Diastereoselective synthesis of an argatroban intermediate, ethyl (2R,4R)-4-methylpipecolate, by means of a Mandyphos/rhodium complex-catalyzed hydrogenation

Article abstract of DOI:10.1016/j.tetasy.2011.09.009

The synthetic antithrombotic argatroban is a dipeptide between the nonproteogenic (2R,4R)-4-methyl-2-piperidine carboxylic acid and l-arginine, in turn bonded to a methyltetrahydroquinoline sulfonyl group. An extensive screening of transition metal-based complexes with different ligands was performed in order to identify the best catalyst for the diastereoselective hydrogenation of a suitable 4,5-dehydropiperidine precursor aimed toward a synthesis of the (2R,4R)-4-methyl piperidine moiety. Copyright

Full text of DOI:10.1016/j.tetasy.2011.09.009

Tetrahedron: Asymmetry 22 (2011) 1626–1631
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Tetrahedron: Asymmetry
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Diastereoselective synthesis of an argatroban intermediate,
ethyl (2R,4R)-4-methylpipecolate, by means of a Mandyphos/rhodium
complex-catalyzed hydrogenation
Patrizia Ferraboschi a, , Maria De Mieri a, Paride Grisenti b, Matthias Lotz c, Ulrike Nettekoven c
⇑
a Dipartimento di Chimica, Biochimica e Biotecnologie per la Medicina, Università di Milano, Via Saldini 50, 20133 Milano, Italy
b EUTICALS SpA, Via Volturno 41/43, 20089 Rozzano (Mi), Italy
c SOLVIAS AG, Synthesis and Catalysis, Mattenstrasse 22, CH-4002 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2011
Accepted 13 September 2011
Available online 26 October 2011
The synthetic antithrombotic argatroban is a dipeptide between the nonproteogenic (2R,4R)-4-methyl-2-
piperidine carboxylic acid and L-arginine, in turn bonded to a methyltetrahydroquinoline sulfonyl group.
An extensive screening of transition metal-based complexes with different ligands was performed in
order to identify the best catalyst for the diastereoselective hydrogenation of a suitable 4,5-dehydropi-
peridine precursor aimed toward a synthesis of the (2R,4R)-4-methyl piperidine moiety.
Ó 2011 Elsevier Ltd. All rights reserved.
12COOH
1. Introduction
NH
O
4
2
7
5
8
6
N
H
NH2
1
N
Argatroban 1 is a synthetic inhibitor of thrombin, the serine
protease that plays a central role in the initiation and propagation
of thrombotic events.1 The most frequently prescribed anticoagu-
lant with antithrombin activity is heparin but limitations due to
its chemical heterogenicity in addition to several adverse events,
such as the heparin-induced thrombocytopenia (HIT), led us to
perform an extensive search for low molecular weight, selective
inhibitors of thrombin. Argatroban is a small molecule (MW 509)
approved in the USA, Europe and Japan for prophylaxis or treat-
ment of thrombosis in patients with HIT that binds reversibly to
and inhibits thrombin, without generation of antibodies or the
degradation of proteases.2,3
In the argatroban structure, three moieties can be recognized:
the 4-methyl-2-piperidine carboxylic acid bonded to the arginine,
which also bears a methyltetrahydroquinoline sulfonyl group on
its amino function. Four stereogenic centers are present in the mol-
ecule: the stereocenter on the tetrahydroquinoline is introduced
via hydrogenation, performed in the last step of the synthesis of
1. This reaction affords a mixture of (21R)- and (21S)-diastereoiso-
mers, for which a respective ratio of 65/35 2 is acceptable.
3
9
HN
11
10
SO2
H
N
N
H
COOCH2CH3
13
14
19
20
15
16
(2R,4R)-2
21
R
18
17
R'
22
1
R = CH3; R' = H (21R)-argatroban
R = H; R' = CH3 (21S)-argatroban
On the contrary each of the other three stereogenic centers
needs to be synthesized with high levels of stereoselectivity. For
the arginine moiety it is sufficient to start from the natural
L
-aminoacid, but more care is required for the synthesis of the
suitable diastereoisomer of 4-methylpipecolate, that is, (2R,4R)-2.
Following a reported procedure,4,5 we prepared intermediate 3 in
order to investigate the best 4,5-double bond hydrogenation condi-
tions (catalyst, solvent, temperature, pressure) with regard to the
stereochemical outcome.
2. Results and discussion
Among the reported syntheses6 of (2R,4R)-4-methyl-2-piperid-
incarboxylate, the good results offered by the reported method of
Agami et al.,7 based on the use of (R)-glycidol as a chiral auxiliary,
⇑
Corresponding author. Tel.: +39 02 50316052; fax: +39 02 50316040.
E-mail address: patrizia.ferraboschi@unimi.it (P. Ferraboschi).
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.09.009

P. Ferraboschi et al. / Tetrahedron: Asymmetry 22 (2011) 1626–1631
1627
CHCOOCH2CH3
4
3
H
H
5
6
N
CH3
H2N
CH3
CHO
2
(S)
COOCH2CH3
(S)
COOCH2CH3
N
(S)
H
PhCH3
CF3COOH / DMF
+ (1'S,2S)-3
1'
CH3
5
4
(1'S,2R)-3
Rh(nbd)2BF4
H2
(10 bar)
SL-M004-1
º
90 C
COOCH2CH3
Pd(OH)2 / C
CH3CH2OH
N
(S)
argatroban 1
H
N
H
COOCH2CH3
CH3
(2R,4R)-2
(1'S,2R,4S)-6
(1'S,2R,4R)-6
98
+
:
2
Scheme 1.
were noteworthy. Nevertheless we still considered an older prepa-
ration4,5 of 2 to be more attractive, not necessarily in terms of
the yield of the final product, but due to the facility of obtaining
the intermediate product 3. In this case, the chiral auxiliary is
the enantiomerically pure 1-phenylethylamine, the choice
between the (R)- or (S)-isomer being addressed by the desired con-
figuration at the 2-position of the piperidine derivative. We started
from commercially available (S)-1-phenylethylamine 4, which in-
duced the formation of the (2R)-2-piperidine carboxylate, over
the course of a Diels–Alder condensation between 2-methyl
butadiene and imine 5 (Scheme 1). In order to establish the diaste-
reoisomeric ratio of the crude Diels Alder products, we developed a
GLC method which showed an 82:18 ratio of the (10S,2R)- and
(10S,2S)-isomers. After separation of the diastereoisomers by silica
gel column chromatography, a comparison with the reported 1H
NMR spectra allowed the assignment desired of the (2R)-configura-
tion to the prevalent diastereoisomer, compound 3.
(2R,4R)-2 via the reduction of 3 with rhodium (I)-[1,4-bis(diphenyl-
phosphino)butane]-(1c,5c-cyclooctadiene) tetrafluoroborate, and
the removal of the chiral auxiliary, as reported in a 1997 German
patent9 constituted an encouraging precedent (62% overall yields
and 93.6 diastereomeric excess, after column chromatography for
each step).
The catalyst screening was performed utilizing P,P- and P,N-
coordinating ligands with rhodium, ruthenium, and iridium cata-
lysts from Solvias. Either isolated metal-complexes or free ligand
combinations with a neutral or cationic metal precursor were
employed.
While iridium complexes bearing P,N ligands afforded low con-
versions and a 1:1 diastereomeric ratio, use of P,P ligands led to the
prevalent formation of the undesired (2R,4S)-cis-isomer. Ruthe-
nium-catalyzed hydrogenations provided low conversions while
the high H2 pressures required to obtain good diastereoselectivities
made these results less attractive from an economical standpoint.
The best results, shown in Table 1, were observed with rhodium
complex-catalyzed hydrogenations (the ligand structures investi-
gated are reported in Fig. 1).
Some preliminary experiments using the achiral ferrocenyl li-
gand 7 (SL-F201-0) revealed that full conversion and 98:2 dr could
be obtained at 100 bar and 80 °C, while at lower pressures and room
temperature, the conversion as well as diastereoselectivity greatly
decreased (entries a and b). Axially chiral ligands enantiomers, 8
(SL-A101-1) and 9 (SL-A101-2), in combination with a neutral rho-
dium precursor, (entries c and d) showed opposite diastereoselec-
tivity. A good dr was observed with the latter ligand, but in favor
of the undesired isomer. The use of 8 with the cationic precursor
[Rh(cod)2]BF4 (bis(1,5-cyclooctadiene)rhodium tetrafluoroborate)
resulted in 85% conversion and a slightly higher diastereoselectivity
of 78:22 (entry e). Good diastereoselectivity (entry f), but with low
conversion, was observed with ligand 10 (SL-M004-1), a chiral ferr-
ocenyl phosphine belonging to the Mandyphos family,10–13 which is
characterized by a C2-symmetrical backbone (Fig. 2). Starting from
these results, an additional screen was planned while taking into
account not only the diastereomeric ratio and the conversion values
but also the reaction conditions [most importantly hydrogen pres-
sure, but also the temperature and substrate/catalyst (S/C) ratio].
The cationic complex bis(norbornadiene) rhodium(I) tetrafluorobo-
In the original synthesis4 the double bond present in (10R,2S)-3
was hydrogenated using 3% Pt/C as a catalyst and afforded an
88:12 (10R,2S,4R)-cis/(10R,2S,4S)-trans isomer mixture with the
minor component being the opposite enantiomer of the compound
required for the argatroban synthesis. The reduction of (10S,2R)-3
was carried out under the same conditions and afforded
(10S,2R,4R)-6 with comparable diastereomeric ratio (dr) (estab-
lished by GLC) and yields (9% after a silica gel column chromatogra-
phy); this was used as the reference reaction to develop the
hydrogenation conditions of the intermediate 3 with improved
yields and stereochemical outcome. Since with platinum on char-
coal as a catalyst, hydrogen addition to the 4,5-double bond takes
place from the less hindered phaseface, the cis-isomer is formed
predominately. It was, however, reasonable to expect a different,
more favorable, result using a different catalyst. After an attempt
with rhodium on alumina that furnished a 1:1 (10S,2R,4S)-cis/
(10S,2R,4R)-trans diastereoisomeric mixture, we turned our atten-
tion to low valent rhodium, ruthenium and iridium complexes,
which are known to be very active and versatile catalysts for homo-
geneous asymmetric hydrogenations.8 Moreover, the preparation of
The respective enantiomers (10R,2S) and (10R,2R) are describes in the literature.4

1628
P. Ferraboschi et al. / Tetrahedron: Asymmetry 22 (2011) 1626–1631
Table 1
Results of preliminary catalyst screening
Entry
Metal precursor
Ligand (L)
S/C
Solvent
p (bar)
T (°C)
Convb (%)
drb (%)
a
a
b
c
d
e
f
[Rh(cod)(7)]BF4
[Rh(cod)(7)]BF4
[Rh(nbd)Cl]2
[Rh(nbd)Cl]2
[Rh(cod)2]BF4
[Rh(nbd)2BF4]
25
25
25
100
25
100
EtOH
EtOH
PhCH3/DCE
PhCH3/DCE
EtOH
100
20
100
80
100
20
80
rt
80
80
80
rt
>99
41
95
77
85
23
98.8:1.2
81:19
68:32
6:94
78:22
84:16
a
8
9
8
10
EtOH
a
Isolated Rh-complex.
Determined by GC.
b
[(CH3)2)CH]2P
O
O
PPh2
PPh2
O
O
PPh2
PPh2
Fe
P[CH(CH3)2]2
9
7
8
H3CO
N(CH3)2
P
N(CH3)2
H
Fe
H
P
Ph
Fe
P
H3CO
Ph
Ph
H
Ph
H
N(CH3)2
P
OCH3
N(CH3)2
OCH3
11
10
Figure 1. Structures of ligands investigated.
rate ([Rh(nbd)2]BF4 and the neutral complex (norbornadiene) rho-
dium(I) chloride dimer ([Rh(nbd)Cl]2) were chosen as rhodium pre-
cursors, and tested with a series of our chiral ligands. Since free
coordinating groups in the substrate, such as the amine functional-
ity present in compound 3, can lead to inactivation of the catalyst
system, methane sulfonic acid (0.5 equiv) was tested as an additive.
Carrying out the reaction at 20 bar and 80 °C with an S/C ratio of 25
for 16 h, more promising results (Table 2) were observed with
[Rh(nbd)2]BF4 as the precursor and two chiral ferrocenyl phos-
phines of the Mandyphos family, namely ligand 11 (SL-M002-1)
and the already tested 10.
Due to these excellent results, 10 was chosen for some addi-
tional experiments (Table 2) in order to investigate if the hydroge-
nation conditions could be improved upon. With an increase of the
S/C ratio upto 400 and a reduction of the hydrogen pressure to
10 bar, the dr remained excellent (98:2) with only a slight decrease
in the conversion (96%). Finally the selected [Rh(nbd)2]BF4/SL-
M004-1 catalyst was used for the preparation of (10S,2R,4R)-6 at
90 °C and 10 bar, in the presence of methanesulfonic acid, on a
gram-scale (50 ml autoclave). This process was successfully scaled
O
up to
a multi-kg scale. The argatroban 1 synthesis was
accomplished, according to a reported method,14 starting from chi-
ral synthon 2, recovered after removal of the chiral auxiliary from
compound 6, by means of a Pd(OH)2/C-catalyzed hydrogenation.4,5
Crude argatroban was purified by silica gel column chromatogra-
phy and subsequent crystallization provided 1 (48% overall yields
from 2) as a mixture of (21R/21S) 65:35 diastereoisomers as estab-
lished by HPLC and NMR15 analyses. The HPLC chromatogram of
argatroban 1 showed two other peaks (0.05% each peak) in addi-
tion to the signals due to the (21R)- and (21S)-diastereoisomers.
LC–MS analysis revealed that these compounds had the same
molecular weight as 1. In order to verify if these impurities corre-
sponded to the (9S,21R/S)-diastereoisomers (C9 of final product is
the C4 of the piperidine moiety), a sample of (9S,21R/S)-argatroban
was prepared starting from the ethyl (2R,4S)-4-methylpipecolate
N
N
P
P
O
H
H
Fe
P
Fe
P
Ph
Ph
Ph
H
Ph
H
N
N
O
O
SL-M002-1
SL-M004-1
Figure 2.

P. Ferraboschi et al. / Tetrahedron: Asymmetry 22 (2011) 1626–1631
1629
Table 2
Hydrogenation of (10S,2R)-3 with [Rh(nbd)2]BF4/L and addition of 0.5 equiv CH3SO3H
Entry
Ligand (L)
S/C
Solvent
p (bar)
T (°C)
Time (h)
Convb (%)
drb (%)
a
b
c
d
e
f
11
10
10
10
10a
10
25
25
100
400
400
400
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH/PhCH3
20
20
10
10
10
10
80
80
90
90
90
90
16
16
18
18
16
16
97
97:3
98:2
98.5:1.5
98:2
98:2
>99
100
90
96
95.5
97:3
a
1 equiv of CH3SO3H instead of 0.5 equiv.
Determined by GC.
b
and a HPLC analysis spiked with these reference compounds con-
firmed this hypothesis.
LC–MS system: HPLC Agilent 1100; Alltima C18 column
m); column temperature 40 °C. Mobile phase:
(150 ꢀ 4.6 mm, 3
l
A: ammonium acetate solution 0.01 M; B: acetonitrile. Elution gra-
dient: A/B 90:10 (10 min), A/B 75:25 (35 min), A/B 50:50 (30 min),
A/B 90:10 (5 min). Flow rate: 1 mL/min. Spectrometer Esquire
300plus using the ESI source with positive ion polarity. Data acqui-
sition and analysis were accomplished with the Bruker Daltonics
DataAnalysis 3.3 software.
Optical rotation values were registered on a Perkin Elmer
instrument (Mod 343) at 589 nm and 25 °C. NMR spectra were
recorded in CDCl3 with a Bruker AVANCE 500 spectrometer.
Chemical shifts are reported on d (ppm) scale from TMS.
3. Conclusion
Argatroban 1 can be regarded as the dipeptide derivative of the
natural L-arginine with the non-proteogenic aminoacid (2R,4R)-4-
methylpipecolic acid, bearing on the arginine amino function a
(3R/S)-3-methyl tetrahydroisoquinoline sulfonyl group. The
straightforward preparation of the intermediate (10S,2R)-3 accord-
ing to a reported method,4,5 prompted us to study an improvement
of its hydrogenation to 6, described in the same papers with global
(cis and trans) 78% yields and a dr of 88:12, in favor of the unde-
sired diastereoisomer. A careful screening of the iridium, ruthe-
nium, and rhodium complexes with P,P- and P,N-ligands allowed
us to identify a hydrogenation catalyst for the intermediate 3
which afforded the desired (2R,4R)-trans-6 with quantitative con-
version and excellent diastereoselectivity (98:2).
We determined that Rh(nbd)2BF4 and Mandyphos ligand 10
afforded the best results when methane sulfonic acid was present
in the reaction mixture, at 90 °C and 10 bar, (see entry f, Table 1
and entries b–f, Table 2) showing that a combination of the sub-
strate and the catalyst control is required in order to obtain high
selectivity.
The screening was performed employing the Solvias catalysts,
which in addition to the observed elevated performances, have
the advantage of being commercially available for laboratory scale
as well as for industrial preparative purposes. Argatroban 1 was
prepared in 48% yield, starting from chiral synthon 2, and showed
excellent purity containing only negligible amounts (0.05%) of the
(9S,21R/S)-diastereoisomers.
4.1. Ethyl[(S)-1-phenylethyl]imino ethanoate 5
To a solution of ethyl glyoxylate (20 mL, 100 mmol) in toluene
(100 mL), (S)-1-phenylethylamine 4 (12.7 mL, 100 mmol) was
added. Water was azeotropically removed by evaporation at atmo-
spheric pressure until the temperature rose to 110 °C. Toluene was
evaporated at reduced pressure. The obtained orange oil (quantita-
tive yields) if not immediately used in the next step, must be kept
under nitrogen at ꢁ20 °C in order to avoid degradation. 1H NMR
(CDCl3) d: 1.38 (t, 3H, J = 7.35 Hz, CH3CH2); 1.65 (d, 3H, J =
6.47 Hz, CH3CH); 4.36 (q, 2H, J = 7.35 Hz, CH3CH2); 4.63 (q, 1H,
J = 6.47 Hz, CHCH3); 7.35 (br s, 5H, Ar); 7.75 (s, 1H, CH@).
4.2. (2R/S)-1-[(10S)-10-Phenylethyl]-2-ethoxycarbonyl-4-methyl-
4,5-dehydro piperidine 3
To a solution of imine 5 (20.11 g, 98 mmol) in N,N-dimethyl-
formamide (70 mL), being stirred at room temperature under a
nitrogen atmosphere, freshly distilled isoprene was added drop-
wise (13.3 g, 195 mmol) over 10 min. The reaction temperature
was brought to 0–5 °C and trifluoroacetic acid (11.25 g, 98.7 mmol)
was added dropwise (60 min) keeping the temperature at 0–5 °C.
The reaction mixture was allowed to reach the room temperature
and stirred for 18–20 h, monitoring the reaction progress by TLC
(n-hexane/ethyl acetate 8:2). The solvent was evaporated under re-
duced pressure and the oily residue was dissolved in toluene
(80 mL). The mixture obtained was sequentially washed with a sat-
urated sodium hydrogen carbonate aqueous solution (80 mL ꢀ 2)
until pH 8 and then with brine (80 mL). The organic phase was
dried over sodium sulfate, filtered, and evaporated under reduced
pressure to obtain a dark oil that was purified by silica gel column
chromatography (30:1). The desired (10S,2R)-isomer was obtained
by elution with n-hexane/ethyl acetate 98:2 with 33% yields. 1H
NMR (CDCl3) d: 1.33 (t, 3H, J = 6.99 Hz, CH3CH2); 1.40 (d, 3H,
J = 6.5 Hz, CH3CH); 1.73 (s, 3H, CH3C@); 2.38 (d, 1H, J = 16.9 Hz,
H-3a); 2.60 (d, 1H, J = 16.2 Hz, H-3b); 2.98 (d, 1H, J = 16.7 Hz, H-
6a); 3.21 (d, 1H, J = 16.7 Hz, H-6b); 4.00 (q, 1H, J = 6.5 Hz, CHN);
4.10 (dd, 1H, J = 5.4, 1.9 Hz, CHCOOEt); 4,22 (q, 2H, J = 6.99 Hz,
CH2CH3); 5.31(br s, 1H, CH@); 7.22–7.45 (m, 5H, Ar). GC Rt 14.23;
for (10S,2S)-isomer Rt 14.05.
4. Experimental
All reagents and solvents were purchased from Sigma–Aldrich,
Milan (Italy). Iridium, ruthenium and rhodium-based complexes
as well as all ligands were obtained from Solvias, Basel
(Switzerland). The GLC system consisted of a VARIAN 3900 while
the detector for capillary columns FID and acquisition data system
was
a
Varian 3900XL. An AT-5 MS (Alltech) column
m) was employed. Carrier: N2 (30 psi);
(60 m ꢀ 0.25 mm ꢀ 0.25
l
injector temperature: 200 °C; detector temperature: 300 °C; oven
temperature: 150 °C (3 min), then from 150 to 250 °C (10 °C/
min), 3 min at 250 °C and then to 300 °C (30 °C /min). Split ratio:
50:1. The HPLC system consisted of an Agilent 1100-series liquid
chromatography, equipped with an auto injector, DAD detector,
and a Chemostation software installed on a PC, for data collecting
and processing. A Zorbax SB-C18 (Agilent) column (100 ꢀ 4.6 mm,
1.8 lm) was employed. Column temperature: 60 °C. Mobile phase:
A: ammonium acetate buffer solution pH 5.5; B: acetonitrile/meth-
anol 1:1. Elution gradient from A/B 70:30 (35 min) to 55:45
(50 min). Flow rate: 1.2 mL/min. Detection UV 260 nm.

1630
P. Ferraboschi et al. / Tetrahedron: Asymmetry 22 (2011) 1626–1631
4.3. Ethyl (2R,4R/S)-1-[(10S)-10-phenylethyl]-4-methylpipecolate 6
4.4. Ethyl (2R,4R)-4-methyl-2-pipecolate 2
To
solution of (10S,2R,4R)-6 (2.5 g, 9 mmol) in ethanol
4.3.1. 5% Pt/C as catalyst
a
To a solution of (10S,2R)-3 (1.13 g, 4.13 mmol) in ethyl acetate
(90 mL) was added 3% Pt/C (0.170 g). The hydrogenation was per-
formed at room temperature and at an atmospheric pressure (48 h)
following the reaction progress by TLC (hexane/diethyl ether 8:2)
and GC. The catalyst was removed by filtration through a Celite
pad; evaporation of the solvent afforded an oil (1.05 g), which
was purified by silica gel column chromatography (10 g). By elu-
tion with hexane/diethyl ether 97:3, pure (10S,2R,4R)-6 (0.101 g,
9%) was recovered. 1H NMR (CDCl3) d 0.92 (d, 3H, J = 5.90 Hz,
CH3-4); 1.08 (m, 1H, H-5a); 1.27 (d, 3H, J = 6.6 Hz, CH3CHN); 1.33
(t, 3H, J = 7.1 Hz, CH3CH2); 1.45–1.57 (m, 3H, H-3a, H-4, H-5b);
2.08 (m, 1H, H-3b); 2.50 (m, 1H, H-6a); 2.86 (dt, 1H, J = 12.6 and
2.8 Hz, H 6b); 3.95–4.02 (m, 2H, H-2 and NCHCH3); 4.20 (m, 2H,
CH2CH3): 7.23–7.52 (m, 5H, Ar). GC Rt 13.76. Elution with hex-
ane/diethyl ether 96:4 afforded the (10S,2R,4S)-isomer 6 (0.34 g,
30%). 1H NMR (CDCl3) d 0.94 (d, 3H, J = 6.10 Hz, CH3-4); 1.19 (qd,
1H, J = 11.9 and 3.5 Hz, H-5a), 1.32 (t, 3H, J = 7.1 Hz, CH3CH2);
1.35(d, 3H, J = 6.9 Hz, CH3CHN); 1.40–1.55 (m, 3H, H-3a, H-4,
H-5b); 1.89 (m, 1H, H-3b); 2.17 (td, 1H, J = 11.2 and 2.3 Hz,
H-6a); 2.51 (dt, 1H, J = 11.2 and 3.2 Hz, H-6b); 3.39 (dd, 1H,
11.2 and 3.2 Hz, H-2); 3.97 (q, 1H, J = 6.9 Hz, NCHCH3); 4.23
(m, 2H, CH2CH3): 7.16–7.44 (m, 3H, Ar); 7.44–7.68 (m, 2H, Ar).
GC Rt 14.44.
(120 mL) 10% Pd(OH)2/C (0.27 g) was added. The hydrogenation
was performed at room temperature and at atmospheric pres-
sure. The reaction progress was monitored by TLC (chloroform/
methanol 9:1). After 4 h, the catalyst was removed by filtration
through a Celite pad. Solvent evaporation at reduced pressure
afforded the crude mixture that was purified by silica gel
(1:20) column chromatography; pure title compound 2 (0.7 g,
45%) was recovered by elution with dichloromethane/methanol
98:2. 1H NMR (CDCl3) d 0.96 (d, 3H, J = 6.50 Hz, CH3-4); 1.15
(m, 1H, H-5a), 1.32 (t, 3H, J = 7.2 Hz, CH3CH2); 1.42–1.54 (m,
1H, H-3); 1.54–1.69 (m, 2H, H-4, H-5b); 2.00–2.11 (m, 2H, H-
3b and NH); 2.87 (m, 2H, H-6); 3.65 (m, 1H, H-2); 4.21 (q, 2H,
J = 7.2 Hz, CH2CH3). ½a D20
ꢂ
¼ ꢁ22 (c 5, ethanol) (lit.4 +24 for the
enantiomer).
Compound (2R,4S)-2 was prepared in the same way starting
from (10S,2R,4S)-6. 1H NMR (CDCl3) d 0.97 (d, 3H, J = 6.50 Hz,
CH3-4); 1.07 (m, 1H, H-5a), 1.29 (m+t, 4H, J = 7.1 Hz, CH3CH2, H-
4); 1.48–1.66 (m, 2H, H-3a, H-5b); 1.78 (br s, 1H, NH); 2.02 (m,
1H, H-3b); 2.46 (td, 1H, J = 2.4 and 12.4 Hz, H-6a); 3.17 (ddd, 1H,
J = 1.9, 3.8, 12.4 Hz, H-6b); 3.31 (dd, 1H, J = 3.0 and 11.9 Hz, H-2);
4.20 (q, 2H, J = 7.1 Hz, CH2CH3).
4.5. Argatroban 1
4.3.2. 5% Rh/Al2O3 as catalyst
Crude 1 prepared from (2R,4R)-2 according to a reported meth-
od14 was purified by silica gel (1:25) column chromatography (elu-
tion with dichloromethane/methanol 8:2), followed by
crystallization (acetone/water 1:1). Pure 1, recovered in 48% yield
from 2, showed chemical and physical properties in agreement
with the reported ones.14 For complete 1H and 13C NMR assign-
ments see the literature.15 HPLC: (21R)-1, Rt 26.87; (21S)-1, Rt
27.44.
To a solution of (10S,2R)-3 (0.64 g, 2.34 mmol) in ethyl acetate
(50 mL), 5% Rh/Al2O3 (0.120 g) was added. The hydrogenation
was performed at room temperature and atmospheric pressure.
After 24 h, the catalyst was removed by filtration and a new
amount of Rh/Al2O3 (0.120 g) was added. Work-up after an addi-
tional 24 h and purification as above afforded the desired
(10S,2R,4R)-6 in 42% yield.
Starting from (2R,4S)-2, diastereomeric (5S,7R,9S,21R/S) arg-
atroban was prepared and analyzed by HPLC and LC–MS using
the methods developed for compound 1: two peaks with Rt 20.76
and 22.04, respectively, were observed. 1H NMR (CDCl3) (selected
signals) d 0.95ꢁ1.02 (d+d, 3H, CH3-21); 1.07 (d, 3H, J = 6.6 Hz,
CH3-7); 3.43 (ddd, 0.4H, J = 11.5, 1.6 and J = 3.3 Hz, H-20a); 3.47
(ddd, J = 0.6H, 11.3, 1.9 and J = 3.5 Hz, H-20a); 6.59 (m, 1H, Ar);
4.3.3. Rh(nbd)2BF4/10 as catalyst
A solution of (10S,2R)-3 (1.45 g, 5.3 mmol) in toluene (10 mL)
was placed in a flask. The solvent was removed under reduced
pressure and the residue dried under high-vacuum. After three
vacuum-argon cycles, the equipment was set under argon. Ethanol
(10 mL), stored over molecular sieves and degassed by bubbling ar-
gon through the solvent, was added with a needle. Methanesulfon-
7.10 (d, 1H, Ar); 7.48 (d, 1H, Ar). ½a D20
ꢂ
¼ þ42 (c 1.02 M, HCl) (lit.16
ic acid (190
l
L, 2.65 mmol, 0.5 eq) was added. In a second flask, the
+43).
metal precursor Rh(nbd)2BF4 (4.97 mg, 0.0133 mmol, S/C 400) was
placed, followed by 10 (15.39 mg, 0.0146 mmol). The flask was set
under argon by three vacuum-argon cycles. Degassed ethanol
(6 mL) was added and the solution obtained was stirred at room
temperature for 20 min. The stainless steel 50 mL autoclave was
set under argon. Under a constant flow of argon, the solution from
the first flask was transferred into the autoclave with a needle, fol-
lowed by the solution from the second flask. The autoclave was
closed, purged with argon (3ꢀ, 10 bar) and then with hydrogen
(3ꢀ, 10 bar). The pressure was adjusted to 7 bar and heating was
started. When the reaction temperature reached 90 °C, the pres-
sure was re-adjusted to 10 bar and stirring was started. The reac-
tion mixture was hydrogenated overnight (16–18 h). Afterward,
heating was stopped, the autoclave was cooled, and vented and
the solution transferred into a flask. Ethanol was evaporated at re-
duced pressure; to the residue, toluene (10 mL) and a 10% sodium
hydrogen carbonate aqueous solution (5 mL) were added. The or-
ganic phase was separated, washed with water and dried by treat-
ment with sodium sulfate. The toluene solution was filtered
through a silica gel pad; after evaporation of the solvent, the title
compound (10S,2R,4R)-6 (1.27 g, 87%, 98:2 dr by GC) was recovered
and used in the next step without further purification.
Acknowledgments
We thank Professor Fulvio Magni (Dipartimento di Medicina
Sperimentale, Università di Milano-Bicocca) for LC–MS spectra.
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