Synthesis and resolution of diethyl (1S,2S)-1-amino-2-vinylcyclopropane-1-phosphonate for HCV NS3 protease inhibitors

Article abstract of DOI:10.1016/j.tetlet.2009.04.044

We herein describe an efficient synthesis of optically active diethyl 1-amino-2-vinylcyclopropane-1-phosphonate (analogous to 1-amino-2-vinylcyclopropane-1-carboxylate). The racemic phosphonate diethyl ester was obtained from an imine derived from aminome

Full text of DOI:10.1016/j.tetlet.2009.04.044

Tetrahedron Letters 50 (2009) 3833–3835
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and resolution of diethyl (1S,2S)-1-amino-2-vinylcyclopropane-
1-phosphonate for HCV NS3 protease inhibitors
*
Hyung-Jung Pyun , Kleem Chaudhary , John R. Somoza, X. Christopher Sheng, Choung U. Kim
Gilead Sciences, 333 Lakeside Drive, Foster City, CA 94404, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We herein describe an efficient synthesis of optically active diethyl 1-amino-2-vinylcyclopropane-1-
phosphonate (analogous to 1-amino-2-vinylcyclopropane-1-carboxylate). The racemic phosphonate
diethyl ester was obtained from an imine derived from aminomethylphosphonate diester and trans-
Received 16 March 2009
Revised 1 April 2009
Accepted 8 April 2009
Available online 14 April 2009
1,4-dibromo-2-butene. Crystallizations of the dibenzoyl-L-tartaric acid salt allowed for separation of
enantiomers. The enantiomerically pure material was used to synthesize an extremely potent tripeptide
phosphonate inhibitor of HCV NS3 protease. X-ray crystal structure of the inhibitor bound to the HCV NS3
protease confirmed the absolute stereochemistry of the title compound.
Ó 2009 Elsevier Ltd. All rights reserved.
One of the approaches to discovering promising drug candi-
dates is the use of a bio-isostere, which utilizes a new functional
group to replace a pharmacophore from a known biologically
active molecule, with the expectation of maintaining or improving
the activity and pharmaceutical properties.1 The phosphonic acid
moiety is considered to be an isostere of the carboxylic acid,
S
N
NH
O
N
O
O
H
N
especially in
a-amino acids and their derivatives. Thus, a-amino-
OH
phosphonic acids and/or their derivatives have been studied exten-
N
H
N
sively as inhibitors and pharmacological agents.2
O
O
O
Hepatitis C virus (HCV) infection is a serious chronic liver dis-
ease which has been known to be linked to liver cirrhosis and
hepatocellular carcinoma.3 It is estimated that over 170 million
people are currently infected with HCV.4 Current standard of ther-
O
1 (BI-2061)
apy is based on a
-interferon (Peg-IntronÒ and PegasusÒ) in combi-
O
O
H2N
H2N
P
OR
OR
nation with ribavirin but it is only partially effective and exhibits
severe side effects.5 Recently, HCV NS3 protease has been proven
to be a promising target for antiviral therapy for HCV. Since the dis-
covery of BI-2061 (1)6 several other promising drug candidates tar-
geting this enzyme have advanced to human clinical trials.7 Among
these candidates, several are mimics of the N-terminal cleavage
product and contain either (+)-(1R,2S)-1-amino-2-vinylcyclopro-
pane-1-carboxylic acid ((+)-2a) moiety or the acylsulfonamide
derivative.8,9
OR
a, R = H
b, R = Me
c, R = Et
(+)-(1R,2S)-2
(+)-(1S,2S)-3
of 1-amino-2-vinylcyclopropane-1-phosphonic acid diethyl ester
(3c) and its resolution.
Based on molecular modeling studies, the phosphonic acid ana-
log 5 was designed as a possible isostere for the carboxylic acid 4
for HCV NS3 protease inhibitors. We report herein the synthesis
Previous reports describe the synthesis of racemic carboxylate
methyl ester, 2b, by alkylation of the imine derived from glycine
methyl ester.10 Similarly, the racemic diethyl phosphonate 3c,
was prepared by the alkylation of commercially available imine 6
as shown in Scheme 1. Thus, imine 6 was treated with trans-1,4-di-
bromo-2-butene (7), in the presence of cesium hydroxide and
phase transfer catalyst.11 After hydrolysis of the imine 8 under
acidic conditions, the racemic diethyl 1-amino-2-vinylcyclopro-
pane-1-phosphonate, ( )-3c, was obtained. An impurity, which
* Corresponding author. Tel.: +1 650 522 5437; fax: +1 650 522 5169.
E-mail addresses: peter.pyun@gilead.com, peterpyun@yahoo.com (H.-J. Pyun).
Present address: Novartis Institutes for BioMedical Research, Inc., 220 Massa-
chusetts Avenue, Cambridge, MA 02139, USA.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.044

3834
H.-J. Pyun et al. / Tetrahedron Letters 50 (2009) 3833–3835
S
N
S
N
NH
NH
O
N
O
O
N
O
O
P
H2N
OEt
OEt
a
+
H
N
OH
R
N
N
H
H
N
O
O
N
O
O
O
O
(+)-3c
O
O
10
4; R = COOH
5; R = PO(OH)2
S
N
NH
O
N
O
was identified as the seven-membered ring 9, was found to have
formed during the reaction. Fortunately, the impurity 9 could be
easily removed by simple acid–base extractions. A subsequent fil-
tration through silica gel provided ( )-3c in 27% yield. It is note-
worthy that only the Z-diastereomer, between phosphonate and
vinyl groups, was isolated in this reaction. A similar seven-mem-
bered ring impurity was also reported in the synthesis of ( )-
2b.10b We speculate that this byproduct 9 is formed via an aza-
Cope rearrangement of the undesired E-diastereomer of the imine
8, which has the cis-vinyl-imine functionality on the strained
cyclopropane ring.
O
P
OR
H
N
OR
N
H
O
O
N
O
O
11; R = Et
5;R = H
b
Scheme 2. Synthesis of HCV NS3-4A inhibitors
5 from (+)-3c. Reagents and
conditions: (a) ClCOOEt, NEt3, THF, À40 °C to rt, 1 h. (b) TMSI, CH3CN, rt, 30 min.
A resolution of racemic 3c was attempted using crystallization
with optically pure acids.12 Our optimized crystallization was
The absolute stereochemistry of (+)-3c was clearly confirmed by
X-ray crystal structure of compound 5 bound to the protease. As
shown in Figure 1, the crystal structure indicates that the phos-
phonic acid makes extensive interactions with the oxyanion hole
of HCV NS3 protease. Thus, the phosphonate was shown to interact
with Ser138, Ser139, and Gly137 backbone amides as well as His57
side chain similarly to BILN 2061 as described in the published
crystal structure.8b Furthermore, the phosphonic acid was found
to make additional H-bonds with the side chains of Ser139 and
Lys136 (see Fig. 1).
achieved with dibenzoyl-
crystallizations, (+)-(1S,2S)-3c could be obtained as the diben-
zoyl-
-tartaric acid salt in >90% ee.13–15
After removing dibenzoyl- -tartaric acid by acid–base extrac-
L-tartaric acid in acetonitrile. After two
L
L
tion, (+)-3c was coupled with the dipeptide precursor 10 as shown
16
in Scheme 2. The diethyl phosphonate ester of the resulting tri-
peptide analog 11 was hydrolyzed using TMSI in acetonitrile to af-
ford the desired phosphonic acid analog 5. We were pleased to find
that the phosphonic acid analog 5 was a potent HCV NS3 protease
inhibitor with IC50 of 0.9 nM. This is three-fold more potent com-
pared to the carboxylic acid analog 4 (IC50 = 3 nM).17
In summary, a procedure has been developed to synthesize the
phosphonic acid analog ( )-3c of 1-amino-2-vinylcyclopropane-1-
carboxylic acid (( )-2). Resolution of the two enantiomers of 3c
was accomplished by crystallization as dibenzoyl-L-tartaric acid
salt. A tripeptide phosphonic acid analog 5 was synthesized from
O
O
a
Ph
N
P OEt
OEt
Br
+
Ph
N
P
OEt
OEt
Br
6
7
8
O
O
O
H2N
P OEt
OEt
BzO
BzO
H2N
P
OEt
OEt
OH
OH
c
O
b
(+)-3c
(+)-(1S,2S)-3c
dibenzoyl-L-tartaric acid salt
+
O
H
Ph
P OEt
OEt
N
9
Scheme 1. Synthesis and resolution of diethyl 1-amino-2-vinylcyclopropane-1-
phosphonate (6). Reagents and conditions: (a) CsOH hydrate, BnNEt3Cl, CH2Cl2, rt,
24 h. (b) 1 N HCl, CH2Cl2, rt, 3 h. (c) 1 equiv dibenzoyl-
L-tartaric acid, MeCN.
Figure 1. X-ray crystal structure of HCV NS3 protease–compound 5 complex.

H.-J. Pyun et al. / Tetrahedron Letters 50 (2009) 3833–3835
3835
N.; Scola, P. M.; Meanwell, N. A.; Wang, X. A.; Sun, L.-Q.; Chen, J.; Good, A. C.;
WO Patent 2006/122188 A2, 2006.; (i) Bailey, M. D.; Forgione, P. Llinas-Brunet,
M.; Poupart, M.-A.WO Patent 2007/009227, 2007.
(+)-3c and was found to be a superior inhibitor of HCV NS3 prote-
ase compared to the carboxylic acid analog 4. Absolute stereo-
chemistry of (+)-3c was further proven by X-ray crystal structure
of 5 bound to HCV NS3 protease. Further studies on the inhibition
of HCV NS3 protease with phosphonic acid analogs will be reported
separately.16
10. (a) Park, K.-H.; Olmstead, M. M.; Kurth, M. J. J. Org. Chem. 1998, 63, 6579; (b)
Beaulieu, P. L.; Gillard, J.; Bailey, M. D.; Boucher, C.; Duceppe, J.-S.; Simoneau,
B.; Wang, X.-J.; Zhang, L.; Grozinger, K.; Houpis, J.; Farina, V.; Heimroth, H.;
Krueger, T.; Schnaubelt, J. J. Org. Chem. 2005, 70, 5869.
11. Procedure for the synthesis of racemic 1-amino-2-vinyl-cyclopropane-
1-phosphonic acid diethyl ester (3c) is as follows:
A mixture of diethyl
(N-benzylideneaminomethyl)-phosphonate (6, 50 g, 196 mmol), trans-1,4-
dibromo-2-butene (7, 50 g, 235 mmol), and benzyltriethylammonium
chloride (4.5 g, 19.6 mmol) in dichloromethane (1.0 L) was stirred at rt with
cesium hydroxide monohydrate (82 g, 490 mmol) using a mechanical stirrer
for 18 h after which another portion of cesium hydroxide monohydrate (82 g,
490 mmol) was added. After stirring for 24 h more, the solids were then
filtered off through Celite pad and the filtrate was allowed to stir with 1 N aq
HCl (500 mL) at rt for 3 h. The resulting mixture was filtered again through
Celite pad and the two phases of the filtrate were separated. The organic phase
was extracted with 1 N aq HCl (250 mL Â 1). The two aqueous phases were
washed with dichloromethane (250 mL Â 1) and were combined. The
combined aq. solution was stirred with ethyl acetate (500 mL) while 84 g
(1 mol) of NaHCO3 was added cautiously. After the aqueous layer was
saturated with NaCl, two layers were separated and the aqueous layer was
extracted further with ethyl acetate (250 mL Â 2). The organic phases were
washed with saturated NaCl solution (250 mL Â 1), combined, dried (MgSO4),
and concentrated to obtain 16.5–17 g of the crude amine. The crude amine was
purified by column chromatography using 165–170 g of silica gel by eluting
ethyl acetate (100%, ꢀ500 mL) followed by 5% methanol in ethyl acetate
(ꢀ1200 mL) to afford 11.5–12 g of ( )-3c.
Acknowledgments
The authors thank Mr. Scott Hluhanich, Ms. Huiling Yang, and
Dr. William Delaney for determining HCV NS3 enzymatic activities.
References and notes
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13. Resolution procedure of 3c is as follows: To the purified ( )-3c (11.5–12 g) was
added a solution of 18.8–19.6 g (1 mol equiv) of dibenzoyl-
L-tartaric acid in
152–158 mL of acetonitrile and the resulting salt was crystallized twice at rt to
obtain 11.5 g of the dibenzoyl-L-tartaric acid salt of (+)-(1S,2S)-3c in >90% ee.
1H NMR (300 MHz, CD3OD) d 8.14 (br, 2H), 8.11 (d, J = 1.2 Hz, 2H), 7.64 (tt,
J = 7.5 and 1.2 Hz, 2H), 7.51 (br t, J = 7.5 Hz, 4H), 5.94 (s, 2H), 5.82 (dt, J = 17.1
and 9.9 Hz, 1H), 5.32 (dd, J = 17.1 and 1.2 Hz, 1H), 5.13 (dd, J = 10.5 and 1.2 Hz,
1H), 4.11–4.26 (m, 4H), 2.11 (m, 1H), 1.33–1.47 (m, 2H), 1.37 (dt, J = 10.2 and
7.2 Hz, 6H); 31P NMR (75 MHz, CD3OD) d 22.55.
14. Analytical sample of (+)-3c was obtained by basic extraction of the salt. The
dibenzoyl-L-tartaric acid salt of (+)-(1S,2S)-3c (10 g) was dissolved in a mixture
of saturated aq. NaHCO3 (200 mL) and saturated aq. NaCl (200 mL), and the free
amine was extracted by dichloromethane (100 mL Â 2). The extracts were
washed once with a mixture of saturated aq NaHCO3 (200 mL) and saturated
aq NaCl (200 mL), dried (MgSO4), and concentrated. The residue was purified
by silica gel column chromatography by eluting ethyl acetate to obtain 3.36 g
7. Chen, S.-H.; Tan, S.-L. Curr. Med. Chem. 2005, 12, 2317.
8. (a) Rancourt, J.; Cameron, D. R.; Gorys, V.; Lamarre, D.; Poirier, M.; Thibeault,
D.; Llinas-Brunet, M. J. Med. Chem. 2004, 47, 2511; (b) Tsantrizos, Y. S.; Bolger,
G.; Bonneau, P.; Cameron, D. R.; Goudreau, N.; Kukoji, G.; LaPlante, S. R.; Llinas-
Brunet, M.; Nar, H.; Lamarre, D. Angew. Chem., Int. Ed. 2003, 42, 1356; (c) Llinas-
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of pure (+)-3c as an oil. [a]
D = +26.6 (c 1, EtOH); 1H NMR (400 MHz, CDCl3) d
5.92 (dt, J = 16.8 and 10.0 Hz, 1H), 5.24 (dd, J = 16.8 and 1.6 Hz, 1H), 5.04 (dd,
J = 10.0 and 1.6 Hz, 1H), 4.05–4.19 (m, 4H), 1.82–1.92 (m, 1H), 1.81 (br, 2H),
1.33 (dt, J = 14.0 and 6.8 Hz, 6H); 1.26–1.36 (m, 2H), 1.17 (ddd, J = 9.2, 6.0, and
5.2 Hz, 6H); 31P NMR (100 MHz, CDCl3) d 27.15.
15. Enantiomeric purity of (+)-3c was determined by NMR analysis of the Mosher
amide.18 Among the various deuterated solvents investigated, DMSO-d6
exhibited the best separation of the 31P signals of the two diastereomers.
16. Sheng, X. C.; Pyun, H.-J.; Chaudhary, K.; Wang, J.; Doerfler, E.; Flurry, M.;
McMurtrie, D.; Chen, X.; Delaney, W.; Kim, C. U. Bioorg. Med. Chem. Lett.,
submitted for publication.
17. Compound 4 was synthesized and its enzymatic activity was tested in house
for comparison.
18. Dale, J. A.; Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512.