Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR PURIFICATION OF WARFARIN ACID, WARFARIN ALKALI METAL SALTS AND
CORRESPONDING CLATHRATES
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a synthesis of pure warfarin acid and derivatives
thereof. In
particular, the invention relates to a commercially feasible, large scale
process for
purification of warfarin acid and the preparation of warfarin sodium, warfarin
sodium 2
propanol clathrate, warfarin potassium and warfarin lithium 2-propanol
clathrate therefrom,
in high, pharmacopoeia) grades of purity.
Background
Warfarin sodium, warfarin sodium 2-propanol clathrate, warfarin potassium and
warfarin lithium 2-propanol clathrate are highly potent anticoagulants,
generally
administered orally and used extensively as active pharmaceutical
ingredients.(APIs). These
compounds are also widely used as rodenticides in different dosages and
formulations.
The commercial production of APIs preferably uses systems and processes
capable of
providing high quality, pure intermediates and products complying with
pharmacopoeia)
requirements. Numerous procedures for the synthesis of warfarin acid, its
salts and
clathrates exist, but there is still a need by the pharmaceutical industry for
methods of
preparing these compounds in high quality as both intermediates and products
and
particularly for the preparation of high purity warfarin acid.
Warfarin acid and its alkali metal salts are coumarinic type derivatives
having
powerful anticoagulant properties. They are useful, established vitamin K
related
anticoagulant pharmaceuticals for the treatment of humans and animals. For
example:
~ Warfarin sodium and its 2-propanol clathrate is marketed under various
commercial names like Coumadin; Marevan; Prothromadin; Tintorane; Warfarin
sodium; Warfilone; Waran.
Warfarin potassium salt is available under the name Athrombin K.
These substances are also the active component in common rodenticide agents.
Warfarin acid, known chemically as racemic 4-hydroxy-3-(3-oxo-phenylbutyl)-2H-
1-
benzopyran-2-one or 3-(oc-acetonylbenzyl)-4-hydroxycoumarin, is typically
prepared by the
Michael addition reaction of commercially available or in situ prepared 4-
hydroxycoumarin
to 4-phenyl-3-buten-2-one (benzalacetone). A wide range of conditions, for
example,
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variations in solvents, variation of acidic and basic catalysts, varying
temperatures, etc., have
been used in synthetic procedures described in the literature. Although
warafarin acid is a
well known chemical entity, a simple, commercially feasible and direct
preparation of
warfarin acid in the pure state has not been significantly addressed.
Earlier purification methods described by Ikawa et al. (U.S. Patent No.
2,427,578, see
also, J. Am. Chem Soc. 1944, 66, 902) or by Siedman et al. (J. Am. Chem. Soc.
1950, 72,
5193), as well as many others, recrystallize the crude acid from acetone-water
mixtures.
U.S. Patent No. 4,113,744 to Badran et al., describes a method for preparation
of
pure, microcrystalline warfarin acid by dissolution of the commercial impure
acid into a
special amine buffered-water system, filtration and reprecipitation of the
pure,
microcrystalline acid by pH readjustment of the filtrate.
Bush and Trage described a procedure for the preparation of pure warfarin
acid,
comprising ether extraction of the crude acid, back-extraction with dilute
sodium hydroxide,
filtration of the alkaline solution, and reprecipitation with SN HCl solution.
(J. of Pharm. Sci.
1983, 72 (7), 830-831.) The pure acid is obtained by further recrystallization
from acetone-
water.
More recent references (for example, Ivanov et al., Arch. Pharm (Weinheim)
1990,
323, 521-522; Uwaydah et al. WO 97/24347) describe
extraction/recrystallization procedures
using glacial acetic acid or ethyl acetate.
Warfarin alkali salts, chemically known as 4-hydroxy-3-(3-oxo-phenylbutyl)2H-1-
benzopyran-2-one alkali salts or 3-(oc-acetonylbenzyl)-4-hydroxycoumarin
sodium,
potassium or lithium salts, are usually prepared by reacting warfarin acid
with a molar
equivalent or less of the respective alkali metal bases in aqueous medium, or
with the metal
alkoxides in lower alkyl alcohols.
U.S. Patent No. 2,765,321 to Schroeder et al. describes a process of preparing
crystalline warfarin sodium by reacting an aqueous sodium hydroxide solution
with an
excess of warfarin acid, followed by removal of the acid excess by addition of
ethanol and
filtration. The final warfarin sodium salt is obtained through a salting out
procedure using
lithium chloride addition into the ethanol-water solution of the warfarin
sodium salt, cooling
and recovering the precipitated warfarin sodium by filtration.
U.S. Patent No. 2,777,859 to Link et al. describes a process of preparing an
aqueous
solution of warfarin alkali metal derivatives by adding an aqueous alkali
metal hydroxide to
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an excess of water wet warfarin acid, warming and removing the excess of
warfarin acid by
filtration.
U.S. Patent No. 3,077,481 to Schroeder et al. describes a process of purifying
colored
warfarin sodium by dissolving the amorphous salt in warm 2-propanol, cooling
the resulting
solution and recovering the crystalline warfarin sodium 2-propanol clathrate
product.
In French Patent 1.397.213, Yates and Thomson describe an improved preparation
of
warfarin sodium by reacting a suspension of warfarin acid in an anhydrous
lower alkyl
alcohol with a solution containing a molar equivalent of sodium hydroxide
dissolved in the
same solvent. The crude product is isolated as a syrup upon solvent
evaporation and must be
further purified and subjected to thorough drying or lyophilization. The
product is normally
not of pharmaceutical quality. The warfarin sodium may be purified by
dissolving in 2
propanol from which the crystalline clathrate is isolated by filtration. The 2-
propanol
trapped may be removed by evaporation to dryness in vacuo of the crystalline
salt solutions
in methanol, ethanol or acetone. Further purification is required in order to
obtain a
pharmaceutical grade substance.
U.S. Pat. No. 3,192,232 to Schroeder and Link describes a process for
preparing
warfarin sodium and warfarin potassium salts by reacting a slurry or warfarin
acid in
acetone-water with less than an equivalent of sodium hydroxide or potassium
hydroxide in
water at room temperature. The solution of the crude salt is purified by
stirnng with active
charcoal and isolating of the salt by evaporation to dryness, spray drying, or
drum drying.
U.S. Pat. No. 3,246,013 to Weiner et al. describes a process for preparing
crystalline
warfarin sodium 2-propanol clathrate through neutralization of a warfarin
acid/2-propanol
slurry to a pH of about 9-10 using sodium hydroxide or, as an alternative to
avoid the color
usually present, using selected sodium alkoxides at about 50-80°C.
Ohnishi et al. have described a method for preparing warfarin alkali metal
salts by
dissolving warfarin acid in an aqueous solution containing an equivalent
amount of the
respective alkali metal hydroxide (lithium, sodium, potassium, rubidium and
cesium). Biosci.
Biotech. Biochem. 1995, 59(6), 995-1006 (cf. CA 123: 105246 (1996)) The
respective salts
are isolated by lyophilization.
In a recent patent publication, WO 97/24347 (published July 10, 1997), Uwaydah
et
al. describe a comprehensive process for warfarin alkali salts (sodium and
potassium) and
clathrate preparation starting from 2-hydroxyacetophenone and a carbonate
ester. The
hydroxycoumarin thus obtained is further reacted with benzalacetone in the
presence of a
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phase transfer catalyst to give warfarin acid in a procedure similar to those
of Ivanov (see
reference above). The latter intermediate is further reacted with sodium or
potassium
hydroxide or carbonate, or preferably with sodium or potassium methoxide or
ethoxide in
anhydrous ethanol or 2-propanol to ultimately yield the desired product.
As can be seen from the description of existing preparations, there is a great
deal of
interest in this field, and a number of synthetic routes for the preparation
of warfarin acid and
its salts exist. However, the need remains for an economical, industrially
feasible procedure
to produce a high quality, pharmacopeial grade of warfarin acid and related
salts.
SUMMARY OF THE INVENTION
Further objectives and advantages will become apparent from a consideration of
the
description, and nonlimiting examples described therein.
The present invention is a process for preparing pure warfarin acid from crude
warfarin acid by suspending the crude acid in a water immiscible solvent,
preferably toluene,
extracting the acid into an aqueous solution of dilute base, separating the
resulting aqueous
phase and diluting it with a lower alkyl alcohol, for example, methanol,
ethanol or,
preferably, 2-propanol. Preferably, the aqueous solution is filtered before
being diluted with
the lower alkyl alcohol. The lower alkyl alcohol is preferably added to a
concentration of
about SO%. The solution is acidified to a pH of about 2 to 5 using a suitable
acid, such as
hydrochloric, sulfuric or phosphoric acid and the like. Sulfuric acid is
preferred. The
resulting suspension is stirred at a temperature of from about 20°C to
about 60°C, preferably
about 40°C to about 50°C, cooling the suspension below room
temperature, preferably to
about S°C to about 10°C, filtering the pure warfarin acid and
drying.
. The crude warfarin acid for use in the purification is preferably prepared
by
hydrolyzing cyclocoumarol using about 2% to about 10% of an acidic catalyst
which is
preferably 4-toluene sulfonic acid, to a solution of cyclocoumarol in a
mixture of a polar
water miscible solvent and water at a temperature of from about room
temperature to about
the boiling point of the solvent. Less than 2% acid catalyst is usually
ineffeicient while more
than 10-15% may impurify the product and be uneconmical. The reaction is
preferably
conducted in butanone or acetone containing about 35% to about 65% of water,
preferably
about 40% to about 50% water, and is most preferably conducting at reflux
temperatures.
In another aspect, the invention is a process for the preparation of warfarin
sodium 2-
propanol clathrate comprising reacting pure warfarin acid with sodium
carbonate in 2-
propanol. The 2-propanol preferably contains between about 0.5% and about S%
of water by
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volume, and more preferably from about 1% to about 3% of water by volume. The
molar
ratio of sodium carbonate to warfarin acid is preferably between about 0.9 and
about 2, and
more preferably between about 1.0 and about 1.3, to ensure yield and quality
of the final
product. The preparation of warfarin sodium 2-propanol clathrate is preferably
conducted at
a temperature of from about room temperature to about the boiling point of the
solvent.
More preferably, the reaction is carned out at reflux temperatures. The
reaction is typically
complete in a time of from about 1 to about 5 hours, and more preferably is
completed in
from about 2 hours to about 3 hours. In a preferred embodiment, the solids
remaining after
the reaction is complete, which consist primarily of unreacted sodium
carbonate, are
removed by filtration. Water is removed by azeotropic distillation by either a
binary or
ternary system. The preferred binary azeotropic system is 2-propanol and
water. Preferred
ternary azeotropic distillation systems are 2-propanol-cyclohexane-water, 2-
propanol
hexane-water or 2-propanol-heptane-water, preferably 2-propanol-hexane-water.
The pure
warfarin 2- propanol clathrate precipitates from the reaction mixture as a
crystalline pure
solid, which is collected by filtration.
In another aspect, the invention is a method for preparing warfarin potassium
by
reacting pure warfarin acid with potassium carbonate in place of sodium
carbonate. The
reaction parameters for potassium salt formation are similar to those for
warfarin sodium 2-
propanol clathrate formation.
In another aspect the invention is a process for preparing warfarin lithium 2-
propanol
clathrate comprising reacting pure warfarin acid with a solution of lithium 2-
propoxide in
anhydrous 2-propanol. Preferred parameters are otherwise identical to those
for the
preparation of warfarin sodium 2-propanol clathrate, although the 2-propanol
must be
anhydrous.
In yet another aspect, the invention is a process for preparing warfarin
sodium
comprising reacting pure warfarin acid with sodium carbonate in a water
miscible polar
solvent other than 2-propanol. Preferably, the water miscible polar solvent is
acetone,
ethanol, acetonitrile or butanone, and is most preferably butanone. The pure
warfarin
sodium is collected by solvent removal or addition of a low polarity solvent,
for example,
hexane, heptane or cycloheptane.
The present invention succeeds where previous efforts have failed by providing
a
convenient synthesis of pure warfarin acid. The invention is particularly
advantageous in
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providing for the preparation of warfarin derivatives from the pure warfarin
acid allowing
use of the derivatives without additional purification.
This invention solves a previously unrecognized problem and provides a method
for
the preparation of pure warfarin acid enabling the production of higher
quality salts and
S clathrates without the need for additional purification.
This invention presents an improved synthesis of warfarin acid and its
derivatives in
a crowded and mature art by providing a method for preparing warfarin acid in
a pure form
not readily obtainable by use of existing methods without additional
purification.
This invention provides advantages that were not previously appreciated such
as the
more efficient synthesis of high purity warfarin alkali metal salts and
clathrates.
The invention provides an advantage in producing warfarin acid in the form of
large
crystals that are more easily filtered, facilitate ease of drying and shorten
drying times.
This invention satisfies a long felt need for a convenient, industrially
useful synthesis
of pure warfarin acid and sodium, potassium and lithium salts and clathrates
prepared
1 S therefrom.
DETAILED DESCRIPTION OF THE INVENTION
In describing preferred embodiments of the present invention, specific
terminology is
employed for the sake of clarity. However, the invention is not intended to be
limited to the
specific terminology so selected. All references cited herein are incorporated
by reference as
if each had been individually incorporated by reference.
"Pure," as used herein, refers to a substance that meets at least one of the
standards of
the U.S. Pharmacopoeia (USP), British Pharmacopoeia, European Pharmacopoeia
(Ph.Eur).
EPCRS refers to the European Pharmacopoeia) Commission of Reference Substances
and
USPRS refers to U.S. Pharmacopoeia) Reference Standards. The EPCRS and USPRS
provide reference standards for warfarin sodium and warfarin acid.
Non-limiting examples of suitable nonpolar solvents include aliphatic and
aromatic
hydrocarbons, for example, hexane, heptane, cyclohexane, toluene and benzene.
Toluene is
particularly preferred.
Non-limiting examples of water miscible solvents include lower molecular
weight
alcohols (methanol, ethanol, isopropanol, etc.), acetonitrile, ketones
(acetone, methylethyl
ketone, etc.), and ethers (tetrahydorfuran, 1,4-dioxane, etc.).
Extensive research has demonstrated that the purity of warfarin acid used to
prepare
derivatives has a critical effect on the quality of the final warfarin salts
and their usefulness
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as APIs. Therefore, a fully reproducible, industrially feasible procedure for
the preparation
of pure warfarin acid as a key intermediate is presented.
The procedure for preparing pure warfarin acid according to the invention
comprises
the following:
1) Preparation of a cyclocoumarol derivative by the Michael addition reaction
of
4-hydroxycoumarin to benzalacetone in methanol at reflux, using known
procedures.
2) Preparation of the crude warfarin acid by acid catalyzed hydrolysis of the
cyclocoumarol in a butanone-water or acetone-water system. A preferred acid
catalyst is 4-
toluene sulfonic acid. The acid concentration is preferably about 2% to about
10% and more
preferably about 3% to about 6%. Less than 2% acid is generally inefficient,
and greater
than 10%-15% leads to increased impurities and may be uneconomical. The
butanone or
acetone contains about 30% to about 60%, preferably about 40% to about 50%,
water. The
reaction may be conducted at a temperature from about room temperature to the
reflux
temperature of the solvent. Preferably, the reaction is conducted at about
reflux temperature.
Although other methods of preparing crude warfarin acid may in principle be
used, the
inventors have found that the purification steps which follow are most
effective when crude
warfarin acid is prepared by this method.
3) Novel, improved preparation of pure warfarin acid by suspension of the
crude
acid in a low polarity, water immiscible solvent, dissolution and extraction
of the warfarin
acid into an aqueous alkaline phase as the warfarin salt, separation, and
optionally filtration,
of the aqueous alkaline solution followed by dilution with a lower alkyl
alcohol, preferably
above room temperature. The low polarity water immiscible solvent is
preferably non-
chlorinated and is most preferably toluene. The alkaline solution is
preferably an aqueous
solution of a dilute base, for example, sodium hydroxide. A preferred lower
alkyl alcohol
solvent is 2-propanol, although other lower alkyl alcohols, for example,
methanol or ethanol,
may be used. The pure, crystalline warfarin acid is preferably isolated by
filtration of the
precipitated substance. The acid used to precipitate the warfarin acid may be
an aqueous
acid selected from hydrochloric acid, sulfuric acid, phosphoric acid and the
like. Sulfuric
acid is preferred.
It has surprisingly been found that the conditions for dilution of the aqueous
salt
solution and precipitation of the purified warfarin acid are instrumental in
obtaining high
purity warfarin acid without the need for additional purification procedures
which tend to be
costly and time consuming. After separating and filtering the aqueous solution
of warfarin
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acid salt, dilution of the aqueous solution with a lower alkyl alcohol,
preferably at or above
room temperature, is critical. Accordingly, prior to the addition of the acid,
the solution is
stirred, preferably at a temperature of from about 20°C to about
60°C, more preferably from
about 40°C to about 50°C. The lower alkyl alcohol, preferably 2-
propanol, is added at a
temperature at or above room temperature. Although the amount of alcohol is
not critical,
the resultant solution preferably contains from about 30% to about 60%
isopropanol, more
preferably between about 40% and about SO% isopropanol, to prevent loss of the
pure acid.
The acid is then added to a pH of about 2-5 at or above room temperature.
Precipitation at or
above temperature is critical and results in the generation of warfarin acid
with higher purity
and larger crystals.
After addition of the acid, the suspension is stirred at or above room
temperature.
Precipitation is further facilitated by cooling the suspension to a
temperature below room
temperature, preferably from about 5°C to about 10°C. The pure,
crystalline warfarin acid is
then isolated by filtration of the precipitated substance.
This invention also relates to procedures for preparing pure warfarin
derivatives
including salts and clathrates. Warfarin derivatives particularly include
warfarin sodium,
warfarin sodium 2-propanol clathrate, warfarin potassium and warfarin lithium
2-propanol
clathrate, from the pure warfarin acid previously described. It will be
appreciated that
warfarin salts of other ions may be formed and clathrates incorporating
solvents other than 2
propanol may be prepared according to the invention.
These procedures comprise the following:
1) Warfarin sodium is prepared by refluxing the pure warfarin acid with an
excess of sodium carbonate in a polar solvent such as acetone or butanone,
which may be
either anhydrous or contain from about 1 % to about 10% water. The unreacted
sodium
carbonate is preferably removed by filtration. The product is isolated by
evaporating the
solvent to dryness or by precipitating the salt by addition of a nonpolar
solvent such as
hexane, cyclohexane, heptane and the like, and filtering and drying the
precipitate.
2) Warfarin sodium 2-propanol clathrate is prepared by refluxing the pure
warfarin acid with an equivalent amount or an excess of sodium carbonate in 2-
propanol.
The 2-propanol may be anhydrous or may contain about 0.5% to about 10% water,
preferably about 1% to about 3% water. The molar ratio of sodium carbonate to
warfarin
acid is between about 0.9 and about 2, and is preferably between about 1.0 and
about 1.3.
The reaction may be conducted at a temperature ranging from room temperature
to the
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boiling point of the solvent for a time of from about 1 hour to about 5 hours.
Preferably, the
reaction is conducted at reflux temperature and the preferred reaction time is
from about 2
hours to about 3 hours. The unreacted carbonate is removed by filtration while
the excess
water is removed by binary (2-propanol/water) or ternary (heptane, cyclohexane
or,
preferably, hexane with 2-propanol and water) azeotropic distillation. The
precipitated pure
warfarin sodium 2-propanol clathrate is further isolated as a pure crystalline
solid by
filtration and drying.
3) Warfarin potassium salt is prepared by refluxing the pure warfarin acid
with
potassium carbonate in 2-propanol. Reaction conditions are similar to those
for warfarin
sodium 2-propanol clathrate. However, when preparing the potassium salt, a
clathrate is not
formed. The salt is precipitated directly from the reaction medium upon
cooling, and is
isolated by filtration and drying.
4) Warfarin lithium-2-propanol clathrate is prepared by reacting pure warfarin
acid with a small excess of lithium-2-propoxide in 2-propanol (prepared in
situ from lithium
strips or rods). The product is isolated by cooling, precipitation, filtration
and drying.
For the described processes, there are possible variations of some of the
conditions
which may provide somewhat different results. Some of the varying conditions
that might
be more preferable are varying time and temperatures, level of stirring, and
reaction vessel
characteristics. These may be further explored for any particular available
equipment and for
the respective final product desired. Varying processes to optimize conditions
for a
particular laboratory setting may be determined by routine experimentation.
Such processes
would be apparent to persons skilled in the art in light of the teachings of
the present
invention.
Warfarin derivatives prepared according to the present invention may be made
into
pharmaceutical dosage forms with appropriate pharmaceutically acceptable
carriers or
diluents. If appropriate, pharmaceutical dosage forms may be formulated into
preparations
including, but not limited to, solid, semi-solid, liquid, or gaseous forms,
such as tablets,
capsules, powders, granules, ointments, solutions, suppositories, injections,
inhalants, and
aerosols, in the usual ways for their respective route of administration.
Methods known in
the art can be utilized to prevent release or absorption of the warfarin
derivative until it
reaches the target organ or to ensure time-release of the composition. A
pharmaceutically-
acceptable form should be employed which does not ineffectuate the
compositions of the
present invention. In pharmaceutical dosage forms, the compositions may be
used alone or
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in appropriate association, as well as in combination with, other
pharmaceutically-active
compounds.
To illustrate the invention and its particular usefulness, the following non-
limiting
examples are presented which exemplify the invention, but do not define its
full scope:
EXAMPLES
EXAMPLE 1: Preparation of Pure Warfarin Acid
The full, two step reaction and purification procedure, starting from
cyclocoumarol
methyl ether is hereinafter described:
A. Crude warfarin acid by cyclocoumarol hydrolysis
Cyclocoumarol methyl ether (1720 g, previously prepared in 72% yield by the
known
procedure, using 1.2 kg of benzalacetone, in methanol) was added to a solution
of butanone
(6.5 L) containing 4-toluene sulfonic acid monohydrate (170 g). Water (4 L)
was added and
the mixture stirred at reflux for about 4 hours.
The reaction solution was then concentrated to about half of its initial
volume by
azeotropic distillation of butanone/water, wherein the crude warfarin acid
began to
precipitate. The suspension was gradually cooled and the crude, wet warfarin
acid was
obtained by filtration and washed with cold butanone and water.
Yield: 1320-1390 g (80-85% from cyclocoumarol) on a dry basis. (The wet
substance contained about 15-25% moisture and was directly used into the next
step.)
Physical properties of the crude warfarin acid were identical to an authentic
sample
(warfarin acid USP standard).
B. Pure warfarin acid from crude warfarin acid
The crude (wet) acid from example 1 was added to toluene (5.5 L) and the
mixture
stirred at room temperature until a homogeneous suspension was achieved. A
solution of 5%
sodium hydroxide (about 4 L) was then added to the stirred suspension until a
basic pH was
reached and an almost clear two-phase solution was obtained. The stirring was
stopped and
the layers allowed to separate. The lower aqueous phase was filtered by
suction and the
upper organic phase discarded.
The aqueous filtrate was returned to the vessel and gently warmed to 40-
50°C. 2-
Propanol (4 L) was added and the solution stirred for about %2 hour while
maintaining the
temperature.
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Sulfuric acid (20% solution in water) was then added slowly in portions to the
warm,
stirred solution, until a constant acidic pH was achieved. A total of about
1.5-2 L of sulfuric
acid solution was needed, while the pure warfarin acid precipitated as a
bright-white
crystalline solid.
The thick suspension was stirred for an additional %2 hour, then cooled to
below 10°C
for another hour.
The cold suspension was filtered, thoroughly washed with demineralized water
and
finally with cold 2-propanol.
The pure warfarin was obtained as a white crystalline powder upon drying in a
vacuum oven for 8-10 hours.
Yield: 1250-1300 g (90-95% from the crude acid)
M.P.: 159.6-161.2°C (lit. 161°C)
Clarity and color: clear and almost colorless.
Color by UV absorption at 385 nm (of the 10% w/v solution in 1N NaOH): < 0.30
AU.
Related substances (TLC): none detected.
Physical properties of the product were identical to warfarin acid USPRS.
EXAMPLE 2: Preparation of Warfarin Sodium
A three-neck, round bottom flask provided with mechanical stirrer, thermometer
and
reflux/distillation condenser was successively charged with pure warfarin
acid, (308 g, 1
mole), sodium carbonate (Merck, art. 106398) (106 g, 1 mole) and butanone
(MEK, CP,
3100 ml).
The mixture was heated to reflux with stirnng for 8 hours, then cooled to room
temperature and filtered.
The clear solution was returned to the flask and the solvent volume reduced to
about
1/3 of the initial volume by distillation.
Heptane (3100 ml) was slowly added in portions to the residue in the flask and
the
mixture stirred at room temperature for an additional 4 hours.
Warfarin sodium was isolated as a white powder by suction filtration, washing
and
drying to constant weight in a vacuum oven at 35-40°C and 1 mm Hg.
Yield: 298 g (90%)
Cl_ arity of the alkaline solution: clear and colorless (complies with Ph.Eur
and USP).
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Color (phenolic ketones) by UV absorption: < 0.1 AU (complies with Ph.Eur and
USP).
~H of 1 % aqueous solution: 7.86 (complies with Ph.Eur and USP).
Related substances (by TLC): not more than 0.1% (complies with Ph.Eur).
Identification (1R): identical with the warfarin sodium EPCR standard.
EXAMPLE 3: Preparation of Warfarin Sodium
As per Example 2 above but using pure warfarin acid (0.1 mole) and hexane (300
ml)
instead of heptane.
Yield: 29 g (88%)
Clarity, color (by UV) and related substances as in example 2 above.
~H: 7.8
Anal,: Identical to EPCR standard.
EXAMPLE 4: Preparation of Warfarin Potassium
A mixture of pure warfarin acid (31 g, 0.1 mole) and potassium carbonate (99%)
(10.4 g, 0.75 mole) in 2-propanol (CP grade, 160 ml) was stirred at 20-
25°C for 2 hours, in a
suitable glass flask under a nitrogen atmosphere.
The unclear solution was filtered using suction and the filtrate was cooled to
below
5°C for 4-6 hours.
The precipitated solid was carefully filtered by nitrogen pressure, washed
with cold
2-propanol and dried in a vacuum oven (40-50°C, 10 mm Hg) for 6-8 hours
to give pure
warfarin potassium as a white, hygroscopic, crystalline solid.
Yield: 24.3 g (70%)
Identification (by IR vs. warfarin acid standard): complies
Potassium test: positive
1H-NMR test: pattern practically identical to warfarin sodium-EPCRS.
Thermal anal (by DSC): no solvent release endotherm observed (in contrast to
warfarin sodium 2-propanol clathrate).
Both 'H-NMR and DSC data strongly suggested that warfarin potassium does not
form clathrates with 2-propanol.
EXAMPLE 5: Preparation of Warfarin Lithium 2-Propanol Clathrate
A solution of lithium 2-propoxide in 2-propanol was prepared by reacting
lithium
rods, (0.42 g, 0.06 at.-g) with excess 2-propanol (CP anhydrous, 10 ml). This
solution was
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added dropwise to a stirred suspension of pure warfarin acid (15.4 g, 0.05
mole) in 2-
propanol (60 ml), under a nitrogen atmosphere.
The clear solution was stirred at room temperature for two hours until a solid
partially precipitated.
The suspension was further cooled to about 0°C for 4 hours, the
white solid
precipitate was carefully filtered under nitrogen pressure, washed with cold 2-
propanol and
dried in a vacuum oven (40-50°C, 10 mm Hg) for 8-12 hours to give the
pure warfarin
lithium 2-propanol clathrate as a white, crystalline powder.
Yield: 15 g (80.2%)
Identification (by IR, vs. warfarin acid standard): complies
1H-NMR test: pattern practically identical to an authentic warfarin sodimn
clathrate
sample, but the integral of the protons relative to the respective methyl
groups suggested an
equimolar content of IPA in clathrate.
IPA content (by GC): 15.1
Thermal analysis (by DSC): typical endotherm, similar to those observed with
an
authentic warfarin sodium 2-propanol clathrate sample appeared around
160°C, suggesting
the existence of a clathrate.
EXAMPLE 6: Preparation of Warfarin Sodium 2-Propanol Clathrate
A suitable glass vessel provided with a distillation column and fraction
separator was
charged with pure warfarin acid (1000 g, 3.25 mole), sodium carbonate (CP, 260
g, 2.45
mole) and 2-propanol (technical grade, 5000 ml).
The mixture was heated to reflux for two hours and then cooled to room
temperature.
Demineralized water (S00 ml) was added and the reaction mixture stirred for %2
hour
at room temperature, cooled to below 10°C and rapidly filtered by
suction.
The clear, uncolored solution was returned to the vessel, hexane (1500 ml) was
added
and the mixture heated to boiling with stirnng.
A ternary azeotropic mixture began to distill and water was gradually removed
from
the mixture. Hexane was then removed by a binary azeotropic distillation and
warfarin
sodium 2-propanol clathrate precipitated.
The solution was filtered with suction, the solids washed with cold 2-
propanol, and
dried for about 8 hours (vacuum oven, 45-50°C, 10 mm Hg) to yield the
pure product as a
white crystalline solid.
Yield: 1030 g (88%)
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~H of 1 % solution: 8.25
Water (by KF): less than 0.1
IPA content (by GC): 8.15%
Clarity of alkaline solution: complies
Color (by UV): < 0.1 AU
Related substances (TLC): not detected
The product complied with the pharmacopeial requirements.
EXAMPLE 7: Preparation of Warfarin Sodium 2-Propanol Clathrate
A suitable glass vessel provided with a distillation column was successively
charged
with 2-propanol (technical grade, 7.7 L), pure warfarin acid, (1.54 kg, 5
mole), sodium
carbonate (0.34 kg , 3.2 mole) and demineralized water (0.22 L).
The~mixture was refluxed for two hours with stirring.
The heating was then stopped and the hot mixture rapidly filtered. The clear
warm
filtrate was returned to the vessel, the solution heated while stirring, and
the water slowly
removed by a binary azeotropic distillation, with a high reflux ratio. The
solvent removed by
distillation was replaced by adding fresh 2-propanol and the suspension
stirred at reflux for
another hour or two.
Heating was stopped and the mixture gradually cooled to below 10°C. The
solid was
filtered, washed with cold solvent and dried in a vacuum oven (45-50°C,
10 mm Hg) for 6
10 hours, to give pure warfarin sodium 2-propanol clathrate as a white,
crystalline powder.
Yield: 1.5 kg (83%)
~H of 1 % solution: 8.3
Clarity of alkaline solution: complies
Color (by UV): < 0.1 AU
Water (by KF): not more than 0.1
IPA content (by GC): 8.2%
Related substances (TLC): not detected
Analysis: Complied with pharmacopeial requirements.
EXAMPLE 8: Preparation of Warfarin Sodium 2-Propanol Clathrate
A suitable glass flask provided with a distillation column was charged with 2-
propanol (8 L, technical grade or recovered) containing about 1 %-2% water
(v/v), sodium
carbonate (CP, 175 g, 1.65 mole) and pure warfarin acid (1 kg, 3.25 mole).
The mixture was heated with stirring and refluxed for two hours.
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The unclear solution was left to cool to around 30°C and rapidly
filtered by suction.
The almost uncolored filtrate was returned to the flask, heating restarted,
and
azeotropic distillation of 2-propanol/water initiated. This distillation was
continued until the
volume in the flask reached about 1/3 of the initial volume and a thick
suspension obtained.
The suspension was cooled to below 10°C, and worked up as previously
described
(examples 4-5) to give the desired pure warfarin sodium 2-propanol clathrate,
as a white
crystalline solid.
Yield: 995 g (85%)
~H of 1 % solution: 7.9-8.0
Clarity of the alkaline solution: complies
Color (by UV): less than 0.1 AU
Water (by KF): 0.1%
Related substances (TLC): not detected
Analysis: Complied with the pharmacopeial requirements.
The embodiments illustrated and discussed in this specification are intended
only to
teach those skilled in the art the best way known to the inventors to make and
use the
invention. Nothing in this specification should be considered as limiting the
scope of the
present invention. All examples presented are representative and non-limiting.
The above-
described embodiments of the invention may be modified or varied, and elements
added or
omitted, without departing from the invention, as appreciated by those skilled
in the art in
light of the above teachings. It is therefore to be understood that, within
the scope of the
claims and their equivalents, the invention may be practiced otherwise than as
specifically
described.
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