Note: Descriptions are shown in the official language in which they were submitted.
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THE PREPARAT10N OF ENANT10MERICALLY-ENRICHED
TFIREO-METHYLPHENIDATE
. Field of the Invention
This invention relates to processes for the preparation of enantiomerically-
enriched
' S threo-methylphenidate, and in particular to bioresolution, to the
separation of the
enantiomers of acid addition salt foams of threo-methylphenidate, and to the
enhancement
of enantiomeric excess (ee) of one enantiomer in a mixture.
Background of the Invention
Methylphenidate is a therapeutic agent that is widely used in the treatment of
attention-deficient hyperactivity disorder. It is a controlled substance.
Methylphenidate was first prepared as a mixture of the erythro and threo
racemates. US-A-29S7880 discloses studies upon the two racemic mixtures, which
revealed that the therapeutic activity resides in the threo disastereoisomer.
It is now
considered that it is the cf threo [or (R,R)] enantiomer that has the
preferred therapeutic
activity. Uses of this enantiomer are disclosed in WO-A-9703671, WO-A-9703672
and
WO-A-9703673, the contents of which are incorporated herein by reference.
The resolution of methylphenidate has been achieved using the expensive
resolving
agent 1,1'-binaphthjrl-2,2'-diyl hydrogen phosphate, a process reported by
Patrick et al,
The Journal of Pharmacology and Experimental Therapeutics, 241:152-15 8 (
1987).
Patrick et al also disclose a modest improvement in the enantiopurity of threo-
methylphenidate hydrochloride, by crystallisation) from 95-97% to 99% ee. It
is now
known that the product is contaminated with resolving agent and/or ritalinic
acid; see WO-
A-9727176.
WO-A-9727176 and WO-A-973285I disclose that the resolution of
methylphenidate has also been achieved, more economically, using either O,O'-
diaroyltartaric acids or menthoxyacetic acid. These resolutions provide d
threo-
methylphenidate in high ee and chemical purity, e.g. containing less than 2%
w/w of
resolving agent and/or ritalinic acid.
~ Generally speaking, the racemate and single enantiomers of a .salt of a
chiral
compound such as threo-methylphenidate have different soiid-state crystalline
forms.
Consequently, in solution, such a salt will have an enantiomeric composition
which
corresponds to its point of maximum solubility (the eutectic composition), and
this is
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dependent upon the solubility of the racemic salt and the single enantiomer
salt. The
solubility ratio a is given by the ratio of the solubility of the racemate
salt divided by the
solubility of the single enantiomer salt.
Summary of the Invention
One aspect of the present invention is based upon the discovery that certain
crystalline salts of threo-methylphenidate, wherein the counterion is achiral,
allow for the
enhancement of enantiomeric excess (ee) by recrystallisation/crystallisation
of partially
enriched material in a suitable solvent. In particular, it has surprisingly
been found that, in
the case of threo-methylphenidate, certain salts of the single enantiomer
showed much
lower solubility than the corresponding racemate in methanol/TBME (tert-butyl
methyl
ether).
A second aspect of the present invention is based on the discovery that (R,R)-
methylphenidate (1) can be conveniently obtained by means of biocatalytic
resolution
of a racemic compound of formula 2 (of which one enantomer is shown, for
convenience), using a range of hydrolase enzymes.
COZMe COyR
H H i H
Fh ~ ~ H (2)
Ph
Description of the Invention
According to the first aspect of this invention, and by approgr;ate use of
partially
enantiomerically-enriched salt, crystallisation can be used to give
essentially enantiopure
threo-methylphenidate. The starting material should be enantiomerically
enriched above
the eutectic point of the threo-methylphenidate salt. In the case of threo-
methylphenidate
hydrochloride salt, the eutectic point has been measured to be 25% ee by
solubility. That
is to say, threo-methylphenidate.HCl salt with a composition of enantiomers
greater than
25% ee will, by crystallisation, yield enriched material. Thus, by contrast
with the
crystallisation reported by Patrick e~ al, sr~pra, threo-methylphenidate salts
of significantly
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lower enantiomeric purity, in the range 25-95%, preferably 50-95%, and more
preferably
70-95%, can be usefully enriched by direct crystallisation. The process of the
invention
is therefore of considerable utility with a feedstock of threo-methylphenidate
of moderate
enantiomeric purity, for example following classical resolution. Any resolving
agent that
' S may be present can be removed, e.g. to a level of 2% w/w or below, e.g. no
more than 0.5
or i% w/w. Preferably, there is no chemical resolving agent, as in
bioresolution or
asymmetric synthesis. In the latter two cases at least, the feedstock is
uncontaminated by
resolving agent. For example, the novel crystallisation process can be used in
combination
with the novel biocatalytic resolution.
By way of example) the novel bioresolution encompasses the following
embodiments:
(i) where R - Me) enandoselective hydrolysis of unwanted (S,S)-
methylphenidate (1-threo) affords (S,S)-ritalinic acid, which is easily
separated from
unreacted (R,R)-methylphenidate by extraction into dilute aqueous alkali.
(ii) where R = Me (or another lower alkyl group), enantioselective
hydrolysis of (R,R)-methylphenidate (d-threo) is followed by isolation of
(R,R)-ritalinic
acid and chemical esterification. To maximise atom utilisation, recycling of
the
unreacted (S,S)-methylphenidate (I-threo) may be carried out according to the
procedure
described in WO-A-9728124.
(iii) where R = H, enantioselective esterification of (R,R}-ritalinic acid
affords (R,R)-methylphenidate directly.
(iv) where R = H, enantioselective esterification of (S,S)-ritalinic acid, is
followed by separation from the (R,R)-antipode, and chemical esterification of
the
latter.
Compared with classical resolution, the bioresolution process of the present
invention provides a number of benefits, including mild reaction conditions
(ambient
temperature, low environmental impact), cost savings by avoidance of
stoichiometric
resolving agents, and easier processing (e.g. simple solvent partitioning in
selected
" cases instead of salt cracking).
As will be apparent from Example 1, suitable biocatalysts can readily be
identified. It is preferred that the biocatalyst provides a sufficient degree
of optical
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enrichment that the desired product can be used effectively, e.g. at least 20%
,
preferably at least 40 % , and more preferably at least SO % , ee, up to
substantially single
enantiomer product, e.g. at least 80% or 90% ee.
The novel crystallisation process is also useful to enhance the ee of material
of high
ee, e.g. at least 95% ee, if that has been produced in chemically-pure form,
using a more
ei~cient resolving agent for this purpose than I,1'-binaphthyl-2,2'-diyl
hydrogen
phosphate. Such processes are described in WO-A-9727176 and WO-A-9732851.
The solvent that is used in the invention can readily be chosen by those of
ordinary
skill in the art. For example, the solvent should be sufficiently polar, e.g.
an alcohol,
optionally together with another solvent such as an ether. An aprotic solvent
such as
acetonitrile or acetone can also be used. A mixture of methanol and TBME is
preferred.
The salt used in the invention may have the formula
coznne
.HX
wherein HX is any achiral acid that forms a suitable salt. The suitability of
any salt for use
in the invention is readily tested in a crystallisation procedure by one of
ordinary skill in
the art. HX is preferably a hydrohalide, and X is more preferably Br or Cl.
For the purposes of comparison, racemic dl threo-methyiphenidate.HCl ( 1.0 g)
was suspended in 10 m1 of I: I methanol:TBME (7.4 g) and stirred at 25~C for
16 hours.
The solid material was collected by filtration, washing the reaction vessel
with 10 ml
TMBE. This gave 0.640 g of solid precipitate. The mother liquors were
evaporated to
dryness to give 0.340 g of a white solid. dl Ihreo-methylphenidate.HCl
therefore has a
solubility of 34 mg per ml of 1:1 MeOH:TBME at 25~C.
This experiment was repeated using 20% ee, 25% ee and single enantiomer d
threo-methylphenidate.HCl. These and other solubility results, obtained using
essentially
the same procedure, are reported in Table 1. In the Tables, MPH = threo-
methylphenidate, PPT = precipitate, and MLS = mother liquor.
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Table 1 shows solubility measurements for threo-methylphenidate hydrochloride
ofdifferent enantiomeric composition (racemate, orenriched in thedenantiomer),
and also
demonstrates one embodiment of the present invention, namely the ee
enhancement
achieved by dissolution of the materials in a 1:1 mixture of methanol:TBME at
ZS~C, and
5 separation of insoluble material. The point of
maximum solubility is at 25% ee, which
defines the eutectic composition and the solubiiity
ratio a = 34.0/ 17.0 = 2Ø Enhancement
of ee is slight when the initial ee is 30%, increasing31.9%, but a progressive
to
enhancement is observed when the initial ee is higher.
Table 1
ee MPH.HCI ee PPT ee MLS Solubility (mg/per
ml)
0% ' - - 34.0
20% 15.9% 25.0% 36.0
25% 19.9% 24.7% 40.0
30% 31.9% 22.6% 33:0
1 S 50% 54.4% 24.3% 27.7
60% 70.7% 25.1 % 26.5
73% 93.0% 46.0% 2S.5
99% - - l7.0
In another embodiment of the present invention, the enrichment procedure may
also be effected by simply treating a solution of ~hreo-methylphenidate free
base above the
eutectic point (>25% ee) with hydrogen chloride in methanol) and isolating the
resultant
precipitate. The results of a series of experiments are given in Table 2.
Table 2
ee MPH ee PPT ee MLS (% yield)
(% yield)
82% -1 threo 97.0% (65%) 36.9% (35%)
87% - I threo 98.0% (69%) 35.7% (23%)
88% - l threo 97.8% (78%) 32.1% (21%)
3 0 91 % - ct threo 99.2% (80%) 43.6% ( 19%)
94% - ct threo 99.7% (86%) 72.3% (8%)
95% - d-threo 99.0% (90%) 36.5% (6%)
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The following Examples illustrate the present invention more specifically.
Example 1
Suitable enzymes for the bioresolution were identified by the following
screening protocol:
100 mg of racemic threo-methylphenidate (free base) was dissolved in 100 mM
phosphate buffer adjusted to pH 7. Approximately 50 mg (or equivalent ml) of
each
candidate enzyme was added and the reactions incubated at 30~C for 24 hours
with
gentle agitation. For assaying purposes, 40 ~cI of the reaction mixture was
dispensed
into a vial and allowed to evaporate over KOH in a desiccator overnight. The
residue
was then dissolved in 1 ml IPA/2 ~ diethylamine solution and undissolved
material
removed by centrifugation. The enantiomeric composition of the unreacted threo-
methylphenidate remaining in solution was analysed by hplc method on the
Chirapak
AD column using 90: l0:0.2 heptane/IPA/diethylamine at 0.5 ml/min and ~. =227
nm.
Any ritalinic acid also present in the solution was found not to interfere
with the
detection of the enantiomeric methyl esters.
A representative result was obtained using a-chymotrypsin (Sigma). Its use
gave a ratio of enantiomers of unreacted substrate (SS:RR) of 79.5:20.5.
Example 2
al thrco-methylphenidate.HCl enriched in theal enantiomer (73% ee) (0.950 g)
was
suspended in 10 ml of 1:1 methanol:TBME (7.7 g) and stirred at 25~C for 16
hours. The
solid material was collected by filtration) washing the reaction vessel with
10 ml TMBE.
This gave 0.725 g of solid precipitate, with an enantiomeric excess of 93.0%.
The mother
liquors were evaporated to dryness to give 0.255 g of a white solid, with an
enantiomeric
excess of46.0%. ct threo-methylphenidate.HC1 (73% ee) therefore has a
solubility of25.5
mg per mi of 1:1 MeOH:TBME at 25~C.
Example 3
cf ~hreo-methylphenidate.HCl enriched in the ct enantiomer (50% ee) (1.00 g)
was
suspended in 10 ml of 1:1 methanol:TBME (7.9 g) and stirred at 25~C for 16
hours. The
solid material was collected by filtration, washing the reaction vessel with
10 ml TMBE.
This gave 0.710 g of solid precipitate, with an enantiomeric excess of 54.4%.
The mother
liquors were evaporated to dryness to give 0.277 g of a white solid, with an
enantiomeric
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excess of 24.3%. al ~hreo-methylphenidate.HCl (S0.0% ee) therefore has a
solubility of
27.7 mg per ml of 1:1 MeOH:TBME at 25~C.
xam le 4
d threo-methylphenidate. HCl enriched in the d-enantiomer (60% ee) ( 1.00 g)
was
suspended in 10 ml of 1:1 methanol:TBME (7.7 g) and stirred at 25~C for 16
hours. The
solid material was collected by filtration, washing the reaction vessel with
10 ml TMBE.
This gave 0.710 g of solid precipitate) with an enantiomeric excess of 70.7%.
The mother
liquors were evaporated to dryness to give 0.2b5 g of a white solid, with an
enantiomeric
excess of25.1%. al threo-methylphenidate.HCl (60% ee) therefore has a
solubility of26.5
mg per ml of 1:1 MeOH:TBME at 25~C.
Example 5
ci threo-methylphenidate.HCl enriched in the c~ enantiomer {30% ee) ( 1.00 g)
was
suspended in 10 ml of 1:1 methanol:TBME(7.7 g) and stirred at 25~C for 16
hours. The
solid material was collected by filtration, washing the reaction vessel with
10 ml TMBE.
This gave 0.655 g of solid precipitate, with an enantiomeric excess of 31.9%.
The mother
liquors were evaporated to dryness to give 0.330 g of a white solid, with an
enantiomeric
excess of 22.6%. ct threo-methylphenidate.HCl (30% ee) therefore has a
solubility of 33.0
mg per ml of 1:1 MeOH:TBME at 25~C.
Example 6
l-lhreo-methylphenidate enriched in the 1 enantiomer (88.3% ee) 15.0 g was
taken
up in 30 ml of methanol) and stirred at 40-50~C whilst bubbling hydrogen
chloride gas
through the reaction mixture for 10 minutes. The reaction mixture was then
heated at
reflux for 5 minutes. After this 30 m1 TMBE was added to the reaction mixture
which was
cooled over one hour to room temperature) and finally at 0~C for 1 hour. The
solid
material was collected by filtration) washing the reaction vessel with 30 m1
TMBE. This
gave 13.50 g (77.8%) of solid precipitate, with an enantiomeric excess of
97.8%. The
mother liquors were evaporated to dryness to give 3.60 g of a yellow/orange
solid
(20.7%), with an enantiomeric excess of 32.1%.
Example 7
d thrco-methylphenidate enriched in the ct enantiomer (9l.3% ee) 11.50 g was
taken up in 23 ml of methanol, and stirred at 40-50~C whilst bubbling hydrogen
chloride
gas through the reaction mixture for 10 minutes. The reaction mixture was then
heated at
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reflux for 5 minutes. After this 23 ml TMBE was added to the reaction mixture
which was
cooled over one hour to room temperature, and finally at 0~C for 1 hour. The
solid
material was collected by filtration, washing the reaction vessel with 23 ml
TMBE. This
gave 10.60 g (79.7%) of solid precipitate, with an enantiomeric excess of
99.2%. The
mother liquors were evaporated to dryness to give 2.50 g of a white solid
(18.8%), with
an enantiomeric excess of 43.6%.
In order to demonstrate the broader applicability of the invention, the
corresponding hydrobromide salt was prepared. Firstly, dl threo-
methylphenidate (0.86
g; 3.71 mmol) and ammonium bromide (0.436 g; 4.45 mmol) were taken up in 10 ml
methanol and stirred at 25~C for 10 minutes. The solvent was removed
undervacuum, this
process being repeated a further two times. The resulting white crystalline
material was
taken up in 25 ml dichloromethane, and filtered through Celite. This gave 0.97
g of a white
solid (83.3%). MP = 205.6~C. IR (KBr) v""x = 1730 cni'.
The resultant racemic d! lhreo-methylphenidate.HBr (0.500 g) was suspended in
5 ml of 1:1 methanol:TBME (3.80 g) and stirred at 25~C for 16 hours. The solid
material
was collected by filtration. This gave 0.355 g of solid precipitate. The
mother liquors were
evaporated to dryness to give 0.140 g of a white solid. dl threo-
methylphenidate.HBr
therefore has a solubility of 28 mg per ml of 1:1 MeOH:TBME at 25~C.
Secondly, c~ threo-methylphenidate (0.86 g; 3.71 mmol) and ammonium bromide
(0.436 g; 4.45 mmol) were taken up in 10 ml of methanol and stirred at 25~C
for 10
minutes. The solvent was removed under vacuum, this process being repeated a
further
two times. The resulting white crystalline material was taken up in 25 ml
dichloromethane)
and filtered through Celite. This gave 0.75 g of a white solid (64.4%). MP =
222.6~C. IR
(KBr) u""x = 1730 cm''.
The resultant single enantiomer c~ threo-methylphenidate.HBr ( 1.0 g) was
suspended in 5 ml of 1:1 methanol:TBME (4.00 g) and stirred at 25~C for 16
hours. The
solid material was collected by filtration. This gave 0.430 g of solid
precipitate. The
mother liquors were evaporated to dryness to give 0.070 g of a white solid. d
threo
methylphenidate.HBr therefore has a solubility of 14.0 mg per ml of 1:1
MeOH:TBME
at 25~C.
