FORM 2

THE PATENTS ACT 1970
(Act 39 of 1970)
COMPLETE SPECIFICATION
(SECTION 10, rule 13)
"PROCESS FOR THE PREPARATION OF RIFAXIMIN"
Glenmark Pharmaceuticals Limited
an Indian Company, registered under the Indian company's Act 1957 and having
its registered office at
Glenmark House,
HDO - Corporate Bldg, Wing -A,
B.D. Sawant Marg, Chakala,
Andheri (East), Mumbai - 400 099
THE FOLLOWING SPECIFICATION DESCRIBES THE NATURE OF THE INVENTION AND
THE MANNER IN WHICH IT IS TO BE PERFORMED

PRIORITY
[0001] This application claims the benefit under Indian Provisional Application No.
738/MUM/2005, filed June 22, 2005, and entitled "PROCESS FOR PREPARATION OF
RIFAXIMIN", the contents of each of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
1. Technical Field
[0002] The present invention generally relates to an improved process for the
preparation of rifaximin. More specifically, the present invention relates to a process which is
a one pot process for the preparation of rifaximin starting from rifamycin S.
2. Description of the Related Art
[0003] Rifaximin, also known as
2
(2S,16Z,18E>20S,2lS,22R,23R,24R,25S)26SJ27S)28E)-5,6,21,23,25-pentahydroxy-27-
methoxy-2,4,11,1620,22,24,26-octamethyl-2,7-(epoxypentadeca-(1, 11,13]trienimino}-
benzofuro[4,5-e]pyrido[l,2-a]-benzimidazole-l,15(2H)-dione,25-acetate, is represented by
the structure of Formula I.


Rifaximin is a semi-synthetic, non-systemic antibiotic. Rifaximin acts by binding to the beta-
subunit of bacterial DNA-dependent RNA polymerase resulting in inhibition of bacterial
RNA synthesis. Rifaximin is commercially sold under the trade name Xifaxan . See, e.g.,
The Merck Index, Thirteenth Edition, 2001, p. 1475-76, monograph 8304,
[0004] U. S. Patent No. 4,341,785 ("the '785 patent"), herein incorporated by
reference, discloses pyrido-imidazo-rifamycins such as rifaximin. The '785 patent further
discloses a process for preparing rifaximin by reacting 3-halorifamycin S with 2-amino-4-
methyl pyridine to provide N-dehydro-4-deoxy-2-imino-4'-methyl-
pyrido[r,2':l,2]imidazo[5,4-c] rifamycin S. This intermediate is then reacted with L-
ascorbic acid to provide rifaximin.
[0005] U. S. Patent No. 4,557,866 ("the '866 patent"), herein incorporated by
reference, discloses a process for preparing rifaximin. This process involves reacting
rifamycin O with 2-amino-4-methyl pyridine to obtain N-dehydro-4-deoxy-2-imino-4'-
methyl-pyrido[1,2':l,2]imidazo[5,4-c] rifamycin S. This intermediate is then reacted with L-
ascorbic acid to provide rifaximin.
[0006] The 3-halorifamycin S compounds are not commercial products and have to be
prepared by halogenating rifamycin S. This is believed to have a negative impact in the
overall yield of rifaximin. Also, rifamycin O needs to be isolated by a fermentation process,
which limits its commercial availability.
[0007] Accordingly, there remains a need for an improved process for preparing
rifaximin that eliminates and reduces the drawbacks of the prior art in a convenient and cost
efficient manner and on a commercial scale.
SUMMARY OF THE INVENTION
[0008] In accordance with one embodiment of the present invention, a process for the
preparation of rifaximin is provided comprising (a) reacting rifamycin S with 2-amino-4-
methyl pyridine in a suitable solvent and in the presence of catalytic iodine; and (b) treating
the product of step (a) with L-ascorbic acid to provide rifaximin.
3

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] In one aspect of the present invention, a process for preparing rifaximin is
provided which includes at least reacting rifamycin S with 2-amino-4-methyl pyridine in a
suitable solvent system and in the presence of catalytic iodine followed by treatment with L-
ascorbic acid to obtain crude rifaximin. The process of the present invention is generally
shown below in Scheme I.

[0010] In step (a) of the process of the present invention, rifamycin S is reacted with
2-amino-4-methyl pyridine in a suitable solvent. Useful solvents include, but are not limited
to, halogenated hydrocarbons, e.g., dichloromethane, 1,2-dichloroethane and the like,
aliphatic alcohols, e.g., methanol, ethanol, isopropanol and the like, water and the like and
mixtures thereof. The reaction may take place at room temperature or at a temperature
ranging from about 20°C to about 30°C. The time period for the reaction will ordinarily
range from about 25 hours to about 100 hours and preferably from about 60 hours to about 90
hours. The 2-amino-4-methyl pyridine can be present in an amount ranging from about 2 to
about 3.5 molar equivalents, and preferably about 2.5 to about 3 equivalents per equivalent of
the rifamycin S. The catalytic iodine can be present in an amount ranging from about 0.1 to
about 0.75 molar equivalents, and preferably about 0.3 to about 0.5 equivalents per equivalent
of the rifamycin S,
4

[0011] In step (b) of the process of the present invention, the product of step (a) is
treated L-ascorbic acid to provide crude rifaximin. The L-ascorbic acid can be present in an
amount of about 1 to about 2 molar equivalents and preferably about 1.5 equivalents per
equivalent of the rifamycin S. The treatment of the product of step (a) with L-ascorbic acid
may take for a time period of about 30 minutes to about 2 hours, and preferably about 1 hour.
[0012] The crude rifaximin thus obtained can then be purified by conventional
techniques, e.g., by column chromatography or crystallization from organic solvents. If
desired, the purified rifaximin may be isolated by conventional techniques, e.g., by treatment
with hydrochloric acid and sodium thiosulfate followed by solvent evaporation and leaching
in diisopropyl ether. The isolated crude rifaximin may then be further purified either by
column chromatography or crystallization from organic solvents. The process of the present
invention advantageously provides rifaximin in relatively high purity, e.g., greater than or
equal to about 95%, and preferably greater than or equal to about 97%.
[0013] The following examples are provided to enable one skilled in the art to practice
the invention and are merely illustrative of the invention. The examples should not be read as
limiting the scope of the invention as defined in the features and advantages.
EXAMPLE 1
[0014] Preparati on of rifaximin
[0015] A solution of 10 g (0.0144 moles) of rifamycin S, 4.67 g (0.0432 moles) of 2-
amino-4-methy] pyridine and 0.9 g (0.00354 moles) iodine in 50 ml dichloromethane was
formed under stirring at 20-25°C for 70 to 75 hours. The reaction mass was quenched by
adding a 20% solution of ascorbic acid and stirring was continued for an additional 30
<• ,
minutes. The layers were separated and the organic layer was washed with a solution of 5%
sodium thiosulfate, IN hydrochloric acid, water, 0.5 N sodium hydroxide solution and finally
with water until it was neutral. The organic layer was treated with silica gel/charcoal (12.0
g/l.Og) and concentrated. The residue was dissolved in chloroform. The solid product was
isolated by adding chloroform solution to n-hexane followed by filtration and drying.
5

[0016] The isolated product was then purified by silica gel column chromatography by
elution with dichloromethane/methanol (7.8:2,2) to provide pure rifaximin. HPLC Purity-
97.1%
EXAMPLE 2
[0017] Preparation of rifaximin
[0018] A solution of 10.0 g (0.0144 moles) of rifamycin S, 4.67 g (0.0432 moles) of
2-amino-4-methy] pyridine and 1.8 g (0.0072 moles) iodine in 50 ml ethanol was formed
under stirring at 20-25QC for 70 hours. The solvent was evaporated and residue dissolved in
dichloromethane. To the organic layer was added a 20% solution of ascorbic acid and stirring
was continued for an additional 30 minutes. The layers were separated and the organic layer
was washed with a solution of 5% sodium thiosulfate., IN hydrochloric acid, water, 0.5 N
sodium hydroxide solution and finally with water until it was neutral. The organic layer was
treated with silica gel/charcoal and concentrated. The residue was dissolved in chloroform.
The solid product was isolated by adding chloroform solution to n-hexane followed by
filtration and drying.
[0019] The isolated product was then purified by silica gel column chromatography by
elution with dichloromethane/methanol (7.8:2.2) to provide pure rifaximin. HPLC Purity-
97.4%
EXAMPLE 3
[0020] Preparation of rifaximin
[0021] A solution of 10.0 g (0.0144 moles) of rifamycin S, 4.67 g (0.0432 moles) of
2-amino-4-methyl pyridine and 1.8 g (0.0072 moles) iodine in 50 ml ethanol: water (1:1) was
formed under stirring at 20 to 25°C for 80 hours: To the reaction mass was added
dichloromethane and ascorbic acid and stirring was continued for an additional 30 minutes.
Next, the layers were separated and the aqueous layer was extracted with dichloromethane
and combined with main organic layer. The organic layer was washed with a solution of 5%
sodium thiosulfate, IN hydrochloric acid, water, 0.5 N sodium hydroxide solution and finally
with water until it was neutral. The organic layer was treated with silica gel/charcoal and
6

concentrated. The residue was dissolved in chloroform. The solid product was isolated by
adding a chloroform solution to n-hexane followed by filtration and drying.
[0022] The isolated product was then purified by silica gel column chromatography by
elution with dichloromethane/methanol (7.8:2.2) to provide pure rifaximin. HPLC Purity-
97.2%
[0023] It will be understood that various modifications may be made to the
embodiments disclosed herein. Therefore the above description should not be construed as
limiting, but merely as exemplifications of preferred embodiments. For example, the
functions described above and implemented as the best mode for operating the present
invention are for illustration purposes only. Other arrangements and methods may be
implemented by those skilled in the art without departing from the scope and spirit of this
invention. Moreover, those skilled in the art will envision other modifications within the
scope and spirit of the features and advantages appended hereto.
Dated this Twenty-Second (22nd) day of June, 2006
(Signed)
VISHAL A. SODHA
SENIOR MANAGER-IPM
GLENMARK PHARMACEUTICALS LIMITED
7

WE CLAIM:
1. A process for preparing rifaximin, the process comprising (a) reacting rifamycin S
with 2-amino-4-methyl pyridine in a suitable solvent and in the presence of catalytic iodine;
and (b) treating the product of step (a) with L-ascorbic acid to provide rifaximin.
2. The process of Claim 1, wherein the solvent comprises at least one solvent selected
from the group consisting of a halogenated hydrocarbon, aliphatic alcohol, water and mixtures
thereof.
3. The process of Claim 1, wherein the solvent comprises at least one solvent selected
from the group consisting of dichloromemane, ethanol, water or mixtures thereof.
4. The process of Claim 1, wherein the 2-amino-4-methyl pyridine is present in an
amount of about 2.0 to about 3.5 molar equivalents per equivalent of the rifamycin S.
5. The process of Claim 1, wherein the 2-ammo-4-methyl pyridine is present in an
amount of about 2.5 to about 3.0 molar equivalents per equivalent of the rifamycin S.
6. The process of Claim 1, wherein the catalytic iodine is present in an amount of
about 0.1 to about 0.75 molar equivalents per equivalent of the rifamycin S.
7. The process of Claim 1, wherein the catalytic iodine is present in an amount of
about 0.3 to about 0.5 molar equivalents per equivalent of the rifamycin S.
8. The process of Claim 1, wherein the reaction of rifamycin S with 2-amino-4-
methyl pyridine is carried out at room temperature.
9. The process of Claim 1, wherein the reaction of rifamycin S with 2-amino-4-
methyl pyridine is carried out at a temperature of about 20°C to about 30°C.
8

10. The process of Claim 1, wherein the time period for the reaction of rifamycin S
with 2-amino-4-methyl pyridine is about 25 hours to about 100 hours.
11. The process of Claim 1, wherein the time period for the reaction of rifamycin S
with 2-amino-4-methyl pyridine is about 60 hours to about 90 hours.
12. The process of Claim 1, wherein the L-ascorbic ^cid is present in an amount of
about 1.0 to about 2.0 molar equivalents per equivalent of the product of step (a).
13. The process of Claim 1, wherein the time period for treatment with L-ascorbic
acid is about 30 minutes to about 2 hours.
14. The process of Claim 1, further comprising the step of purifying the rifaximin.
15. The process of Claim 14, wherein the purified rifaximin has a purity of greater
than or equal to about 95%,
16. The process of Claim 14, wherein the purified rifaximin has a purity of greater
than or equal to about 97%.
] 7. The process of Claim 1, wherein the rifaximin is thereafter converted to a
pharmaceutically acceptable salt thereof.
18. Purified rifaximin having a purity of greater than or equal to about 95%, prepared
in accordance with the process of Claim 1.
19. The purified rifaximin of Claim 18, having a purity of greater than or equal to
about 97%.
9

20. Purified rifaximin having a purity of greater than or equal to about 95%, prepared
in accordance with the process of Claim 14.
10
Dated this Twenty-Second (22nd) day of June, 2006