figure II shown herein above. The peak at retention time of 8.79 represents an unknown impurity formed as a prominent impurity during acidification when sulfuric acid is used for the purpose.


Figure III Inventors of the present invention also disclose a novel compound of formula VII which is formed as an impurity during the preparation of compound of Formulae I and IA. The said novel compound of Formula VII is neither a condensation product of N-carbamoyl-tert-leucine and tert-leucine nor it is a reaction product formed by the degradation of an isocyanic acid compound termed as bis urea in '294 patent. This impurity of formula VII is an isomer of compound of formula I that is formed when a positional isomer of tert-leucine reacts with isocyanate as depicted herein below. The mass is confirmed as 230 by LCMS and is further characterized by its infrared spectroscopy and NMR.


Inventors of the present invention have characterized the impurities termed in prior art as dipeptide and bis urea impurities and have assigned the following structures:

In a genera] embodiment disclosed herein is a process for the preparation of compound of formula I, comprising the steps:
a) in-situ preparation of isocyanate of formula IV comprising contacting aqueous solution of amine of formula II with solution of phosgene of formula III in an organic solvent; thereby generating a binary phase,
b) separating the organic layer containing isocyanate of formula IV;
c) contacting solution of isocyanate of formula IV from step b with aqueous solution of alkali metal salt tert-leucine of formula V thereby generating a binary phase;
d) filtering the reaction mass to remove the bis impurity of formula IX (wherein R| is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms) and collecting the aqueous phase;
e) contacting aqueous layer from step d with monobasic acid like
hydrochloric acid, hydrobromic acid and the like to precipitate the
compound of formula I; in case when compound of formula I is not

isolated as solid it can be extracted and isolated by standard means
reported in the prior art. Amine of the formula II is selected from the group comprising primary, secondary or tertiary amine and the like.
Phosgene of the formula III is selected from the group selected from monophosgene, diphosgene, triphosgene.
Organic solvent is any organic solvent that is water immiscible and is selected from aliphatic hydrocarbon, aromatic hydrocarbon, halogenated hydrocarbons, ethers and the like.
Alkali metal salt of compound of formula V is selected from lithium, potassium, sodium salt and the like.
Monobasis acid is selected from group comprising hydrochloric acid, hydrobromic acid and the like.
In another general embodiment disclosed herein is for the process for the preparation of compound of formula I, the steps comprising:
a) contacting dilute solution of isocyanate of formula IV made in an
organic solvent with aqueous solution of amine of formula II thereby
generating a binary phase;
b) filtering the reaction mass to remove N,N'-di(tert-butyl) urea impurity represented by general formula IX if any; (wherein R| is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms) and collecting the aqueous phase;
c) contacting aqueous layer from step b with monobasic acid like hydrochloric acid, hydrobromic acid and the like to precipitate the compound of formula I;

d) isolating the compound of formula I by filtration or by standard processes known in the prior art.. In case if the compound of the formula I is not obtained as solid it can be extracted and isolated by the standard established means known in the prior art,
In a preferred embodiment disclosed herein is a process for the preparation of compound of formula I A, the steps comprising:
a) in-situ generation of tert-butyl isocyanate comprising contacting
aqueous solution of tert-butyl amine with solution of triphosgene
represented by formula III (wherein n is 3) in toluene as an organic
solvent thereby generating a binary phase;
b) separating the organic layer containing tert-butyl isocyanate;
c) contacting the said organic layer containing tert-butyl isocyanate with aqueous solution of sodium salt of tert-leucine thereby generating a binary phase;

d) filtering the reaction mass to remove N,N'-di(tert-butyl) urea represented by general formula IX (wherein R1 is tert-butyl) and collecting the aqueous phase;
e) contacting aqueous layer from step d with hydrochloric acid to precipitate the compound of formula I A;
a) isolating the compound of formula IA by filtration. In case if the compound of the formula IA is not obtained as solid it can be extracted and isolated by the standard established means known in the prior art.
In another preferred embodiment disclosed herein is a process for the preparation of compound of formula 1, the steps comprising:

a) contacting dilute solution of tert-butyl-isocyanate made in toluene with aqueous solution of alkali metal salt of tert-leucine of formula V, thereby generating a binary phase;
b) filtering the reaction mass to remove the impurity of formula IX if any;(wherein R1 is tert-butyl) and collecting the aqueous phase;
c) contacting aqueous layer from step b with hydrochloric acid to
precipitate the compound of formula ];
d) isolating the compound of formula I by filtration or by standard
processes known in the prior art. In case if the compound of the
formula I is not obtained as solid it can be extracted and isolated by
the standard established means known in the prior art.
The following scheme explains the general embodiment of the present invention:

In a preferred embodiment of the present invention aqueous alkaline solution of tert-butyl amine of formula 1 is contacted with solution of triphosgene represented by formula III (wherein n is 3) in toluene to obtain tert-butyl isocyanate. The said in-situ generated tert-butyl isocyanate in toluene is then contacted with aqueous solution of sodium metal salt of tert-leucine at pH in the range of about 9 to about 13. Reaction is monitored and after completion

the reaction mass is filtered off to remove the impurity of bis tert-butyl urea.
Aqueous layer is collected and then taken for the acidification using
hydrochloric acid to isolate the compound of formula IA. The compound is
filtered off and dried under vacuum.
The following scheme explains the preferred embodiment of the present
invention:

In an another embodiment of the present invention disclosed herein is a process for the preparation of novel compound of formula VII formed as an impurity and characterization of the said impurity comprising contacting positional isomer of tert-leucine represented by formula A with tert-butyl isocyanate as shown in below given scheme.

The word contacting used hereinabove and herein below refers to reacting,
heating, refluxing, stirring and the like.
The present invention can be best understood by the following but non
limiting examples.
All experiments are conducted under Nitrogen atmosphere:

Example 1: Preparation of compound of formula IA as per present invention
Solution containing tert-butyl amine 50g (0.68mol) 500 ml of saturated sodium bi-carbonate was charged with solution of triphosgene 66.91 g (0.22 mol) is charged in lit toluene under stirring at 20-25°C. Reaction mass is stirred till the completion of the reaction generating tert-butyl isocyanate. Solution of tert-butyl isocyanate is then further added to an aqueous solution of (S)-2-amino-4,4-dimethylpentanoic acid (L-tert-leucine)solution in 500 ml of water containing 15 gm of NaOH and 50 gm of (0.77mol) of (S)-2-amino-4,4-dimethylpentanoic acid) keeping the temperature 0-10 °C and pH in the range of 9-9.5. The reaction is continued till the completion. Aqueous layer after filtration is separated and then taken for the acidification using hydrochloric acid (pH 1-1.5) to isolate the target product of formula I A. The product is filtered off and dried under vacuum. 84-86 gm (>98%) HPLC purity of isolated crude product: 99.10%. Online HPLC analysis of reaction mass for the preparation of product of formula IA indicating 99.13% of product of formula IA.


using Arch process (Crude):
Example 2: Preparation of compound of formula IA as per present invention using dilute solution of isocyanate:
380 ml solution containing 34g tert-butyl isocyanate is added to an aqueous solution of (S)-2-amino-4,4-dimethylpentanoic acid (L-tert-leucine) in 500 ml of water contains 15 gm of NaOH and 50 gm(0.77mol) of (S)-2-amino-4,4-dimethylpentanoic acid) keeping the temperature 0-10 °C and pH in the range of 9-9.5, The reaction is continued till the completion. Aqueous layer after filtration is separated and then taken for the acidification using hydrochloric acid (pH 1-1.5) to isolate the target product of formula I A. The product is filtered off and dried under vacuum, yield obtained is 80g. Example 3 (As negative data): Preparation of compound of formula IA using neat tert-butyl isocyanate as per the process disclosed in '294 patent:
Solution containing (2.5 It water and 250 g L- tert-leucine) is added a 48% sodium hydroxide at 15-20°C to adjust the pH in the range of 11.3-11.5

HPLC analysis of crude material:
under stirring. The reaction mass is further cooled to 0-15nC and neat tert-butyl isocyanate is added keeping the temperature below 10°C. The reaction is continued for its completion (online chromatograph as given below). The reaction mass is then acidified with hydrochloric acid and is extracted with ethyl acetate in three lots using 2lt. Ethyl acetate is distilled of and concentrated mass is the added 500 ml toluene to isolate the product of formula IA. The product is filtered off and dried under vacuum. Dry wt of the crude product is 383 gm. (71.0%) yield, with inferior HPLC purity of 97.92% with impurity of 1.11% at RT11.36 (chromatograph is given below) Online HPLC analysis of reaction mass indicating the preparation of 88% of product of formula I A, 4.7% of dipeptide impurity of formula VIII and 4.6% of novel impurity of formula VII.

Pit t RT(Mln) Name Height Area Are* Ti RRI
1 5.S5 2003*14 19137837 97.92 0.00
2 7.29 f?21 15459 0.O8 0.00
3 10.17 w 7222 0.04 0.00
4 10.41 12716 1139S1 o.sa 0.00
5 10.72 977* 49642 0.2S 0.00
6 10.99 0 1719 0.01 0.00
7 11.36 33i».< 217854 1.1] 0.00
Too* 2062)11 19S436B4 100.0O
Example 4: Total synthesis and characterization of isomeric impurity of formula VII:
Solution containing tert-butyl amine 50g (0.68mol) 500 ml of saturated sodium bi-carbonate was charged with solution of triphosgene 66.91 g (0.22 mol) is charged in lit toluene under stirring at 20-25°C. Reaction mass is stirred till the completion of the reaction generating tert-butyl isocyanate. Solution of tert-butyl isocyanate is then further added to an aqueous solution contains 50 gm of (S)-2-amino-4-methylpentanoic acid (isomeric L-tert-leucine), 15 gm NaOH and 50gm (0.77mol) of (S)-2-amino-4,4-dimethylpentanoic acid) in 500 ml of water keeping the temperature 0-10 "C and pH in the range of 9-9.5. The reaction is continued till the completion. Aqueous layer after filtration is separated and then taken for the acidification using hydrochloric acid (pH 1-1.5) to isolate the target impurity of formula

VII. The impurity is filtered off and dried under vacuum. The isolated
impurity is characterized found below:
Analytical data:-
Melting Point >250 OC
,H NMR (CDCI-,): 0.921 (6H, m); 1.55 (9H, S); 1.65(1H, m), 1.67-1.73(2H,
m); 4.26(1 H, m); 6.56-6.67 (2H bs, D2o exchangeable); 9.85 (1H, bs, D20
ex.)
13C NMR (CDC13): 21.77, 22.85; 24.69; 29.32; 40.30; 50.66; 52.50; 76.67;
77.00; 77.31; 159.00; 177.20
Mass by LCMS: M+l = 231.04
MASS SPECTRA:


NMR of synthesized isomeric dipeptide of formula VII:

IR of synthesized novel isomeric dipeptide of formula VII:

Claims:
We claim:
1. A process for the preparation of substantially pure N-carbamoyl-tert
leucine of the formula 1

comprising the steps of:
a) a) contacting aqueous solution of amine of formula II with solution of phosgene represented by formula III in an organic solvent, thereby generating a binary phase to obtain isocyanate of formula IV;

b) separating the organic layer containing isocyanate of formula IV;


c) contacting solution of isocyanate of formula IV from step b with aqueous solution of alkali metal salt of tert-leucine of formula V, thereby generating a binary phase ;

d) filtering the reaction mass to remove the impurity of formula IX; if any; (wherein R, is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms) and separating the aqueous layer;

e) contacting aqueous layer from step d with monobasic acid to precipitate the compound of formula I;
f) isolating the compound of formula I by filtration or by standard processes known in the prior art.
2. A process for the preparation of substantially pure N-carbamoyl-tert leucine of the formula I


comprising the steps of:
a) contacting dilute solution of isocyanate of formula IV made in an organic solvent with aqueous solution of alkali metal salt of tert-leucine of formula V, thereby generating a binary phase;

b) filtering the reaction mass to remove the impurity of formula IX if any; (wherein R1 is a hydrogen atom, an optionally substituted alkyl group having 1-10 carbon atoms or an optionally substituted aryl/heteroaryl group having 1-6 carbon atoms) and separating the aqueous layer,


c) contacting aqueous layer from step b with monobasic acid to precipitate the compound of formula I;
d) isolating the compound of formula I by filtration or by standard processes known in the prior art.

3. Organic solvent as claimed in claims la and 2a is selected from the group comprising aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, ester, ethers that will form the binary phase when mixed with aqueous solvent.
4. Alkali metal salt as claimed in claims lc and 2a is selected from group comprising lithium, sodium and potassium.
5. Monobasic acid as claimed in claim le and 2c is selected from the hydrohalide comprising hydrochloric acid, hydrobromic acid and like.
6. A process for the preparation of compound of formula I A, comprising the steps of:

a) contacting aqueous solution of tert-butyl amine with aqueous solution of triphosgene (n is 3 in formula III) in toluene as an organic solvent thereby generating a binary phase to obtain tert-butyl isocyanate;


b) separating the organic layer containing tert-butyl isocyanate ( when R1 is tert-butyl in formula IV);
c) contacting the said organic layer containing tert-butyl isocyanate with aqueous solution of sodium salt of tert-Ieucine thereby generating a binary phase;
d) filtering the reaction mass to remove N.N'-di(tert-butyl) urea impurity represented by formula IX (wherein R1 is tert-butyl) and collecting the aqueous phase;

e) contacting aqueous layer from step d with hydrochloric acid to precipitate the compound of formula I A;
f) isolating the compound of formula IA by filtration or by standard processes known in the prior art.
7. A process for the preparation of compound of formula I A, comprising the steps of:


a) contacting dilute solution of tert-butyl isocyanate represented by formula IV; (wherein R1 is tert-butyl) in an organic solvent with aqueous solution of alkali metal salt of tert-leucine of formula V, thereby generating a binary phase;

b) filtering the reaction mass to remove N,N'-di(tert-butyl) urea when R1 is tert-butyl in formula IX) if any; (wherein R1 is tert-butyl) and separating the aqueous layer,

c) contacting aqueous layer from step b with hydrochloric acid to precipitate the compound of formula I;
d) isolating the compound of formula I by filtration or by standard processes known in the prior art
8. A compound of the formula VII

wherein R1 is tert-butyl
9. A process for the preparation of compound of formula VII comprising: a) contacting aqueous solution of tert-butyl amine with aqueous solution of triphosgene (n is 3 in formula III) an organic solvent thereby generating a binary phase to obtain tert-butvl isocyanate;

b) separating the organic layer containing tert-butyl isocyanate ( when R1 is tert-butyl in formula IV);

c) contacting the said organic layer containing tert-butyl isocyanate with aqueous solution of sodium salt of positional isomer of tert-leucine of formula A thereby generating a binary phase;

d) separating the aqueous layer
e) contacting aqueous layer from step d with monobasic acid to precipitate the compound of formula VII;

f) isolating the compound of formula VI) by filtration.
10. Organic solvent as claimed in claims 10a is selected from the group comprising aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, ester, ethers that will form the binary phase when mixed with aqueous solvent.
11. Alkali metal salt as claimed in claims 10a is selected from group comprising lithium, sodium and potassium.