Novel chiral auxiliary for attempted resolution of key roxifiban intermediate: A simple diastereoselective coupling approach for the synthesis of roxifiban

Article abstract of DOI:10.1080/00397910701575012

This article describes a simple method for the synthesis of roxifiban, a potent glycoprotein GP IIb-IIIa receptor antagonist, by a diastereoselective coupling approach to give >99.9% optical purity. We have also described an attempt to resolve the key syn

Full text of DOI:10.1080/00397910701575012

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Novel Chiral Auxiliary for Attempted
Resolution of Key Roxifiban Intermediate: A
Simple Diastereoselective Coupling Approach
for the Synthesis of Roxifiban
Ashok K. Gangopadhyay a , Gopal V. Gole a , Ravindra D. Jadhav a & Bansi Lal
a
a Nicholas Piramal India Ltd. , Mumbai, India
Published online: 10 Dec 2007.
To cite this article: Ashok K. Gangopadhyay , Gopal V. Gole , Ravindra D. Jadhav & Bansi Lal (2007) Novel
Chiral Auxiliary for Attempted Resolution of Key Roxifiban Intermediate: A Simple Diastereoselective Coupling
Approach for the Synthesis of Roxifiban, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 37:23, 4157-4171, DOI: 10.1080/00397910701575012
To link to this article: http://dx.doi.org/10.1080/00397910701575012
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Synthetic Communicationsw, 37: 4157–4171, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701575012
Novel Chiral Auxiliary for Attempted
Resolution of Key Roxifiban Intermediate:
A Simple Diastereoselective Coupling
Approach for the Synthesis of Roxifiban
Ashok K. Gangopadhyay, Gopal V. Gole, Ravindra D. Jadhav,
and Bansi Lal
Nicholas Piramal India Ltd., Mumbai, India
Abstract: This article describes a simple method for the synthesis of roxifiban, a potent
glycoprotein GP IIb-IIIa receptor antagonist, by a diastereoselective coupling approach
to give .99.9% optical purity. We have also described an attempt to resolve the key
synthetic intermediate by diastereomeric ester formation. Although we have been
able to separate two diastereomeric esters, the removal of the chiral auxiliary led to
partial racemization.
Keywords: diastereoselective coupling, 1,9-dideoxy forskolin, dynamic kinetic resol-
ution, glycoprotein IIb-IIIa receptor, roxifiban
INTRODUCTION
Development of glycoprotein GP IIb-IIIa (GP IIb-IIIa) (integrin aIIb-b3)
receptor antagonists has been the focus for antithrombotic research for the past
several years.[1] Several orally active development candidates earlier failed in
clinical trials, possibly due to poor to moderate bioavailability and/or short
half life.[2] An orally active low-molecular-weight GP IIb-IIIa receptor antagon-
ist roxifiban (1) was under clinical development with promising pharmacokinetic
properties.[3] In this article, we describe the synthesis of roxifiban with .99.9%
diastereomeric excess.
Received April 30, 2007
Address correspondence to Ashok K. Gangopadhyay, Nicholas Piramal Research
Centre, Nicholas Piramal India Ltd., Goregaon (East), Mumbai 400 063, India.
E-mail: agangopadhyay@nicholaspiramal.co.in
4157

4158
A. K. Gangopadhyay et al.
Figure 1. Key chiral intermediate for roxifiban sythesis.
RESULT AND DISCUSSION
We have followed essentially the synthetic protocol described by Xue et al.[4]
However the major difficulty was the resolution of the key acid intermediate
(2) (Fig. 1). These authors reported the resolution of (2) by using chinchoni-
dine with 29% yield and 98% ee.[5] Enzymatic hydrolysis of butyl esters with
lipase P30 gave 41.5% yield and 98% ee.[4] In our hands, we could not achieve
more than 64% ee for resolution after three crystallizations and 42% ee for
enzymatic hydrolysis.
We envisaged a new approach for resolution of acid 2. Thus acid 2 with ee
50% was treated with 1,9-di-deioxy forskolin[6] in the presence of DCC-
DMAP[7] to give the required 3a in 58% yield and the undesired 3b in 9.9%
yield (Scheme 1). Pure 3a was subjected to removal of the chiral auxiliary
(Table 1). The best result was achieved by using iodosilane generated in
situ to provide 2 with 66% ee.
While this study was under investigation, Pesti et al.[12] reported the
synthesis of the R-acid 2 with 80.4% yield and .99.9% ee after crystallization
by following enzymatic hydrolysis. This attracted our attention to the use of
thioethyl ester of (+) acid 2. Thus (+) acid 2 was reacted with ethanethiol
Scheme 1.

Roxifiban Synthesis
4159
Table 1. Study of different reactions to 3a to obtain enantiomerically pure 2
Reaction
condition
[a]2D5 in
degree
Ee excess
(%)
Sr. no.
1
Reagent
Yield (%)
56
Ref.
5% Aqueous. TFA 75–808C,
1.5 h
261.81
265.22
276.1
294.56
276
44
47
54
66
52
2
3
4
5
6N HCl:
dioxane (1:1)
6N HCl:
dioxane (1:1)
Rt, 72 h
30
18
60
60
508C, 16 h
Reflux, 3 h
Reflux, 2 h
Me3SiCl, NaI,
CH3CN
[8]
Me3SiCl, NaI,
Cu2O, Mol.
sieve 4A8,
CH3CN
[8]a
6
7
Me3SiCl, NaI,
Na2SO4,
CH3CN
Reflux, 2 h
30
—
265.22
47
—
[8]a
[8]a
Me3SiCl, NaI,
CaCO3,
Na2SO4,
Reflux, 48 h
—
CH3CN
8
9
(nBu3Sn)2O,
AIBN, ether
(nBu3Sn)2O, dry
toluene
9 h at rt
2 h, 508C
Reflux, 72 h
—
12
—
—
—
—
243.48
—
—
30
—
—
—
[9]
[10]
10
11
12
nBuSn (O) OH,
H2O, CH3CN
nBuSn (O) OH,
MeOHb
Reflux, 16 h
Reflux, 54 h
1008C, 72 h
[11]
—
[11]a
[11]a
nBuSn (O) OH,
MeOHb
—
aModification of reported procedure.
bTransesterification attempted with an idea to hydrolyze enzymatcally.
in presence of dicyclohexyl carbodiimide (DCC) and 4-NN-dimethyl-amino
pyridine (DMAP) to give ester 7 in 91% yield (Scheme 2), which was then
subjected to enzymatic hydrolysis according to the protocol described by
Pesti et al. to give 2 with 80% yield and 60% ee.
While further studies on the optimization of enzymatic resolution were in
progress, we thought of exploring the possibility of diastereoselective coupling
of partially resolved acid 2 with S-amine-A. As a model experiment, acid 2 (ee
28%) was coupled with amine-A using mixed anhydride method to give 4a in
75% yield as described in Scheme 3. The chiral high pressure liquid chromato-
graphy (HPLC) column showed the desired isomer (RS) was obtained in 58%

4160
A. K. Gangopadhyay et al.
Scheme 2. 1) Hydrolysis by amano lipase P30;[4] 2) EtSH, DCC, DMAP, EtOAc; 3)
amano lipase P30, NaH2PO4 buffer, Me3N, triton X;[12] 4) NH2OH HCl, MeOH;
.
5) Ac2O, AcOH, H2, 10% Pd-C.
Scheme 3. 1) NMM, Isobutyl chloroformate, Et3N, DMF, 2208C, 1 h, acid–
amine ¼ 1:1.1 eqv.; 2) Chinchonidine, repeated crystallization from acetone; 3) MeOH,
.
dry HCl; 4) NH4OAc; 5) NH2OH HCl, MeOH; 6) Ac2O, AcOH, H2, 10% Pd-C.

Roxifiban Synthesis
4161
Table 2. Reaction of acid with amine as described in Schemes 2 and 3 by using
NMM, isobutyl chloroformate, and Et3N in DMF
Ratio of acid
and amine
used
Starting R:S
Sr. no. Comp. no. ratio of acid 2
Diastereomeric
Yield (%) (de) excess (%)a
1b
2b
3b
4c
5d
4a
4b
8
64:36
82:18
82:18
76:24
77:23
1:1
75
80
58
90
1:1.1
1.2:1
1.25:1
1.25:1
66.4
73
.99.9
.99.9
.99.9
8
8
70
aThe diastereomeric ratio was determined by chiral HPLC on chiralcel OD column,
solvent system; hexane: EtOAc (90:10) þ 0.1% diethyl amine, flow rate, 1 mL/min;
Rt ¼ 28.64 (RS); Rt ¼ 22.06 (SS).
b0.3 g to ,1 g scale.
c.1 g to 5 g scale.
d.15 g scale.
de. If we compare this value (58% de) with the starting acid (28% ee), it is clear
that significant diastereomeric enrichment (ꢀ30%) had occurred. We started
exploiting this finding to develop an alternative synthesis of roxifiban. We
repeated the experiment with the acid 2 (64% ee) and coupling with 1.1 equiv-
alent of amine to get 4b in 79% yield with 90% de. Compound 4b was converted
into roxifiban 5a by a reported method[5] to give a chiral purity of 93.2% de.
We designed a number of experiments, the results of which are summarized
in Table 2. It is clear from the table depending upon the starting R:S ratio in 2 that
itisessentialtotakeexcessofacid(thepercentageofSacidpresentandadditional
10% of acid) or, in other words, to get 100% chirally pure 8 for n mmol of amine,
the requirement of acid (with X% R þ Y% S) should be [1.1 n þ n ꢀ Y/
100] mmol. The rest of the process is the same as described by earlier authors.
We also observed that amide 4b with 90% de was converted into oxime
6 in 58% yield with the same chiral ratio. The same material was hydrogenated
over 5% Pd-C after treatment with Ac2O in AcOH, as described earlier.[14]
Roxifiban (5b) was obtained in 33% yield with .99.9% chiral purity.
Thus the final synthetic protocol was described in Scheme 2.
The racemic acid 2 can be resolved partially by a chinchonidine method
or better by dynamic kinetic resolution of thioethyl ester 7 followed by
coupling with the amine (Table 1) to get optically pure 8, which was
converted to roxifiban as described earlier with chiral purity .99.9%.
CONCLUSION
We have been able to successfully separate the diastereomers obtained by the
reaction of partially resolved 2 with 1,9-di-deoxy forskolin. However, the

4162
A. K. Gangopadhyay et al.
removal of the chiral auxiliary led to partial racemization. From a trial exper-
iment, we had identified the preference of R-acid 2 toward S-amine in mixed
anhydride coupling procedure as used by us. It can be anticipated that other
coupling reagent may have similar selectivity. This finding led us to
discover a novel diastereoselective coupling approach that involve use of
excess partially resolved acid to get very high optical purity of the desired
intermediate. This intermediate was converted to roxifiban with .99.9%
chiral purity. We have shown the reproducibility of the process by
gradually scaling up the reaction up to 15-g batch. We are currently
exploring the application of this method to synthesize other biologically
active diastereomers.
EXPERIMENTAL
General Procedures
MeltingpointsweredeterminedwithaKoflerhot-stageapparatusandareuncor-
rected. IR spectra were determined with a Perkin-Elmer 157 spectrophotometer
1
as KBr film unless otherwise mentioned. H NMR spectra were recorded in
CDCl3 unless otherwise mentioned on a Jeol FT-90 spectrometer or Bruker
ACP 300 spectrometer with TMS as internal standard, and coupling constant
values are expressed in hertz. Light petroleum refers to the fraction of bp 60–
808C. For flash-column chromatography, silica gel (finer than 0.08-mm
particle size) was used. For reverse-phase medium pressure liquid chromato-
graphy (MPLC) purification, RP-18 (30-m particle size) was used. Precoated
(silica gel 60 F254) thin-layer chromatography (TLC) plates were used for
checking purity of compounds. UV light at l 366 nm and 254 nm as well as
spray reagents such as 0.2% ninhydrin in acetone, Draggendroff, and I2 vapor
was used to detect the spots on TLC. All compounds were homogeneous on
TLC and gave proper spectral characteristics.
[(R)-3-(4-Cyano-phenyl)-4,5-dihydro-isoxazol-5-yl]-acetic acid
6-Hydroxy-3,4a,7,7,10a-pentamethyl-1-oxo-3-vinyl-dodecahydro-
benzo[f]chromen-5-yl ester (3a) and [(S)-3-(4-Cyano-phenyl)-4,5-
dihydro-isoxazol-5-yl]-acetic acid 6-hydroxy-3,4a,7,7,10a-
pentamethyl-1-oxo-3-vinyl-dodecahydro-benzo[f]chromen-5-yl
ester (3b)
In a round-bottomed flask fitted with a guard tube, a solution of 5,6-dihydroxy-
3,4a,7,7,10a-pentamethyl-3-vinyl-dodecahydro-benzo[f]chromen-1-one (1,9-di-
deoxy forskolin) (3.45 g; 10.27 mmol) and DCC (1.86 g; 9.03 mmol) in EtOAc
(40 mL) were chilled in an ice box to 08C. To this, a solution of
[3-(4-cyano-phenyl)-4,5-dihydro-isoxazole-5-yl]-acetic acid (52% ee) (1.7 g;

Roxifiban Synthesis
4163
7.39 mmol) in EtOAc (10 mL) and DMF (5 mL) was added under vigorous
stirring. After 10 min, DMAP (0.15 g; 1.25 mmol) was added. The reaction
mixture was stirred for 1 h at room temperature followed by 16 h at room tempera-
ture. The reaction mixture was kept at 2108C for 2 h, and DCU was filtered off and
washed with cold EtOAc (2 ꢀ 10 mL). The combined filtrate was washed with
10% NaHCO3 (2 ꢀ 10 mL) followed by water (3 ꢀ 10 mL) and brine (5 mL).
The EtOAc layer was dried over anhydrous Na2SO4. The solvent was removed
in a rotary evaporator and purified by flash chromatography over silica gel with
10 to 20% EtOAc in light petroleum to give 3a (2.35 g; 58%) and 3b (0.4 g; 9.9%).
Data
3a: mp, 1038C; IR (KBr): 3528 (br), 2926 (br), 2229, 1744, 1719, 1399,
1190 cm21; mass: ESIþ, 549 (Mþ þ 1), ESI–, 547 (M 2 1)21, EI, 548, 533,
515, 480, 465, 462 (100%), 391, 378, 360, 347 (100%), 336; 1H NMR
(300 MHz): 0.95 (s, 3H), 1.20 (m, 8H), 1.40 (m, 4H), 1.48 (s, 3H), 1.65–1.80
(m, 2H), 1.88 (br, 1H), 2.42 (m, 1H), 2.53 (d, 1H, Jgem ¼ 18, 2-Hb), 2.59
(d, 1H, Jgem ¼ 18, 2-Ha), 2.76 (s, 1H, 10b-H), 2.82 (dd, 1H, Jgem ¼ 15.3,
J ¼ 6.9), 3.0 (dd, 1H, Jgem ¼ 15.3, J ¼ 6.6), 3.18 (dd, 1H, Jgem ¼ 16.8,
J ¼ 7.5), 3.57 (dd, 1H, Jgem ¼ 16.8, J ¼ 10.5), 4.39 (br, 1H, 6-H), 5.07
(d, 1H, Jcis ¼ 10.8, 2-Hcis vinylic), 5.10 (d, J ¼ 7.4, 5-H), 5.12 (m, 1H, 5-H,
dihydroisoxazol), 5.26 (d, 1H, Jtrans ¼ 17.4, 2-Htrans vinylic), 5.96 (dd, 1H,
Jtrans ¼ 17.4, Jcis ¼ 10.8, 1-H vinylic), 7.69 (d, 2H, J ¼ 8.4, ArH), 7.76
(d, 2H, J ¼ 8.4, ArH); 13C NMR: 16.9, 18.4, 23.8, 24.2, 31.7, 33.1, 34.3,
37.7, 39.7, 40.13, 41.3, 43.6, 50.0, 55.1, 65.4, 69.5, 75.1, 78.2, 78.3, 82.1,
112.8, 113.7, 127.1, 132.5, 133.6, 146.3, 155.4, 169.1, 205.7; 13C DEPT 135:
CH2: 18.4, 39.7, 40.1, 41.2, 43.6, 50.1, 112.9; CH & CH3: 16.9, 23.8, 24.3,
31.7, 33.2, 55.1, 65.4, 69.55, 78.3, 82.1, 127.2, 132.6, 146.3.
1
3b: mp, 91–948C; H-NMR (300 MHz): mass: ESIþ, 549 (Mþ þ 1), ESI2,
547 (M 2 1)21, EI, 548, 533, 515, 480, 465, 462 (100%), 360, 336; 0.80
(s, 3H), 1.24 (s, 3H), 1.42 (s, 3H), 1.58 (s, 3H), 1.65–1.80 (m, 2H), 1.88
(br, 1H), 2.42 (m, 1H), 2.54 (d, 1H, Jgem ¼ 18, 2-Hb), 2.68 (d, 1H,
Jgem ¼ 18, 2-Ha), 2.78 (s, 1H, 10b-H), 2.90 (m, 2H), 3.28 (dd, 1H,
Jgem ¼ 16.8, J ¼ 7.5), 3.53 (dd, 1H, Jgem ¼ 16.8, J ¼ 10.5), 4.24 (br,
1H, 6-H), 5.07 (d, 1H, Jcis ¼ 10.8, 2-Hcis vinylic), 5.10 (d, J ¼ 7.4, 5-H),
5.12 (m, 1H, 5-H, dihydroisoxazol), 5.26 (d, 1H, Jtrans ¼ 17.4, 2-Htrans
vinylic), 5.96 (dd, 1H, Jtrans ¼ 17.4, Jcis ¼ 10.8, 1-H vinylic), 7.69 (d, 2H,
J ¼ 8.4, ArH), 7.76 (d, 2H, J ¼ 8.4, ArH) (additional peaks appeared in the
regions 0.8–2.0 and 2.4–2.5, which is due some DCU contamination).
Removal of Chiral Auxiliary from 3a to Get 2 (Table 1)
1. A solution of 3a (0.274 g; 0.5 mmol) in 5% aqueous TFA (2.5 mL) was
heated at 70–808C for 1.5 h. The starting material disappeared. The

4164
A. K. Gangopadhyay et al.
solvent was removed. The residue was dissolved in 10% NaHCO3 (5 mL)
and extracted with ether (3 ꢀ 5 mL). The aqueous layer was acidified
with 1N HCl to pH 2; the solid was filtered and dried. It was further
purified by flash chromatography over silica gel with 2% MeOH in
CHCl3. The pure product 2 was crystallized from hot EtOAc–light
petroleum to yield 0.063 g (55%). [a]2D5 ¼ 261.818 (C ¼ 0.18 in
CHCl3) (44% ee, based on optical rotation).
2. A solution of 3a (0.2 g; 0.37 mmol) was dissolved in dioxane (3 mL) and
6N HCl (3 mL). The resulting reaction mixture was stirred at room temp-
erature for 72 h. The reaction mixture was poured on 10% NaHCO3
(10 mL) and extracted with ether (3 ꢀ 5 mL). The aqueous layer was
acidified with 1N HCl to pH 2. The solid was filtered and dried. It was
crystallized from hot EtOAc–light petroleum to get a pure solid 2,
0.025 g (30%); [a]2D5 ¼ 265.228 (C ¼ 0.184 in CHCl3) (46% ee, based
on optical rotation).
3. A solution of 3a (0.4 g; 0.74 mmol) was dissolved in dioxane (5 mL) and
6N HCl (5 mL). The resulting reaction mixture was heated at 508C for
16 h. The reaction mixture was poured on 10% NaHCO3 (10 mL) and
extracted with ether (3 ꢀ 5 mL). The aqueous layer was acidified with
1N HCl to pH 2. The solid was filtered and dried. It was purified by
flash chromatography over silica gel with 5% MeOH in CHCl3
followed by flash chromatography over RP-18 with MeOH–water
(1:1). It was crystallized from hot EtOAc–light petroleum to get a pure
solid 2 (0.030 g; 18%); [a]D25 ¼ 276.18 (C ¼ 0.182 in CHCl3) (54% ee;
based on optical rotation).
4. A 25-mL, dry, round-bottomed flask fitted with a condenser with guard
tube and dry N2 inlet, 3a (0.2 g; 0.36 mmol) was dissolved in dry
CH3CN (1 mL). To this solution, trimethylsilyl chloride (0.139 mL;
1.08 mmol) and NaI (0.165 g; 1.08 mmol) were added under stirring.
The resulting reaction mixture was stirred under reflux for 3 h. It was
diluted with water (10 mL) and extracted with EtOAc (3 ꢀ 5 mL). The
organic layer was washed with water and brine. It was dried over
anhydrous Na2SO4. The solvent was removed, and the residue was
purified by flash chromatography with 5% MeOH in CHCl3. The pure
product was crystallized from hot EtOAc–light petroleum to give
0.50 g (60%) of 2. [a]2D5 ¼ 294.568 (C ¼ 0.188 in CHCl3) (66% ee;
based on optical rotation).
5. A 25-mL, dry, round-bottomed flask fitted with a condenser with guard
tube and dry N2 inlet, 3a (0.2 g; 0.36 mmol) was dissolved in dry
CH3CN (1 mL). To this solution, trimethylsilyl chloride (0.139 mL;
1.08 mmol), NaI (0.165 g; 1.08 mmol), and Cu2O (0.02 g; 0.14 mmol)
and powdered 4 A8 molecular sieve (0.05 g) were added under stirring.
The resulting reaction mixture was stirred under reflux for 2 h. It was
filtered and washed with EtOAc (3 ꢀ 7 mL). The organic layer was
washed with water and brine. It was dried over anhydrous Na2SO4. The

Roxifiban Synthesis
4165
solvent was removed, and the residue was purified by flash chromato-
graphy with 5% MeOH in CHCl3. The pure product was crystallized
from hot EtOAc–light petroleum to give 0.50 g (60%) of 1.
[a]2D5 ¼ 2768 (C ¼ 0.184 in CHCl3); R:S ¼ 77:23 (54% ee; based on
optical rotation).
6. A 25-mL, two-necked, dry, round-bottomed flask fitted with a condenser
with guard tube and dry N2 inlet, 2a (0.2 g; 0.36 mmol) was dissolved in
dry CH3CN (2 mL). To this solution, trimethylsilyl chloride (0.139 mL;
1.08 mmol), NaI (0.165 g; 1.08 mmol), and flame-dried powdered
Na2SO4 (0.25 g) were added under stirring. The resulting reaction
mixture was stirred under reflux for 2 h. It was filtered and washed with
EtOAc (3 ꢀ 7 mL). The organic layer was washed with water and
brine. It was dried over anhydrous Na2SO4. The solvent was removed,
and the residue was purified by flash chromatography with 5% MeOH
in CHCl3. The pure product was crystallized from hot EtOAc–light
petroleum to give 0.50 g (60%) of 1. [a]2D5 ¼ 265.228 (C ¼ 0.184 in
CHCl3) (based on optical rotation).
7. The reaction in step 6 was repeated with the addition of fused CaCO3
(0.055 g;0.547 mmol) and refluxedfor 48 h without any formationof acid2.
8. A 25-mL, two-necked, dry, round-bottomed flask fitted with a condenser
with guard tube and dry N2 inlet was dried with a hot gun under dry N2
atm. Compound 3a (0.1 g; 0.18 mmol) was dissolved in ether (10 mL).
To the solution, bis (tributyl-tin) oxide (0.22 g; 0.19 mmol) and AIBN
(0.01 g) were added. The reaction mixture was stirred at room tempera-
ture for 16 h, followed by refluxing for 2 h. No hydrolysis to compound
2 was detected.
9. A 25-mL, two-necked, dry, round-bottomed flask fitted with a condenser
with guard tube and dry N2 inlet was dried with a hot gun under dry N2
atm. Compound 3a (0.2 g; 0.36 mmol) was dissolved in toluene
(2.6 mL). To the solution, bis (tributyl-tin) oxide (0.44 g; 0.38 mmol)
was added. The reaction mixture was refluxed for 72 h. The reaction
mixture was cooled to room temperature and extracted with 10%
NaHCO3 (2 ꢀ 5 mL). The aqueous layer was acidified with citric acid,
and the solid separated was extracted with EtOAc (3 ꢀ 4 mL). The
EtOAc layer was washed with water (3 ꢀ 2 mL). It was dried over
anhydrous Na2SO4. The solvent was removed, and the residue was
crystallized from hot EtOAc and light petroleum to give 0.03 g (18%)
of 2. [a]2D5 ¼ 243.488 (C ¼ 0.17 in CHCl3) (30% ee; based on optical
rotation).
10. A 25-mL, two-necked, dry, round-bottomed flask fitted with a condenser
with guard tube and dry N2 inlet, 3a (0.2 g; 0.36 mmol) was dissolved in
toluene (3 mL). To this solution, MeOH (0.2 mL; 2.3 mmol) and butyl-
stannonic acid (0.019 g; 0.09 mmol) were added. The resulting reaction
mixture was stirred under reflux for 24 h; there was a trace of nonpolar
product formation. The product was not isolated.

4166
A. K. Gangopadhyay et al.
11. A 25-mL, two-necked, dry, round-bottomed flask fitted with a condenser
with guard tube and dry N2 inlet, 3a (0.2 g; 0.36 mmol) was dissolved in
toluene (3 mL). To this solution, benzyl alcohol (0.23 mL; 2.3 mmol)
and butylstannonic acid (0.019 g; 0.09 mmol) were added. The resulting
reaction mixture was stirred under reflux for 24 h; there was a small
amount of nonpolar product formation with lot of decomposed
material. The product was not isolated.
(S)-2-Butoxycarbonylamino-3-f2-[3-(4-cyano-phenyl)-4,5-dihydro-
isoxazol-5-yl]-acetylaminog-propionic acid methyl ester (4a)
A solution of 2 (28% ee; 0.301 g; 1.3 mmol) was dissolved in DMF (4 mL) and
chilled to 2208C in a cold bath. To this solution, NMM (0.144 mL; 1.3 mmol)
and isobutylchloroformate (0.175 mL; 1.3 mmol) were added successively
under vigorous stirring. After 2 min, (S)-3-amino-2-butoxycarbonylamino-
propionic acid methyl ester hydrochloride (0.33 g; 1.3 mmol) was dissolved
in DMF (3 mL), neutralized with Et3N (0.18 mL; 1.3 mmol), and added to
the reaction mixture under stirring. The resulting reaction mixture was
stirred at 2208C for 1 h and kept in the freezer at 258C for 16 h. The
solvent was removed under reduced pressure, and the residue was taken in
EtOAc (15 mL). It was washed successively with 1N HCl (2 ꢀ 5 mL),
water (3 ꢀ 3 mL), 1N NaHCO3 (2 ꢀ 3 mL), and water (3 ꢀ 3 mL). The
EtOAc layer was dried over anhydrous Na2SO4. The solvent was removed.
The residue was purified by flash chromatography with 2% MeOH in
CHCl3 and finally crystallized from EtOAc to give 0.424 g (75%; 58% de)
of 4a; chiracel OD; solvent system: hexane–EtOH–diethyl amine ¼
10:90:0.1; flow rate 1.7 mL/min; Rt of RS ¼ 28.64; Rt of SS ¼ 22.06; mp
145–1478C; IR (KBr): 3314 (br), 2958, 2227 (CN), 1745, 1734, 1690,
1
1654, 1552 (br), 1437, 1275; H NMR: 0.9 (9t, 3H, J ¼ 7.2, CH2CH3), 1.35
(m, 2H, CH2), 1.60 (m, 2H, CH2), 2.56, 2.70 (2 ꢀ dd, 2H, Jgem ¼ 15.0,
J ¼ 6.0 & 7.5, CH2CONH), 3.21, 3.50 (2 ꢀ dd, 2H, Jgem ¼ 15.2, J ¼ 7.3 &
10, 4-CH2), 3.65 (m, 2H, NHCH2CH), 3.78 (s, 3H, OCH3), 4.05 (t, 2H,
J ¼ 7.4, OCH2CH2), 4.42 (m, 1H, CaH), 5.15 (m, 1H, 6-H), 5.69 (br, 1H,
NH), 6.35 (br, 1H, NH), 7.68, 7.76 (2 ꢀ d, 4H, J ¼ 8.5, ArH).
1. The acid 2 (64% ee) was treated with (S)-3-amino-2-butoxycarbonyla-
mino-propionic acid methyl ester hydrochloride in the ratio of 1:1.1
molar equivalent (2-g scale) by the same procedure as described for the
synthesis of 4a. The product 4b was obtained in 79% yield (90% de);
chiracel OD; solvent system: hexane–EtOH–diethyl amine ¼
10:90:0.1; flow rate 1.7 mL/min; Rt of RS ¼ 28.64; Rt of SS ¼ 22.06;
mp 150–1528C; mass (ESþ): 431.56 (Mþ þ 1), 458.56 (M þ Naþ); IR
(KBr): 3314 (br), 2958, 2227 (CN), 1745, 1734, 1690, 1654, 1552 (br),
1
1437, 1275; H NMR: 0.9 (9t, 3H, J ¼ 7.2, CH2CH3), 1.35 (m, 2H,

Roxifiban Synthesis
4167
CH2), 1.60 (m, 2H, CH2), 2.56, 2.70 (2 ꢀ dd, 2H, Jgem ¼ 15.0, J ¼ 6.0 &
7.5, CH2CONH), 3.21, 3.50 (2 ꢀ dd, 2H, Jgem ¼ 15.2, J ¼ 7.3 & 10,
4-CH2), 3.65 (m, 2H, NHCH2CH), 3.78 (s, 3H, OCH3), 4.05 (t, 2H,
J ¼ 7.4, OCH2CH2), 4.42 (m, 1H, CaH), 5.15 (m, 1H, 6-H), 5.69 (br,
1H, NH), 6.35 (br, 1H, NH), 7.68, 7.76 (2 ꢀ d, 4H, J ¼ 8.5, ArH).
2. In a two-necked, round-bottomed flask fitted with a guard tube and
passing tube, a solution of 4b (1 g; 2.32 mmol) in dry MeOH (30 mL)
was prepared. The solution was chilled to 08C in an icebox, and dry
HCl gas was passed through the solution for 1 h. The resulting solution
was stirred at 08C for 5 h. The solvent was removed under reduced
pressure at 408C in a rotary evaporator. The residue was dried in
vacuum desiccators over KOH for 3 h. The crude material was
dissolved in dry MeOH (15 mL), and dried ammonium acetate (0.89 g;
11.6 mmol) was added under stirring at room temperature. The reaction
mixture was stirred for 16 h at room temperature. The solvent was
removed, and the residue was purified by flash chromatography over
RP-18 with 0.2% AcOH–MeOH ¼ 70:30. The pure fractions were
pulled together and evaporated to dryness under reduced pressure in a
rotary evaporator. The residue thus obtained was crystallized from dry
MeOH and ether to give 0.3 g (25.5%) roxifiban 5a; mp 181–1838C;
mass (ESþ): 448.67 (Mþ þ 1); IR (KBr): 3339 (br), 2958, 2932, 1733,
1
1686 (br), 1649, 1539 (br), 1412, 1276, 1242; H NMR (DMSO-D6):
0.86 (t, 3H, J ¼ 7.2, CH2CH3), 1.29 (m, 2H, CH2CH2CH3), 1.48
(m, 2H, CH2CH3), 1.69 (s, 3H, CH3COO), 2.99 (m, 2H), 3.22 (m, 2H),
3.55 (m, 2H), 3.61 (s, 3H, OCH3), 3.92 (t, 2H, J ¼ 6.3, OCH2CH2),
4.14 (br, 1H, CaH of A2Pr), 5.0 (m, 1H, 5-H), 7.5 (br, 1H, NH), 7.80,
7.84 (2 ꢀ d, 4H, J ¼ 8.4, ArH), 8.23 (br, 1H, NH); HPLC, Rt ¼ 15.18
(3.4%), Rt ¼ 19.87 (96.6%); chiral OD column; solvent system EtOH–
hexane–TFA ¼ 15:85:0.1, flow rate 1.2 mL/min.
(S)-2-Butoxycarbonylamino-3-(2-f3-[4-(N-hydroxycarbamimidoyl)-
phenyl]-4,5-dihydro-isoxazol-5-ylgacetylamino)-propionic acid
methyl ester (6)
In a 25-mL, round-bottomed flask fitted with a guard tube 4b (0.121 g;
0.281 mmol) was dissolved in dry MeOH (5 mL). To the solution,
.
NH2OH HCl (0.049 g; 0.701 mmol) was added, followed by Et3N
(0.098 mL; 0.701 mmol) at room temperature under vigorous stirring. The
stirring was continued for 16 h. The solvent was removed. The crude thus
obtained was purified by flash chromatography over silica gel with 5%
MeOH in CHCl3. Crystallization from hot EtOH–light petroleum gave
0.116 g (90%) of 6; mp 194–1968C; mass (ESþ): 464.57 (Mþ þ 1), 486.27
(M þ Naþ); 1H NMR (DMSO-D6): 0.95 (t, 3H, J ¼ 7.3, CH2CH3), 1.3
(m, 2H, CH2CH3), 1.53 (m, 2H, CH2CH2CH3), 2.35, 2.55 (2 ꢀ m, 2H,

4168
A. K. Gangopadhyay et al.
CH2CONH), 3.15–3.35 (m, 2H, 4-CH2), 3.55 (m, 2H, NHCH2CH), 3.65
(s, 3H, OCH3), 3.98 (t, 3H, J ¼ 7.1, OCH2CH2), 4.20 (m, 1H, CaH), 4.95
(m, 1H, 5-H), 5.86 (s, 2H, NH2), 7.50 (d, 1H, J ¼ 7.5, NHCOO), 7.67, 7.84
(2 ꢀ d, 4H, J ¼ 8.5, ArH), 8.18 (br, 1H, NH), 9.98 (s, 1H, NHOH).
The compound 6 (0.075 g; 0.162 mmol) was dissolved in glacial AcOH
(10 mL) in
a small hydrogenation bottle, and Ac2O (0.017 mL;
0.178 mmol) was added. The reaction mixture was stirred for 1 h under a
dry N2 blanket. To the solution, 5% Pd-C was added and hydrogenated at
15 psi for 30 min. The catalyst was filtered off. The filtrate was evaporated
under reduced pressure. The crude material was purified as described for
the synthesis of 5a to give 0.07 g (85%) Roxifiban 5b; mp 2008C (d)–
2108C; HPLC, Rt ¼ 19.89 (.99.9%); chiral OD column; solvent system
EtOH–hexane–TFA ¼ 15:85:0.1, flow rate 1.2 mL/min.
[3-(4-Cyano-phenyl)-4,5-dihydro-isoxazol-5-yl]-thioacetic acid ethyl
ester[13] (7)
To a solution of racemic [3-(4-cyano-phenyl)-4,5-dihydro-isoxazol-5-yl]-
acetic acid (21.2 g; 92 mmol) and ethanethiol (10.2 mL; 137 mmol) in
EtOAc (200 mL) and DMF (5 mL), a solution of DCC (20.86 g; 101 mmol)
in EtOAc (50 mL) was added at 08C. After 10 min, DMAP (1.59 g;
13 mmol) was added. The reaction mixture was stirred at 08C for 30 min,
and then the ice bath was removed. The reaction mixture was stirred at
room temperature for 16 h. The reaction mixture was kept in the freezer at
258C for 1 h. The DCU was filtered off. The filtrate was washed with
water (2 ꢀ 50 mL). It was dried over anhydrous Na2SO4. Solvent was
removed. The residue was purified by flash chromatography with 10–20%
EtOAc in light petroleum. It was crystallized from EtOAc and light
petroleum to give 23 g (91.2%); mp 92–948C. All other physicochemical
data agreed with reported values.
[3-(4-Cyano-phenyl)-4,5-dihydro-isoxazol-5-yl]-acetic acid (2)[12]
Finely powdered 7 (27.4 g; 100 mmol) was suspended in 0.65 N NaH2PO4
(406 mL), trimethyl amine (25 g in 100 mL water), Triton X 100 (2.3 g),
and amino lipase PS 30 (2.3 g), and the mixture was stirred at 408C. A
solution of 6N NaOH (200 mL) was added at intervals such that the pH of
the mixture was maintained at 8.9 to 9.5. The addition of NaOH solution
was completed in 20 h. It was further stirred for 16 h (90% cv. checked by
HPLC). The reaction mixture was filtered through a pad of Celitew and was
extracted with dichloromethane (2 ꢀ 100 mL). The aqueous layer was
acidified with 4N HCl to pH 2. The solid was filtered (20.7 g; 90%). The
solid was recrystallized from EtOAc–light petroleum to give 18 g (79%) of

Roxifiban Synthesis
4169
compound 2; [a]2D5 ¼ 285.1 (60% ee). The rest of the physicochemical data
matched with the reported compound.
Diastereoselective Synthesis of (S)-2-Butoxycarbonylamino-3-f2-[3-
(4-cyano-phenyl)-4,5-dihydro-isoxazol-(R)-5-yl]-acetylaminog-
propionic acid methyl ester (8)
1. Acid 2 (64% ee) and (S)-3-amino-2-butoxycarbonylamino-propionic acid
methyl ester hydrochloride were reacted in a 1.2:1 ratio in a 1-g scale by
following exactly the same procedure described for the synthesis of 4a to
give 8 in 66.4% yield. Mp 158–1608C; IR (KBr): 3314 (br), 2958, 2227
(CN), 1745, 1734, 1690, 1654, 1552 (br), 1437, 1275; HPLC .99.9%
chiracel OD; solvent system: hexane–EtOH–diethyl amine ¼ 10:90:0.1;
flow rate 1.7 mL/min; Rt of RS ¼ 29.7.
2. Acid 2 (R:S ¼ 75:25) and (S)-3-amino-2-butoxycarbonylamino-propionic
acid methyl ester hydrochloride were reacted in a 1.25:1 ratio in a 2-g scale
by following exactly the same procedure described for the synthesis of 4a
to give 8 in 73% yield. Mp 158–1608C; HPLC .99.9% chiracel OD;
solvent system: hexane–EtOH–diethyl amine ¼ 10:90:0.1; flow rate
1.7 mL/min; Rt of RS ¼ 28.54.
3. Acid 2 (R:S ¼ 80:20) obtained from dynamic kinetic resolution of
thioethyl ester 7 and (S)-3-amino-2-butoxycarbonylamino-propionic
acid methyl ester hydrochloride were reacted in a 1.25:1 ratio in a 20-g
scale by following exactly the same procedure described for the
synthesis of 4a to give 8 in 80% yield. Mp 159–1608C; HPLC
.99.9% chiracel OD; solvent system: hexane–EtOH–diethyl amine ¼
10:90:0.1; flow rate 1.7 mL/min; Rt of RS ¼ 28.34.
(S)-2-Butoxycarbonylamino-3-(2-f3-[4-(N-hydroxycarbamimidoyl)-
phenyl]-4,5-dihydro-isoxazol-(R)-5-ylgacetylamino)-propionic acid
methyl ester (9)
This compound was prepared from 8 following the same procedure as
described for the synthesis of 6 in 88% yield using a batch size of more
than 15 g. Mp 205–2068C; IR (KBr): 3496 (br), 3390, 3362, 2958, 1721,
1690, 1670, 1533 (br), 1347, 1277, 1218 cm21
.
Roxifiban
The compound 8 was converted to roxifiban following the procedure
described for the synthesis of 5b in 89% yield using a batch size of more

4170
A. K. Gangopadhyay et al.
than 10 g of 9. Mp 200 (d)–210 (m), IR (KBr): 3339 (br), 2958, 2932, 1733,
1686 (br), 1649, 1539 (br), 1412, 1276, 1242; mass (ESþ): 448.58 (Mþ þ 1);
1H NMR (DMSO-D6): 0.86 (t, 3H, J ¼ 7.2, CH2CH3), 1.29 (m, 2H,
CH2CH2CH3), 1.48 (m, 2H, CH2CH3), 1.69 (s, 3H, CH3COO), 2.99
(m, 2H), 3.22 (m, 2H), 3.55 (m, 2H), 3.61 (s, 3H, OCH3), 3.92 (t, 2H,
J ¼ 6.3, OCH2CH2), 4.14 (br, 1H, CaH of A2Pr), 5.0 (m, 1H, 5-H), 7.5 (br,
1H, NH), 7.80, 7.84 (2 ꢀ d, 4H, J ¼ 8.4, ArH), 8.23 (br, 1H, NH); 13C
NMR: 14.1, 19.0, 25.1, 31.1, 41.1, 52.5, 54.1, 64.4, 78.9, 127.2, 128.5,
131.5, 133.9, 156.6 (2 ꢀ C), 165.7, 169.6, 171.6, 176.9; 13C-135-DEPT
(DMSO-D6): 14.1, 19.0 (CH2), 25.1, 31.1 (CH2), 39.4 (CH2), 40.2 (CH2),
41.1 (CH2), 52.5, 54.1, 64.4 (CH2), 78.9, 127.2, 128.5. HPLC, Rt ¼ 23.93
(.99.9%); chiral OD column; solvent system EtOH–hexane–TFA ¼ 10:85:0.1,
flow rate 1.2 mL/min.
ACKNOWLEDGMENTS
We gratefully acknowledge Swati A. Piramal for her interest and encourage-
ment and Swati Bal-Tambe and her group for providing analytical data.
Finally we acknowledge A. Ranade for providing useful literature input and
fruitful discussion.
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