Claims:We Claim:
1. A process for the preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula -I(a), comprising the steps of:

Formula-I(a)
(i) reacting 4,4’,4’’-trimethoxytrityl alcohol of formula-I(e) with chlorinating agent in organic solvent medium

Formula-I(e)
to afford 4,4’,4’’-trimethoxytrityl chloride of formula-I(f),

Formula-I(f)

(ii) reacting 4,4’,4’’-trimethoxytrityl chloride of formula-I(f) with L-cysteine hydrochloride in the presence of an organic solvent medium to afford S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g),

Formula-I(g)
(iii) optionally purifying the S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) using organic solvent / organic solvent mixture,
(iv) reacting S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) with 2,2,2-trichloroethoxycarbonyl chloride (Troc-Cl) in an organic solvent medium to afford N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxy phenyl)methyl]-L-cysteine of formula-I(a),
(v) optionally purifying N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxy phenyl)methyl]-L-cysteine of formula-I(a) by chromatographic techniques to afford pure compound of formula-I(a).
2. The process as claimed in claim 1, wherein step (i) the chlorinating agent is selected from thionyl chloride, oxalyl chloride, phosphorous oxychloride, phosphorous pentachloride, 2-chloro-4,6-dimethoxy-1,3,5-triazine, hydrochloric acid.

3. The process as claimed in claim 1, wherein step (i) the organic solvent is selected from toluene, methylene chloride, chloroform, cyclohexane.

4. The process as claimed in claim 1, wherein step (ii) the organic solvent is selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl pyrrolidinone.

5. The process as claimed in claim 1, wherein step (iii)the organic solvent used in purification of S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) is selected from methanol, isopropyl alcohol, acetone, ethylacetate, toluene, acetonitrile, cyclohexane, diisopropylether or mixture thereof.

6. The process as claimed in claim 1, wherein step (iv) the organic solvent is selected from ethyl acetate, methylene chloride, chloroform, diisopropylether, toluene.

7. The process as claimed in claim 1, wherein step (v) N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a) is optionally purified from chromatographic techniques like column chromatography, flash chromatography.
, Description:FIELD OF THE INVENTION

The present invention relates to a novel process for the preparation of one of the Trabectedin key intermediate of formula-I(a) which is used in multistep process of Trabectedin of formula-I. The chemical name of Trabectedin key intermediate of formula-I(a) is N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl) methyl]-L-cysteine. Trabectedin key intermediate of formula-I(a) is an orange to red coloured glassy foam.

Formula-I(a)
Formula-I

BACKGROUND OF THE INVENTION
Trabectedin (Ecteinascidin 743 or ET-743) has a complex tris(tetrahydroisoquinolinephenol) structure of the following formula.

Formula-I
It was approved under the brand name YONDELIS® and indicated for the treatment of patients with unresectable or metastatic liposarcoma or leiomyosarcoma.

In a patent application, WO2007045686, the preparation of N-[(2,2,2-tri chloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a) was outlined (scheme-1). According to the process, (R)-S-trityl cysteine of formula-I(b) is reacted with Troc-Cl in the presence of sodium bicarbonate in aqueous dioxane medium and the protected derivative of formula-I(c) is selectively de-protected in the presence of triethylsilane in trifluoroacetic acid medium. The resulting thiol derivative of formula-I(d) is finally reacted with 4, 4’, 4”-trimethoxytrityl chloride in methylene chloride to afford N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a).

Scheme-1
The process disclosed in WO2007045686 suffers from the following disadvantages outlined below:
a) No experimental procedure disclosed in the document.
b) No purity details are disclosed.
c) Unnecessary de-protection followed by protection increases the cost of the product as well as pollution load to the environment.
However, in spite of having literature method for the preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a), there is still a need to have a simple, cost effective, and commercially viable process for the preparation of compound of formula-I(a) on large scale operations since, the above disadvantages are limiting factors for commercial production of compound of formula-I(a) as this is one of the key intermediates required in multi kilogram quantities in the multistep synthesis of Trabectedin of formula (I).
As this key intermediate is required in multi kilogram quantities for the commercial production of Trabectedin, the present process is not suitable for commercial manufacturing of compound of formula-I(a). Hence there is an urgent need for a simple, cost effective and commercially viable process for the preparation of compound of formula-I(a) on large scale to meet the market demand for Trabectedin.
Accordingly, the process should be circumventing the following disadvantages like:
a) Identifying a convenient route for the synthesis of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a), which can obviate the drawbacks associated with literature process.
b) Avoiding unnecessary de-protection thereby reducing the pollution load and cost of the product.
c) Analysing the quality of the isolated product using analytical techniques like HPLC and other methods.
OBJECTIVE OF THE INVENTION
Keeping in view of the above disadvantages associated with the literature process for the preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a), the inventors of the present invention have developed a simple, economical and commercially viable process for the production of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxy phenyl)methyl]-L-cysteine of formula-I(a) by circumventing the above mentioned disadvantages.
Accordingly, the main objective of the present invention is to provide novel process for the preparation of compound of formula-I(a) as shown in scheme-2, which comprises simple, economical and commercially viable process which surpasses the above mentioned disadvantages.
Accordingly, another main objective of the present invention is to provide an alternate process for the preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a), which comprises by converting commercially available 4,4’,4’’-trimethoxytrityl alcohol of formula-I(e) to 4,4’,4’’-trimethoxytrityl chloride of formula-I(f) using with thionyl chloride followed by condensing it with L-cysteine hydrochloride in dimethyl formamide to afford S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g). Thereafter, compound of formula-I(g) is then protected with Troc-chloride to afford N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a) as orange to red coloured glassy powder (scheme-2).
Accordingly, another main objective of the present invention is to provide novel process for the preparation of compound of formula-I(a), where the isolated product is optionally purified by chromatographic techniques.
Accordingly, another main objective of the present invention is to provide a novel process for the preparation of compound of formula-I(a), wherein the purified compound is analysed by analytical techniques like HPLC and other methods.
The below is schematic representation for the preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a).

Scheme-2

SUMMARY OF THE INVENTION
The main aspect of the present invention provides a novel process for the preparation of compound of formula-I(a), comprising the steps of:

Formula-I(a)
(i) reacting 4,4’,4’’-trimethoxytrityl alcohol of formula-I(e) with chlorinating agent in organic solvent medium

Formula-I(e)
to afford 4,4’,4’’-trimethoxytrityl chloride of formula-I(f),

Formula-I(f)

(ii) reacting 4,4’,4’’-trimethoxytrityl chloride of formula-I(f) with L-cysteine hydrochloride in the presence of an organic solvent medium to afford S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g),

Formula-I(g)
(iii) optionally purifying the S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) using organic solvent or organic solvent mixture,
(iv) reacting S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) with 2,2,2-trichloroethoxycarbonyl chloride (Troc-Cl) in an organic solvent medium to afford N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a),
(v) optionally purifying N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxy phenyl)methyl]-L-cysteine by chromatographic techniques to afford pure compound of formula-I(a).

DETAILED DESCRIPTION OF THE INVENTION:
The main embodiment of the present invention provides a novel process for the preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl) methyl]-L-cysteine of formula -I(a), comprising the steps of:

Formula-I(a)
(i) reacting 4,4’,4’’-trimethoxytrityl alcohol of formula-I(e) with chlorinating agent in organic solvent medium

Formula-I(e)
to afford 4,4’,4’’-trimethoxytrityl chloride of formula-I(f),

Formula-I(f)

(ii) reacting 4,4’,4’’-trimethoxytrityl chloride of formula-I(f) with L-cysteine hydrochloride in the presence of an organic solvent medium to afford S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g),

Formula-I(g)
(iii) optionally purifying the S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) using organic solvent or organic solvent mixture,
(iv) reacting S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) with 2,2,2-trichloroethoxycarbonyl chloride (Troc-Cl) in an organic solvent medium to afford N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a),
(v) optionally purifying N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxy phenyl)methyl]-L-cysteine by chromatographic techniques to afford pure compound of formula-I(a).
In step (i) of the present invention, 4,4’,4’’-trimethoxytrityl chloride of formula-I(f) is prepared by reacting 4,4’,4’’-trimethoxytrityl alcohol of formula-I(e) with chlorinating agent in an organic solvent medium.
In step (i) of the present invention, the chlorinating agent is selected from thionyl chloride, oxalyl chloride, phosphorous oxychloride, phosphorous pentachloride, 2-chloro-4,6-dimethoxy-1,3,5-triazine, hydrochloric acid or any other suitable chlorinating agent preferably thionyl chloride.
In step (i) of the present invention, the organic solvent selected from toluene, methylene chloride, chloroform, cyclohexane, preferably methylene chloride.
In step (i) of the present invention, the temperature at which reaction is conducted between 0° to 115°C.
In step (i) of the present invention, the compound of formula-I(f) is directly used in the next reaction without isolation, or optionally isolated by an organic solvent.
In step (i) of the present invention, the organic solvent used for the isolation of compound of formula-I(f) is selected from hexane, heptane, cyclohexane or any other organic solvent preferably hexane.
In step (ii) of the present invention, 4,4’,4’’-trimethoxytrityl chloride of formula-I(f) is reacted with L-cysteine hydrochloride monohydrate or L-cysteine hydrochloride anhydrous in an organic solvent medium.
In step (ii) of the present invention, the organic solvent is N,N-dimethylformamide, N,N-dimethyl acetamide, N-methylpyrrolidinone or any other suitable organic solvent preferably N,N-dimethylformamide.
In step (ii) of the present invention, the temperature at which the reaction is conducted between 0°C to 75°C preferably at 20-35°C.
In step (iii) of the present invention, the isolated compound of S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) is optionally purified from organic solvent or organic solvent mixture.
In step (iii) of the present invention, the organic solvent used in purification of S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) is selected from methanol, isopropyl alcohol, acetone, ethylacetate, toluene, acetonitrile, cyclohexane, diisopropylether, or mixture thereof.
In step (iv) of the present invention, N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a) is prepared by reacting S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) with 2,2,2-trichloroethoxy carbonyl chloride in the presence of organic solvent.
In step (iv) of the present invention, the organic solvent is selected from ethyl acetate, methylene chloride, chloroform, diisopropylether, toluene or any other suitable organic solvent preferably ethyl acetate.
In step (v) of the present invention, N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a) is optionally purified from chromatographic techniques like column chromatography, flash chromatography preferably flash chromatography.
In step (v) of the present invention, N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine of formula-I(a) is purified by flash chromatography using organic solvents selected from methylene chloride, ethyl acetate, hexane, chloroform, methanol or mixture thereof preferably mixture of ethyl acetate and hexane.

Advantages:
(i) The process of the present invention is a straight forward and no un-necessary de-protection is required.
(ii) The starting materials and reagents used in the process are commercially available.
(iii) The process of the present invention successfully circumvents the demerits of the literature process.
(iv) The purity of the isolated product is >95% which is sufficient to convert it to Trabectedin.
(v) The overall yield is above 70%.
(vi) The process of the present invention is commercially viable and industrially applicable.
The present invention is further illustrated in detail with reference to following examples. It is desired that the examples be considered in all respect as illustrative and are not intended to limit the scope of the invention in any way.

EXAMPLES:
EXAMPLE-1: Preparation of 4,4’,4’’-trimethoxytrityl chloride I(f):
4,4’,4’’-trimethoxytrityl alcohol-I(e) (265g; 1.0 m.eq.) was dissolved in methylene chloride (800 mL). To this solution, thionyl chloride (136g; 1.5 m.eq) was added and heated to reflux temperature. After completion of reaction, the excess solvent was distilled out completely under vacuum. The resulting reddish orange solid was slurried in hexane, filtered and dried.
Yield: 262g (94%)
IR (? cm-1): 3037.90, 3015.69, 2963.22, 2840.74, 1252.77, 1178.12, 1025.27, 775.57.
1H NMR (CDCl3, 400MHz; ppm): 7.25-7.23 (6H, d, 8.8 Hz), 6.94-6.91 (6H, d, 9.2 Hz), 3.88 (9H, s).
13C NMR (CDCl3, 400MHz; ppm): 161.99, 136.27, 134.10, 113.87, 81.09, 55.78.
Mass (m/z): 333.1 [M-Cl]+.
EXAMPLE-2: Preparation of 4,4’,4’’-trimethoxytrityl chloride I(f):
4,4’,4’’-trimethoxytrityl alcohol-I(e) (5g; 1.0 m.eq.) was dissolved in methylene chloride (15 mL). To this solution, oxalyl chloride (2.7g; 1.5 m.eq) was added and heated to reflux temperature. After completion of reaction, the excess solvent was distilled out completely under vacuum. The resulting reddish orange solid was slurried in hexane, filtered and dried.
Yield: 4.0g (76.2%)

EXAMPLE-3: Preparation of 4,4’,4’’-trimethoxytrityl chloride I(f):
4,4’,4’’-trimethoxytrityl alcohol-I(e) (3g; 1.0 m.eq.) was dissolved in toluene (30 mL). To this solution, thionyl chloride (1.53g; 1.5 m.eq) was added and heated to reflux temperature. After completion of reaction, the excess solvent was distilled out completely under vacuum. The resulting reddish orange solid was slurried in toluene, filtered and dried.
Yield: 1.2g (38%)

EXAMPLE-4: Preparation of 4,4’,4’’-trimethoxytrityl chloride I(f):
4,4’,4’’-trimethoxytrityl alcohol-I(e) (5g; 1.0 m.eq.) was dissolved in chloroform (15 mL). To this solution, thionyl chloride (2.55g; 1.5 m.eq) was added and heated to reflux temperature. After completion of reaction, the excess solvent was distilled out completely under vacuum. The resulting reddish orange solid was slurried in hexane, filtered and dried.
Yield: 4.5g (86.5%)

EXAMPLE-5: Preparation of S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g):
4,4’,4’’-trimethoxytrityl alcohol-I(e) (25g; 1.0 m.eq.) was dissolved in methylene chloride (75 mL). To this solution, thionyl chloride (12.8g; 1.5 m.eq) was added and heated to reflux temperature. After completion of reaction, the excess solvent was distilled out completely under vacuum. The resulting reddish orange solid of formula-I(f) (~26g) was further reacted with L-cysteine hydrochloride 12.7g (1.04 m.eq.) in N,N-dimethylformamide (60 mL) at 25-30°C for about 24h. After completion of reaction, about 10% sodium acetate solution (500 mL) was added and filtered. The wet product was washed with water followed by acetone and suck dried. The product of formula-I(g) was dried under vacuum.
Yield: 23g (71.09%); HPLC purity: >97%
IR (? cm-1): 3425.76, 3258.59, 3061.25, 293153, 1505.52, 1250.21, 1176.12, 103172.
1H NMR (DMSO-d6, 400MHz; ppm): 7.61 (2H, br), 7.20-7.18 (6H, d, 8.8 Hz), 6.88-6.85 (6H, d, 9.2 Hz), 3.73 (9H, s), 2.98-2.95 (1H, dd, 9.2Hz, 4.0Hz), 2.62-2.58 (1H, dd, 12.8Hz, 4.4Hz), 2.43-2.37 (1H, dd, 12.4Hz, 9.2Hz).
13C NMR (DMSO-d6, 400MHz; ppm): 168.53, 157.62, 136.84, 130.20, 113.23, 64.67, 55.04, 53.36, 33.44.
Mass (m/z): 451.90 [M-H]-

EXAMPLE-6: Preparation of S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g):
4,4’,4’’-trimethoxytrityl alcohol -I(e) (25g; 1.0 m.eq.) was dissolved in methylene chloride (75 mL). To this solution, thionyl chloride (12.8g; 1.5 m.eq) was added and heated to reflux temperature. After completion of reaction, the excess solvent was distilled out completely under vacuum. The resulting reddish orange solid of formula-I(f) (~26g) was further reacted with L-cysteine hydrochloride 12.7g (1.04 m.eq.) in N,N-dimethylformamide (60 mL) at 25-30°C for about 24h. After completion of reaction, about 10% sodium acetate solution (500 mL) was added and filtered. The wet product was washed with water followed by methanol and suck dried. The product of formula-I(g) was dried under vacuum.
Yield: 21g (65%); HPLC purity: >97%

EXAMPLE-7: Preparation of S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g):
4,4’,4’’-trimethoxytrityl chloride-I(f) (45g; 1.0 m.eq.) was reacted with L-cysteine hydrochloride 21.5g (1.0 m.eq.) in N,N-dimethylformamide (107 mL) at 25-30°C for about 24h. After completion of reaction, about 10% sodium acetate solution (900 mL) was added and filtered. The wet product was washed with water followed by acetone and suck dried. The product was dried under vacuum.
Yield: 45.4g (82.0%); HPLC purity: >97%

EXAMPLE-8: Preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine I(a):
S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) (100g, 1.0 m.eq.) was reacted with Troc-Cl (46.6g; 1.0 m. eq.) in ethyl acetate (5L) at 75-80°C for about 6h. After completion of reaction, the ethyl acetate was distilled out completely under vacuum to afford thick pasty material. The product was isolated from hexane to afford orange red coloured glassy powder.
Yield: 136.2g (98.2%); HPLC purity: >95%

EXAMPLE-9: Preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine I(a):
S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) (1g, 1.0 m.eq.) was reacted with Troc-Cl (0.47g; 1.0 m. eq.) in ethyl acetate (55 mL) at 25-30°C for about 10h. After completion of reaction, the ethyl acetate was distilled out completely under vacuum to afford thick pasty material. The product was isolated from hexane to afford orange red coloured glassy powder.
Yield: 1.2g (86.9%); HPLC purity: >95%

EXAMPLE-10: Preparation of N-[(2,2,2-trichloroethoxy)carbonyl]-S-[tris(4-methoxyphenyl)methyl]-L-cysteine I(a):
S-(4,4’,4’’-trimethoxyphenylmethyl)-L-cysteine of formula-I(g) (15g, 1.0 m.eq.) was reacted with Troc-Cl (8.74g; 1.25 m. eq.) in ethyl acetate (750 mL) at 70-80°C for about 6h. After completion of reaction, the ethyl acetate was distilled out completely under vacuum to afford thick pasty material. The product was further purified from flash chromatography.
Yield: 13.5g (65%); HPLC purity: >97%
IR (? cm-1): 3405.76, 3035.06, 2954.10, 2836.10, 1737.19, 1606.00, 1250.44, 1177.08, 1033.86, 816.78.
1H NMR (CDCl3, 400MHz; ppm): 7.28-7.26 (6H, d, 9.2Hz), 6.81-6.79 (6H, d, 8.8Hz), 3.78 (9H, s), 5.43-5.41 (1H, d, 8.0Hz), 4.70 (2H, s), 4.37-4.32 (1H, m), 2.75-2.73 (2H, m).
13C NMR (CDCl3, 400MHz; ppm): 174.51, 158.15, 153.94, 136.73, 130.46, 113.26, 95.18, 74.70, 65.97, 55.22, 52.90, 33.66.
Mass (m/z): 625.82 [M-H]-, 627.84, 629.85 and 631.81 are diagnostic for trichloro pattern.