FIELD OF THE INVENTION

The present invention relates to a process for the preparation of compound formula I,

Formula I

wherein, R 1 and R 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or R 1 and R 2 together form a moiety derived from an alpha-hydroxy carboxylic acid compound or a beta-hydroxy carboxylic acid compound, wherein the atom attached to boron in each case is an oxygen atom; or R 1 and R 2 together form the boronate esters of boronic acid.

More particularly, the present process relates to preparation of Ixazomib citrate of compound of formula la,

Formula la

Preferably, the process comprises the use of diimidazole or ditriazole based coupling reagent for amide and peptide bond formation. The present process is more economical and less time consuming than processes described in the literature due to easier isolation of intermediates and Ixazomib of compound of formula,

Ixazomib

The present invention also relates to preparation and isolation of an amorphous form of Ixazomib citrate of formula la.

BACKGROUND OF THE INVENTION

Ixazomib citrate is boronic ester compound and marketed as Ninlaro ^® , it is structurally known as compound of formula la,

The Ixazomib citrate is an antineoplastic agent and works as pro drug. After oral administration it rapidly hydrolyzes to its biologically active form, Ixazomib of formula,

Ixazomib

The ixazomib citrate is R- stereoisomer and chemically known as 1,3,2- dioxaborolane-4,4-diacetic acid, 2-[(lR)-l-[[2-

[(2,5dichlorobenzoyl)amino]acetyl]amino]-3-methylbutyl]-5 -oxo. It is indicated in combination with lenalidomide and dexamethasone for the treatment of patients having multiple myeloma and who have received at least one prior therapy.

The boronic ester compounds, particularly the Ixazomib citrate as represented by formula la, is disclosed in WO2009/154737A1 (hereinafter referred as WO 737). The synthetic scheme described in WO2009/154737A1 is depicted below as scheme - 1 :

Scheme -1 In first step, the amide bond is formed by reacting 2,5-dichlorobenzoyl chloride with glycine under basic environment at 0+1 °C. The preparation of 2,5- dichlorobenzoyl chloride is not disclosed in WO 37; however, commonly known reagents like thionyl chloride or oxalyl choride are extensively used in industry for the formation of acid chloride from their acid precursors. The reagents like thionyl chloride or oxalyl choride are hazardous and not advisable at large scale production.

In second step, the peptide bond is formed by reacting 2,5- [(dichlorobenzoyl)amino] acetic acid with trifluoroacetic acid salt of (lR)-3- methyl- l-[(3aS,4S,6S,7aR)-3a,5,5-trimethylhexahydro-4,6-methano- 1,3,2- benodioxaborol-2-yl]butan-l-amine in presence of coupling reagent O- (Benzotriazol-l-yl)-N,N,N N-tetramethyluronium tetrafluoroborate (TBTU) and diisopropylethylamine (DIPEA) base in dimethylformamide. After completion of the reaction the mixture is diluted with ethyl acetate then washed multiple times, with sodium chloride, potassium carbonate and phosphoric acid.

The resulting organic layer is concentrated to a thick oil, diluted with heptane and evaporated to yield 2,5-dichloro-N-[2-({(lR)-3-methyl-l-[(3aS,4S,6S,7aR)- 3a,5,5- trimethylhexahydro-4,6-methano-l,3,2-benzodioxaborol-2- yl]butyl}amino)-2-oxoethyl]benzamide. The obtained benzamide compound is further dissolved in methanol/hexane mixture and stirred overnight in presence of (2-methylpropyl)boronic acid ((iBuB(OH) ₂ ) to form Ixazomib (a free boronic acid derivative) or its boroxine derivative. The isolation of these derivatives requires long acid-base processing of the reaction mixture, which is tedious and time consuming. The isolated Ixazomib or its boroxine derivative is further reacted with citric acid in ethyl acetate (EtOAc) to form Ixazomib citrate, which is isolated in two different crystalline forms I and II by controlling the process of cooling of the reaction mixture.

The process disclosed in \νθ 737 suffers from following disadvantages;

1) The 2,5-dichlorobenzoyl chloride is liquid above 30°C, which is difficult to handle at large scale production.

2) The reaction of 2,5-dichlorobenzoyl chloride with glycine is performed at 0+1 °C, which is tedious to maintain at large scale production. 3) The use of coupling reagent such as 0-(Benzotriazol-l-yl)-N,N,N N→ tetramethyluronium tetrafluoroborate (TBTU) during the peptide bond formation is not advisable, the TBTU and its by-products such as hydroxybenzotriazole (HOBt) are not water soluble. Hence removal of these from the reaction mixture requires very long and tedious processing steps like multiple washings with sodium chloride, potassium carbonate and phosphoric acid, which is time consuming and equally not economical.

4) The use of costly reagents such as (2-methylpropyl)boronic acid ((iBuB(OH) ₂ ) for the hydrolysis to form Ixazomib is also not advisable. The use of (2-methylpropyl)boronic acid requires long acid-base treatment of the reaction mixture before the isolation of Ixazomib.

WO2016/165677A1 (hereinafter referred as WO ^" 677) discloses an amorphous form of Ixazomib citrate characterized by an X-ray pattern and glass transition temperature Tg=104.1°C. The WO~677 further discloses the process of preparation of amorphous form of Ixazomib citrate by lyophilisation of a freeze- dried solution.

The process disclosed in WO ^" 677 involves the use of special instrument for lyophilisation of the freeze-dried solution, which is difficult to use and not viable at industrial scale production.

As evident from the above discussion the disclosed processes of WO ^* 737 and WO ^" 677 suffer from many disadvantages; hence, there is a need to develop a process for the preparation of boronic ester compound such as Ixazomib citrate and its amorphous form.

The inventors of the present application have developed a process which preferably involves a diimidazole or ditriazole based coupling reagent for the formation of amide and peptide bonds to prepare Ixazomib citrate of compound of formula la. The use of a diimidazole or ditriazole based coupling reagent is advantageous, which devoid the use of hazardous reagent, use of low temperature condition and reduces the long processing time of the reaction mixture for the isolation of intermediates and Ixazomib. Further, the process of present invention is cleaner, more economical, less time consuming and does not require any special instrument as compared to processes described in the literature.

SUMMARY OF THE INVENTION

In a first aspect, the present invention relates to a process for the preparation compound of formula I,

Formula I wherein, R 1 and R 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or R 1 and R 2 together form a moiety derived from an alpha-hydroxy carboxylic acid compound or a beta-hydroxy carboxylic acid compound, wherein the atom attached to boron in each case is an oxygen atom; or R 1 and R 2 together form the boronate esters of boronic acid comprising the steps of: a) reacting a compound of formula Ila,

Formula Ila wherein, X is imidazole or triazole with glycine in suitable solvent to obtain a compound of formula III,

Formula III

b) optionally, converting the compound of formula III to a compound of formula Ilia,

Formula Ilia

wherein, X is imidazole or triazole c) reacting the compound of formula III or formula Ilia with a compound of formula IV or its salt in suitable solvent,

Formula IV

wherein, R 1 and R 2 are same as defined for the compound of formula I

to obtain the compound of formula I. Another aspect of the present invention relates to a process for the preparation of compound of formula I,

Formula I

wherein, R 1 and R 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or R 1 and R 2 together form a moiety derived from an alpha-hydroxy carboxylic acid compound or a beta-hydroxy carboxylic acid compound, wherein the atom attached to boron in each case is an oxygen atom; or R 1 and R 2 together form the boronate esters of boronic acid comprising the reaction of a compound of formula Ilia,

Formula Ilia

wherein, X is imidazole or triazole with a compound of formula IV or its salt in suitable solvent,

Formula IV

wherein, R 1 and R 2 are same as defined for the compound of formula I

to obtain the compound of formula I. Another aspect of the present invention relates to a process for the preparation of a compound of formula Ila,

Formula Ila

wherein, X is imidazole or triazole by reacting a compound of formula II,

Formula II

with diimidazole or ditriazole based coupling reagent in suitable solvent.

Another aspect of the present invention relates to a process for the preparation of a compound of formula Ilia,

Formula Ilia

wherein, X is imidazole or triazole by reacting a compound of formula III,

Formula III

with diimidazole or ditriazole based coupling reagent in suitable solvent.

Another aspect of the present invention relates to use of compound of formula Ila and/or Ilia,

Formula Ila Formula Ilia wherein, X is imidazole or triazole

for the preparation of the compound of formula I.

Yet another aspect of the present invention relates to an amorphous

Ixazomib citrate of a compound of formula la,

Formula la Still yet another aspect of the present invention relates to preparation amorphous form of compound of formula la,

Formula la

comprising the steps of: a) dissolving a compound of formula la in suitable solvent, b) removing the solvent to isolate the amorphous form of compound of formula la.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an X-ray powder diffraction (PXRD) pattern of an amorphous form of compound of formula la

FIG. 2 is an illustration of a Differential Scanning Calorimetry (DSC) of an amorphous form of compound of formula la FIG. 3 is an illustration of a Thermogravimetric analysis (TGA) of an amorphous form of compound of formula la

FIG. 4 is a schematic representation of preferred embodiments of the process of preparation of a compound of formula I and la

DETAILED DESCRIPTION OF THE INVENTION

DEFINITIONS

The following definitions are used in connection with the present invention unless the context indicates otherwise. The term ^" Amorphous _ as used herein, refers to a solid state form of a compound of formula la wherein the three dimensional structure positions of the molecules relative to one another are essentially random, [for example, see Hancock et al. "Characteristics and significance of the amorphous state in pharmaceutical systems" J. Pharm. Sci. Vol. 86, pp. 1-12 (1997)]. As a result, amorphous material will have only liquid-like short range order, and, when examined by X-ray diffraction (XRPD), will generally produce broad, diffuse scattering will result in peak intensity sometimes centered on one or more amorphous halos. Thus, XRPD analysis of amorphous material will provide a 2-theta pattern with one or more broad bands with no distinctive peaks.

The ^" Amorphous _ compound of formula la may sometimes be characterized by its glass transition temperature (Tg), which defines a pseudo second order phase transition in which a super cooled melt of formula la yields, on cooling, a glassy structure with properties similar to those of crystalline compound of formula la. However, since Tg is a kinetic parameter, its value will be dependent on the melt cooling rate and the measurement conditions used for its determination (e.g., the slower the melt cooling rate, the lower Tg will be).

The glass transition temperature (Tg) for a sample of amorphous form of compound of formula la may be obtained by differential scanning calorimetry (DSC).

The terms ^" amide bond and peptide bond _ are distinguish in the context that amide bond results in amide derivative, which is any derivative of oxoacid in which the hydroxyl group has been replaced with an amino or substituted amino group while peptide bond results in a derivative of at least two amino acids in which the amine of one is reacted with the carboxylic acid of the next to form a peptide bond. The term ^" alpha-hydro xy carboxylic acid compound _ refers to a compound which contains a carboxylic acid functional group and hydroxy functional group separated by one carbon atom.

The term ^" beta-hydro xy carboxylic acid compound _ refers to a compound which contains a carboxylic acid functional group and hydroxy functional group separated by two carbon atoms.

The terms ^" alpha- hydroxy carboxylic acid compound _ and ^" beta-hydroxy carboxylic acid compound _ are not intended to be limited to compounds having only one hydroxyl group and one carboxylic acid group.

The term "moiety derived from an alpha-hydroxy carboxylic acid compound" refers to a moiety formed by condensation of carboxylic acid and hydroxyl group of an alpha-hydroxy carboxylic acid compound, wherein hydroxyl group of an alpha-hydroxy carboxylic acid compound is present in an alpha position relative to the carboxylic acid group.

The term "moiety derived from a beta-hydroxy carboxylic acid compound" refers to a moiety formed by condensation of carboxylic acid and hydroxyl group of a beta-hydroxy carboxylic acid compound, wherein hydroxyl group of a beta- hydroxy carboxylic acid compound is present in a beta position relative to the carboxylic acid group.

The terms ^" about, general, generally _ and the like are to be construed as modifying a term or value such that it is not an absolute. Such terms will be defined by the circumstances and the terms that they modify, as those terms are understood by those skilled in the art. This includes, at very least, the degree of expected experimental error, technique error and instrument error for a given technique used to measure a value. As used herein, the terms ^" comprising _ and ^" comprises _ mean the elements recited, or their equivalents in structure or function, plus any other element or elements which are not recited.

The terms ^" having _ and ^" including _ are also to be construed as open ended. All ranges recited herein include the endpoints, including those that recite a range between two values. Whether so indicated or not, all values recited herein are approximate as defined by the circumstances, including the degree of expected experimental error, technique error, and instrument error for a given technique used to measure a value.

The term ^" optional _ or ^" optionally _ is taken to mean that the event or circumstance described in the specification may or may not occur, and that the description includes instances where the event occurs and instances where it does not.

One aspect of the present invention relates to a process for organic synthesis, more particularly for the synthesis of amide and peptide bonds, and new intermediates of compound of formula I,

Formula I

wherein, R 1 and R 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or R 1 and R 2 together form a moiety derived from an alpha-hydroxy carboxylic acid compound or derived from a beta-hydroxy carboxylic acid compound, wherein the atom attached to boron in each case is an oxygen atom; or R ¹ and R together form the boronate esters of boronic acid. Preferably, the boronate esters of boronic acid of formula I contain a derivative which may be formed by reacting the acid groups of the boronic acid with a hydroxy compound. Preferred hydroxy compounds are dihydroxy compounds, especially pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3-propanediol, 2,3-butanediol, glycerol or diethanolamine.

The present invention involves efficient methods for formation of amide and peptide bonds to prepare compound of formula I.

In first stage, the amide bond is formed by reacting a compound of formula Ila,

wherein, X is imidazole or triazole with glycine to form a compound of formula III; Preferably, in second stage, the peptide bond is formed by reacting a compound of formula Ilia,

Formula Ilia

wherein, X is imidazole or triazole with compound of formula IV or its salt to obtain a compound of formula I. The compound of formula Ila and Ilia are activated carboxylic acid derivatives. These derivatives are typically formed by reacting their corresponding acid compound of formula II or III,

Formula II Formula III with diimidazole or ditriazole based coupling reagent in suitable solvent.

The activation of carboxylic acid group of compound of formula II or compound of formula III with a suitable diimidazole or ditriazole based coupling reagent forms following derivatives,

These derivatives may be isolated before subjecting to formation of amide or peptide bond, which is also a part of preferred aspect of the present invention. The present process resides in a novel and improved method of producing activated carboxylic acid group with diimidazole or ditriazole based coupling reagent. The diimidazole or ditriazole based coupling reagent may be selected from the group consisting of 1,1 ^" carbonyldiimidazole (CDI), 1,1 ^" -carbonyldi- (1,2,4-triazole), 1,1 ^" thiocarbonyldiimidazole and 1,1 ^" oxalyldiimidazole. Preference is given to diimidazole based coupling reagents. Particular preference is given to 1,1 ^" carbonyldiimidazole (CDI).

The other coupling reagents may also be utilized for producing the activated carboxylic acid group of the compounds of present invention. These coupling reagent may be selected from the group consisting of carbodiimide such as Ν,Ν'- dicyclohexylcarbodiimide (DCC), Ν,Ν'-diisopropylcarbodiimide (DIC), 1-ethyl- 3-(3-dimethylaminopropyl)carbodiimide (EDC) and the like.

The compounds II or III of the present invention react readily with diimidazole or ditriazole based coupling reagent to form carbon dioxide and imidazole or triazole as by-products. The carbon dioxide is gaseous by-product, which is easily removed from the reaction mixture wherein imidazole or triazole is soluble in water hence does not require any cumbersome methods such as multiple washings with sodium chloride, potassium carbonate and phosphoric acid to remove them from the reaction mixture.

The use of diimidazole or ditriazole based coupling reagent for the preparation of activated carboxylic acid compound of formula Ila also avoids the use of hazardous chlorinating reagent such as thionyl chloride or oxalyl chloride for the preparation of acid chloride derivative, 2,5-dichlorobenzoyl chloride. The problem of handling liquid 2,5-dichlorobenzoyl chloride and maintaining reaction temperature at 0+1 °C during its reaction with glycine are also solved by the process of the present invention. The activation of carboxylic acid group of formula II is typically performed by reacting it with diimidazole or ditriazole based coupling reagent at suitable temperature in suitable solvent, optionally isolating the compound of formula Ila, then reacting it with glycine, preferably for 30 minutes to 5 hours at 0°C to 20°C, particularly preferably for 1 to 2 hours at 0°C to 10°C. It is advantageous to use aqueous alkaline solution of glycine, preferably aqueous sodium hydroxide solution of glycine. After completion of the reaction compound of formula III can be isolated by any known methods of the art.

The compound of formula III can be converted to compound of formula I as per the disclosed method of \νθ ^* 737.

Preferably, the carboxylic acid group of compound of formula III,

Formula III

is further activated by reacting it with diimidazole or ditriazole based coupling reagent at suitable temperature in suitable solvent, optionally isolating the compound of formula Ilia, then reacting it with compound of formula IV or its salt such as hydrochloric acid salt, hydrobromic acid salt, p-toluene sulfonic acid salt, phosphoric acid salt or trifluoro acetic acid salt or the like,

Formula IV wherein, R 1 and R 2 are each independently hydroxy, alkoxy, aryloxy, or aralkoxy; or R 1 and R 2 together form a moiety derived from an alpha-hydroxy carboxylic acid compound or a beta-hydroxy carboxylic acid compound, wherein the atom attached to boron in each case is an oxygen atom; or R 1 and R 2 together form the boronate esters of boronic acid.

The boronate esters of boronic acid of formula IV may be formed by reacting the acid groups of the boronic acid with a hydroxy compound. Preferred hydroxy compounds are dihydroxy compounds, especially pinacol, perfluoropinacol, pinanediol, ethylene glycol, diethylene glycol, 1,2-cyclohexanediol, 1,3- propanediol, 2,3-butanediol, glycerol or diethanolamine.

Preferably, the salt of compound of formula IV is trifluoro acetic acid salt of pinanediol derivative of boro leucine of compound of formula IVa,

Formula IVa

The reaction mixture of compound of formula Ilia and IV or IVa is preferably stirred for 30 minutes to 5 hours at 0°C to 20°C, particularly preferably for 1 to 2 hours at 0°C to 10°C. After completion of the reaction, the compound of formula I can be isolated by any known methods of the art.

Preferably, the isolation of resulting compound I, compound of formula Ila and Ilia may be achieved by filtration of precipitated solid by gravity or by suction, distillation, centrifugation, cooling, crystallization, anti-solvent addition, removal of solvent by evaporation or the like. During the preparation of compound of formula I,

Formula I

wherein, R 1 and R 2 are other than hydroxyl group or compound of formula I is not a compound of formula la, the resulting compound of formula I may be further hydro lysed with suitable reagent based on the nature of R 1 and R 2.

Preferably, the resulting compound of formula I is pinanediol boronate ester of compound of formula lb,

Formula lb

which may be hydrolysed by acid in suitable solvent to obtain Ixazomib or its boroxine derivative,

Boroxine derivative

The suitable acid for the hydrolysis may be selected from the group consisting of hydrochloric acid, boric acid, isobutyl boronic acid, paratoluenesulphonic acid (PTSA), phosphoric acid; and mixtures thereof. The strength and concentration of the acids may be varied as per the hydrolysis condition to achieve the desired result.

Preferably, the Ixazomib or its boroxine derivative is further reacted with citric acid in suitable solvent to obtain Ixazomib citrate of formula la.

The suitable solvent may be selected from the group consisting of acetone, methyl ethyl ketone, isobutyl methyl ketone; dimethyl ether, diisopropyl ether, tetrahydrofuran, dioxane, dimethoxyethane; methanol, ethanol, propanol, isopropyl alcohol, n-butanol; ethyl acetate; acetonitrile, propionitrile; dimethylformamide, dimethylacetamide, dimethyl sulfoxide or mixtures thereof.

Preferably, the solvent is acetonitrile during the activation of the carboxylic acid group of compound of formula II or III. The hydrolysis of pinanediol boronate ester of formula lb is preferably carried out in acetonitrile or diisopropyl ether or mixture thereof. The formation of citrate salt of Ixazomib is preferably carried out in acetone.

A further aspect of the present invention relates to an amorphous form of Ixazomib citrate of formula la and process of preparation thereof. The amorphous form of Ixazomib citrate of formula la is characterized by the X- ray powder diffraction (PXRD) pattern as shown in Fig. l and/or characterized by the Differential scanning calorimetry (DSC) as shown in Fig.2 and/or characterized by the Thermogravimetric analysis (TGA) as shown in Fig.3.

The amorphous form of Ixazomib citrate of formula la is further characterized by glass transition temperature Tg = 112.06°C.

The preparation of an amorphous form of Ixazomib citrate of formula la comprising steps of: a) dissolving a compound of formula la in suitable solvent, b) removing the solvent to isolate the amorphous form of compound of formula la.

The suitable solvent for the preparation of amorphous form of a compound of formula la may be selected from the group consisting of methanol, ethanol, propanol, isopropyl alcohol, n-butanol; acetone, methyl ethyl ketone, methyl iso butyl ketone (MIBK); ethyl acetate; dichloromethane, chloroform; dimethyl ether, diisopropyl ether, tetrahydrofuran, dioxane, dimethoxyethane; acetonitrile, propionitrile or mixtures thereof. Preferably, the solvent is methanol or its mixture with dichloromethane or acetone or ethyl acetate. Particularly preferably, the solvent is methanol or a methanol/dichloromethane mixture or a methanol/acetone mixture.

In step a), optionally undissolved particles, if any, may be removed suitably by filtration, centrifugation, decantation, and any other known techniques. The solution can be filtered by passing through paper, glass fiber, or other membrane material, or a clarifying agent such as celite. Depending upon the equipment used and the concentration and temperature of the solution, the filtration apparatus may need to be preheated to avoid premature isolation.

In step b), the removal of the solvent from solution can be performed by suitable techniques which may be selected from the group consisting of evaporation, evaporation under vacuum, flash evaporation, simple evaporation, rotational drying, agitated nutsche filter drying, pressure nutsche filter drying or any other technique known in the art.

The resulting amorphous compound may be optionally further dried. Drying can be carried out in a tray dryer, vacuum oven, air oven, cone vacuum dryer, rotary vacuum dryer, fluidized bed dryer, spin flash dryer, flash dryer, or the like. The drying can be carried out at temperatures of less than about 70°C, or any other suitable temperatures; at atmospheric pressure or under a reduced pressure; as long as the amorphous Ixazomib citrate is not degraded in its quality. The drying can be carried out for any desired times until the required product quality is achieved.

The X-ray Powder Diffraction (XRPD): XRPD analysis of amorphous Ixazomib citrate of formula la is conducted on a Panalytical, Model-Empyrean X-Ray powder diffractometer. The instrumental parameters are mentioned below, and graph XRPD graph is shown in Fig. 1:

Start position [ 2Theta] 3.0 End position [ 2Theta] 40.0 Step size [ 2Theta] 0.013 Scan step time (s) 39.27 Anode material Cu Generator setting 40mA, 45 KV Spinning Yes Goniometer theta: theta Sample stage Reflection-transmission spinner Sample mode Reflection

Sample specimen preparation Sample back loading technique

The thermal analysis of amorphous Ixazomib citrate of formula la is conducted by differential scanning calorimetry (DSC) (TA Instruments- DSC Q-2000) equipped with refrigerated cooling system (RCS90). Analysis is performed by taking 2 to 3 mg of sample encapsulated into aluminum sample pan with crimped lid. The thermogram is recorded from 25°C to 250°C under nitrogen atmosphere of 50 mL/min by using following methodology, a) Equilibrate at 25 °C b) Modulate (+) 1°C every 60 seconds c) Isothermal for 5 minutes d) Ramp 5°C/minutes to 250°C

The glass transition temperature (Tg) is observed at 112.06°C. The corresponding thermogram is shown in Fig.2.

The thermal gravimetric analysis (TGA Q-500) of amorphous Ixazomib citrate of formula la is conducted on TA Instruments model Q-500. The sample (about 10- 30 mg) is placed in a platinum pan previously tared. The weight loss of the sample is determined by heating the sample from room temperature to 300°C at a heating rate of 10°C/minutes under nitrogen atmosphere of 60 mL/min. The weight loss of Ixazomib citrate of formula la from RT to ~150°C is found to be 2.52 % w/w and the corresponding thermogram is shown in Fig.3.

EXPERIMENTAL

Detailed experimental parameters according to the present invention are provided by the following examples, which are intended to be illustrative and not limiting of all possible aspects of the invention. EXAMPLES

PREPARATION OF IXAZOMIB CITRATE

Example 1: Synthesis for N-(2,5-dichlorobenzoyl)glycine (Formula III)

1,1 ^" Carbonyldiimidazole (CDI) (127.3 g) was added to a mixture of 2,5- dichlorobenzoic acid (100 g) in acetonitrile (500 mL) under nitrogen atmosphere at ambient temperature. The reaction mixture was stirred for 1 hour at the same temperature to obtain compound of formula Ila ¹ . After stirring, the reaction mass was added to a mixture of water (1500 mL), sodium hydroxide (NaOH; 31.4 g) and glycine (58.9 g) at 0-10°C. The obtained reaction mass was stirred for 1 hour at 0-10°C. After completion of the reaction, the reaction mixture was maintained at pH 2-3 with hydrochloric acid solution to obtain a solid residue. The solid residue was filtered to obtain N-(2,5-dichlorobenzoyl)glycine. (Yield: 97.7 %)

Example 2: Synthesis of 2,5-dichloro-N-[2-({(lR)-3-methyl-l-[(3aS,4^,6^,7a/?)- 3a,5,5-trimethylhexa- hydro-2H-4,6-methano-l,3,2-benzodioxaborol-2- yl]butyl}amino)-2-oxoethyl]benzamide (Formula lb)

1,1 ^" -Carbonyldiimidazole (CDI) (15.7 g) was added to a mixture of N-(2,5- dichlorobenzoyl)glycine (20 g) in acetonitrile (100 mL) at 20-30°C to obtain compound of formula Ilia ¹ . The reaction mixture was cooled to 0-10°C then added trifluoro acetic acid salt of pinanediol derivative of boroleucine of formula IVa (29.0 g). The reaction mixture was stirred for 1 hour at the same temperature. After completion of the reaction, methanol (100 mL) and demineralised water (300 mL) was added to the reaction mass and stirred for 3 hours at 20-30°C. The solid was collected by filtration to obtain the title compound. (Yield: 93.7 %)

Example 3: Synthesis of Ixazomib

The pinanediol boronate ester (5 g) of compound of formula lb was mixed with a mixture of concentrated hydrochloric acid (5 mL) and boric acid (1.9 g) in diisopropyl ether (100 mL) at ambient temperature. The reaction mixture was stirred for 5 hours and solid was filtered and washed with diisopropyl ether (25 mL). The isolated solid was dried under vacuum to obtain Ixazomib. (Yield: 86.5 %)

Example 4: Synthesis of Ixazomib citrate of formula la

The Ixazomib (5 g) was added in a solution of citric acid (2.93 g) in acetone (100 mL) at ambient temperature. The reaction mixture was stirred for 30 minutes and filtered. The filtrate was stirred for 4-5 hours. The solid was collected by filtration and dried to give Ixazomib citrate. (Yield- 95 %)

Example 5: Preparation of amorphous form of Ixazomib citrate The Ixazomib citrate (2 g) was dissolved in methanol (30 mL) at ambient temperature. The solvent was removed under vacuum at 50-55°C and further degassed for 2-3 hours at the same temperature to obtain amorphous form of Ixazomib citrate.

Example 6: Preparation of amorphous form of Ixazomib citrate

The Ixazomib citrate (2 g) was dissolved in methanol (15 mL) and dichloromethane (55 mL) at ambient temperature. The solvent was removed under vacuum at 40-50°C and further degassed for 2-3 hours at the same temperature to obtain amorphous form of Ixazomib citrate.

Example 7: Preparation of amorphous form of Ixazomib citrate

The Ixazomib citrate (2 g) was dissolved in methanol (4 mL) and acetone (40 mL) at ambient temperature. The solvent was removed under vacuum at 50-55°C and further degassed for 2-3 hours at the same temperature to obtain amorphous form of Ixazomib citrate.