FORM-2
THE PATENTS ACT, 1970 (39 of 1970)
&
THE PATENTS RULE, 2003
COMPLETE SPECIFICATION [See section 10, rule 13]
An improved process for the preparation of highly pure bis(phenylacetyl) disulfide
APPLICANT:
HERBERT BROWN
PHARMACEUTICAL & RESEARCH LABORATORIES
W-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 an improved process for the preparation of highly pure and stable bis(phenylacetyl) disulfide of Formula I.

Formula I
More particularly, it relates to an improved process for the preparation of bis(phenylacetyl) disulfide of Formula I having HPLC purity of more than 99%.
BACKGROUND OF THE INVENTION
Oligonucleotide and nucleotide phosphorothioates are of great interest in field of pharmaceuticals and as tools in molecular biology and diagnostics. Oligonucleotide and nucleotide phosphorothioates are modified oligonucleotides and nucleotides respectively, in which one of non-bridging oxygen atoms of the internucleotidlc phosphate group is replaced by sulfur atom, generally by using sulfur transfer agent.
Bis(phenylacetyl) disulfide having Formula I, is found to be an efficient sulfur transfer agent used for this purpose.
Various processes for the preparation of bis(phenylacetyl) disulfide are reported in the literature.
Journal of Chemical Society, Chemical Communication, Pg. 838 (1978) reports that variety of electrophiles like alkyl or acyl halides on treatment with solution of lithium sulfide or lithium disulfide in anhydrous tetrahydrofuran yields sulfide or disulfide

derivatives of electrophile used. The yields obtained for bis(phenylacetyl) disulfide by using this process was found to be very low.
Synthesis 8, Pg. 637 (1981) discloses a process for preparation of bis(acyl) disulfide by reacting acyl chloride with aqueous solution of sodium disulfide in presence of hexadecyltributyl phosphonium bromide in benzene at 0°C. The yields obtained for bis(phenylacetyl) disulfide by using this process were comparatively high. However the process utilizes a rare and an expensive phase transfer catalyst like hexadecyltributyl phosphonium bromide thereby increasing overall cost of the process.
JP2001039947 discloses a process for preparation of bis(phenylacetyl) disulfide by reacting phenyl acetyl chloride with aqueous solution of sodium disulfide in presence of phase transfer catalyst like tetrabutylammonium bromide in aprotic solvent like toluene, xylene or trimethyl benzene. After completion of reaction the organic layer is separated from aqueous layer and is subjected to evaporation. The residue thus obtained, is crystallized using methanol to obtain bis(phenyl acetyl) disulfide. The purity of the product obtained is not discussed in this patent application. Use of methanol for crystallizing bis(phenylacetyl) disulfide affects stability of the compound. Furthermore it was also observed that heating bis(phenylacetyl) disulfide with methanol while crystallization results in degradation of bis(phenylacetyi) disulfide.
Journal of Chemical Research. (8). Pg. 547 (2006) discloses a process for preparation of diacyl disulfide using samarium disulfide prepared by reducing sulfur with samarium diiodide. In this process samarium disulfide is reacted with acyl chloride in presence of hexamethylphosphoramide to afford diacyl disulfide. The reagents used in the process are hazardous and expensive and hence this process less feasible on industrial scale.

Some of the other references relating to preparation of diacyl disulfides are Synthetic Communications, 28(7), Pg. 1275 (1998), Synthetic Communications, 25(6), 889 (1995), Monatshefte fuer Chemie, 139(12), Pg. 1453 (2008).
Like any other synthetic compound, the bis(phenylacetyl) disulfide obtained from above discussed processes also contains process impurities, unreacted starting material, chemical derivatives of impurities contained in starting material, synthetic by-products and degradation products. It is important to control these impurities in order to obtain pure bis(phenylacetyl) disulfide. The purity of bis(phenylacetyl) disulfide has a crucial role as it is widely used as sulfur transfer agent in field of oligonucleotide / pharmaceuticals. Bis(phenylacetyl) disulfide is a low melting solid and is known to be unstable during storage and under certain conditions like exposure to moisture, air etc. However, none of the above mentioned processes discuss purity or stability of the bis(phenylacetyl) disulfide obtained.
Hence there remains a need for developing simple, industrially feasible and environment friendly process for the preparation of highly pure and stable bis(phenylacetyl) disulfide. The inventors of the present invention have rationally designed an improved process for the preparation of highly pure and stable bis(phenylacetyl) disulfide by identifying the parameters that influence the amount of impurities formed during process and by controlling these process impurities to great extent.
OBJECT OF THE INVENTION
1. An object of the present invention is to provide an improved process for the preparation of highly pure and stable bis(phenylacetyl) disulfide.

2. Another object of the present invention is to provide in high yield of bis(phenylacetyl) disulfide having HPLC purity of more than 99%.
3. Another object of the present invention is to provide an economical and environment safe process for the preparation of bis(phenylacetyl) disulfide by avoiding use of costly and hazardous reagent.
4. Yet another object of the present invention is to provide a process for the preparation of highly pure and stable bis(phenylacetyl) disulfide wherein the solvents used in the process are recovered and reused.
5. Yet another object of the present invention is to provide easy to operate and industrially feasible process for the preparation of highly pure and stable bis(phenylacetyl) disulfide.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided an improved process for the preparation of highly pure and stable bis(phenylacetyl) disulfide of Formula I

Formula I
comprising,
a) reacting sodium sulfide with sulfur in water at 95-l05°C for 0.5 to 2 hours to obtain aqueous solution of sodium disulfide

b) insitu acylating sodium disulfide with phenyjacetyl chloride in presence of a phase transfer catalyst and a water immiscible organic solvent at 0-25°C for 1 to 4 hours
c) separating an organic layer and an aqueous layer
d) distilling the organic layer to obtain an oily mass
e) treating the oily mass with a C3-C2 alcohol to obtain bis(phenylacetyl) disulfide of Formula 1 having HPLC purity in the range of 97 to 99%
f) subjecting the bis(phenylacetyl) disulfide obtained in step e) to crystallization using a C6-C8 cycloalkane as a solvent and isolating highly pure and stable bis(phenylacetyl) disulfide
According to another aspect of the present invention, there is provided a process for the preparation of bis(phenylacetyl) disulfide of Formula I having HPLC purity of more than 99%.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improved process for the preparation of highly pare and stable bis(phenylacetyl) disulfide of Formula I

Formula I
comprising,
a) reacting sodium sulfide with sulfur in water 3t 95-105°C for 0.5 to 2 hours to obtain aqueous solution of sodium disulfide

b) insitu acylating sodium disulfide with phenylacetyl chloride in presence of a phase transfer catalyst and a water immiscible organic solvent at 0-25°C for 1 to 4 hours
c) separating an organic layer and an aqueous layer
d) distilling the organic layer to obtain an oily mass
e) treating the oily mass with a C3-C12 alcohol to obtain bis(phenylacetyl) disulfide of Formula I having HPLC purity in the range of 97 to 99%
f) subjecting the bis(phenylacetyl) disulfide obtained in step e) to crystallization using a C6-C8 cycloalkane as a solvent and isolating highly pure and stable bis(phenylacetyl) disulfide
The improved process for the preparation of highly pure and stable bis(phenylacetyl) disulfide of Formula I can be depicted in Scheme I
Scheme I


According to an embodiment of the present invention, the molar ratio of the sulfur used with respect to the sodium sulfide is in the range of 0.7 to 1.2, preferably in the range of 0.8 to 1.0.
The amount of water used with respect to sodium sulfide is in the range of 4 to 8 volumes, preferably 5 to 7 volumes.
According to another embodiment of the present invention, the phase transfer catalyst used is selected from a quaternary ammonium salts like tetrabutylammonium bromide or a quaternary phosphonium salts like tetrabutylphosphonium bromide or a polyethylene glycol, preferably the phase transfer catalyst used is tetrabutylammonium bromide.
The amount of the phase transfer catalyst used with respect to phenylacetyl chloride is in the range of 5 to 20% w/w, preferably 8 to 12% w/w.
The water immiscible organic solvent used in step b) is selected from aliphatic halogenated solvents like methylene chloride, chloroform or aromatic halogenated solvents like monochlorobenzene, o-dichlorobenzene or ketones like methyl isobutyl ketone or acetates like ethyl acetate, preferably monochlorobenzene is used.
The amount of water immiscible organic solvent used with respect to phenylacetyl chloride is in range of 1 to 5 volumes, preferably 2 to 3 volumes.
According to an embodiment of the present invention, in step e) the C3-C12 alcohol used is selected from n-propanol, isopropanol, n-butanol, tert-butanol, pentanol, cyclohexanol. dodecanol and the like, preferably n-butanol is used.

The inventors of present invention contemplated the use of number of alcohols in step e) to facilitate high purity and high yield of bis(phenylacetyl) disulfide. An appropriate alcohol for the present invention is the one that efficiently yields highly pure bis(phenyiacetyl) disulfide of Formula I. The observations using different alcohols in step e) of the present invention are summarized in Table I
Table I

Sr.
No. Alcohol used in step e) HPLC purity of bis(phenylacetyl) disulfide of Formula I obtained in step e) Yield of bis(phenylacetyl) disulfide of Formula I obtained in step e) (based on assay)
1. Methanol 98.05% 76.13%
2. ' Ethan ol 96.26% 77.70%
3. n-Propanol 97.78% 79.75%
4. Isopropanol 97.4% 83.2%
5. n-Butanol 98.42% 82.00%
6. tert-Butanol 98.95% 64.21%
7. Dodecanol 99.95% 77.60%
8. Cyclohexanol 99.40% 51.67%
From the above observations, it can be concluded that in step e) the treatment of oily mass with n-butanol gives better purity and yield of bis(phenylacetyl) disulfide of Formula I.
Furthermore, It is also established in Table II that heating of pure bis(phenylacetyl) disulfide of Formula I with lower alcohols like methanol degrades the compound and makes it unstable as compared to heating with higher alcohols like n-butanol.

Table II

Initial Purity of
Bis(phenylacetyl)
disulfide Purity of Bis(phenylacetyl) disulfide after heating at 60°C for 1-3 hours with methanol Purity of Bis(phenylacetyl) disulfide after heating at 60°C for 1-3 hours with n-butanol
HPLC Purity 99.98% 89.71% 99.50%
HPLC
Assay 101.84% 88.24% 96.78%
The inventors of present invention have therefore skillfully selected use of less reactive C3-C12 alcohol in step e) to control impurity formation and degradation of the desired compound. The preferred C3-C12 alcohol used in step e) is n-butanol as it is easily available commercially and gives better results.
The amount of C3-C12 alcohol used with respect to phenylacetyl chloride is in the range of 1 to 5 volumes, preferably 2 to 4 volumes.
According to another embodiment of the present invention, the C6-C8 cycloalkanes used in step f) for crystallization is selected from cyclohexane. cycloheptane and cyclooctane, preferably cyclohexane is used.
It was observed that use of cyclohexane for crystallization of bis(phenylacetyl) disulfide avoids degradation and gives moisture-free, stable and free flowing crystalline powder of bis(phenylacetyl) disulfide
The amount of C6-C8 cycloalkane used with respect to phenylacetyl chloride is in the range of 1 to 5 volumes, preferably 2 to 4 volumes.

According to another embodiment of the present invention, the bis(phenylacetyl) disulfide of Formula I obtained has HPLC purity of more than 99%.
According to yet another embodiment of the present invention, the solvents used in the process like the water immiscible organic solvent, the C3-12 alcohol and the C6-
C8cycloalkane are individually recovered to the extent of 85 to 95% with purity of more than 98% and are reused.
The present invention controls formation of phenylacetyl sulfide of Formula II during reaction, an analogue of bis(phenylacetyl) disulfide of Formula I, to the extent of less than 1%.

Formula II
It is observed that after final crystallization of bis(phenylacetyl) disulfide in step f) with cyclohexane, the impurity phenylacetyl sulfide of Formula II in desired product is removed completely.
The details of the invention provided in the following example are given by the way of illustration only and should not be construed to limit the scope of the present invention.

EXAMPLE
Preparation of highly pure and stable bis(phenylacetvl) disulfide
To 238.75g (Assay: 57.6%) of sodium sulfide were added 51,91g of sulfur powder ~nd 800ml of water. The reaction mixture thus obtained was heated to 100-105°C for 1-2 hours, the mixture was then cooled to 30°C and 1356ml of monochlorobenzene and 50g of tetrabutylammonium bromide were added. The mixture was further cooled to 0-5oC and 500g of phenylacetyl chloride was added to it at the same temperature. The temperature of reaction mixture was then slowly increased to 25°C. After completion of reaction, organic layer was separated from aqueous layer and distilled under reduced pressure to obtain an oily mass. To the oily mass was added ] 500ml of n-butanol and the mixture was cooled to 0-5°C. The solid precipitated was filtered to obtain of bis(phenylacetyl) disulfide having HPLC purity of 98.71%. The bis(phenylacetyl) disulfide obtained after treatment with n-butanol, was further purified by crystallization using 1500ml of cyclohexane as a solvent to obtain 375g (i.e. 76.69% yield) of bis(phenylacetyl) disulfide having HPLC purity of 99.8%. Furthermore, 24.5g (i.e. 5% yield) of bis(phenylacetyl) disulfide with similar purity was isolated from cyclohexane mother liquor, as a second crop.

We claim
I. An improved process for the preparation of highly pure and stable
bis(phenylacetyl) disulfide of Formula I

comprising,
a) reacting sodium sulfide with sulfur in water at 95-105°C for 0.5 to 2 hours to obtain aqueous solution of sodium disulfide
b) insitu acylating sodium disulfide with phenylacetyt chloride in presence of a phase transfer catalyst and a water immiscible organic solvent at 0-25°C for 1 to 4 hours
c) separating an organic layer and an aqueous layer
d) distilling the organic layer to obtain an oily mass
e) treating the oily mass with a C3-C12 alcohol to obtain bis(phenytacetyl) disulfide of Formula I having HPLC purity in the range of 97 to 99%
f) subjecting the bis(phenylacetyl) disulfide obtained in step e) to crystallization using a C6-C8 cycloalkane as a solvent and isolating highly pure and stable bis(phenylacetyl) disulfide
2. The process as claimed in claim 1, wherein the bis(phenylacetyl) disulfide obtained has HPLC purity of more than 99%

3. The process as claimed in claim 1, wherein the molar ratio of the sulfur used with respect to the sodium sulfide is in the range of 0.7 to 1 .2, preferably in the range of 0.8 to 1.0
4. The process as claimed in claim 1, wherein the phase transfer catalyst used is selected from quaternary ammonium salts like tetrabutylammonium bromide, quaternary phosphonium salts like tetrabutylphosphonium bromide and polyethylene glycol, preferably the phase transfer catalyst used is tetrabutylammonium bromide.
5. The process as claimed in claim 1, wherein the water immiscible organic solvent used in step b) is selected from aliphatic halogenated solvents tike methylene chloride, chloroform or aromatic halogenated solvents like monochlorobenzene, o-dichlorobenzene or ketones like methyl isobutyl ketone or acetates like ethyl acetate, preferably monochlorobenzene is used

6. The process as claimed in claim 1, wherein the amount of water immiscible organic solvent used with respect to phenylacetyl chloride is in range of I to 5 volumes, preferably 2 to 3 volumes
7. The process as claimed in claim 1, wherein the C3-C12 alcohol used is selected from n-propanol, isopropanol, n-butanol, tert-butanol, pentanol, cyclohexanol, dodecanol and the like, preferably n-butanol is used.
8. The process as claimed in claim 1, wherein the amount of C3-C12 alcohol used with respect to phenylacetyl chloride is in the range of 1 to 5 volumes, preferably 2 to 4 volumes.

9. The process as claimed in claim 1, wherein the C6-C8 cycloalkane used in step f)
for crystallization is selected from cyclohexane, cycloheptane and cyclooctane,
preferably cyclohexane is used.
10. The process as claimed in claim 1, wherein the solvents used in the process like
the water immiscible organic solvent, the C3-C12 alcohol and the C6-C8 cycloalkane
are individually recovered to the extent of 85 to 95% with purity of more than 98%
and are reused