Field of the invention

The present invention is pertains to an improved process for the preparation of (15, 45, 7Z, 105, 165, 217?)-7-ethylidene-4,21-bis(l-methylethyl)-2-oxa-12,13-dithia-5J 8, 20, 23-tetraazabicyclo[8.7.6]tricos-16-ene-3, 6, 9, 19, 22-pentone, which is represented by the following formula I

Background of the invention

(15, 45, 7Z, 105, 16£, 217?)-7-ethylidene-4,21-bis(l-methylethyl)-2-oxa-12,13-dithia-5, 8, 20, 23-tetraazabicyclo[8.7.6]tricos-16-ene-3, 6, 9, 19, 22-pentone of formula I is commonly known as "Romidepsin".

Romidepsin is a natural product and it is belongs to a class of histone deacetylase (HDAC) inhibitor, bicyclic depsipeptide. Romidepsin is approved for the treatment of cutaneous T-cell lymphoma (CTCL) and peripheral T-cell lymphoma (PTCL) in patients who have received at least one prior systemic therapy and it is marketed under the brand name of Istodax®

United States Patent US 4977138 assigned to Fujisawa is first disclosed the Romidepsin which is produced by fermantation of Chromobacterium Vilolaceum.

(£)-5-(Tritylthio)pent-2-enal of formula (VIII) and (6i?, 95, 125, \3R) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4, 7, 10-trioxo-9-(triphenylmethyl thiomethyl)-3-oxa-5, 8, ll-triazatetradecane-12-carboxylate of formula (XII) are key materials used in the preparation of intermediates of formulae II and III which are directly involved in the synthesis of Romidepsin.

Journal of Organic Chemistry 2008, 73, 9353-9361 reported a process for the

preparation of Romidepsin from key components VIII and XII which are further converted to formulae II and III as shown below:

The main drawback of the above process is the use of benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP) as a coupling agent. It remains in the reaction mixture without being completely consumed. Its removal from the product requires repeated column purifications which is commercially not viable.

Moreover, low temperatures are preferred for the condensation reactions (if the starting materials are chiral) which can control the racemization during the reaction. PyBOP and its analogue reagent (benzotriazol-l-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP) required high temperature for completion of the reaction, hence the use of condensation agents like BOP and PyBOP are not preferred for the condensation of chiral intermediates.

Journal of American Chemical Society 1996, 7237-7238 reported a process for the preparation of Romidepsin comprising reacting the methyl pentadienoate with tritylthiol (also known as tritylthiol) in the presence CS2CQ3 followed by purification from flash chromatography to produce (2£)-5-tritylthio-2-pentenoic acid methyl ester, which is further reducing with diisobutyl aluminium hydride (DIBAL) and purified by flash chromatography

to produce (2£)-5-tritylthio-2-pentenol. This (2£)-5-tritylthio-2-pentenol is oxidizing with oxalyl chloride in presence of DMSO followed by purification from flash chromatography to produce (£)-5-(tritylthio)pent-2-enal. This (£)-5-(tritylthio)pent-2-enal is reacted with O-benzyl, O-TMS ketene in presence of Ti(IV) catalyst to produce the aldol product which is hydrolyzed followed by condensation with D-valyl-D-cysteinyl-CiS-trity^^^-dehydrobutinyr-Z-valine, methyl ester to yield bis S-trityl ester product. This bis S-trityl ester product is hydrolyzed and reacted with DIAD in presence of PPI13 and TSOH.H2O to produce bis-(S-trityl) lactone which is further reacted with iodine in MeOH to produce Romidepsin.

The major drawback with the above prior art process is the use of BOP as a coupling agent which generates carcinogenic by-product hexamethylphosphoramide (HMPA). Therefore the handling of the BOP and the absence study for its by-product is highly critical. It is not suitable for a large scale process, therefore there is a need to develop an alternate process by avoiding the use condensing agents like BOP.

Another disadvantage of the above process is the formation of unwanted P,y isomer i.e. (3£)-5-tritylthio-3-pentenoic acid methyl ester as shown below
Repeated purifications are required to remove this P, y-isomer impurity which is very time consuming process and results decrease in the yield and increasing the production cost.

Another disadvantage of the above process is oxidation involving oxalyl chloride in presence of DMSO and triethylamine in DCM solvent at -78°C under nitrogen atmosphere. Handling of this reaction at very low temperature is highly critical and the by-product (Dimethyl sulfide gas) evolved in this reaction which is unbearable and highly pungent smell.

Organic Biomolecular Chemistry 2011, 9, 3825-3833 reported a process for the preparation of (£)-5-(tritylthio)pent-2-enal of formula (VIII) comprising reaction of acrolein with tritylthiol to produce 3-tritylthiopropanal of formula (IX) which is further reacting with monoethyl malonate to produce the mixture of ethyl (£)-5-(tritylthio)pent-2-enoate of formula (X) and ethyl (£)-5-(tritylthio)pent-3-enoate of formula (Xa) followed by reducing with DIBAL to get the mixture of (£)-5-(tritylthio)pent-2-enol of formula (XI) and (£)-5-

(tritylthio)pent-3 -enol of formula (XIa) and finally oxidizing with oxalyl chloride in presence of DMSO to produce (£)-5-(tritylthio)pent-2-enal of formula (VIII). The process is schematically shown as follows:

The major disadvantages with the above prior-art process are the formation of mixture of isomers such as ethyl (£)-5-(tritylthio)pent-3-enoate (X) and ethyl (£)-5-(tritylthio)pent-2-enoate (Xa); and (£>5-(tritylthio)pent-3-enal (XI) & (E)-5-(tritylthio)pent-2-enal (XIa). Moreover, in this article, these isomers are separated by using column chromatography which is not suitable for industrial scale preparations.

Synlett 2012, 23(5), 783-787 reported a process for the preparation of (E)-5-(tritylthio)pent-2-enal of formula VIII comprising, reaction of acrolein with tritylthiol to give 3-(tritylthio)propanal of formula IX which is condensed with ethyl 2-(triphenylphosphoranylidene) acetate to produce (£)-ethyl 5-(tritylthio)pent-2-enoate of formula X followed by reducing with DIBAL-H and further oxidizing by Dess-Martine

periodinane to produce (£)-5-(tritylthio)pent-2-enal of formula VIII. The process is schematically shown as follows:

The major disadvantage with the above prior-art process is the formation of byproduct impurity i.e. ethyl (Z)-5-(tritylthio)pent-2-enoate of formula (Xb).

Impurity of formula (Xb) is generated along with required isomer ethyl (E)-5-(tritylthio)pent-2-enoate of formula (X).

This unwanted isomer or impurity of formula Xb is carried over in to the next stages and caused to the increase in impurities. Accordingly, the yield of required (E)-5-(tritylthio)pent-2-enal compound is reduced and also more purification steps are required to remove the impurities.

Removing TPPO impurity is very difficult as it is not soluble in most of organic solvents. TPPO impurity and unwanted isomer of formula Xb are soluble in alcohol solvents.

All the above prior art processes involves the use of column and flash chromatography methods to remove the TPPO and unwanted (Z)-5-(tritylthio)pent-2-enoate isomer of formula (Xb), which is tedious, laborious, requiring repeated purifications and also involving the use of large quantities of solvents. Hence it is not suitable at industrial scale operations.

Hence, there is a need to develop an improved process which is industrially feasible, eco-friendly, cost effective for the preparation of (ZT)-5-(tritylthio)pent-2-enal of formula X.

The present inventors had developed an improved process for the preparation of Romidepsin by avoiding all the aforementioned drawbacks in the prior art processes.

The present invention is also related to an improved process for the preparation of (£)-5-(tritylthio)pent-2-enal of formula (VIII) and (67?, 95, 125, 137?) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4, 7, 10-trioxo-9-(triphenylmethyl thiomethyl)-3-oxa-5, 8, 11-triazatetradecane-12-carboxylate of formula (XII) which are key components used in the preparation of vital intermediates of formulae (II) and (III) of Romidepsin. Brief description of the invention

The first aspect of the present invention is to provide an improved process for the preparation of Romidepsin of formula (I).

The second aspect of the present invention is to provides an improved process for the preparation of (£)-5-(tritylthio)pent-2-enal of formula (VIII).

The third aspect of the present invention is to provide an improved process for the preparation of 3-(tritylthio)propanal of formula (IX)

The fourth aspect of the present invention is to provide an improved process for the preparation of (67?, 95, 125, 137?) methyl 13-hydroxy-6-isopropyl-2?2-dimethyl-4,7,10-trioxo-9-(triphenylmethylj thiomethyl)-3-oxa-5,8,ll-triazatetradecane-12-carboxylate of formula (XII).

Detailed description of the invention

The first aspect of the present invention provides an improved process for the preparation of (15, 45, 7Z, 105, 16£, 217?)-7-ethylidene-4,21-bis(l-methylethyl)-2-oxa-12,13-dithia-5,8,20,23-tetraazabicyclo[8.7.6]tricos-16-ene-3,6,9,19,22-pentone of formula (I), comprising the steps of;

condensing (5,£)-2-(trimethylsilyl)ethyl 3-(((5)-2-amino-3-methylbutanoyl) oxy)-7-(tritylthio)hept-4-enoate of formula (II) with (57?,85,Z>l-(9//-fluoren-9-yl)-5-isopropyl-3,6,9-trioxo-8-(tritylthiomethyl)-2-oxa-4,7,10-triazatridec-11 -ene-11 -carboxylic acid of formula (III) in presence of a condensing agent and an organic base in a organic solvent to provide (57?, 85, 145, Z)-(5,£)-1 -oxo-1 -(2-(trimethylsilyl)ethoxy)-7-(tritylthio)hept-4-en-3-yl ll-ethylidene-l-(9H-fluoren-9-yl)-5,14-diisopropyl-3,6,9,124etraoxo-8-((tritylthio)methyl)-2-oxa-4,7,10,13-tetraaza- pentadecan-15-oate of formula (IV)

wherein, the condensing agent is selected from (l-[bis(dimethylamino) methylene]-lH-l,2,3-triazolo[4,5-b]pyridinium3M3xidhexafluoro phosphate (HATU), hydroxy benzotriazole (HQBt), dicyclohexylcarbodiimide (DCC), l-hydroxy-2,5-pyrrolidinedione (HOSu). deprptecting the compound of formula (IV) with alkylamine in presence of a polar aprotic solvent to get (S,£)-2-(trimethylsilyl)ethyl 3-(((5)-2-((Z)-2-((S)-2-((i?)-2-amino-3-methylbutanamido)-3-(tritylthio)propanamido)but-2-enamido)-3-methyl butanoyl)oxy) -7-(tritylthio)hept-4-enoate of formula (V)
wherein, the alkylamine is selected from diethylamine, triethylamine,
diisopropylethylamine, tripropylamine, n-butylamine or mixtures thereof; and polar
aprotic solvent is selected from acetonitrile, dimethyl sulfoxide, dimethylacetamide,
dimethylformamide, tetrahydrofuran, N- methylpyrrolidinone.
wherein, the organic base is selected from diisopropylethylamine (DIPEA), triethylamine
(TEA), N-methylmorpholine (NMM) or mixtures thereof; the organic solvent is selected from
dichloromethane, chloroform, acetonitrile, tetrahydrofuran, ethyl acetate or mixtures thereof.
wherein, the organic solvent is selected from dichloromethane, chloroform, acetonitrile, tetrahydrofuran, ethyl acetate or mixtures thereof.

In the present aspect of the invention, the condensation of compound of formula (II) in dichloromethane with the compound of formula (III) is carried out using HATU and HOBt at 0-5°C in presence of DIPEA to get the compound of formula (IV).

The main advantage of the present invention is condensing of chiral intermediate (II) with intermediate (III) at low temperatures which controls the racemization, whereas by use of condensation agents like BOP and PyBOP required higher temperature and lead to formation of racemic impurities and poor yield of the desired product.

Deprotection of Fmoc protecting group from compound of formula (IV) using cyclic organic base such as piperidine (prior-art process) results the formation of higher level of impurities, whereas by using alkylamine base preferably diethylamine, the impurities or side products were formed to a lesser extent and provided the desired compound with high purity.

Converting the compound of formula V to the compound of formula I can be carried out by known methods in the art as follows:

a. the compound of formula (V) is treated with TBAF in THF to produce
(3S,6S,12S,157?,Z)-15-amino-9-e%^^
tetra oxo-3-((£)-4-(tritylthio)but-1 -en-1 -yl)-12-((triphenylmethylthio)methyl)-4-oxa-7, 10,13-triazaheptadecan-l-oic acid of formula (VI)
b. the compound of formula VI is internally condensed in presence of HATU,
DIPEA in dichloromethane to produce (3S,95,12i?,165,Z)-6-ethylidene-3,12-
diisopropyl-16-((£)-4-(tritylthio)but-1 -en-1 -yl)-9-((tritylthio)methyl)-1 -oxa-
4,7,10,13-tetraaza cyclohexadecane-2,5,8,ll,14-pentaone of formula (VII)
c. the compound of formula (VII) is reacted with iodine in methanol to produce
the compound of formula (I)

The second aspect of the present invention provides a process for the preparation of
compound of formula I comprising: |
a. treating the compound of formula (IV) with tetrabutylammonium fluoride
(TBAF) in an organic solvent to provide the compound of formula (VI);
b. converting the compound of formula VI to the compound of formula I.
In the present aspect of the invention the (57?, 85, 145, Z)-(S,£)-l-oxo-l-(2-(trimethylsily l)ethoxy)-7-(tritylthio)hept-4-en-3 -y 1 11 -ethylidene-1 -(9H-fluoren-9-yl)-5,14-diisopropyl-3,6,9,124etraoxo-8-((tritylthio)methyl)-2-oxa-4,7,10,13-tetraaza-pentadecan-15-oate of formula (IV) (NH-Fmoc and COO-Si diprotected compound) in organic solvent treated with tetrabutylammonium fluoride (TBAF) in a polar aprotic solvent at 0-5°C provides amino acid compound of formula (VI).
The third aspect of the present invention provides an improved process for the preparation of (£)-5-(tritylthio)pent-2-enal compound of formula (VIII), comprising of:
a) reacting 3-(tritylthio)propanal of formula (IX) with ethyl 2-(triphenylphosphoranylidene)acetate in an organic solvent and purification to get (£)-ethyl 5-(tritylthio)pent-2-enoate of formula (X)

b) reducing pure (£)-ethyl 5-(tritylthio)pent-2-enoate of formula (X) using a suitable reducing agent and purification to get (£)-5-(tritylthio)pent-2-ene-l-ol of formula (XI)
c) oxidizing pure (£)-5-(tritylthio)pent-2-ene-l-ol of formula (XI) using MnC>2 in presence of organic solvent and purification to get pure (£)-5-(tritylthio)pent-2-enal of formula (VIII)
wherein, the purification in step-a), step-b) and step-c) is carried out as
recrystallization from a suitable solvent which is selected from polar protic, polar
aprotic and non polar solvents or mixtures thereof.
The polar protic solvent is selected from alcohols for example methanol, ethanol, n-propanol, iso-propanol, n-butanol, isobutanol, tertiary butanol, trifluoroethanol, methoxy ethanol, ethylene glycol; and water and the like.

The polar aprotic solvent is selected from esters such as methyl acetate, ethyl acetate, isopropyl acetate; ketones such as acetone, butanone, pentanone; nitriles such as acetonitrile, propionitrile; dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), dimethyl acetamide (DMAc), N-methyl morpholine (NMP) and the like.

The non polar solvent is selected from hexane, heptane, cyclohexane, toluene, trifluorotoluene, chlorobenzene, ter^-butyl -methyl ether, cyclopentylmethyl ether, dichloromethane and the like.

The suitable reducing agent is selected from DIBAL-H, sodium borohydride, lithium aluminum hydride and the like, more preferably DIBAL-H.

The step-a), step-b) and step-c) are carried out in presence of a organic solvent which is selected from halo hydrocarbons such as chloromethane, dichloromethane, chloroform, carbon tetrachloride, ethylene dichloride, chlorobenzene; esters such as ethyl acetate, methyl acetate, propyl acetate, isopropyl acetate; nitrile solvents such as acetonitrile, propionitrile; DMF, DM Ac, THF, methyl tertiary butyl ether, N-methyl pyrrolidinone, DMSO.

The 3-(tritylthio)propanal of formula (IX) is reacted with ethyl 2-(triphenylphosphoranylidene)acetate in dichloromethane at 30°C and purified by recrystallization from methanol to provide pure (E)-ethyl 5-(tritylthio)pent-2-enoate of formula (X).

The pure compound of formula X is reduced by using DIBAL-H at room temperature to get the (£)-5-(tritylthio)pent-2-ene-l-ol of formula (XI) which is purified by recrystallization from toluene to provide the pure compound (£)-5-(tritylthio)pent-2-ene-l-ol of formula (XI).

The pure compound of formula (XI) is oxidized by using Mn02 in dichloromethane to get a compound of formula (VIII). The compound of formula (VIII) is purified by recrystallization from isopropanol.

The main advantage of the present invention is that, it greatly reduced the amount of impurities such as TPPO, P, y-isomer of formula (Xa) and Z-isomer of formula (Xb) in required compound (E)-ethyl 5-(tritylthio)pent-2-enoate of formula (X) when purified by recrystallization from organic solvents, preferably from methanol and accordingly the use of flash and column chromatographic methods are avoided. This has a great impact during industrial scale-up.

The compound of formula (VIII) can be converted to the compound of formula (II) by known methods in the prior arts.The fourth aspect of the present invention provides an improved process for the

preparation of 3-(tritylthio)propanal of formula (IX), comprising the steps of:

a) 3-(tritylthio)propionic acid of formula (XIX) is reduced by using a suitable reducing agent in an organic solvent to provide 3-(tritylthio)propan-l-ol of formula (XX);
b) purification of the compound of formula (XX) by recrystallization from a suitable solvent to provide pure 3-(tritylthio)propan-l-ol of formula (XX);
c) oxidation of pure compound of formula (XX) using oxalyl chloride in presence of DMSO to provide 3-(tritylthio)propanal of formula (IX)
wherein, the suitable solvent for the purification is selected from alkanes such as
cyclohexane, cyclopentane, cyclobutane n-hexane etc. Preferably cyclohexane.
The suitable reducing agent is selected from sodium borohydride, boron trifluoride etherate, boron etherate, lithium aluminum hydride or mixtures thereof and the like. More preferably sodium borohydride and boron trifluoride etherate.
3-(Tritylthio) propionic acid of formula (XIX) in tetrahydrofuran is reduced by using sodium borohydride and boron trifluoride.etherate at 0-5°C to get 3-(tritylthio)propan-l-ol of formula XX. This crude compound is purified by recrystallization from cyclohexane to get pure product. This pure product is oxidized by using oxalyl chloride in presence of DMSO at room temperature in presence of triethylamine to .provide 3-(tritylthio)propanal of formula (IX).
The fifth aspect of the present invention provides an improved process for the preparation of (67?, 95, 125, 13R) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4,7,10-trioxo-9-(tritylthiomethyl)-3-oxa-5,8,ll-triazatetradecane-12-carboxylate of formula (XII), comprising the steps of:

a) reacting (D)-tritylthio cysteine of formula (XIII) with fluorenylmethoxycarbonyl
chloride (Fmoc-Cl) in presence of a base in polar aprotic solvent to provide

fluorenylmethoxycarbonylamino (Z))-tritylthio cysteine of formula (XIV)
b) fluorenylmethoxycarbonylamino (Z))-tritylthio cysteine of formula (XIV) is condensed with (2S, 37?) methyl 2-amino-3-hydroxy butanoate hydrochloride of formula (XV) in presence of a condensing agent and base in organic solvent to produce methyl 2-((5)-2-(((9//-fluoren-9-yl)methoxy)carbonylamino)-3-(tritylthio) propanamido)-3-hydroxy butanoate of formula (XVI) which is further in-situ treatment with organic base in polar aprotic solvent to produce (25, 37?)-methyl 2-((S)-2-amino-3-(tritylthio)propanamido)-3-hydroxy butanoate of formula (XVII)

wherein, the condensing agent is selected from the group comprising of DCC, HOBt, N-hydroxylsuccinimide(HOSu), 3-hydroxy-l?2,3-benzotriazin-4-[3/7]-one; an organic base is selected from triethylamine, diethylamine, diisopropylethylamine, n-butylamine, pyridine, c) (25, 37?) Methyl 2-((5)-2-amino-3-(tritylthio)propanamido)-3-hydroxy butanoate of formula (XVII) is condensed with fluorenylmethoxycarbonylamino-D-valine of formula (XVIII) in presence of a condensing agent in an organic solvent to produce (67?, 95, 125, 137?) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4,7,10-trioxo-9-(tritylthiomethyl)-3-oxa-5,8,l l-triazatetradecane-12-carboxylate of formula (XII)

The preferred embodiment of the present invention provides (Z))-tritylthio cysteine of
formula (XIII) is reacted with fluorenylmethoxycarbonyl chloride (Fmoc-Cl) in presence of
sodium bicarbonate in tetrahydrofuran to produce fluorenylmethoxycarbonylamino (Z))-
tritylthio cysteine of formula (XIV). This compound of formula (XIV) is condensed with (25,
37?) methyl 2-amino-3-hydroxy butanoate hydrochloride of formula (XV) in presence of
dicyclohexylcarbodiimide, hydroxybenzotriazole, N-methylmorpholine to get methyl 2-((iS)-
2-(((9//-fluoren-9-yl)methoxy)carbonylamino)-3-(tritylthio) propanamido)-3-hydroxy
butanoate of formula (XVI) which on in-situ treatment with triethylamine in acetonitrile to produce (2S, 37?)-methyl 2-((S)-2-amino-3-(tritylthio)propanamido)-3-hydroxy butanoate of formula (XVII). The compound of formula (XVII) is condensed with the compound of formula (XVIII) in presence of dicyclohexylcarbodiimide, hydroxybenzotriazole, N-methylmorpholine to get (67?, 95, 125, 137?) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4,7,10-trioxo-9-(tritylthiomethyl)-3-oxa-558,ll-triazatetradecane-12-carboxylate (XII).

The compound of formula (XII) is further converted to the compound of formula (III) by known methods in the prior art.

The best mode of carrying out the present invention is illustrated by the below mentioned examples. These examples are for illustrative purposes only and in no way limit the embodiments of the present invention. Examples: Example-1: Preparation of (£)-ethyl 5-(tritylthio)pent-2-enoate of formula (X)

Toluene (500 ml), water (500 ml) and triphenylphosphine (100 gm) were charged in a clean and dry round bottom flask at 25-30°C and stirred for 5-10 minutes. Ethyl-2-bromo acetate (70 gm) was added to the reaction mixture and stirred for 5-6 hours at 100-105°C and cooled the reaction mixture to 25-30°C. Aqueous layer and organic layer both are separated and the aqueous layer extracted with dichloromethane (100 ml). Aqueous NaOH solution (45.8 gm of NaOH in 300 ml of water) was slowly added to the reaction mixture at below

10°C for about 10-15 minutes. Organic and aqueous layers were separated and organic layer was cooled to 0-5°C. 3-(TrityIthio)propanal (101.4 gm) in dichloromethane was added to the above pre-cooled organic layer at 0-5°C and stirred for 2 hours at same temperature. Thereafter, the dichloromethane was distilled under reduced pressure to provide a crude compound. The obtained compound was recrystallized from methanol to get pure white crystalline title compound. Yield: 87.6 gms; melting range: 70-72°C. Example 2: Preparation of (E)-5-(tritylthio)pent-2-en-l-ol of formula (XI)

(£)-Ethyl 5-(tritylthio)pent-2-enoate (50 gm), toluene (250 ml) were charged into a dry and clean round bottom flask under nitrogen atmosphere and stirred for 10-15 minutes at 25-30°C and cooled to 0-5°C. DIBAL-H (44.16 gms) was added to the reaction mixture and stirred for 2-3 hours at 0-5°C. The reaction mixture was quenched with ethyl acetate at 0-5°C. Water was added to the above reaction mixture 0-5 °C and further raised the temperature to 25-30°C for 2-3 hours. Hyflow was added to the reaction mixture, filtered and washed with ethyl acetate (50 ml). The organic layer and aqueous layer both are separated from the filtrate and distilled off the solvent completely from the organic layer. Toluene was added to the above obtained residue. The reaction mixture was cooled to 0-5°C and stirred for 1 hour at same temperature. Filtered the solid and washed with toluene. Yield: 38.8 gms; Meiting range 105-110°C. Example 3: Preparation of (2^-5-(tritylthio)pent-2-enaI of formula (VIII)

(£)-5-(Tritylthio)pent-2-en-l-ol (50 gms) was dissolved in dichloromethane (1000 ml) in a clean and dry round bottom flask at 25-30°C. To this solution, MnC>2 (241.2 gms) was added and slowly heated the reaction mixture to 40±5°C and stirred for 3-4 hours. The reaction mixture was cooled to 25-30°C and filtered. Distilled off the solvent under reduced pressure to get a crude solid. IPA was added to the obtained solid and stirred for 1 hour. Filtered the solid and washed with IPA. Yield: 42.4 gms; Melting range: 116-120°C.

Example 4: Preparation of (S, £)-3-hydroxy-l-((/f)-4-isopropyl-2-thioxothiazolidin-3-yl)-7-(tritylthio)hept-4-en-l-one (7?)-l-(4-Isopropyl-2-thioxothiazolidin-3-yl)ethanone (9 gms) and dichloromethane (250 ml) were charged into a round bottom flask at 25-30°C and cooled the reaction mass to 0-

5°C. To this resulting reaction mixture, TiCU (9.2 gms) was added and stirred for 10-15 min. The reaction mixture was cooled to -78°C. Diisopropylethylamine (8.48 ml) was added to the reaction mixture and stirred for 2 hours at -78°C. (£)~5-(Tritylthio)pent-2-enal (9.5 gms) in dichloromethane (250 ml) was slowly added to the reaction mixture and stirred for about 20-30 mins at -78°C. Aqueous ammonium chloride solution was added to the reaction mixture and the temperature was raised to 25-30°C. Organic and aqueous layers were separated and the organic . layer was washed with water and distilled under reduced pressure to get titled compound. Example 5: Preparation of (S, £)-3-hydroxy-7-(trityIthio)hept-4-enoic acid

(S, £)-3-Hydroxy-l-((i?)-4-isopropyl-2-thioxothiazolidin-3-yl)-7-(tritylthio)hept-4-en-1-one (25 gms) and tetrahydrofiiran (250 ml) were charged in a round bottom flask at 25-30°C and the resulting solution was cooled to 0-5°C. To this solution, LiOH.IrbO (9.33 gms) in water (250 ml) was added and stirred the reaction mixture for 12 hours at 25-30°C. Distilled off the solvent from the reaction mixture. To the obtained reaction mixture, methyl tertiary butyl ether (125 ml) was added and organic and aqueous layers were separated. Dichloromethane was added to the aqueous layer and acidified with 2N hydrochloric acid at 0-5°C and stirred for 25-30 min at 0-5°C. Organic layer was separated and distilled off the solvent completely to provide a title compound.
Example 6: Preparation of (S, £)-2-(trimethylsilyl)ethyl 3-hydroxy-7-(tritylthio)hept-4-enoate (S, £)-3-Hydroxy-7-(tritylthio)hept-4-enoic acid (2 gms) and dichloromethane (20 ml) were dissolved in a round bottom flask at 25-30°C and cooled to 0-5°C. To this reaction solution, 2-(trimethylsilyl)ethanol (2.15 ml) and dimethylaminopyridine (0.12 ml) were added at 0-5°C and stirred for 10 mins at same temperature. To this reaction mixture, DCC solution (1.24 gms of DCC dissolved in 10 ml of dichloromethane) was added and stirred at 0-5°C and raised the temperature to 25-30°C. Filtered the reaction mixture and washed with dichloromethane (4 ml). Aqueous sodium bicarbonate solution (2 gm of sodium carbonate dissolved in 20 ml of water) was added to the obtained filtrate and stirred for 20 mins. The organic layer was separated, washed with water (20 ml). Distilled off the solvent from the organic layer under reduced pressure to obtain the title compound.

Example 7: Preparation of (S, £)-2-(trimethylsilyl)ethyI 3-(((^-2-((((9/^Huoren-9-yl) methoxy)carbonyl)amino)-3-methyI butanoyl)oxy)-7-(tritylthio)hept-4-enoate The compound of example 6 was dissolved in dichloromethane (25 ml) in a round bottom flask at 25-30°C and stirred for 5-10 mins. The reaction mixture was cooled to 0-5°C and Fmoc-L-Valine (2.2 gms) and dimethylaminopyridine (0.058 ml) were added and stirred for 10-15 min. To this reaction mixture, DCC solution (1.39 gms of DCC dissolved in 5 ml of dichloromethane) was slowly added at 0-5°C. The reaction mixture temperature was raised to 25-30°C and stirred for about 9 hours. The reaction mixture was filtered and washed with dichloromethane (10 ml). Aqueous sodium bicarbonate solution was added to the reaction mixture and stirred for 20 mins. Separate the both aqueous and organic layers and solvent from organic layer was distilled off under reduced pressure to get the title compound. Example 8: Preparation of the compound of formula (II)

(S, E)-2-(trimethylsilyI)ethyI 3-(((S)-2-((((9H-fluoren-9-yl) methoxy) carbonyl) amino)-3-methyl butanoyl)oxy)-7-(tritylthio)hept-4-enoate (7 gms) and acetonitrile (50 ml) were dissolved in a round bottom flask at 25-30°C and stirred for 5-10 mins. To this solution, triethylamine (6.20 ml) was added and stirred for about 8 hours at 25-30°C. Distilled off the solvent from the reaction mixture under reduced pressure. To the obtained compound, water (50 ml) and dichloromethane (50 ml) were added and stirred for about 15-20 mins. The organic layer and aqueous layer were separated and distilled off the solvent completely from the organic layer to get the title compound. Example 9: 3-(Tritylthio) propionic acid of formula (XIX)

To a solution of trityl chloride (100 gm) in toluene (500 ml), 3-mercapto propionic acid (41.9 gms) was added and stirred for 6 hours at 25-30°C. 500 ml of water was added and stirred for 15-20 mins at 25-30°C. Filtered the precipitated solid and dried to get title compound as a white coloured powder. Yield: 124.9 gms; Melting range 200-205°C. Example 10: 3-(Tritylthio)propan-l-ol of formula (XX)
3-(Tritylthio) propionic acid (100 gms) was dissolved in tetrahydrofuran (700 ml) and the resulting solution was cooled to 0-5°C. To this solution, NaBFL* (4.4 gms) was added lot wise at 0-5°C and stirred for 10-15 mins. To this reaction mixture, BF3. etherate (56.9 ml) was

slowly added at 0-5°C for each lot addition. The temperature of the reaction mixture was raised to 25-30°C and stirred for about 3-4 hours. Cooled the reaction mixture to 0-5°C and water (700 ml) and dichloromethane (800 ml) were added to the reaction mixture and stirred for about 15-25 mins. Organic and aqueous layers both were separated and washed with NaCl solution and stirred for about 10-15 mins. Organic layer was separated and distilled off the solvent completely from the organic layer under reduced pressure and co-distilled with cyclo hexane. 300ml of cyclohexane (100 ml) was added to the obtained compound at 25-30°C and heated the reaction mixture to 55-60°C. The reaction mixture was stirred for 45 mins at 55-60°C. The reaction mixture was cooled to 25-30°C and stirred for 30 mins. Filtered the precipitated solid and dried the material to get the title compound. Yield: 79.5 gms Example 11: 3-(Tritylthio)propanal of formula (IX)

Dichloromethane (300 ml) and oxalyl chloride (22.7 ml) were charged to the round bottom flask at 25-30°C and cooled the reaction mixture to -75 to -70°C. To this reaction mixture, DMSO (38.3 ml) was added slowly at -75 to -70°C and stirred for 1-2 hours at the same temperature. A solution of 3-(tritylthio)propan-l-ol (50 gms) in dichloro methane (350 ml) slowly added to the above reaction mixture and stirred for about 2-3 hours at -75 to -70°C. To this reaction mixture, triethylamine (125 ml) was added at -75 to -70°C and stirred the reaction mixture for 2 hours at 0-5°C. Water (350 ml) was added to the reaction mixture at 0-5°C. The temperature was raised to 25-30°C. Organic and aqueous layers both were separated and the organic layer was washed with aqueous sodium thiosulfate solution (25 gms of sodium thiosulfate was dissolved in 250 ml of water). Distilled off the solvent completely from organic layer under reduced pressure. Cyclohexane (200 ml) was added to the obtained compound at 25-30°C and stirred for 30-40 min at same temperature. Filtered and dry the material to get the title compound. Yield: 44.35 gms

Example 12: Preparation of (5)-2-(((9/^-fluoren-9-yl)methoxy)carbonylamino)-3-(tritylthio) propanoic acid. (iS)-2-Amino-3-mercaptopropanoic acid (50 gms) was dissolved in dimethylformamide (150 ml) at 25-30°C. To this reaction mixture, trityl chloride (137.8 gms) was added and heated to 40-50°C and stirred for 3-4 hours at same temperature. The reaction mixture was cooled to 25-30°C, and slowly added the aqueous sodium acetate solution (135 gms of sodium

acetate was dissolved in 1000 ml of water) and stirred for 25-30 mins. The solid formed in the reaction mixture was filtered and washed with water. Methyl tertiary butyl ether (1000 ml) was added to the obtained compound.

Aqueous sodium bicarbonate solution (69 gms in 1000 ml of water) and 1250 ml of tetrahydrofuran were added to the obtained compound at 25-30°C. Cooled the reaction mixture to 0-5°C and Fmoc-Cl (53 gms) was added and stirred for 1 hour at 0-5°C. Acidified the reaction mixture with dilute hydrochloride solution to separate the both organic and aqueous layers and the aqueous layer was extracted with MTBE. Combined the total organic layers and distilled off the solvent completely under reduced pressure. The obtained compound was dissolved in 200 ml of dichloromethane at 25-30°C. Pet.ether (1050 ml) was added to the reaction mixture at 25-30°C and stirred for 6 hours at the same temperature. Filtered the precipitated solid and dried the material to get the title compound.. Yield: 180 gms

Example 13: Preparation of (25", 37?)-methyl 2-((5)-2-amino-3-(trityIthio)propanamido)-3-hydroxy butanoate Dichloromethane (500 ml) was added to the (25, 37?)-methyl-2-amino-3-hydroxy butanoate hydrochloride at 25-30°C and cooled the reaction mixture to 0-5°C. To this reaction mixture, (5)-2-(((9//-fluoren-9-yl)methoxy)carbonylamino)-3-(tritylthio)propanoic acid (100 gms), N-methyl morpholine (37.42 ml), hydroxy benzotriazole (4.61 gms), DCC solution (42.27 gms of DCC in 100 ml of dichloromethane) was added and stirred for 20-30 mins at 0-5°C. Raised the temperature of reaction mixture to 25-30°C and stirred for 3-4 hours at the same temperature. The reaction mixture was distilled out under atmospheric pressure.
To the above obtained compound, acetonitrile (250 ml) and triethylamine (71.68 ml) were added and stirred the reaction mixture for 30 mins at 25-30°C. The obtained solid precipitate was filtered and washed with acetonitrile (50 ml). The filtrate was taken into a round bottom flask and stirred for 12 hours at 25-30°C followed by distilled off the solvent completely from reaction mixture under reduced pressure. Dichloromethane (300 ml) was added to the obtained compound at 25-30°C and cooled to 0-5°C, stirred for 25-30 mins. The reaction mixture was filtered through hiflow bed and washed with dichloromethane (50 ml). Separated the both aqueous and organic layers and solvent from the organic layer was distilled

off under the atmospheric pressure and finally reduced pressure to get a residue compound. To this residue compound, methyl tertiary butyl ether was added and stirred the reaction mixture for 3 hours at 0-5°C. To the obtained compound dichloromethane was added and stirred for 30 mins. at 25-30°C. Filtered the reaction mixture and distilled off the solvent from filtrate. Finally stirred the reaction mixture in a mixture of dichloromethane and methyl tertiary butyl ether provided crystalline solid compound. Yield: 55 gms

Example 14: (57?, 85, 115, 12i?)-MethyI l-(9i7-fluoren-9-yl)-12-hydroxy-5-isopropyl-3, 6, 9-trioxo-8-(tritylthiomethyl)-2-oxa-4, 7,10-triazatridecane-ll-carboxylate (25, 37?)-Methyl 2-((5)-2-amino-3-(tritylthio)propanamido-3-hydroxy butanoate (100 gms) is dissolved in dichloromethane (1300 ml) at 25-30°C and cooled to 0-5°C. To this reaction mixture, Fmoc D-valine (69.3 gms) was added at 0-5°C and stirred for 10-15 min. at the same temperature. To the above reaction mixture, N-methyl morpholine (34.4 ml), hydroxybenzotriazole (5.6 gms), dicyclohexylcarbodiimide solution (51.6 gms of DCC dissolved in 200 ml of dichloromethane) were added at 0-5°C and stirred for 25-30 mins at the same temperature. The reaction mixture temperature was raised to 25-30°C and stirred for 3 hours. The obtained reaction mixture was filtered and washed with dichloromethane (200 ml). The filtrate was washed with aqueous sodium bicarbonate solution (10 gms of sodium bicarbonate was dissolved in 200 ml of water) followed by aqueous sodium chloride solution (20 gms of NaCl in 200 ml of water). Distilled off the solvent completely from the organic layer under atmospheric pressure. Ethyl acetate (700 ml) was added to the above obtained distillate at 25-30°C and stirred for 25-30 mins. The reaction mixture was cooled to 0-5°C and stirred for 1-2 hours. The obtained compound was filtered, washed with ethyl acetate (100 ml) and dried to produce the title compound. Yield: 190 gms.

Example 15: (5tf, 85, 115, 12tf)-Methyl l-(9/T-fluoren-9-yl)-12-methanesuIfonyloxy-5-isopropyl-3, 6, 9-trioxo-8-(tritylthiomethyl)-2-oxa-4, 7,10-triazatridecane-ll-carboxylate (67?, 95, 125, 137?) Methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4, 7,10-trioxo-9-(tritylthiomethyl)-3-oxa-5,8,ll-triazatetradecane-12-carboxylate (160 gms) is dissolved in dichloromethane (1600 ml) at 25-30°C and cooled to 0-5°C. To this reaction mixture, dimethylaminopyridine (0.97 gms), triethylamine (42 ml), methanesulfonyl chloride (19.16 ml) were added at 0-5°C and stirred for 2-3 hours at the same temperature. The obtained

reaction mixture was washed with aqueous ammonium chloride solution followed by aqueous sodium chloride solution. Distilled off the dichloromethane solvent completely from the reaction mixture under reduced pressure and co-distilled with cyclohexane (80 ml). Cyclohexane (640 ml) was added to the above distillate and stirred for 30 mins at 25-30°C. The obtained compound was filtered and washed with cyclohexane (80 ml) and dried to produce the title product. Yield: 153 gms

Example 16: Preparation of (Z)-methyl 2-((iS)-2-((7f)-2-amino-3-methylbutanamido)-3-(trityIthio)propanamido)but-2-enoate . (57?, 85, 115, 12tf)-Methyl l-(9//-fluoren-9-yl)-12-hydroxy-5-isopropyI-3, 6, 9-trioxo-8-(tritylthiomethyl)-2-oxa-4, 7, 10-triazatridecane-ll-carboxylate (165 gms) was dissolved in dichloromethane (1650 ml) at 25-30°C and cooled to 0-5°C. To this reaction mixture, 1, 4-diazabicyclo [2.2.2]octane (DABCO) (210 gms) was added and stirred for 2-3 hours at 0-5°C. The reaction temperature was raised to 25-30°C and stirred for 8 hours at the same temperature. The reaction mixture was filtered and washed with dichloro methane (330 ml) and again washed the filtrate with water (660 ml). Distilled off the solvent completely from the organic layer under reduced pressure. Acetonitrile (660 ml) was added to the above obtained compound and stirred for 1-2 hours. The reaction mixture was washed with cyclohexane. Distilled off the acetonitrile solvent completely from the reaction mixture under reduced pressure to get the title compound. Example 17: Preparation of the compound of formula (III)
The title compound of example 16 (50 gms) was dissolved in tetrahydrofuran (600 ml) and cooled to 0-5°C. To this reaction mixture, aqueous LiOH was added at Q-5°C and stirred for 5-6 hours. Acidifying the reaction mixture using IN HC1 solution. The compound was extracted with ethyl acetate. Distilled off the solvent completely from the organic layer under reduced pressure. The obtained compound was co-distilled with methyl tertiary butyl ether (MTBE). MTBE was added to the above distillate and stirred for 30-45 mins. The obtained compound was filtered and washed with MTBE.

The above obtained compound was dissolved in tetrahydrofuran (600 ml) at 25-30°C. Sodium bicarbonate solution was added to the above reaction mixture and cooled to 0-5°C. To this reaction mixture, Fmoc-Cl was added and stirred for 30-45 mins. Acidifying the reaction

mixture using IN HC1 solution. Ethyl acetate (125 ml) was added the reaction mixture and both aqueous and organic layers were separated. Distilled off the solvent completely from the organic layer under reduced pressure and co-distilled with dichloromethane. Dichloromethane (150ml) was added to the reaction mixture and heated to 40-45°C and stirred for 15-20 mins at same temperature. Filtered the reaction mixture through highflow bed and washed with dichloromethane. Diisopropyl ether (400 ml) was added to the filtrate at 25-30°C and stirred for 2-3 hours at same temperature. The obtained material was filtered, washed with diisopropyl ether and dried to get the title compound. Yield: 22 gms Example 18: Preparation of the compound of formula (IV)

The compound of formula II (2 gms) and the title compound of example 17 (3.3 gms) were dissolved in dichloromethane (20 ml) and stirred for 5-10 mins at 25-30°C. To this reaction mixture, HATU (1.70 gms), HOBt (0.87 gms) and DIPEA (1.41 ml) were added and stirred for 1-13 mins at 25-30°C. Water (20 ml) was added to the above reaction mixture at 25-30°C and stirred for 10-15 mins at the same temperature. The organic and aqueous layers both were separated and distilled off the solvent completely from the organic layer under reduced pressure to produce the title compound. Example 19: Preparation of the compound of formula (V)

The compound of formula IV (7 gms) was dissolved in acetonitrile (70 ml). To this reaction mixture, diethylamine (1.9 gms) was added and stirred for 12 hours at 25-30°C. Distilled off the solvent completely from reaction mixture under reduced pressure to get the title compound. Yield: 4.2 gms

Example 20: Preparation of the compound of formula (VI)
The compound of formula V (1.5 gms) was dissolved in tetrahydrofuran (50 ml). To this reaction mixture, tetrabutyl ammonium fluoride (0.683 gms) was added at 25-30°C and stirred the reaction mixture for 15-20 hrs at the same temperature. Distilled off the solvent completely from the reaction mixture under reduced pressure to get the title compound. Yield: 1.2 gms Example 21: Preparation of the compound of formula (VII)

The compound of formula VI (2 gms) was dissolved in dichloromethane (200 ml). To this reaction mixture, HATU (2 gms) and DIPEA (1.4 gms) were added and stirred for 2 days

at 25-30°C. Quenched the reaction mixture with aqueous ammonium chloride solution and stirred for 30 mins at 25-30°C. The organic and aqueous layers were separated and the organic layer was washed with water (50 ml). Distilled off the solvent completely from organic layer under reduced pressure to get a title compound. Yield: 1.37 gms Example 22: Alternate process for the preparation of the compound of formula (VII)

The compound of formula IV (2 gms) was dissolved in acetonitrile (70 ml) at 0-5°C. To this reaction mixture, tetrabutyl ammonium fluoride tri hydrate (3.8 gms) was added at room temperature and stirred for 1-2 hours. Distilled the solvent completely from the reaction mixture under reduced pressure to get the crude amino acid compound. This crude amino acid mixture was dissolved in dichloromethane (200 ml) followed by HATU (2 gms), diisopropyl ethylamine (1,4 gms) were added at room temperature and stirred for 20 hours. Quenched the reaction mixture with aqueous ammonium chloride solution and stirred for 30 mins at 25-30°C. The organic and aqueous layers were separated and the organic layer was washed with water (50 ml). Distilled off the solvent completely from organic layer under reduced pressure to get a title compound. Yield: 0.75 gms Example 23: Preparation of Romidepsin (Formula I)

A solution of compound of formula VII (2 gms) was added to the mixture of dichloromethane (200 ml) and methanol (200 ml). At 25-30°C, to the reaction mixture, iodine (0.1 gms), dichloromethane (1000 ml), methanol (1000 ml) were added and stirred for 2 hours at same temperature. Aqueous sodium thiosulphate solution was added to the above reaction mixture at 0-5°C and stirred for 10-15 mins. Organic and aqueous layers both were separated and organic layer was washed with water. Distilled off the solvent completely from reaction mixture under reduced pressure. The obtained compound was purified by using column chromatography on silica gel using dichloromethane and methanol mixture as mobile phase. The obtained solid was isolated by using n-hexane. Yield: 0.4 gms.

We claim:

1. An improved process for the preparation of (15, 45, 7Z, 105, 16£, 21i?)-7-ethylidene-
4,21-bis(l-methylethyl)-2-oxa-12,13-dithia-5, 8, 20, 23-tetraazabicyclo[8.7.6]tricos-
16-ene-3, 6, 9, 19, 22-pentone of formula (I), comprising:
a) reacting (5*, £)-2-(trimethylsilyl)ethyl 3-(((5)-2-amino-3-methylbutanoyl) oxy)-
7-(tritylthio) hept-4-enoate of formula (II) with the (57?,85,Z>l-(9//-fluoren-9-
yl)-5-isopropyl-3,6,9-trioxo-8-(tritylthiomethyl)-2-oxa-4,7,10-triazatridec-11 -
ene-11-carboxylic acid of formula (III) in presence of a condensing agent and
an organic base in a organic solvent to provide the (57?, 85, 145, Z)-(5,£)-l-oxo-
1 -(2-(trimethylsilyl)ethoxy)-7-(tritylthio)hept-4-en-3-yl 11 -ethylidene-1 -(9H-
fluoren-9-yl)-5,14-diisopropyl-3,6,9, 12-tetraoxo-8-((tritylthio)methyl)-2-oxa-
4,7,10,13-tetraaza-pentadecan-15-oate of formula (IV),
wherein, the condensing agent is selected from HATU, HOBt, DCC and HOSu or mixtures thereof,
b) deprotecting the compound of formula (IV) using alkylamine in presence of a polar aprotic solvent to provide the (5, £)-2-(trimethyIsilyl)ethyl 3-(((5)-2-((Z)-2-((5)-2-((/?)-2-amino-3-methylbutanamido)-3-(tritylthio)propanamido)but-2-enamido)-3-methylbutanoyl)oxy) -7-(tritylthio)hept-4-enoate of formula (V), wherein, the alkylamine is selected from diethylamine, triethylamine, diisopropylethylamine, tripropylamine and n-butylamine or mixtures thereof; the polar. aprotic solvent is selected from acetonitrile, dimethyl sulfoxide, dimethylacetamide, dimethylformamide, tetrahydrofuran, N-methylpyrrolidinone
c) converting the compound of formula (V) to the compound of formula I.
2. An improved process for the preparation of (15, 45, 7Z, 105, 16£, 217?)-7-ethylidene-
4,21-bis(l-methylethyl)-2-oxa-12,13-dithia-5, 8, 20, 23-tetraazabicyclo[8.7.6]tricos-
16-ene-3, 6, 9, 19, 22-pentone of formula (I), comprising:
a) reacting (5, £)-2-(trimethylsilyl)ethyl 3-(((5)-2-amino-3-methylbutanoyl) oxy)-7-(tritylthio) hept-4-enoate of formula (II) with the (57?,85,Z>l-(9//-fluoren-9-yl)-5-isopropyl-3,6,9-trioxo-8-(tritylthiomethyl)-2-oxa-4,7,10-triazatridec-ll-

ene-11-carboxylic acid of formula (III) in presence of a condensing agent and an organic base in a organic solvent to provide the (57?, 85, 145, Z)-(S,E)-l-oxo-1 -(2-(trimethylsilyl)ethoxy)-7-(tritylthio)hept-4-en-3-yl 11 -ethylidene-1 -(9//-fluoren-9-yl)-5,14-diisopropyl-3,6,9, 12-tetraoxo-8-((tritylthio)methyl)-2-oxa-4,7,10,13-tetraaza-pentadecan-15-oate of formula (IV), wherein, the condensing agent is selected from HATU, HOBt, DCC and HOSu or mixtures thereof,
b) reacting the compound of formula (IV) with tetrabutylammonium fluoride in polar aprotic solvent to provide the compound of formula (VI),
c) converting the compound of formula (VI) to the compound of formula (I).

3. According to claim 1, the organic base is selected from diisopropylethylamine (DIPEA), triethylamine (TEA), N-methylmorpholine (NMM) or mixtures thereof; the organic solvent is selected from dichloromethane, chloroform, acetonitrile, tetrahydrofuran, ethyl acetate or mixtures thereof.
4. An improved process for the preparation of compound of formula (V), comprising the steps of:

a) reacting the compound of formula (II) with the compound of formula (III) in presence of HATU, HOBt and DIPEA in dichloromethane to provide the compound of formula (IV)
b) deprotecting the compound of formula (IV) with ethylamine in presence of acetonitrile to provide the compound of formula (V).
5. According to claim 1, a process for the preparation of compound of formula (II)

comprising:

a) reacting the compound of formula (VIII) with (7?)-l-(4-isopropyl-2-thioxothiazolidin-3-yl) ethanone in presence of TiCU and DIPEA to provide the (5, £)-3-hydroxy-l-((i?)-4-isopropyl-2-thioxothiazolidin-3-yl)-7-(tritylthio)hept-4-en-l-one;
b) hydrolyzing the product of step-a) to provide the (S,E)-3 -hydroxy-7-(tritylthio)hept-4-enoic acid;

c) condensing the product of step-b) with 2-(trimethylsilyl)ethanol to provide (S9E)-2-(trimethylsilyl)ethyl 3-hydroxy-7-(tritylthio)hept-4-enoate;
d) reacting the product of step-c) with Fmoc-D-valine to get (S,E)~2-(trimethylsilyl)ethyl 3-(((iS)-2-((((9//-fluoren-9-yl) methoxy)carbonyl) amino)-3-mcthyl butanoyl)oxy)-7-(tritylthio)hept-4-enoate following by deprotection using triethylamine in acetonitrile to provide the compound of formula (II).
6. According to claim 1, a process for the preparation of compound of formula (III)
comprising:
a) reacting the compound of formula (XII) with mesityl chloride (MsCl) in presence of DMAP in dichloromethane to provide (2S,3./?)-methyl 2-amino-3-hydroxybutanoate (5R,8S,Z) methyl l-(9H-fluoren-9-yl)-5-isopropyl-3,6,9-trioxo-8-(tritylthiomethyl)-2-oxa-4,7,10-triazatridec-11 -ene-11 -carboxylate;
b) reacting the product of step-a) with DABCO in DCM to provide (Z)-methyl 2-((S)-2-((R)-2-amino-3-methylbutanamido)-3-(tritylthio)propanamido)but-2-enoate;
c) hydrolyzing the product of step-b with suitable base in a suitable solvent followed by in-situ reacting with Fmoc-Cl to provide the compound of formula (III).
7. An improved process for the preparation of (£)-5-(tritylthio)pent-2-enal compound of

formula (VIII), comprising:

a) reacting the 3-(tritylthio)propanal of formula (IX) with ethyl 2-(triphenylphosphoranylidene)acetate in an organic solvent and purifying the obtained compound to provide the pure (£)-ethyl 5-( trity!thio)pent-2-enoate of formula (X),
b) reducing the product of step-a with a suitable reducing agent in a suitable solvent followed by purifying the obtained compound to provide the pure (£)-5-(tritylthio)pent-2-ene-l-ol of formula (XI),
c) oxidizing the product of step-b using MnC>2 in presence of suitable organic solvent and purifying the obtained compound to provide pure (£)-5-(tritylthio)pent-2-enal of formula (VIII),

wherein, the purification is carried out as the recrystallization from an organic solvent which is selected from polar protic solvents, polar aprotic solvents and non polar solvents or mixtures thereof.
8. An improved process for the preparation of compound of formula (XII) comprising of:
a) reacting (Z))-tritylthio cysteine of formula (XIII) with fluorenylmethoxycarbonyl chloride (Fmo9-Cl) in presence of a base and polar aprotic solvent to provide fluorenylmethoxycarbonylamino (D)-tritylthio cysteine of formula (XIV),
b) condensing the product of step-a with (2S, 37?) methyl 2-amino-3-hydroxy butanoate hydrochloride of formula (XV) in presence of a condensing agent and N-methylmorpholine to produce methyl 2-((S)-2-(((9/7-fluoren-9-yl)methoxy)carbonylamino)-3-(tritylthio) propanamido)-3-hydroxy butanoate of formula (XVI) which is further in-situ treatment with organic base in polar aprotic solvent to provide (2S, 37?)-methyl 2-((S)-2-amino-3-(tritylthio)propanamido)-3-hydroxy. butanoate of formula (XVII),
wherein, the condensing agent is selected from DCC, HOBt, N-hydroxylsuccinimide (HOSu), 3-hydroxy-l,2,3-benzotriazin-4-[3/7]-one; an organic base is selected from triethylamine, diethylamine, diisopropylethylamine, n-butylamine, pyridine,
c) (25, 37?) Methyl 2-((S)-2-amino-3-(tritylthio)propanamido)-3-hydroxy butanoate
of formula (XVII) is condensed with fluorenylmethoxycarbonylamino-D-valine of
formula (XVIII) in presence of a condensing agent in an organic solvent to provide
(67?, 9S, 125, 137?) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4,7,10-trioxo-9-
(tritylthiomethyl)-3-oxa-5,8,l l-triazatetradecane-12-carboxylate of formula (XII).
9. An improved process for the preparation of (£)-5-(tritylthio)pent-2-enal compound of

formula (VIII), comprising:

a) reacting the 3 -(tritylthio)propanal of formula (IX) with ethyl 2-(triphenylphosphoranylidene)acetate in dichloromethane and recrystallizing the obtained compound from methanol to provide pure (£)-ethyl 5-( tritylthio)pent-2-enoate of formula (X)
b) reducing the product of step-a using DIBAL-H in toluene and recrystallizing the

obtained compound from toluene to provide the pure (.E)-5-(tritylthio)pent-2-ene-1 -ol of formula (XI) c) oxidizing the product of step-b using Mn02 in dichloromethane and recrystallizing the obtained compound from isopropanol to provide pure (E)~5-(tritylthio)pent-2-enal of formula (VIII).
10. An improved process for the preparation of compound of formula (XII) comprising:
a) reacting (Z))-tritylthio cysteine of formula (XIII) with fluorenylmethoxycarbonyl chloride (Fmoc-Cl) in presence of sodium bicarbonate and tetrahydrofuran to provide fluorenylmethoxycarbonylamino (D)-tritylthio cysteine of formula (XIV),
b) condensing the product of step-a with (25, 37?) methyl 2-amino-3-hydroxy butanoate hydrochloride of formula (XV) in presence of dicyclohexylcarbodiimide, hydroxy benzotriazole and N-methylmorpholine to produce methyl 2-((iS)-2-(((9//-fluoren-9-yl)methoxy)carbonylamino)-3-(tritylthio) propanamido)-3-hydroxy butanoate of formula (XVI) which is further in-situ treatment with triethylamine in acetonitrile to provide (25, 37?)-methyl 2-((5)-2-amino-3-(tritylthio)propanamido)-3-hydroxy butanoate of formula (XVII),
c) condensing the product of step-b with fluorenylmethoxycarbonylamino-D-valine of formula (XVIII) in presence of dicyclohexylcarbodiimide,' hydroxy benzotriazole in dichloromethane to provide (67?, 95, 125, 137?) methyl 13-hydroxy-6-isopropyl-2,2-dimethyl-4,7,10-trioxo-9-(tritylthiomethyl)-3-oxa-5,8,ll-triazatetradecane-12-carboxylate of formula (XII).