Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NOVEL PROCESS
The present invention relates to novel process for the preparation of
prostaglandin
compounds which are useful as medicaments.
s
Prostaglandins (PGs) are a family of 20-carbon fatty acids found in virtually
all mammalian
cells and are biosynthesised from C-20 polyunsaturated fatty acids via
cyclooxygenase
enzyme system (S. Bergstroem., Science, 157, 382, 1967). For several decades,
PGs have
been the focus of extensive efforts in synthetic chemistry. Because of the
potential
~o therapeutic advantages, increasing attention is being focused on PG
analogs. The non-
availability of suitable natural source coupled with their potential drug
utility has led to the
clinical development of a number of synthetic PG analogs. Among them,
particularly
interesting, both pharmacologically and clinically, are analogs incorporating
methyl groups
into the prostaglandin skeleton at C-1.5 (a). E. Yankee et al., J. Amer. Chem.
Soc., 96,
~s 5865, 1974) b) E. Yankee et al., J. Amer. Chem. Soc., 94, 3651, 1972).
c). E. Yankee et al., J. Amer. Chem. Soc., 96, 5875, 1964) and references
cited
therein). The most rapid mode of metabolism (deactivation) of the natural PGs
in man bas
been shown to be the oxidation of the allylic C-15 alcohol, followed by very
rapid
reduction of the 13-14 double bond. The enzyme responsible for the oxidation,
15-hydroxy
2o prostaglandin dehydrogenase has been isolated from a variety of tissue
preparations (B.
Samuellson et al., Advanc. Biosci., 9, 7, 1973).
Of particular interest is the 15-methyl PGF2a (carboprost, Upjohn). Carboprost
is
clinically listed for treatment of post partum haemorrage as an alternative to
a methylergometrine or where the latter has produced inadequate response.
Intramuscular
injection of 250 mg of carboprost in sterile aqueous solution is the best
therapy now
available, very successful in cases ready for uterectomy due to bleeding not
stoppcd by
methergin etc. This has been registered for this indication in Sweden and
India (available
under the brand names Prostinefem in Sweden and Prosodin in India).
The development of a total synthetic route for carboprost is highly desirable
because of its
promising clinical potential. A large number of strategies towards the
synthesis of PGs
have been reported (E.J. Corey et al., J. Amer. Chem. Soc., 9(1, 3245 and
3247, 1968).
However, known synthetic procedures often involve a multistep linear synthesis
approach
3s leading to high cost and a low overall yield of the final product. Noyori
et al reported a
convergent three-component coupling process, wherein the entire carbon
framework is
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2
assembled stereoselectively by tandem alkylation of an appropriate optically
active enone
(R. Noyori et al., Angew. Chem. Int. Edn.(Eng.)., 23, 847, 1984 and references
cited
therein). The prostaglandin PGEz has also been synthesised using solid phase
chemistry (S
Chen, IBC International Conference on Combinatorial synthesis of natural
products,
December 1997).
The most recent approach which is frequently used for the syntheses of PGs
involves the
conjugate addition of an organometallics to an a-substituted 4-hydroxy 2-
cyclopentenone
(M.P.L. Caton in "New Synthetic Routes to Prostaglandins "., Academic Press
N.Y., pg
~0 105, 1982 and F. Sato et al." J. Org Chem., 59, 6153, 1994). The two-
component process
consists of two independent but complementary routes: introduction of a o-side
chain to an
endo-enone bearing an a-side chain and introduction of a-chain to an exo-enone
bearing an
o-side chain.
~s The present invention involves the use of enantiomerically pure endo-enone
(II) bearing an
a-sidechain and the desired o-chain was introduced via conjugate addition of a
higher order
cuprate generated using an enantriomerically pure ~-chain iodide (VI). The
delineated
process describes the synthesis of endo-enone (II) in an excellent optical
purity (> 99°lo ee),
following the alkylation chemistry of the stabilised carbanion and its
resolution using the
zo technique of ultrasound mediated enzymatic irreversible
transesterification. Herein, we
have described a process for the preparation of enantiomerically pure (i-chain
alcohol (>
99% ee) and its conversion to component B of very high optical purity. The
reported
procedures for the two component coupling process involve the conjugate
addition of a
higher order cuprate to the optically pure enone and are often encountered
with an
zs incomplete addition. This involves the recovery of the unreacted starting
enone by
exhaustive column chromatographic separation. In the present process we have
demonstrated the use of Lewis acids such as BF3-etherate to activate the enone
at
temperature such as -78°C, followed by the conjugate addition of the
higher order cuprate,
resulting in the complete consumption of the enone. This procedure circumvents
the
so chromatographic purification of the coupled product and avoids the loss
during
chromatographic purification. Further post-coupling operations and the
purification of the
product in the last step affords carboprost methyl ester of USP
specifications.
In a first aspect the invention therefore provides a process for the
preparation of a
ss compound of formula (I):
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3
,,~~ ~OMe
O
HO ~~, / Me
~OH
in which R is CH=CH or CH2CH2, and R~ and R2 are both hydrogen or together
form a
bond, which comprises coupling a compound of formula (II];
s
O
R (CH2~-C02Me
PO
in which R is as defined in formula (I) and P is a protecting group, with a
compound of
io formula (III):
M / CSH~~
1
OP (~)
is in which Pl is a protecting group and M is a cuprate to give a compound of
formula (IA):
,.~~-R-(CHZ)3 C02Me
PO'~', / , CsH~i
OP'
in which P and Pl are as defined above, and there after in any order:
~ optionally reducing the compound (IA)
~ . removing any protecting groups.
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Suitably R is CH=CH or CH2CH2, preferably R is CH=CH.
Suitably Rl and R2 are both hydrogen or together form a bond such that the
groups ORS
and R2 form a carbonyl group. Preferably RI and R2 are both hydrogen.
The groups P and Pl, which can be the same or different, can be any suitable
oxygen
protecting groups, for example tetrahydropyran and, in particular, silyl
protecting groups
such as t-butyldiphenylsilyl, dimethylphenylsilyl, triethylsilyl, t-
butyldimethylsilyl
(TBDMS), trimethylsilyl (TMS) and triisopropylsilyl . Preferably P is TBDMS
and P~ is
~o TMS.
The reaction of compounds of formula (II) and (III) is carried out in a
suitable solvent such
as THF-hexane, THF-ether, preferably THF/ether. Preferably the reaction is
carried out at
reduced temperature, for example at about -78°C. Preferably the
reaction is carried out in
is the presence of a Lewis acid, particularly in the presence of BF3.OEt2. It
has been found
that the use of BF3.OEt2 not only activates the enone at low temperatures and
drives the
reaction to completion, but also circumvents the problem of dehydration at the
C-15
position. The higher order cuprate of formula (III) is preferably generated
using anorgano-
lithium reagent, preferably butyl lithium.
Reduction of a compound of formula (IA) can be carried out using known
reducing agents.
For example reduction of the cyclopentanone is preferably carried out using a
selective
reducing agent such as K-selectride to give the desired cyclopentanol isomer.
Reduction of
the triple bond can be carried out using conventional techniques such as
Lindlar
2s hydrogenation.
Removal of protecting groups can be carried out using conventional procedures.
For
example when protecting groups P and Pt are both silyl groups deprotection of
both groups
can be achieved using a fluoride reagent such as TBAF in a suitable solvent
such as 'TIC.
3o Preferably the cyclopentanone moiety is reduced, followed by deprotection
of the groups P
and Pl followed by reduction of the triple bond to furnish the desired
compounds of
formula (1).
Compounds of formula (II) can be prepared from compounds of formula (IV):
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O
L
PO (N)
in which P is as defined in formula (II) and L is a leaving group with a
compound of
formula (V}:
Hal-CH2 (CH2)3 C02Me (V)
in which Hal is halogen in the presence of an organolithium reagent.
Preferably L is a
group such as a -SaPh to stabilise the carbanion generated and Hal is bromo or
iodo,
io preferably iodo.
Compounds of formula (IV} in which L is SePh can be prepared from the
corresponding
enone and phenylselenyl chloride using literature procedures. The enone can be
prepared
by protecting the corresponding alcohol, for example by treating with TBDMS
chloride
is conventional conditions.
Compounds of formula (V) can be prepared by halogenation of the con~esponding
alcohol
which in turn is prepared using known procedures as exemplified herein.
zo The compound of formula (~ is a suitable cuprate, in particular a higher
order cuprate and
preferably a thienyl cuprate, in particular dilithium [3-methyl, 3-
(trimethylsilyl~xy octyl}-
2-thienyl cyanocuprate of formula (BIA):.
~N
Cu ~ CsH» Li2
S
OP
2s
which is prepared from the corresponding halide of formula (Vn:
Hal / CSH»
.:
OP'
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in which Hal is halogen, in particular iodo, and PI is a protecting group as
defined in
formula (III), preferably aTMS group. This compound can be prepared according
to the
procedures exemplified herein.
Novel intermediates form a further aspect of the invention.
The invention is illustrated by the following Examples.
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Intermediate 1
SYNTHESIS OF METHYL 7-IODO HEPTA-5-YONATE
a) Methyl-7-hydroxy hept-5-ynoate.
s
The title compound was prepared according to the method of R.J.K. Taylor et
al.,
Tetrahedron, 42, 5849-56 (1986) as an oil, by 130 - 132°C at O.Smm
Hg.
1H NMR (CDCI3): 8 4.I5 (m, 2H), 3.6 (s, 3H), 3.0 (bs, IH, exchanges with D20),
2.35 (t,
2H), 2.25 (t, 2H) and I.75 (q, 2H).
~o
b) Methyl-7-iodo hepta-5-yonate:
The title compound was prepared from the above a-alcohol according to the
method of
Carl Johnson et al., JACS,110, 4726-35 (1988) as an oil, by 110 - 113°C
at 0.9 mm Hg.
~s
1H NMR (CDCI3~: b 3.6 - 3.7 (bs,SH), 2.4 (t, 2H), 2.25 (t, 2H), 1.75 (q, 2H).
Intermediate 2.
SYNTHESLS OF RAGEMIC 4-HYDROXY CYCLOPENTENONE.
The title compound was prepared according to the method of M. Minai, Jap.
Patent., Kokai
No. 62236,1982.
1H NMR (CDCI3); 8 7.55 (dd, J = 6Hz, I.SHz, 1H), 6.2 (d, J = 6 Hz, 1H), 5.05
(m, IH),
2s 2.75 (dd, J = 17 Hz, 6 Hz, 1H), 2.25 (dd, J = 17 Hz, 6 Hz, 1H) and 2.0 (bs,
1H, exchanges
with D20).
Intermediate 3
Synthesis of 1-lodo(S)-methyl-3-(trimethylsilyloxy)-oct-2-ene.
a) Racemic 3-methyl-octa-1-yn-3-of
A 3-necked 3litre r.b. flask was cooled under a stream of nitrogen gas and was
fitted with
mechanical stirrer through the centre neck. Side arm was connected to a socket
with gas
3s inlet and a cold, finger condenser. Another side arm was connected to a
nitrogen inlet
under static pressure. The cold finger condenser was charged with dry ice-
acetone mixture
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(-78°C) and the reaction flask was kept in an insulated bath. Anhydrous
ammonia gas
(dried through a wash bottle containing KOH pellets) was bubbled at a steady
rate through
the gas inlet in an atmosphere of nitrogen, whereupon ammonia condenses into
the reaction
flask. It took around 2 hr. to condense ammonia gas to obtain around 1.6 Litre
of liquid
s ammonia. At this stage the condensation of ammonia was stopped and in the
place of
ammonia bubbler was connected the nitrogen inlet. The other side arm was
stoppered.
Sodium metal was cut into small pieces and washed with anhydrous hexane. After
the
addition of the first piece while stirring mechanically, the colour of the
reaction mixture
turned blue as the metal dissolved. As soon as the blue colour persists,
ferric nitrate was
io added through the side arm, whereupon the reaction becomes exothermic and
upon stirring
the blue colour discharges to afford the greyish white sodium amide. The
addition of
sodium metal was continued. Upon addition of sodium metal, the appearance of
blue
colour was observed and as the stirring is continued, the greyish white
coloration is
observed. After the complete addition of the metal (it took 1 hr. for the
completion of
is addition of sodium metal), the reaction mixture was stirred for about 0.5
hr to ensure the
complete formation of sodium amide. During the whole process a static pressure
of
nitrogen gas and the temp. of -78°C in the cold finger condenser was
maintained. Through
one side arm, a gas inlet tube was inserted and bubbling of acetylene gas was
continued at a
steady rate (acetylene was bubbled through a paraffin trap and an empty trap
kept at -78'C)
20 over a period of 5 hr. to ensure the complete formation of sodium
acetylide. Acetylene
bubbling was stopped and a pressure equalising dropping funnel was fitted to
one side arm
of the reaction flask and was charged with 2-heptanone (287g, 2.511 mol) in
anhydrous
ether (75 ml). The contents were then added dropwise while stirring over a
period of 1 hr.
(exothermic reaction!). The dropping funnel was rinsed with ether {40 ml) and
the
is washings were added to the reaction flask. The contents in the reaction
flask were stirred
vigorously and the bubbling of acetylene gas was continued for another 3 hr.
and allowed
to stand overnight without stirring. The temp. in the cold finger condenser
was raised to
ambient temp. slowly the facile evaporation of ammonia gas.
3o Workup: The mechanical stirrer and the cold finger condenser was dismantled
and the 3-
necked flask was cooled to 0°C under nitrogen using an ice-bath and to
this was added
NH4CI solution in water in small lots of 50 ml each. The reaction mixture was
stirred at
0°C for a further 0.5 hr. and warmed to RT and stirred at RT for 2 hr.
The resultant
solution was filtered over a celite bed using a G-3 sintered funnel. The
residue was washed
ss with pet ether (2 x 150 ml). The contents were transferred to a separatory
funnel and the
aqueous layer was separated. Aqueous layer was extracted with pet ether (3 x
500 ml).
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Combined PE portion was washed with water (2 x 500 ml), brine ( 100 ml) and
dried over
anhydrous Na2SO4. Removal of volatiles in vacuo afforded 352 gm of crude ~i-
chain
alcohol. This was further purified by distillation in vacuum (bp 76-
78°C at 17mm of Hg)
to get 330 gm of (3-chain alcohol (2.360 Mols, 94% yield). Initial forerun in
the distillation
s furnished unreacted 2-heptanone (bp. 67-69°C at l7mm of Hg). This was
recycled with
next batch for the preparation of racemic octynol.
Yield of racemic (3-chain alcohol: 330 gm, 2.360 mols, 94%. Purity of > 99%
was
ascertained by GC analysis: column: OV-101, 90pC, RT for octynol: 5 min: for
heptanone:
io 2.5 min.).
b). 3-Methyl-3-carboxy-oct-1-yn-3-of hydrogen phthalate.
i). Recrystallisation of phthalic anhydride - representation procedure.
is
A precooled 5 litre r.b. flask with a magnetic stirring bar was charged with
570 gm of
phthalic anhydride and 2.3 litre of chloroform. The contents were refluxed on
a water bath
for 0.5 hr and filtered hot on a sintered funnel. Undissolved residue is
phthalic acid. The
filterate was left in the refrigerator in a tightly stoppered flask to get the
crystalline phthalic
2o anhydride. This was filtered, washed with hexane and dried in a desiccator.
mp 130-
131°C.
ii) 3-Methyl-3-carboxy-oct-1-yn-3-of hydrogen phthalate.
is A pre-cooled 5 litre r.b. flask with a magnetic stirring bar was charged
with the racemic
octynol (0.92kg, 6.57 mol), phthalic anhydride (0,979kg, 6.606 mol), DMAP
(O.O8kg,
0.658mo1) and triethylamine (0.67kg, 6.617mo1). The reaction mixture was
refluxed at
80°C for 6 hr and allowed to cool to RT and stirred overnight. The
progress of the reaction
was monitored by TLC (solvent system: 20% EtOAc in PE) for the disappearance
of the
so starting octynol. The reaction mixture was left overnight at RT.
Worlcup: A 20 Litre r.b. flask with a flange and a mechanical stirring
assembly was
charged with 7.4 Litre of water and 0.731 litre of cone. HCI and cooled to OpC
using an ice
bath. To this mixture was added the hemiphthalate reaction mixture while
stirring,
3s followed by the addition of chloroform (2.3 litre). The stirring was
continued for 1 hr. The
organic layer was separated and the aqueous layer was extracted with
chloroform (3 x 3
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litre). Combined organic layer was washed with water (2 x 500 ml) and brine
(500 ml).
This was dried over anhydrous Na2S04 and volatiles were removed using a rotary
evaporator and dried in high vacuum.
s In another 20 Litre r.b. flask with a flange and a mechanical stirring
assembly was taken
petr. ether S.5 Litre) and cooled to 0°C. Hemiphthalate from the
earlier step was poured
into PE while stirring and stirred for 1 hr., whereupon the desired
hemipthalate ester
crystallises. This was filtered using a buchner funnel, residue washed with
cold PE (500
ml) and dried in suction. This residue was air dried (m p 61-62°C,
0.915 Kg,). The mother
io liquor from filitration was concentrated in vacuo using a rotary evaporator
and kept in the
refrigerator to get an additional lot of hemiphthalate ester (2"d crop, 0.43
Kg, m p 61-
62°C). However, by repeating the sequence for the third crop, the
compound obtained
(0.1 S 1 Kg) showed higher m p and hence was kept aside and taken up for the
recovery of
racemic octynol.
a Yield of hemiphthalate ester: 1.348 Kg, 71 %.
c). Brucine phthalate of 3-Methyl-oct-1-yn-3-ol.
A pre-cooled 20 litre flanged r.b. flask with a mechanical stirnng assembly
was charged
with the hemiphthalate ester of racemic octanol (0.94kg, 3.262 mol), brucine (
1.432kg,
3.327 mol) and anhydrous acetone (3.751). The reaction mixture was heated at
SOPC for 1
2o hr and allowed to cool to RT and stirred overnight at RT.
Wockup: The precipitated solid was filtered using a buchner funnel and washed
with cold
acetone (2 x 250 ml) to get rid of the contamination of undesired
diastereomer. The
resulting solid was air dried.
Yield: 0.739 Kg, 1.028 mols, 31.5°!0. m p 158-159°C, [a]D = -
12.0° t 0.5(c 0.88, EtOH).
zs The mother liquor was kept aside for the recovery of brucine.
d). Hemiphthalate ester of (S)-octynol from brucine salt: (S)- 3-Methyl-3-
carboxy-oct-
1-yn-3-of hydrogen phthalate.
3o A pre-cooled 20 litre flanged r.b. flask with a mechanical stirring
assembly was charged
with brucine salt (0.827kg, 1.15 mol) and ether (8.21). To this was added
conc. HCI (0.83
1) in small lots at 0°C while stirring. The reaction mixture was
stirred at 0°C for 2 hr and
allowed to warm to RT.
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Workup: The contents were transferred to a separatory funnel and the ether
layer was
separated. Aqueous layer was extracted with ether (2 x 500 ml). Combined ether
layer
was washed with water (2 x 250 ml), brine (250 ml) and dried over anhydrous
Na2S04.
s Removal of volatiles in vacuo afforded the desired hemipthalate ester of (S)-
octynol as a
viscous material. The aqueous layer was discarded.
Yield of hemiphthalate ester of (S)-octynol: 0.332 Kg (near quantitative).
This was used as
such without purification for further reaction.
~o e). (S)-3-Methyl-oct-1-yn-3-ol.
A pre-cooled 20 litre flanged r.b. flask with a mechanical stirring assembly
and a reflux
condenser was charged with the hemiphthalate ester of (S)-octynol (0.332 kg,
1.151 mol)
and to this was added NaOH solution (0.504 kg in 3.61 water} at RT. The
reaction mixture
~s was refluxed on a water bath for 2 hr. The reaction mixture was cooled to
RT, whereupon
the octynol layer separates. The contents were transferred to a separatory
funnel and the
supernatent layer was separated. The aqueous layer was extracted with
petroleum ether (3
x 500 ml) and mixed with the octynol fraction. Combined organic portion was
washed
with distilled water (3 x 500 ml) till the pH is 7. The organic portion was
dried over
2o anhydrous Na2S04 and removal of volatiles furnished 155 gm (96%) of crude
(S)-octynol
which was further purified by distillation in vacuo. b p. 73-76°C at 14-
15 mm of Hg.
Yield: 128.87 gm, 0.921 mols, 80%, [a]D = -2.3° t 0.5(c 2.7, EtOH).
Reported [a]D =
2.33° (c 2..625, EtOH).
f7. (S)-3-Methyl-3-(trimethylsilyl~xy-oct-I-yne.
a
A pre-cooled 1 litre r.b. flask with a magnetic stirring bar was charged with
(S}-octynol
(29g, 0.207 mol) and DMF. The reaction mixture was cooled to 0°C (ice-
bath) under
nitrogen atmosphere. To the reaction mixture was added imidazole (39.44g, 0.58
mol) in
small portions (of - 5 gm) at 0°C and stirred to 15 min. To the
reaction mixture was
3o added TMS chloride ( 33.72g, 0.310 mol) dropwise by means of a syringe over
a period of
45 minutes and then stirred at 0°C for 1 hr. The reaction mixture was
allowed to warm to
RT and stirred at RT overnight. The progress of the reaction was monitored by
TLC
(solvent system: 10% EtOAc in petroleum ether) for the disappearance of the
starting
alcohol.
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Workup: The reaction mixture was poured into ice water (750 ml) and extracted
with
ether. The supernatent ether layer was separated and the aqueous layer was
extracted with
ether (3 x 250 ml). Combined organic portion was washed with water (2 x 100m1)
and
brine (50 ml). This was dried over anhydrous Na2S04 and volatiles were removed
using a
s rotary evaporator and dried in high vacuum. This was purified by
distillation under
reduced pressure to get the title product:
bp. 87-90°C at 12-15 mm of Hg.
Yield: 35.09 gm, 165.5 mmols, 80%.
~o g). (S)-3-Methyl-3-(trimethylsilyl)oxy-1-(n-tributylstannyl)-oct-1-ene (E).
A pre-cooled 500 ml r.b. flask with a magnetic stirring bar was charged with
TMS ether of
(S)-octynol 7.SOg, 35.37 mmol), TBTH ( 16.31 g, 35.4 mmol) and AIBN (0.4g,
2.47 mmol).
The reaction mixture was evacuated and flushed with nitrogen gas. The flask
was then
immersed in a preheated oil-bath at 130°C, whereupon a vigorous
initiation reaction took
is place with the evolution of hydrogen gas. (a suitable vent is desirable).
The reaction
mixture was heated in an oil bath at 150°C for 3 hr and cooled to RT
under nitrogen
atmosphere. To the reaction mixture was added petr. ether (150 ml) and allowed
to stir at
RT for 15 min. The reaction mixture was filtered over a celite bed using a G-3
sintered
funnel. The residue was washed with anhydrous PE (60 ml) and the combined PE
layer,
zo upon removal of volatiles afforded desired stannane derivative in
quantitative yield.
Note: This derivative was stored in a tightly stoppered flask covered with an
aluminium
foil, under nitrogen. This is highly moisture and light sensitive! Large scale
preparation
would involve mixing of reactants followed by the addition of AIBN, once the
operation
temperature is attained.
zs h) 1-Iodo (S)-3-methyl-3-(trimethylsilyoxy)-oct-2-ene.
A pre-cooled 500 ml r.b. flask with a magnetic stirring bar was charged with
stannane
derivative (61.06g, 0.121 mol) and to this was added freshly distilled THF (
170 ml). The
reaction mixture was cooled to -78°C using dry ice-acetone bath and to
this was added a
3o solution of N-iodo succinimide (27.31 g, 0.121 mol) in THF ( 100 ml) by
means of a
cannula at -78°C under nitrogen over a period of 45 min. The reaction
mixture was stirred
at -78°C for 1 hr. The temp. of the bath was gradually raised to RT and
stirred at RT for 15
min. The progress of the reaction was monitored by TLC (solvent system: Petr.
ether) for
the disappearance of starting material.
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Workup: The reaction mixture was poured over ice and filtered over celite bed
using a G-
3 sintered funnel. The filitreate was extracted with PE (4 x I00 mI). Combined
PE layer
was washed with water ( 100 ml), 10% sodium thiosulphate solution ( 150 ml)
and finally
s with water (SO mI): PE portion was dried over anhydrous Na2S04 and removal
of volatiles
in vacuo afforded desired iodide (0.072 Kg). This was purified by high vacuum
distillation
(bp.79-82°C at 0.1 mm of Hg, 0.040 Kg, 0.118 Mols, 97%.
[a]D = -24.0° ~ 0.5(c, 1.61: CHCI3): reported [a]D = -22.5° (c,
1.2: CHCI3).
~ o Intermediate 4
Preparation of 4-[(tert-butyldimethylsilyl)oxy]-2-cyclopenten-1-one.
Prepared according to the method of R. Noyori., et al., Tetr. Lett., 28, 4719-
20 (1987)
from 4-hydroxycyclopentanone (intermediate 2, 10.1 g, 0.103 mmol}, 4-DMAP (
1.21 g,
is 0.00993 mol) and triethylamine (11.46g, 0.113 mmol).
Yield: 24.03 gm (pale brownish in colour). The material had some colouring
impurities
which were removed via distillation under reduced pressure. (bp. 82-
86°C at lmm of Hg).
Yield of the desired product: 20.02 gm, 0.158 mots, 91.7%.
2o iH NMR (CDCI3); 8 7.4 (m, d, 1H), 6.1 (d, 1H), 4.9 {m, 1H) 2.6 (dd, 1H),
2.15 (d, 1H),
0.8 (s, 9H), 0.1 (s, 6H). The purity of the material was established by HPLC
using a
chiracel-OD column using 1.5% isopropan-2-Ol in n-hexane.
Intermediate 5
as a) Preparation-of racemic 4-[(t-butyldimethylsilyl)oxy)-2-(phenylseleno)-2-
cyclopenten-1-one
Prepared according to the method of T. Toru, et al., J.Org. Chem., 57, 4719-20
(1992)
from TBS-derivative 4-[(tert-butyldimethylsilyl~xy]-2-cyclopenten-1-one.
30 (23.84g, 0. I 12 mol), phenyl selenyl chloride (32.32g, 0.169 mol) and
pyridine ( 14.69g,
0.186 mol).
Yield 37.14g, 90%.
1H NMR (CDCI3): 8 7.3-7.7 (m, SH), 4.85 {m, 1H}, 2.8-2.9 (dd, 1H) 2.35 (d,
1H), 0.8 (s,
ss 9H) and 0 (s, 6H).
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14
b). Preparation of Methyl 7-(3-Hydroxy-5-oxo-1-cyclopentene-1-yl)-5-heptynoate
via
a two-component coupling process.
s Prepared according to the method of T. Toru, et al., J.Org. Chem., 57, 3145-
3152 (1992)
from selenyl derivative 4-[(tert-butyldimethylsilyl~xy]-2-(phenylseleno)-2-
cyclopenten-1-
one.
(16.658, 45.3 mmol), bis(tributylstannane (28.948, 49.91 mmol), n-BuLi (35.6
ml of 1.4M
in hexane, 49.91 mmol), iodine (25.358, 95.28 mmol) and HMPA (26m1) in THF.
~o
The product was purified by silica gel chromatography eluting with ethyl
acetate/petroleum
ether. Yield of the pure product: 11.11 gm, 31.76 mmols, 70°k.
1H NMR (CDCI3); b 7.3 (s, 1H), 4.9 (m, 1H), 3.65 (s, 3H), 3.0 (bm, 3H), 2.75
(dd, 1H)
is 2.45 (t, 2H), 2.25 (bt, 2H), 1.8 (q, 2H), 0.8 (s, 9H) and 0.1 (s.6H).
c). Methyl 7-(3-Hydroxy-5-oxo-1-cyclopentene-1-yl)-5-heptynoate
Prepared according to the method of E.J. Corey et al, JACS., 94, 6190-91
(1972) from the
zo TBS derivative (6.028, 17.22 mmol) , Methyl-7-[-3-tert
butyldimethylsilyl~xy]-5-oxo-1-
cyclopenten-1-yl]-kept-S-ynoate in a solution of AcOH: THF: water (3:1:1).
Yield 2.44 gm, 10.33 mmols, 60%. TLC: solvent system: 70% EtOAc in petroleum
ether.
1H NMR (CDCI3); $ 7.4 (bs, 1H), 5.0 (bs, 1H), 3.65 (s, 3H), 3.0 (bm, 3H), 2.85
(dd, 1H)
zs 2.45 (m, 2H), 2.25 (m, 2H), and 1.8 (q, 2H).
d) ENZYMATIC IRREVERSIBLE TRANSESTERIFICATION USING Lipase IN
VINYL ACETATE IN A SONICATOR BATH:
Prepared according to the methods of K.A. Babiak, et al., J. Org.Chem., 55,
3377-81,
so (1990) and G Lin, et al., Tetr.Lett., 36, 6067-68 (1995) from the hydroxy-
enone (4.78,
19.91 mmol, Methyl-7-[(3-hydroxy)-S-oxo-1-cyclopentene-1-yl]-hept-5-ynote and
PPLipase and/or HPLipase (crude, 7.Sg) in vinyl acetate with sonication.
Yield of desired acetate 2.408, 43%.
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WO 99/44990 PCT/SE99/00318
1H NMR (CDCI3); 8 7.4 (bs, 1H), 5.75 (m, 1H), 3.65 (s, 3H), 3.05 (bm, 3H),
2.85 (dd,
1H), 2.4 (t, 2H), 2.25 (m, 2H), 2.1 (s, 1H) and 1.8 (q, 2H).
e} DEACETYLATION USING GUANIDINE IN METHANOL
s Preparation of a stock solution of Guanidine in Methanol:
Prepared according to the method of K.A. Babiak, et al., J. Org.Chem., 55,
3377-81,
(1990) from guanidine carbonate (28.48, 0.158 mol), sodium metal (3.56g, 0.155
mol) in
methanol (0.3081).
io
Methyl-7-(((R)-3-hydroxy)-5-oxo-1-cyclopentene-1-yl]-kept-5-ynote.
Prepared according to the method of : K.A. Babiak, er al., J. Org.Chem., 55,
3377-81,
(1990) from the (R) acetate ( 1.905g, 6.85 mmol) and guanidine in methanol.
is
Note: it is recommended to carry out the deacetylation on a small scale before
performing
deacetylation on a rather big scale because of the sensitive nature of (3-
hydroxy ketone.
Yield: 1.130 gm, 4.788 mmols, 70% yield, (90 optically pure). This was used
for further
2o upgradation of optical purity using PPLipase and/or HPLipase as described
below.
Procedure: Same as the earlier enzymatic resolution using a sonicator bath.
Enrichment
was complete in 5 days. Desired product was isolated by flash chromatography
in 40-50%
EtOAc in PE eluate. Yield of (R)-Acetate: optical purity: > 99.9%. This was
further
2s deacetylated using guanidine in methanol to get 1.553 gm of optically pure
alcohol (5.586
mmols, > 99.9% optically pure by chiral HPLC analysis.
f) MITSUNOBU INVERSION TO CONVERT UNDESIRED ENANTIOMER TO
THE DESIRED ENATIOMER:
Prepared according to the methods of K.A. Babiak, et al., J. Org.Chem., 55,
3377-81,
(1990).
A precooled 250 ml. flask with a septum inlet and a magnetic stirring bar was
charged with
(S)-alcohol (2.S8g, 10.93 mmol of 88% optical purity), Ph3P {5.74g, 21.86
mmol) and
3s freshly distilled THF. The reaction mixture was cooled to 10°C and
to this was added
formic acid by means of a syringe, followed by the addition of
diisopropylazodicarboxylate
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16
(DIAD, 4.42g, 21.86 mmol) and slowly warmed to RT. The pale yellow reaction
mixture
was stirred at RT for 12 hr. in an atmosphere of nitrogen gas. The progress of
the reaction
was monitored by TLC (solvent system: 70% EtOAc in PE).
s Worlcup: The solvent was removed in vacuo under pressure using a rotary
evaporatorand
the resulting brownish residue was dissolved in ether (70 ml) and triturated
with n-hexane
( 16S ml) to precipitate phosphorous salts. The mixture was stirred at RT for
30 min. and
filtered over a G-3 sintered funnel, washed with ether (2 x SO ml). Combined
organic
portion was transferred to another r.b. flask and volatiles were removed in
vacuo using a
io rotavapor. The resulting residue was dissolved in MeOH (120 ml) and to this
was added
alumina (activated, neutral, 100 gm) and stirred at RT overnight. The progress
of the
reaction was monitored by TLC (solvent system: 70% EtOAc in PE). The reaction
mixture was filtered on a G-3 sintered funnel and the residual alumina was
repeatedly
washed with MeOH (3 x 10(? ml), combined filterate was subjected to flash
evaporation
is using a rotary evaporator to give 13.805 gm of the crude product. This was
further purified
by flash chromatography over a column of silicagel (400 gm, 200-400 mesh).
Initial
elution with a gradient of 1S-60% EtOAc in petroleum ether gave undesired 1,2-
diisopropyl dicarboxyhydrazine and the desired alcohol was obtained in 80-9S%
EtOAc in
PE. The HPLC analysis using Chiracel-OD column indicated clean inversion at
the chiral
2o centre (no noticeable racemisation) with an optical purity of 88% of the
(R) isomer.
This product was further subjected to enzymatic transesterfication using HPL
in a
sonicator to furnish desired (R)-acetate of >99% optical purity. Yield: 1.891
gm, 6.802
mmols, 70%. This was further deacetylated using guanidine in methanol, to get
the desired
as (R)-alcohol in > 99% optical purity. Yield: 1.166 gm, 4.94 mmols, 73%
yield.
g). Methyl-7-[(R)-3-tert-butyldimethylsilyl)oxy]-5-oxo-1-cyclopenten-1-yl]-
hept-5-
ynoate.
3o To a precooled 2S0 ml r.b. flask with a septum inlet and a magnetic
stirring bar was taken
TBDMS chloride ( 1.90g, 12.64 mmol) and dichloromethane. The solution was
cooled to
0°C using an ice-bath and to this was added imidazole ( 1.60g, 23.66
mmol) in one portion,
followed by the addition of DMF. The reaction mixture was stirred at
0°C for 1 S min. In
another pre-cooled flask was taken (R)-enone alcohol ( 1.99g, 8.4 mmol) in
anhydrous
3s CH2CIz (3 ml) and this was transferred to the reaction flask under nitrogen
by means of a
cannula. The washings ( 1 mI) were transferred to the reaction flask and
stirred at 0°C for 2
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I7
hr. The reaction mixture was allowed to warm to RT and stirred further
overnight (- 10
hr.). The progress of the reaction was monitored by TLC (solvent system:
petroleum.ether:EtOAc, 3:7).
s Workup: The reaction mixture was poured over ice-water and extracted with
dichloromethane (3 x 100 ml). Combined organic layer was washed with water (30
ml)
and with brine. Organic portion was dried over anhydrous Na2S04, removal of
voladles
in vacuo furnished 3.80 gm of oily material, which was further purified by
flash
chromatography on a column of silica gel ( 150 gm, 200-400 mesh) in 15-25%
EtOAc in
to pet. ether eluate. Yield of the desired product: 2.90 gm, 4.66 mmols, 70%.
TLC: solvent
system: 70% EtOAc in petr. ether.
1H NMR (CDCI3); 8 7.4 (bs, 1H), 5.0 (bs, 1H), 3.65 (s, 3H), 3.0 (bm, 3H), 2.85
(dd, 1H),
2.45 (m, 2H), 2.25 (m, 2H) and 1.8 (q, 2H). The purity of the material was
established by
is HPLC using a chiracel-OD column using 7% isopropan-2-of in n-hexane.
Example 1.
a). 2-ThienylUt6ium
Procedure: In a pre-cooled 100 ml r.b. flask with a septum inlet and a magnet
stirring bar
was taken freshly distilled THF (30 ml) and thiophene ( 1.59 ml, 20 mmols).
The reaction
mixture was cooled to -60°C (dry ice-CHCI3 bath) and to this was added
n-BuLi ( 16.7 ml,
1.2M in hexane) dropwise by means of a syringe over a period of 15 min. The
reaction
zs mixture was stirred at -60°C for 1 hr. to ensure the completion of
metallation. The colour
of thienyllithium in THF was pale yellow.
b) Preparation of vinyllithium:
so Procedure: A pre-cooled 100 ml r.b. flask with a septum inlet and a
magnetic stirring bar
was charged with B-chain iodide TMS ether (6.8g, 20 mmol) in 30 ml of THF
(freshly
distilled, anhydrous) and the contents cooled to -78°C {dry ice-acetone
bath) under
nitrogen. To this was added n-BuLi (21 mmol, 17.5 ml) dropwise by means of a
syringe at
-78°C over a period of 30 min. The reaction mixture became turbid with
pale yellow
ss precipitate of vinyllithium indicating the completion of reaction. The
contents were further
stirred at -78°C or 1 hr.
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18
c) TWO-COMPONENT COUPLING PROCESS:
Procedure: To a precooled 500 ml r.b. flask with a septum inlet was added
Cu(I)CN (20
s mmol, 1.79gm) and a magnetic stirring bar. The flask was capped with a
rubber septum
and heated with a heat gun under high vacuum to remove any traces of moisture,
allowed
to cool and purged with nitrogen. THF (30 ml) was added and the suspension
cooled to
-22°C (dry ice-CCI4 bath) under nitrogen. To this was added the
solution of preformed
2-thienyllithium (step a) dropwise by means of a cannula over a period of 10
min. The
~o contents were washed with THF (5 ml) and added to the flask, whereupon the
solution
became homogenous (pale yellow colour) to afford the desired lower order
cuprate. The
reaction mixture was further stirred at -22°C for 1 hr.
To the above solution of the lower order cuprate was added the solution of
vinyl lithium
~s (step b) dropwise by means of a cannula over a period of 10 min at -
22°C, washed with
THF (5 ml) and the solution stirred at -22°C for a further 1 hr. to
provide a homogenous
higher order cuprate (clear solution, pale yellowish orange colour). The
resulting cuprate
solution was cooled to -78°C (dry ice-acetone bath).
2o In a precooled 100 ml pear shaped flask was taken (R)-enone TBDMS ether (>
99%
optically pure, 3.5 gm, 10 mmols) and to this was added anhydrous ether (40
ml) by means
of a syringe. The contents were cooled to -78°C using a dry ice-acetone
bath for 10 min.
To this was added BF3:OEt2 ( 1.29 ml, 10.5 mmols) dropwise while stirring, by
means of a
syringe at -78°C and left at this temp. for 5 min. This solution was
then added dropwise
2s (very slow dropwise addition!) to the solution of the higher order cuprate
(obtained in step-
c) at -78°C under nitrogen by means of a cannula over a period of 45
min. The flask of
enone was washed with ether (Sml) and the washings transferred to the reaction
flask over
a period of 45 min. The stirring was continued at -78°C for 1.5 hr
(progress of the reaction
monitored by TLC, solvent system: pet. ether -10% EtOAc, for the disappearance
of the
3o starting enone). The reaction mixture was quenched with saturated aqueous
ammonium
chloride solution (20 ml) at -78°C and warmed to RT. The reaction
mixture was poured
into a mixture of 150 ml distilled water and 300 ml of ether. The aqueous
layer was
extracted with ether (3 x 100 ml) and the combined organic portion was washed
with brine,
dried over anhydrous Na2S04. Removal of volatilities in vacuo afforded an oil
( 11.35
ss gm). This product was used as such for the further sequence without
chromatographic
purification. TLC: solvent system: pet ether-10% EtOAc: A part of the sample
was
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WO 99/44990 PCT/SE99/00318
19
purified by flash chromatography (silicagel, 1:20 ratio, 200-400 mesh) and the
desired
product was obtained in 20% EtOAc in PE eluate for characterisation, Rg 0.31,
solvent
system: pet.ether-10% EtOAc:
IH NMR (CDCI3): b 0.05 (s, 6H); 0.8-0.9 (m, 12H); 1.1-1.6 (m, lOH); 1.7-1.8
(m, 3H);
s 2.0-2.3 (m, 6H); 2.4 (t, 2H, J = 7.4 Hz); 2.6-2.9 (m, 3H); 3.65 (s, 3H); 4.1
(m, 1H); 5.3-5.5
(m,2H).
Example 2.
(5,6-Didehydro-11-O-(tert-butyldimethylsilyl)-15-O-(trimethylsilyl-13-(S)-
methyl -
i o PGF~ methyl ester
Procedure: To a flame dried 250 ml r.b. flask with a septum inlet and a
magnetic stirring
bar, was added the crude TCC product [5,6-Didehydro-11-O-tent
butyldimethylsilyl)-15-O-
(trimethylsilyl)-13-(5~-methyl - PGE2 methyl ester]. (11.35 gm, from 10 mmol
of the
~s enone) in THF (anhydrous, 35 ml). The solution was cooled to -78°C
and treated with a
solution of K-selectride (32.85 ml, 30 mmol, 0.9M solution in THE dropwise by
means of
a syringe. The reaction mixture was allowed to stir at -78°C for 1.5
hr. and to this was
added dropwise a 30% aqueous solution of H202 (4.54 ml, 40 mmol). Stirring was
continued for another I S min, the cooling bath was removed and the reaction
mixture
Zo warmed to room temperature. Water (20 ml) was added and the organic phase
separated.
The aqueous phase was extracted with EtOAc (3 x 50 ml). The combined organic
layer
was washed with brine (2 x 20 ml) and dried over anhydrous Na2S04. Removal of
solvent
in vacue furnished 13.516 gm of the desired crude product. This was used as
such for
further desilylation. TLC solvent system: 20% EtOAc in PE, Rf 0.2.
is
A small portion of the selectride reduction product obtained in the earlier
step was purified
by flash chromatography using silicagel (60 gm, 200-400 mesh). The desired
product was
obtained in pet ether-30% EtOAc eluate: Rg 0.2 (solvent system pet.ether-20%
EtOAc). 1H
NMR (CDCI3): S 0.05 (s,6H); 0.8-0.9 (m, 12H); 1.2-1.6 (m, lOH); 1.7-1.9 (m,
3H); 2.I-2.3
30 (m, lOH); 2.4 (t, 2H J = 7.4 Hz); 3.65 (s, 3H); 3.9-4.0 (bs, 1H); 4.2-4.3
(bs, 1H); 5.3-5.5
(m, 2H).
Procedure: To a precooled 250 ml r.b. flask with a septum inlet and a magnetic
stirring
bar was added the crude selectride reduced product ( 13.52 gm) and THF
(anhydrous, 35
ss ml). The solution was cooled to 0°C and to this was added a solution
of tetra-n-butyl
ammonium flouride (30 mmols, 30 ml of 1M soln. In THF, 3 equiv.) dropwise by
means
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of a syringe over a period of 5 min. Stirring was continued for 0.5 hr. at OPC
and 5 hr. at
room temperature until the completion of reaction by TLC analysis (solvent
system:
EtOAc). THF was removed in vacao and water (20m1 ) was added, extracted with
EtOAc
(3 x 30m1). The combined EtOAC layer was washed with water (2 x 10 ml), brine
(2 x
s lOml) and dried over anhydrous Na2S04. Concentration in vacuo afforded 5.409
gm of the
crude product. This was further purified by flash chromatography over a column
of silica
gel (350 gm, 200-40(? mesh). The desired product was obtained in EtOAc eluate
as a pale
brown oiI ( 1.94 gm, 5.10 mmols), 51 % yield based on the starting (R) eonone:
Rf 0.5
(solvent system EtOAc). This was dissolved in anhydrous ethyl acetate (50 ml)
and to this
io was added 0.400 gm of the activated charcoal and stirred in an atmosphere
of nitrogen at
RT for 1 hr. This solution was filtered over celite bed using a G-4 sintered
funnel, washed
with EtOAc (30 ml): Removal of volatiles in vacuo afforded 1.95 gm of the
crude product,
which was purified by flash chromatography over a column of silica gel ( 150
gm, 200-400
mesh) and the desired product was obtained in ethyl acetate eluate as a
colourless viscous
~s oil, which solidified on storing at -22°C as a colourless solid
(chloroform-methanol
gradient system can also be used for the flash chromatographic purification).
Yield: 1.90
gm, 5 mmols 50%. 1H NMR (CDCI3): 8 0.85 (t, 3H, J = 6.8 Hz); 1.1-1.5 (m, 8H);
1.6-1.9
(m, 7H); 2.1-2.5 (m, lOH); 3.65 (s, 3H); 3.85-4.0 (bs, 1H); 4.3-4.4 (bs, 1H)
5.3-5.6 (m,
2H): 13C NMR (CDCI3): 8 173.64, 139.50, 127.92, 79.56, 79.48, 77.2, 72.28,
72.02,
20 54.53, 51.4, 48.84, 42.56, 42.0, 32.69, 32.0 26.51, 23.87, 23.68, 22.35,
17.98, 16,97 and
13.78.
Note For the desilylation reaction it is essential to use anhydrous TBAF in
THF. Use of
TBAF.3 H20 does not give the desired product in good yield.
Preparation of carboprost methyl ester, (15S)-15- Methyl PGFZa methyl ester or
IUPAC nomenclature as mentioned in USP as Prosta-5,13-diem-1-oic methyl ester,
9,11,15-trihydroxy-15-methyl-(5Z,9a,11a,13E,15S) or (Z)-7-[1R,2R,3R,SS)-3-5-
Dihydroxy-2-[(E)-(3S)-3-hydroxy-3-methyl-1-octenyl]cyclopentyl]-5-heptenoic
methyl
3o ester.
Selective Lindlar hydrogenation was performed in batches of 0.500 gm up to 1
gm scale.
The following procedure is representative.
3s A precooled 2-necked 1000 ml r.b. flask was charged with the desilylated
product ( 1.065
gm, 2.80 mmols) and to this was added a preformed mixture of benzene ( 100 ml)
and
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21
cyclohexane (300 ml) by means of a cannula under nitrogen. To the reaction
mixture was
added 0.315 gm of Pd over CaC03 poisoned with lead under nitrogen. The
reaction
mixture was evacuated in vacuo and flushed with hydrogen gas and through the
reaction
mixture was bubbled hydrogen gas while stirring at a rate of 45 bubbles per
minute over a
s period of 90 min: An aliquot was taken out at regular time intervals and the
progress of the
reaction was monitored by 13C NMR analysis (approx. 1.5 hrs, 13C NMR analysis
should
indicate the total disappearance of alkynyl carbon at 8 80 ppm). The reaction
mixture was
filtered using a G-4 sintered funnel, the residue washed with EtOAc. (The
catalyst
recovered could be recycled). Concentration of the organic phase in vacuo
furnished the
~o desired carbonprost methyl ester as a viscous colourless oil ( lg). This
was further purified
by chromatography over silica gel in EtOAc eluate (chloroform-methanol
gradient system
can also be used for the flash chromatographic purification). 1H NMR (CDCI3 :
8 0.85 (t,
3H, J ~ 6.8 Hz); 1.1-1.5 (m, 8H); 1.6-1.9 (m, 7H); 2.1-2.5 (m, lOH); 3.65 (s,
3H); 3.85-4.0
(bs, 1H); 4.3-4.4 (bs, 1H); 5.3-5.6 (m, 4H): 13C NMR (CDCI3): 8 173.79, 139.0,
129.26,
is 129.0, 128.5, 77.9, 72.6, 72.5, 55.6, 51.4, 50.4, 42.58, 33.16, 32.03,
27.23, 26.33, 25.29,
24.55, 23.62, 22.4 and 13.82.
This product was further purified by preparative reversed phase HPLC using a
preparative
ODS (C-18) SHIMPAK column (20 x 250 mm) and using a mobile phase of
acetonitrite-
2o water ( 1:1 ): UV detection: 210 nm. The preparative HPLC was done in
batches of 0.170
gm and the recovery of the pure carbonprost methyl ester of USP specifications
was --
65%.
