FORM-2
THE PATENT ACT, 1970
(39 of 1970)
&
THE PATENT RULE, 2003
COMPLETE SPECIFICATION
[See section 10, rule 13]
Process for the preparation of Cytarabine & its intermediates
APPLICANT:
HERBERT BROWN PHARMACEUTICAL & RESEARCH
LABORATORIES
W-256/257/258A, M.I.D.C. Phase II, Shivaji Udyog Nagar, Dombivli (E)-421203, District-Thane, Maharashtra, India
Indian Company incorporated under the Companies Act 1956
The following specification particularly describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The present invention relates to a novel process for the preparation of Cytarabine (1-β-D-Arabinofuranosyl cytosine) of Formula I from cytidine via 2, 2'-Cyclocytidine hydrochloride intermediate of Formula II.

More particularly the present invention relets to a novel process for the preparation of Cytarabine having HPLC purity >99.7%, avoiding use of ion exchange resin for the purification of 2,2'-Cyclocytidine hydrochloride which is key raw material for Cytarabine.
BACKGROUND OF THE INVENTION
Cytarabine of Formula I, a specific nucleoside compound, is a known antineoplastic and antiviral agent. Cytarabine, which is also known as 4-amino-l-β-D-arabino-pentofuranosyl-2(lH)-pyrimidinone, 1-β-D-arabino-pentofuranosylcytosine and β-cytosinearabinoside, has the following chemical structure;


Cytarabine or cytosine arabinoside (Cytosar-U or Depocyt) is a chemotherapetic agent used mainly in the treatment of cancers of white blood cells such as acute myeloid leukemia (AML) and non-Hodgkin lymphoma. It is also known as Ara-C (Arabinofuranosyl Cytidine). It kills cancer cells by interfering with DNA synthesis. It is called cytosine arabinoside because it combines a cytosine base with an arabinose sugar. Cytosine normally combines with a different sugar, deoxyribose, to form deoxycytidine, a component of DNA. Certain sponges, where it was originally found, use arabinoside sugars to form a different compound (not part of DNA). Cytosine arabinoside is similar enough to human cytosine deoxyribose (deoxycytidine) to be incorporated into human DNA, but different enough that it kills the cancer cell. This mechanism is used to kill cancer cells. Cytarabine is the first of a series of cancer drugs that altered the sugar component of nucleosides. Other cancer drugs modify the base. Cytarabine is mainly used in the treatment of acute myeloid leukaemia, acute lymphocytic leukaemia (ALL) and in lymphomas, where it is the backbone of induction chemotherapy
The known processes for the preparation of Cytarabine (1-β-D-arabinofuranosylcytosine) are given below.
Japanese Patent 47025356B: Cytidine was converted to l-(2, 2'-anhydro-β -D-

arabinofuranosyl)cytosine and hydrolyzed to give the title product. Thus, intermediate in ethyl acetate was refluxed for 2 hrs with aq. POCl3, the mixture, concentrated under vacuum and the residue stirred for 24 hrs with 2N NaOH to give 37% yield.
Cytarabine synthesis reported in US patent 3116282 in which Uracil arabinoside was acetylated and treated with phosphorous pentasulfide followed by deacetylated by ammonia.
US Patent 3595853 decribes the synthesis of Cytarabine in which cytidine was nitrated with by 90% fuming nitric acid at -20°C, for one to two hours in presence of dehydrating agent such as phosphorus pentoxide, kept it for one to two hours at -20°C quenched in ice water precipitate was filterd. Filtrate was concentrated up to half quantity and its pH was adjusted 3 by 5N sodium hydroxide solution, it was concentrated white precipitate was filtered and recrystallized in ethanol, and reduction of nitro group was carried out by using 5% palladium on BaSo4 to give Cytarabine, yield and purity is not mentioned.
US Patent US5610292 describes the synthesis of Cytarabine in which cytidine was converted into of 2,2'-cyclocytidine hydrochloride by using dibutyl tin oxide. Methanol, cytidine and dibutyl tin oxide suspension was refluxed for five hours, stirred at room temperature for twelve hours. Then treated with triethyl amine and p-toluene sulfonyl chloride, for twelve hours at room temperature. Concentrated and suspension was refluxed in chloroform. White precipitate filtered dried to get crude 2,2'-cyclocytidine hydrochloride. Crude 2,2'-cyclocytidine hydrochloride was purified by using ethanol to provide pure 2,2'-cyclocytidine hydrochloride (29%).
2,2'-0-cyclocytidine hydrochloride was dissolved in water at 80°C followed by addition of t-butylamine and the mixture stirred for 2 hours. Thereafter, the solvent was evaporated under vacuum and purified in ethanol at room temperature for 12

hours. Filtration of the resulting precipitation of pure Cytarabine gave 83% yield after drying.

US Patent 3856777 describe a procedure in which mixture of cytidine hydrochloride and ethylene carbonate was heated at 150°C for 50 min. The reaction mixture was dissolved in water and the solution was passed through a column packed with active charcoal. By passing water through column to elute 2,2,-anhydrocytidine hydrochloride, evaporated to dryness. The residue was recrystallized from 1:1 mixture of water ethanol. The columnar crystals obtained consisted of pure 2,2'-anhydrocytidine hydrochloride in 51% yield. Purity was not mentioned.

US patent No. 3755296; Cytidine was reacted with mixture of thionyl chloride and DMF at 30°C for 5 hrs. The reaction solution was poured into water and introduced into a column of 100ml Dowex-50W4 (pyridinium type). And eluted with a 0.1M formic acid -pyridine buffer solution (pH: 5). The fraction was dried under reduced pressure, dissolved in water and introduced into Dowex-1 x 2(C1- type). The eluate

was distilled to dryness and crystals of the 2,2'-cyclocytidine hydrochloride were obtained from ethanol (Yield 65%). Ion exchange resin used for the purification. Purity of final product was not mentioned.
US patent US3755296 describes a process for 2,2'-cyclocytidine hydrochloride, where cytidine in ethyl acetate was reacted with ice-cooled mixture of phosphorus oxychloride water and it was heated to 50-70 C. concentrated and the residue obtained dissolved in water and introduced into a column Dowex-50W (H+ type) which was then, eluted with 2% solution of sodium chloride. Fraction was concentrated. Sodium chloride separated by filtration and the filtrate was concentrated to dryness. The residue was extracted with 50% ethanol and ethanol was distilled out. The remaining solution was treated with a column of Dowex-50W (H+ type). The compound was eluted with a 0.5N formic acid-pyridine buffer solution and evaporated to dryness. A water solution of the residue was passed through a column Dowex-1 (Cl- type) and then concentrated. Crystallization from water gave 2,2'-cyclocytidine hydrochloride (Yield 77.88%). Ion exchange resin used for the purification.
Preparation of Cytarabine is also published in Tet. Lett., 869, 1970; J. Org. Chem, 37, 284, 1972; J. Med. Chem., 19, 654, 1979; J. Org. Chem., 1965,.30 (3), 835; J. Org. Chem., 1967, 32 (3), 816; Carbohyd. Res., 24, 210, 1972; Nucleic Acid Chemistry. Vol. 1, 249; J. Org. Chem.. 32, 816, 1967; J. Org. Chem., 37, 284, 1972; Bull. Chem. Soc. Jap, 48, 505, 1975; Proc. Chem. Soc, 84, 1959; J. Org. Chem, 32, 1462, 1967.
The aforementioned prior art techniques for the preparation of 2,2'-cyclonucleosides are deficient in that they require multiple steps with inherent loss of yield and or they require silica, resin, charcol, columns for isolation and purification. Furthermore, the prior art processes for the production of Cytarabine and its analogues are deficient in

that the purified product is obtained in a relatively low yield and/or the process is complicated requiring a series of steps including the use of ion exchange resins.
Furthermore in all above described process, the aqueous solution of crude 2,2'-Cyclocytidine hydrochloride is purified by ion exchange resin. Use of ion exchange resin suffers from several drawbacks are listed below.
the large volume of water is required for loading, washing and eluting the desired product.
the large volume of eluent comprising desired product is to be concentrated.
the huge amount of energy is required to concentrate the eluet comprising desired.
the need to regenerate the resin column after every use in order to maintain the ion exchange capacity
the time cycle for ion exchange resin chromatography is long.
All above discussed drawbacks increases the over all time cycle and cost of the process and also indicates that the process is not environment friendly as it generates huge amount of effluent and less feasible on industrial scale.
There are also enzymatic processes for the preparation of Cytarabine discussed in J. Org. Chem., 1967, 32 (3), pp 816-817. However these enzymatic processes are
difficult to handle in large scale production.
•
There are some processes for preparation of cyclocytidine hydrochloride without using resin, charcol and silica column, yield obtained from these processes is very low (10-29%), handling of reagents used is less feasible on industrial scale.

It is difficult to put these known processes into commercial practice since most of these processes have some troublesome steps such as the introduction of a protective substituent and release there off are required, the reagents used in the reaction are very expencive and the yield of the desired 2,2'-Cyclocytidine hydrochloride is very low.
None of the processes in prior art, gives an alternate process for the preparation of Cytarabine with high HPLC purity (>99.7%) via 2,2'-Cyclocytidine hydrochloride intermidiate avoiding ion exchange resin for purification there is, therefore, an unfulfilled need for developing alternative process for preparation of Cytarabine and its intermidiate 2,2'-Cyclocytidine hydrochloride avoiding the use of tedious ion exchange resin for purification and allowing industrial preparation of this product in simplified and economic manner.
OBJECT OF THE INVENTION
1. An object of present invention is to provide a novel process for the preparation of Cytarabine and its intermidiate, 2,2'-Cyclocytidine hydrochloride with substantial purity avoiding the use of ion exchange resin for purification.
2. Another object of present invention is to provide a Cytarabine having HPLC purity of >99.7%.
3. Yet another object of present invention is to provide a Cytarabine free from chloride content.
4. Yet another object of present invention is to provide a 2,2'-Cyclocytidine hydrochloride having HPLC purity of more than 99.5%, and which is free from inorganic salts.

5. Yet another object of present invention is to provide a 2',3'-O-SulfinyIcytidine hydrochloride having HPLC purity of more than 99%.
6. Yet another object of present invention is to provide a improved process for the preparation 2,2'-Cyclocytidine hydrochloride from 2',3'-O-Sulfinylcytidine hydrochloride by using weak base sodium bicarbonate.
7. Yet another object of present invention is to provide a improved process for the preparation 2,2'-Cyclocytidine hydrochloride from 2',3',-O-Sulfinylcytidine hydrochloride where reaction is carried out under mild vacuum.
8. Yet another object of present invention is to provide simple, safe, environment friendly and industrially feasible process for the preparation of Cytarabine.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided a novel process for the preparation of highly pure Cytarabine of Formula I, having HPLC purity of >99.7%, from 2,2'-Cyclocytidine hydrochloride of Formula II, avoiding use of ion exchange resin for purification.


comprising,
a) reacting 2,2'-Cyclocytidine hydrochloride of Formula II with base in the presence water solvent at 10-15 °C
b) stirring reaction mixture at 25-30°C for 1 hr, followed by charcolisation to obtain charcolised solution
c) evaporating charcolised solution at 45-50°C under vacuum
d) isolating crude Cytarabine by using mixture of water and alcoholic solvents to obtain crude Cytarabine
e) purifying the crude Cytarabine by using mixture of water in alcoholic solvent to obtain pure Cytarabine of Formula I, having HPLC purity more than 99.7%
According to another aspect of present invention there is provided an improved process for the preparation of 2,2'-Cyclocytidine hydrochloride of Formula II, used as a starting material for the preparation of Cytarabine of Formula I


comprising,
a) addition of aqueous solution of base in 2',3'-O-Sulfinylcytidine hydrochloride of Formula III in water till pH of the mixture is reached between 3 to 3.5
b) heating the reaction mixture in step a) at 70-75°C under mild vacuum by maintaining pH 3 to 3.5
c) distilling off water from reaction mixture in step b) under vacuum till 0.5 to 1.0 volume with respect to 2',3'-0-Sulfinylcytidine hydrochloride of Formula III
d) cooling the reaction mass of step c) at room temperature
e) adjusting the pH of concentrated solution in step d) to 1.5 to 2.5 using dil. hydrochloric acid
f) adding mixture of organic solvents in the mixture from step e) at 5-10°C to isolate crude 2,2'-Cyclocytidine hydrochloride
g) purification of crude 2,2,-Cyclocytidine hydrochloride obtained in step f) using alcoholic solvents, to obtain pure 2,2'-Cyclocytidine hydrochloride of Formula II, having HPLC purity more than 99.5%

According to another aspect of present invention there is provided a process for the preparation of 2',3'-0-Sulfinylcytidine hydrochloride of Formula III used as a starting material for the preparation of 2,2'-Cyclocytidine hydrochloride of Formula II.

comprising,
a) reacting cytidine with thionyl chloride in acetonitrile solvent at 0-15°C
b) quenching the reaction mixture in to ice cold water to precipitate out 2',3'-0-Sulfinylcytidine hydrochloride of Formula III
c) filtering & washing the wet cake with alcoholic solvent to obtain pure 2',3'-0-Sulfinylcytidine hydrochloride of Formula III having HPLC purity of more than 99%.
DETAILED DESCRIPTION OF INVENTION
The present invention relates to an improved, commercially viable and simple process for the preparation of Cytarabine of Formula I and its intermediates. The process of

the present invention eliminates the use of ion exchange resin and the risk of handling hazardous chemicals, thus making the process more environment friendly and industrially viable.
According to an aspect of the present invention, there is provided a novel process for the preparation of highly pure Cytarabine of Formula I having HPLC purity of >99.7%, from 2,2'-Cyclocytidine hydrochloride of Formula II, avoiding use of ion exchange resin for purification.

comprising,
a) reacting 2,2'-Cyclocytidine hydrochloride of Formula II with base in the presence water solvent at 10-15 °C
b) stirring reaction mixture at 25-30°C for 1 hr, followed by charcolisation to obtain charcolised solution
c) evaporating charcolised solution at 45-50°C under vacuum
d) isolating crude Cytarabine by using mixture of water and alcoholic solvents to obtain crude Cytarabine

e) purifying the crude Cytarabine by using mixture of water in alcoholic solvent to obtain pure Cytarabine of Formula L having HPLC purity more than 99.7%
In an embodiment of the present invention, the base used such as, sodium hydroxide, ammonia, sodium bicarbonate, sodiumcarbonate, tri-ethyl amine, diisopropyl amine, tert- butyl amine preferably tert- butyl amine is used.
The molar ratio of the tert-butyl amine used in step a) with respect to 2,2'-Cyclocytidine hydrochloride Formula II is in the range of 1 to 1.5. preferably 1.3 mole equivalent is used.
According to yet another an embodiment of the present invention, the combination of water alcohol used in step d) is selected from, combination of water-n-butanol, water-methanol, water-n-butanol-methanol, preferably combination of water-n-butanol-methanol is used.
According to yet another an embodiment of the present invention, for purification of crude product Cytarabine of Formula I is carried out by using , combination of 5% water in methanol and 10% water in methanol, preferabJy combination of 5% water in methanol is used.
According to another aspect of the present invention there is provided an improved process for the preparation of 2,2'-Cyclocytidine hydrochloride of Formula II, used as a starting material for the preparation of Cytarabine of Formula I


comprising,
a) addition of aqueous solution of base in 2,,3'-0-Sulfinylcytidine hydrochloride of Formula III in water till pH of the mixture is reached between 3 to 3.5
b) heating the reaction mixture in step a) at 70-75°C under mild vacuum by maintaining pH 3 to 3.5
c) distilling off water from reaction mixture in step b) under vacuum till 0.5 to 1.0 volume with respect to 2',3'-0-Sulfmylcytidine hydrochloride of Formula III
d) cooling the reaction mass of step c) at room temperature
e) adjusting the pH of concentrated solution in step d) to 1.5 to 2.5 using dil. hydrochloric acid
f) adding mixture of organic solvents in the mixture from step e) at 5-10°C to isolate crude 2,2'-CycIocytidine hydrochloride
g) purification of crude 2,2'-Cyclocytidine hydrochloride obtained in step f) using alcoholic solvents, to obtain pure 2,2'-Cyclocytidine hydrochloride of Formula II, having HPLC purity more than 99.5%

In an embodiment of the present invention, the base used such as, sodium hydroxide, ammonia, sodium bicarbonate, sodiumcarbonate, preferably sodium bicarbonate is used.
According to an embodiment of the present invention, in step a) the pH of the reaction mixture is adjusted to 1.5, 2.0, 3.0, 3.5, 4.0, 5.0 preferably pH is adjusted in the range of 3.0 to 3.5.
According to an embodiment of the present invention, amount of water used in step a) with respect to 2',3'-0 -Sulfinylcytidine hydrochloride of Formula III is in range of 3.5 to 5 volumes, preferably 3.5 volume is used.
According to an embodiment of the present invention, reaction is carried out at temperature range 70°C, 80°C, 90°C, preferably 70-75°C temperature is used.
According to an embodiment of the present invention, reaction is carried out by nitrogen bubbling and under vacuum to remove sulphur dioxide gas formed during reaction.Which is responsible for generation of sulphurous acid in reaction mass, preferably mild vcuum is applied.
According to yet another an embodiment of the present invention, for isolation of crude product the 2,2'-Cyclocytidine hydrochloride of Formula III, combination of water, alcohol, dichloromethane used in step g) is selected from, combination of water-isoproanol, water-methanol, water-methanol-dichloromethane, preferably combination of water-methanol-dichloromethane is used.
According to yet another embodiment of the present invention, for purification of crude product 2,2'-Cyclocytidine hydrochloride of Formula III, methanol and

combination of water-methanol is used in step h) is selected from, combination of water- isoproanol, water-methanol, water-methanol-dichloromethane, preferably combination of water-methanol-dichloromethane is used.
According to another aspect of present invention there is provided a process for the preparation of 2',3,-0-Sulfinylcytidine hydrochloride of Formula III used as a starting material for the preparation of 2,2'-Cyclocytidine hydrochloride of Formula II.

comprising,
a) reacting cytidine with thionyl chloride in acetonitrile solvent at 0-15°C
b) quenching the reaction mixture in to ice cold water to precipitate out 2',3'-0-Sulfinylcytidine hydrochloride of Formula III
c) filtering & washing the wet cake with alcoholic solvent to obtain pure 2',3'-0-Sulfinylcytidine hydrochloride of Formula III having HPLC purity of more than 99%.

The molar ratio of the thionyl chloride used in step a) with respect to cytidine is in the range of 2.5 to 5.0, preferably 3.5 equivalent is used.
According to an embodiment of the present invention, amount of acetonitrile solvent used in step a) with respect to cytidine is in the range of 4 to 7 volumes, preferably 5 volumes are used.
The process for the preparation of Cytarabine of Formula I via Cyclocytidine intermediate of Formula II, can be depicted in scheme I
SCHEME I


According to another aspect of present invention, the Cytarabine of Formula I is prepared by the process of present invention has HPLC purity more than 99.7% and uracil arabinoside impurity less than 0.1%.
The Cytarabine of Formula I; obtained by the present invention is free from chloride
content.
It was observed by inventors of present invention, that in the present process for the
preparation of Cytarabine, formation of impurities like uracil arabinoside and uracil
are controlled to the great extent.
The Cytarabine of Formula I, obtained from the process of present invention complies with the quality standard established by the British, European and U.S. Pharmacopoeia.
EXAMPLES
Example 1: Preparation of 2',3'-0-Sulfinylcytidine hydrochloride
To a suspension of 300g cytidine in 1500ml of acetonitrile was slowly added 513.60g
of thionyl chloride at temperature of about 0-5°C. After addition, temperature of
reaction mixture was slowly raised to 15°C and maintained for 20 to 25 minutes with
stirring. Reaction mixture was added into 1710ml ice cold water under stirring. Solid
product precipitated out was filtered, washed with 150ml water followed by 300ml
methanol. Wet weight: 337g, KF: 2.30%
Dry weight: 329.24g (81.93%)
HPLC Purity: 99.59%.
Melting Point: 253-254°C
ESI Mass: 289.93 (free base), 226.00 and 112.07

Example 2: Preparation of 2,2'-Cyclocytidine hydrochloride
To a wet cake of 327.53g (KF: 2.3%) 2,,3'-O-Sulfinylcytidine hydrochloride 1120ml
water was added. The pH of reaction mixture was adjusted to 2.5 to 3.5 by using
aqueous sodium bicarbonate. Reaction mixture was heated at 70-75°C under mild
vacuum, (simulteneously distillation of water from reaction mixture was started) for
2 to 3 hrs. After completion of reaction, high vacuum was applied and water was
distilled out by keeping 0.5 to 1.0 vol of water in reaction mass, pH of reaction
mixture was adjusted to 1.5 to 2.5 by using dilute hydrochloric acid. Then the mixture
of 640ml dichloromethane and 640ml methanol was added, it was stirred at 0-5°C for
30 minutes and filtered. Wet cake was crystallised in 320 ml of methanol & filtered.
Wet weight: 162.78g, LOD: 4.24%,
HPLC Purity: 99.76%,
Dry weight: 156.65g (60.97% yield on dried basis)
Melting Point: 260-262°C
1H-NMR in DMSO-d6 (ppm): 9.64-9.23 (bs, 2H, D2O exchangeable, NH2), 8.29-8.27
(dd, 1H, Cytosine nucleus, H-6), 6.65-6.63 (m, 1H, H-l'), 6.55-6.54 (m, 1H, Cytosine
nucleus, H-5), 6.22-6.21 (bs, 1H, D2O exchangeable, 5'-OH), 5.40-5.38 (m, 1H, H-
2'), 5.09-5.07 (bs, 1H, D2O exchangeable, 3'-OH), 4.47-4.46 (m, 1H, H-3'), 4.22 (m,
1H, H-4'), 3.40-3.38 (m, 2H, H-5'a & b).
GC-MS: 225 (free base), 195,178, 164, 148, 136, 112, 111, 95, 83, 69,44 and40
Example 3: Preparation of Cytarabine
To a 104.24 g (dry wt. lOOg) wet cake of 2,2'-Cyclocytidine hydrochloride in 500ml of water was slowly added 36.28g of tert-butyl amine at temperature of about 10-15°C. After addition, tempreature of reaction mixture was slowly raised to 25-30 C and maintained for 30 minutes with stirring. After completion of reaction, it was subjected to charcolisation, filtered & charcolised solution was evaporated under

vacuum to dryness. To the residue was added 80ml of water, 100ml of methanol and
200 ml of n-butanol, the white solid precipitated out, was filtered and purified in
water methanol mixture (Mixture of 10.35ml water and 196.65ml methanol). Finally
pure product was dried under vacuum at 50-60°C.
Dry weight: 65.85gm
HPLC Purity: 99.95%, Uracil arabinoside impurity: 0.04%
Sulphated ash: 0.10%
Titrimeteric assay: 99.18%
Chloride content: <500ppm
Melting Point: 212.5°C
1H-NMR in DMSO-d6 (ppm): 7.58-7.56 (dd, 1H, Cytosine nucleus, H-6), 7.11-7.06 (bs, 2H, Cytosine nucleus-NH2), 6.03-6.02 (m, 1H, H-l'), 5.67-5.65 (dd, 1H, Cytosine nucleus, H-5),5.41-5.39 (bs, 2H, 3'-OH & 5'-OH), 5.03-5.0 (bs, 1H, T-OH), 3.95-3.92 (m, 1H, H-2'), 3.89-3.86 (m, 1H, H-3'), 3.74-3.71 (m, 1H, H-4'), 3.61-3.54 (m,2H,H-5'a,b).
13C-NMR in DMSO-d6: 165.66 [-N=C-NH2, Cytosine nuclues (4)], 155.25 (-C=0, Cytosine nuclues), 142.97 [-N-C=C-, Cytosine nuclues (6)], 92.47 (-N-C=C-, Cytosine nuclues (5)], 85.91 [-N-C-O, (l')]s 84.90 [-OC-CH2- (4')], 76.42 [HO-C-C-N-, (2')]> 74.84 [HO-C-C-, (3')] and 61.23 ppm [HO-CH2-C, (5')]
Specific optical rotation: +156.2 (Lit: +154 to +160)

We claim
1. According to an aspect of the present invention, there is provided a novel process for the preparation of highly pure Cytarabine of Formula I, having HPLC purity of >99.7%, from 2,2'-Cyclocytidine hydrochloride of Formula II, avoiding use of ion exchange resin for purification

comprising,
a) reacting 2,2'-Cyclocytidine hydrochloride of Formula II with base in the presence water solvent at 10-15°C
b) stirring reaction mixture at 25-30°C for 1 hr, followed by charcolisation to obtain charcolised solution
c) evaporating charcolised solution at 45-50°C under vacuum
d) isolating crude Cytarabine by using mixture of water and alcoholic solvents to obtain crude Cytarabine
e) purifying the crude Cytarabine by using mixture of water in alcoholic solvent to obtain pure Cytarabine of Formula I, having HPLC purity more than

99.7%
2, According to another aspect of present invention there is provided an improved process for the preparation of 2,2'-Cyclocytidine hydrochloride of Formula II, used as a starting material for the preparation of Cytarabine of Formula I

comprising,
a) addition of aqueous solution of base in 2,,3,-0-Sulfinylcytidine hydrochloride of Formula III in water till pH of the mixture is reached between 3 to 3.5
b) heating the reaction mixture in step a) at 70-75°C under mild vacuum by maintaining pH 3 to 3.5
c) distilling off water from reaction mixture in step b) under vacuum till 0.5 to 1.0 volume with respect to 2,,3'-0-Sulfinylcytidine hydrochloride of Formula III
d) cooling the reaction mass of step c) at room temperature
e) adjusting the pH of concentrated solution in step d) to 1.5 to 2.5 using dil. hydrochloric acid

f) adding mixture of organic solvents in the mixture from step e) at 5-10°C to isolate crude 2,2,-Cyclocytidine hydrochloride
g) purification of crude 2,2'-Cyclocytidine hydrochloride obtained in step f) using alcoholic solvents, to obtain pure 2,2'-Cyclocytidine hydrochloride of Formula II, having HPLC purity more than 99,5%
3. According to another aspect of present invention there is provided a process for the preparation of 2,,3'-O-SulfinyIcytidine hydrochloride of Formula III used as a starting material for the preparation of 2,2'-Cyclocytidine hydrochloride of Formula II.

comprising,
a) reacting cytidine with thionyl chloride in acetonitrile solvent at 0-15°C
b) quenching the reaction mixture in to ice cold water to precipitate out 2',3'-0-Sulfinylcytidine hydrochloride of Formula III
c) filtering & washing the wet cake with alcoholic solvent to obtain pure 2',3'-0-Sulflnylcytidine hydrochloride of Formula III having HPLC purity of more

than 99%.
4. The process as claimed in claim 1, wherein the base used in step a) are sodium hydroxide, potassum hydroxide, sodium carbonate, sodium bicarbonate, triethyl amine, diisopropyl amine, tert-butyl amine etc, preferably tertiary butyl amine is used.
5. The process as claimed in claim 1, wherein the combination of water alcohol used for isolation of crude Cytarabine in step d) is selected from, combination of water-n-butanol, water-methanol, water-n-butanol-methanol, preferably combination of water-n-butanol-methanol is used.
6. The process as claimed in claim 1, wherein the purification of crude product Cytarabine of Formula I in step e) is carried out by using combination of 5% water in methanol and 10% water in methanol, preferably combination of 5% water in methanol is used.
7. The process as claimed in claim 2, wherein the base used in step a) is sodium
hydroxide, potassium hydroxide, Sodium carbonate, sodium bicarbonate & ammonia,
preferably sodium bicarbonate is used.
8. The process as claimed in claim 2, wherein the reaction in step b) is carried out
under mild vacuum at temperature range of 70°C, 80°C, 90°C, preferably 70-75°C
temperature is used.
9. The process as claimed in claim 2, wherein the isolation of crude 2,2'-
Cyclocytidine hydrochloride Formula III in step f) & purification in step g) is carried
out using mixture of water & organic solvent such as mixture of water-isoproanol,
water-methanol, water-methanol-dichloromethane, preferably mixture of water-

methanol-dichloromethane is used.
10. The process as claimed in claim 3, wherein the molar ratio of the thionyl chloride used in step a) with respect to cytidine is in range 2.5 to 5, preferably 3.5 mole equivalent is used.