TECHNICAL FIELD OF THE INVENTION
The present invention relates to a novel synthetic process for the preparation of a 1,4-
diphenylazetidinone.
BACKGROUND OF THE INVENTION
The azetidinone of the below formula, which is the subject of the present invention, is a
therapeutically useful compound. It is disclosed in the product patent RE3 7721, which is
are-issue ofUS5767115. Different processes for its synthesis are also described therein.
This compound is a biologically active molecule and research has shown it to have the
useful property of inhibiting the absorption of cholesterol from the intestine (J Med.
Chern. 1998, 41(6), 973). Since then this azetidinone - 1-(4-fluorophenyl)-3(R)-[3(S)-
hydroxy-3-( 4-fluorophenyl)propyl)]-4(S)-( 4-hydroxyphenyl)-2-azetidinone - has been
approved by the USFDA and other regulatory authorities worldwide for use in the
lowering of cholesterol in humans. It is at present marketed under the names of Zetia™,
EzetroFM, Ezemibe™, Zient™ et cetera.
Many processes for the preparation of this cholesterol absorption inhibitor have been
reported: most are total chemical synthesis and others have a few enzymatic steps. The
product patent itself discusses various synthetic strategies. In general, the difference in
the routes lies in the introduction of the 4-fluorophenyl group, which is at the end of the
aliphatic side-chain, with respect to the formation of the azetidinone ring. The
azetidinone ring is obtained by cyclization, which could be carried out after the three
substituted-phenyl groups have been incorporated into the molecule. However, in an
alternate method the same cyclization may be carried out first and then the 4-
fluorophenyl group at the end of the side chain or, sometimes, the whole side chain is
attached onto this ring. In the processes described in US6207822 and W02007/119106
the 4-fluorophenyl moiety is part of the molecule which is then cyclized to form the 4-
membered ring, whereas, US5886171 and US5856473 describe processes wherein the 4-
fluorophenyl group is introduced later.
Of these two strategies, it has been observed that within the former approach also there
are two different methods that revolve around the secondary hydroxyl group. In the
synthesis of this 1,4-diphenylazetidinone, the hydroxyl group is always brought in by the
reduction of the corresponding ketone. In certain processes the reduction of the ketone to
form the alcohol is done in the first step itself, while in others the reduction to alcohol is
preferred in the final stages. However, regardless of whether the reduction is carried out
initially or later in the synthesis, the moiety therein has to be protected in order to avoid
the formation of by-products that lead to lowering in the yield of the desired product.
When reduction is done at the beginning then the resulting alcohol is protected by a
1l suitable protecting group and if done later, then the pro-moiety, i.e. the ketone, is
protected suitably.
The process that forms the present invention too follows the former strategy wherein the
molecule comprising the three substituted phenyl groups is cyclized to create the
azetidinone ring. Further, the ketone moiety on the side chain is protected until formation
of the I3-Iactamring and then deprotected and reduced stereospecifically to yield the
desired product.
OBJECT OF THE INVENTION
Herein we report novel intermediates in the synthesis of this 1,4-diphenyl azetidinone.
In one aspect is presented the novel intermediate of formula (II).
The novel intermediate of formula (IV) forms the second aspect of this invention.
In the third aspect is presented the intermediate of formula (V).
The fourth aspect of this invention is a novel process for the synthesis of the 1,4-diphenyl
azetidinone of formula (VIII).
In a fifth aspect is provided a chemical process for the synthesis of 1,4-diphenyl
azetidinone of formula (VIII), using the above intermediates and thereby minimizing the
loss in yield.
SUMMARY OF THE INVENTION
The present process for the synthesis of 1-(4-fluorophenyl)-3(R)-[3(S)-hydroxy-3-(4-
fluorophenyl)propyl)]-4(S)-( 4-hydroxyphenyl)-2-azetidinone involves the condensation
of an imine of formula (III) with a ketone of formula (I), wherein the ketone functionality
is protected as an oxime of formula (II). Thus, the compounds of formula (III) and (II) are
reacted to give a novel compound of formula (IV).
The intermediate of formula (IV) is a ~-(substituted-amino) amide which is cyclized in
the following steps to form the azetidinone ring. The resulting compound is also a novel
intermediate of formula (V).
The steps that follow involve the deprotection of the ketone group, (i.e. removal of the
oxime functionality), reduction of the resulting ketone and deprotection of the phenolic
protecting group to obtain the desired final product.
This process has been developed for manufacture on a large scale and the parameters of
economy of synthesis, excellent yield and ease of operation can be ascribed to it. The full
details of the process are presented below.
DESCRIPTION
As mentioned hereinabove, the present synthesis follows the widely used strategy of
incorporating the three substituted-phenyl groups into the chiral compound of formula
(IV), which is then cyclized to form the azetidinone. The reaction scheme is as shown
below.
The synthesis begins with the compound of formula (I). In most instances this is the
preferred starting material and can be prepared by any suitable means of synthesis. Thus
the ketone (I), comprising the 4-phenyloxazolidinone chiral auxiliary, is treated with a
hydroxylamine or its O-alkyl derivative to protect the ketone as an oxime. The starting
material can be dissolved in any suitable solvent, preferably an alcohol, and is reacted
with a hydroxylamine or its derivative. The reaction is carried out in the presence of a
base, wherein an inorganic or an organic base may be used. Periodic sampling to
determine the amount of unreacted ketone could be done in order to determine reaction
completion. However the reaction was found to be complete after 8 - 12 hours.
If hydroxylamine is used to make the oxime, then the free hydroxyl group of the oxime
may be optionally protected. Any suitable hydroxyl protecting group could be used such
as an alkyl or silyl protecting group and this compound may be taken forward for further
reactions.
The oxime (II) can be carried forward for further steps. When the oxime was isolated it
was found to exist predominantly as the E-isomer. 92.6% E-isomer for the oxime formed
with O-methyl hydroxylamine. However, for purposes of this invention, as the oxime
only serves to protect the ketone functionality, the percentage of E/Z isomers is
immaterial to the present synthesis. Accordingly the separation of these isomers is not
envisaged and both isomers are taken forward for subsequent steps in the manufacturing
process.
The oxime is then reacted with the imine (III), wherein the phenolic group is protected by
any suitable protecting group known in the art, to form the J3-substituted amino amide
(IV). This reaction is carried out in the presence of TiCl4 and Titanium isopropoxide in
any suitable solvent. Preferred solvents are dichloromethane and methyl tertiary butyl
ether (MTBE). The product obtained IS then treated with N,Obis(
trimethylsilyl)acetamide (BSA) and tetrabutylammonium fluoride (TBAF) in a
solvent medium to form the p-Iactam ring. The solvent medium that is preferred for this
step is toluene, dichloromethane or MTBE. The product of this reaction step is the
azetidinone of formula (V). This compound is deprotected to bring back the ketone
functionality. The deprotection may be carried out under acidic conditions and if desired
the phenolic protecting group may also be removed at this stage.
In the next step, the ketone (VI) is reduced stereospecifically. Many such reductions are
known and have been reported for the synthesis of the azetidinone (VIII). One such
reduction is with chiral borane that is already disclosed in the product patent RE37721
and which the present inventors have used herein. The resulting alcohol (VII) is then
subjected to a final deprotection of the phenolic group if it had not already been done
before to yield the 1,4-diphenylazetidinone (VIII). This 1,4-diphenylazetidinone (VIII)
that is obtained can be purified by crystallization or any other purification technique.
Needless to say, a person skilled in the art would contemplate different oxime protections
for the ketone and different protecting groups for the phenolic -OR. This invention
includes all such variations and procedural modifications. The process of the invention is
further illustrated in the following examples. These examples are not to be construed to
be limiting in any way.
100g of (48)-3-[5-( 4-fluorophenyl)-5-oxopentanoyl)]-4-phenyl-l ,3-oxazolidin-2-one was
added to 450mL of denatured ethanol. To this solution was added 30.5g of the
hydrochloride salt of a-methyl hydroxylamine and 51.3mL of triethylamine. This
reaction mixture at room temperature was heated to 80 - 85°C and maintained at this
temperature for about 10 hrs. At this time the reaction was found to be complete and the
reaction mixture was allowed to cool to 45 - 50°C after which the solvent was distilled
off under vacuum. The solid mass that was left behind was dissolved in 500mL of
dichloromethane and washed twice with water (200mL). The organic solvent containing
the product was concentrated to an oil and 400mL of hexane was added to it. This
mixture was stirred for about an hour and then filtered to give the title compound as a
mixture ofE and Z isomers (92.6:7.3), (95g) m.p.: 73°C.
If desired the E and Z isomers may be separated from the mixture by column
chromatography (e.g. silica gel, (60-120mesh) with hexane-ethyl acetate 9:1 as the
eluant). Identification of the isomers was done by NMR.
IH NMR (DMSO-d6 200MHz)
07.85-7.55 (m, 2H), 7.50-7.10 (m, 7H), 5.45 (dd, J=3.01 and 5034Hz, IH), 4.72 (t, J=
8.66Hz, IH), 4.17(dd, J=10.63and3.22Hz, IH), 3.87(s, 3H), 2.91(t, J= 6.94Hz, 2H),
2.71 (t, J= 7.61Hz, 2H), 1.67 (quintet, J= 7037Hz, 2H).
MS = 385.22 (M+ 1)
IR(KBr): 1781,1765,1702,1512, 1396,1331cm-1
50g of (4S)-3-[5-( 4-fluorophenyl)-5-oxopentanoyl)]-4-phenyl-l ,3-oxazolidin-2-one was
added to 100mL of isopropanol. To this solution was added 19.4g of hydroxylamine
hydrochloride and 58.4g of anhydrous potassium carbonate. This reaction mixture at
room temperature was heated to reflux and stirred at this temperature for about 2 - 4 hrs.
At this time the reaction was found to be complete and the reaction mixture was allowed
to cool to 45 - 50°C after which the solvent was distilled off under vacuum. The solid
mass that was left behind was washed with water and crystallized using 250mL of
isopropanol.
60g of the oxime (product of Example 1) was dissolved in dichloromethane (l20mL). To
this solution was added TiCl4 (l8.0mL) and titanium isopropoxide (15.0mL) in
dichloromethane (780mL) in an atmosphere of nitrogen. The reaction mixture was stirred
for 30 - 40 min. at -5 to O°C, diisopropylethylamine (58.5mL) was added and stirring
was continued for 1 hr at the same temperature. This was then cooled further to about -
25°C and 4-benzyloxybenzylidine-(4-fluoro)aniline (l03.4g) was added to it. The
reaction mixture was stirred for another 5 - 6 hrs at the same temperature after which a
mixture of acetic acid (60mL) and dichloromethane (120mL) was added dropwise over
20 - 25min. This mixture was warmed to O°C and 2N H2S04 (300ml) was added
dropwise over 20 - 25min. The mixture was then warmed to 20 - 25°C and stirring was
continued for 1 hr, the layers were separated and the organic layer was washed with water
(300ml) and brine (20% soln., 300ml). The solvent was distilled off U.v. to leave an oil
which was crystallized from a mixture of ethyl acetate (240ml) and n-heptane (l200ml)
to give a solid product (70g). m.p.: 135 - 137°C.
lH NMR (DMSO-d6, 200MHz)
8 7.54-7.11 (m, 16H), 6.95-6.70 (m, 4H), 6.68-6.50(m, 2H), 5.97(d, IH), 5.60 (dd, J =
4.45 & 3.63, IH), 5.02 (s, 2H), 4.78 (t, J = 8.73, IH), 4.48-4.30 (m, 2H), 4.20-4.08 (m,
IH), 3.83 (s, 3H), 2.85-2.52 (m, 3H), 1.67 (bs, IH).
MS = 690.94 (M+1)
IR (KBr): 3388, 1755, 1697, 1608, 1509, 1386cm-1
25g of the oxime (product of Example 2) was dissolved in dichloromethane (375mL) and
the imine (III) (29g) was added to it. The solution was cooled to -10 to -5°C.
Diisopropylethyl amine (61.3ml) was added slowly to this solution followed by trimethyl
silyl chloride (28.5ml). The temperature of the reaction medium was maintained at -10
to -5°C and stirred till completion of the reaction. After completion of the reaction, the
reaction mass was cooled to -30 to -25°C and titanium tetrachloride (802ml) was added
slowly. The reaction mixture was stirred for 3 - 6 hrs at -30 to -25°C. After completion
of the reaction glacial acetic acid was added to it at -20 to -25°C. The reaction mixture
was quenched in 7% of tartaric acid solution (430mL) at ooe and the temperature was
raised to 25 - 30°C. The reaction mixture was stirred for 2 - 3 hrs at this temperature and
20% sodium bisulphate solution (125ml) was added to it. Stirring was continued for 2 - 3
hrs at room temperature and then the dichloromethane layer was separated and washed
with 125ml of water. Dichloromethane was distilled out under vacuum completely and
fresh dichloromethane was added. N,O-bis(trimethylsilyl)acetamide (BSA) 23.5ml was
added to the DCM solution, which was then refluxed for 1 hr. The DCM was recovered, a
mixture of ethyl acetate (25ml) and n-heptane (125ml) was added to the reaction mass
and stirred at room temperature for 1 - 2 hrs. The solid was filtered, washed with 25ml of
n-heptane and dried.
25g of the product of example 4 was added to 325ml methyl tertiary butyl ether. 30ml of
N,O-bis(trimethylsilyl)acetamide and TBAF solution in tetrahydrofuran (5 mole %) was
added to it and the reaction mixture was stirred at room temperature for 2 - 4 hrs. After
completion of the reaction, glacial acetic acid was added to it and the solvent was
distilled off under vacuum to get an oil. 2N sulphuric acid (25ml) and isopropyl alcohol
(250ml) were added to the above oil. The reaction mass was stirred at room temperature
for 1 - 3 hrs. The solid was filtered, washed with 10ml of IPA and dried.
23g of the product of example 3 was dissolved in toluene (345mL) and warmed to 50°C.
To this solution was added 46mL of N,O-bis(trimethylsilyl)acetamide (BSA). The
reaction mixture was stirred for 1 hr at 500e after which 3.35g of tetrabutylammonium
fluoride (1M in THF) was added. After 4 hrs the reaction mixture was cooled to 15 -
20oe, 25mL of MeOH was added to it followed by 46mL of IN Hel, the layers were
separated and the organic layer was washed with water (92mL) and brine (20%, 92mL).
The solvent was distilled off u.v. to leave an oil that was purified by flash
chromatography to afford a liquid product. (15g) b.p.: 105°C
IH NMR (200MHz) in CDCh
87.67 -7.60(m, 2H), 7.44-7.2~ (m, 10H), 7.l0-6.9l(m, 6H), 5.10 (s, 2H) 4.63 (d, J =
2.0, lH), 3.90 (s, 3H), 3.20 (t, J= 6.20, lH), 2.93 (t, J= 7.8, 2H), 2.14 (m, 2H)
Mass spectra 527.11(M+l)
IR (KBr): 2924, 1745, 1610, 1585, 1506cm-1
500g of the product obtained in example 6 was dissolved in a mixture of acetone and 1,4-
dioxane (1 :1, 1OmL). 5M HCl (1mL) was added to it and the reaction mixture was heated
for 6 hrs at 90 - 95°C. The reaction mixture was cooled to 45 - 50°C and the solvent was
distilled off u.v. to give an oil that was dissolved in dichloromethane, washed with water
(2mL) and brine (10%, 2mL). Again solvent was distilled off and the resulting oil was
purified by flash chromatography.
To a 20ml solution of ethanol and water (1: 1) was added 2g of compound of formula (V).
0.85ml of formic acid was added to it followed by sodium bisulphate (3Ag). The reaction
mixture was refluxed for 2 - 4 hrs. After completion of the reaction, the solvent was
distilled off and the reaction mass was extracted with ethyl acetate. Ethyl acetate was
then distilled off to yield the product.
Borane-dimethyisuifide complex in THF (O.5mL, 5.2mmole) was cooled to about -lOOC
and (R)-tetrahydro-I-methyl-3,3-diphenyl-IH,3H-pyrrolo[I ,2-C] [1,3,2]oxazaborolidine
(O.2mL, 1M in toluene) was added to it slowly. After stirring for 15 - 20 min. the ketone
(VI) (2g) was added to it and the reaction mixture was stirred for another 3 hrs at 0 - 5°C.
The reaction was quenched with MeOH (2mL) and IN HCI (5mL) was added after 0.5
hr. After stirring for another 0.5 hr at 5 - 10°C, the product was extracted with ethyl
acetate (50mL). This solution as washed with brine (20ml x 2) and concentrated to an oil,
which was purified by flash chromatography.
Ig of the alcohol (VII) was taken in MeOH (25mL) and ammonium formate
(l.5g) and 10% Pd/C (l50g) was added to it. The pH was adjusted to 3 - 4 with glacial
acetic acid (2mL) and the reaction mixture was warmed to 55 - 60°C. After stirring it at
this temperature under nitrogen for 3 hrs, the mixture was filtered through celite. The
filtrate was concentrated to an oil, which was added to a mixture of ethyl acetate (25mL)
and water (lOmL). The organic layer was separated, washed with brine (20%, 5mL x 2)
and distilled off. The oil obtained was crystallized with aq. EtOH and re-crystallized with
isopropanol- water.
We claim
1. A compound of formula (II), wherein R = H, alkyl.
2. The compound of formula (II) adapted to act as intermediate in the synthesis of
the compound of formula (VIII).
3. A compound of formula (IV), wherein R = H or alkyl and R' = H or any phenolic
protecting group.
4. The compound of formula (IV) adapted to act as intermediate in the synthesis of
the compound of formula (VIII).
5. A compound of formula (V), wherein R = H or alkyl and R' = H or any phenolic
protecting group.
6. The compound of formula (V) adapted to act as intermediate in the synthesis of
the compound of formula (VIII).
7. A compound of formula (II), wherein R = CH3
8. The compound of claim 7 adapted to act as intermediate in the synthesis of the
compound of formula (VIII).
9. A compound of formula (IV), wherein R = CH3
10. The compound of claim 9 adapted to act as intermediate in the synthesis of the
compound of formula (VIII).
11. A compound of formula (V) wherein R = CH3
12. The compound of claim 11 adapted to act as intermediate in the synthesis of the
compound of formula (VIII).
13. A process for the preparation of the compound of formula (VIII),
the process comprising the steps of
a) reacting hydroxylamine or its derivative with a ketone of formula (1) to obtain
an oxime of formula (II) wherein R = H, alkyl or silyl;
b) reacting the oxime of formula (II) with an imine of formula (III), wherein R' =
H or any phenolic protecting group;
c) cyclizing the ~-(substituted-amino)amide of formula (IV) to form an
azetidinone of formula (V);
d) converting the oxime functionality in the compound of formula (V) to a
ketone of formula (VI);
e) reducing the ketone functionality of a compound of formula (VI) to the
alcohol of formula (VII);
f) removing the phenolic protecting group at any desired stage in the above
synthesis.
14. A process for the preparation of the compound of formula (VIII),
the process comprising the steps of
a) reacting the oxime of formula (II), wherein R = H, alkyl or sHyl; with an
imine of formula (III), wherein R' = H or any phenolic protecting group;
b) cyclizing the P-(substituted-amino)amide of formula (IV) to form an
azetidinone of formula (V);
c) converting the oxime functionality in the compound of formula (V) to a
ketone of formula (VI);
d) reducing the ketone functionality of a compound of formula (VI) to the
alcohol of formula (VII);
e) removing the phenolic protecting group at any desired stage in the above
synthesis.
15.A process for the preparation of the compound of formula (VIII),
the process comprising the steps of
a) cyclizing the ~-(substituted-amino)amide of formula (IV), wherein R = H,
alkyl or silyl and R' = H or any phenolic protecting group, to form an
azetidinone of formula (V);
b) converting the oxime functionality in the compound of formula (V) to a
ketone of formula (VI);
c) reducing the ketone functionality of a compound of formula (VI) to the
alcohol of formula (VII);
d) removing the phenolic protecting group at any desired stage in the above
synthesis.
16.A process for the preparation of the compound of formula (VIII),
the process comprising the steps of
a) converting the oxime of formula (V), wherein R = H, alkyl or silyl and R' =
H or any phenolic protecting group, to a ketone of formula (VI);
b) reducing the ketone functionality of a compound of formula (VI) to the
alcohol of formula (VII);
removing the phenolic protecting group at any desired stage in the above synthesis.
17. The process according to any of the claims 13 to 16 wherein the cyclization is
carried out in the presence of a silylating agent.
18. The process according to any of the claims 13 to 16 wherein the reduction of the
ketone functionality is carried out stereospecifically.
19. A process according to any of the claims 13 to 16 wherein the oxime of formula
(V) is converted into a ketone of formula (VI) by using an acid;
20. A process according to any of the claims 13 to 16 wherein the hydroxylamine
derivative that is used is a-methyl hydroxylamine.
21. A process according to any of the claims 1.3 to 16 wherein hydroxylamine is used
to protect the ketone functionality.