Development of an oxazolopyridine series of dual thrombin/factor Xa inhibitors via structure-guided lead optimization

Article abstract of DOI:10.1016/j.bmcl.2005.07.022

Thrombin-inhibitor X-ray crystal structures, in combination with the installation of binding elements optimized within the pyrazinone series of thrombin inhibitors, were utilized to transform a weak triazolopyrimidine lead into a series of potent oxazolop

Full text of DOI:10.1016/j.bmcl.2005.07.022

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4411–4416
Development of an oxazolopyridine series of dual thrombin/factor
Xa inhibitors via structure-guided lead optimization
James Z. Deng,a Daniel R. McMasters,b Philippe M. A. Rabbat,a Peter D. Williams,a
Craig A. Coburn,a Youwei Yan,c Lawrence C. Kuo,c S. Dale Lewis,d Bobby J. Lucas,d
Julie A. Krueger,d Berta Strulovici,e Joseph P. Vacca,a Terry A. Lylea
and Christopher S. Burgeya,*
aDepartment of Medicinal Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
bDepartment of Molecular Systems, Merck Research Laboratories, West Point, PA 19486, USA
cDepartment of Structural Biology, Merck Research Laboratories, West Point, PA 19486, USA
dDepartment of Biological Chemistry, Merck Research Laboratories, West Point, PA 19486, USA
eDepartment of Automated Biotechnology, Merck Research Laboratories, West Point, PA 19486, USA
Received 12 April 2005; revised 19 July 2005; accepted 20 July 2005
Available online 30 August 2005
Abstract—Thrombin-inhibitor X-ray crystal structures, in combination with the installation of binding elements optimized within
the pyrazinone series of thrombin inhibitors, were utilized to transform a weak triazolopyrimidine lead into a series of potent oxaz-
olopyridines. A modification intended to attenuate plasma protein binding (i.e., conversion of the P3 pyridine to a piperidine) con-
ferred significant factor Xa activity to this series. Ultimately, these dual thrombin/factor Xa inhibitors demonstrated excellent
in vitro and in vivo anticoagulant efficacy.
Ó 2005 Elsevier Ltd. All rights reserved.
In an attempt to overcome the monitoring and safety
liabilities associated with current anticoagulation
therapies, an intensive effort to develop orally bio-
available inhibitors of critical coagulation enzymes
has been undertaken. Among the potential targets,
thrombin (factor IIa) and factor Xa have received
the most attention. A recent disclosure on the clinical
within our thrombin inhibitor program, the Merck sam-
ple repository was screened for activity versus thrombin;
this survey revealed the triazolopyrimidine 2a to be a
weak inhibitor (IC50 ꢀ 1 lM, Fig. 1).4 Herein, we detail
an optimization process in which inhibitor-enzyme
X-ray crystal structures were utilized to convert this lead
into a series of potent dual thrombin/factor Xa
inhibitors.
efficacy of
a
small-molecule thrombin inhibitor,
ximelagatran, in the prevention of stroke in patients
with atrial fibrillation has helped validate this strate-
gy.1 Recently, there has been considerable interest in
developing dual inhibitors of both thrombin and fac-
tor Xa that could afford highly efficacious antithrom-
botic agents.2
Determination of the X-ray crystal structure of 2a com-
plexed to thrombin helped elucidate the following gener-
al binding features (Fig. 2): the 5-chloro-2-methylaniline
occupies the S1 specificity pocket, the methyl of the
triazolopyrimidine fills the S2 insertion loop, and the bu-
tyl group binds to the distal hydrophobic pocket. The
antiparallel b-sheet hydrogen bonding motif, common
to many thrombin inhibitors,5 is maintained between
the triazolopyrimidine and Gly-216: the substrate is
Previous work from our laboratories has primarily fo-
cused on the design, development, and optimization of
the pyrazinone acetamide series of thrombin inhibitors
(e.g., 1).3 In an effort to expand the structural diversity
˚
strongly hydrogen bonded to the C@O (dN–O = 2.26 A)
and NH (dN–N = 2.72 A) of Gly-216, although the
˚
acceptor angles are not optimal.6 The inhibitor aniline-
NH is possibly engaged in a weak H-bond with Ser-
Keywords: Thrombin; Factor Xa; Structure-guided design.
*
˚
195 (dN–O = 3.45 A). Another notable feature is that
the P3 butyl group does not take advantage of the
Corresponding author. Tel.: +1 215 652 2382; fax: +1 215 652
7310; e-mail: christopher_burgey@merck.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.07.022

4412
J. Z. Deng et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4411–4416
inhibitor H-bonding interactions with Gly-216. These
Cl
N
considerations led to the design and preparation of
benzoxazole 3 (Fig. 1), in which the additional modifica-
tions of P3 2,2-difluoro-2-(2-pyridyl)ethyl ligand3 incor-
poration, and P1 and P2 methyl group deletion have
been made. These extensive structural changes yielded
N
O
N
O
N
H
F
F
F
O
N
H
1
Ki = 2.3 nM
a
(Ki = 670 nM), from which optimization studies could
moderately potent thrombin inhibitor,
3
2xaPTT = 0.40 µM
be readily developed.
Me
simplify
bicyclic core
N
Guided by the X-ray structure (Fig. 2), efforts to explore
the nature of a P1–P2 linker were initiated. Molecular
modeling studies have revealed that, to achieve the
conformation necessary for thrombin binding, the
aniline P1 phenyl and P2 triazole rings must break
away from the coplanar arrangement that is favored in
the unbound state of this extended conjugated system
(DE(bound–unbound) = 7.3 kcal/mol). Elimination of this
preference for coplanarity by insertion of a methylene
group between the NH and the 3-chlorophenyl group
resulted in a significant improvement in potency (Ta-
ble 1, 4). This is presumably due, at least in part, to a
smaller energy difference between the bound and un-
bound states (DE(bound–unbound) = 4.7 kcal/mol). Alter-
natively, replacement of the aniline NH with a CH2
R
N
N
H
N
H
N
O
N
F
F
install
optimized P3
N
N
NH
NH
Cl
Cl
Me
2a : R = CH2CH3 : IC50 ~1 µM
2b : R = Ph : Ki = 290 nM
3
Ki = 670 nM
Figure 1.
afforded
a 27-fold improvement in potency (5,
Ki = 25 nM); this analog combines both the correct
linker length (compare 6: Ki = 16 lM) and angle
(i.e., sp3 hybridization) required to access the S1 sub-
site via a low-energy conformer (DE(bound–unbound)
3.5 kcal/mol).
=
Having established the preferred P1–P2 linker,8 the P1
aryl group SAR was investigated next (Table 1). As
anticipated, installation of the 2,5-dichlorophenyl P1
ligand afforded a substantial improvement in potency
(Table 1, 7).3 Incorporation of the 1-phenyl-1,2,4-tria-
zoles and tetrazoles, P1 binding elements that afforded
extremely potent pyrazinone thrombin inhibitors,9
brought about an improvement in potency (8–9). The
triazole and m-chloro substituents were combined to
afford 10, an 80 pM thrombin inhibitor.
The preceding studies afforded a series of potent throm-
bin inhibitors; however, the enzyme potency did not
translate into good in vitro anticoagulant efficacy, as
measured by the 2xaPTT assay in human plasma.7 For
example, inhibitor 8 exhibited a good binding potency
of 2.1 nM but a weak functional potency (2xaPTT) of
3.2 lM. This discrepancy between binding and clotting
potency is presumably due to the high lipophilicity
and protein binding of this series of benzoxazole inhib-
itors (e.g., 8: clogP = 3.0).10 In an effort to reduce the
overall lipophilicity, the analogous oxazolopyridine ser-
ies was prepared (Table 2). Although compounds within
this series generally exhibited a slightly diminished
inherent potency, the shift in anticoagulant efficacy
(2xaPTT assay) was reduced in comparison to the benz-
oxazoles. For example, 8 and 11 possess the same anti-
coagulant potency, even though 8 is an 8-fold more
potent thrombin inhibitor. In accord with the pyrazi-
none series of thrombin inhibitors, conversion of the
P3 pyridine to its N-oxide confers an improvement in
Figure 2. X-ray crystal structures of 1 (blue) and 2a bound to the
thrombin active site.
potential p-interactions (with Trp-215) available within
the S3 pocket. Replacement of the P3 butyl with a phen-
ethyl group exploits these interactions to bring about a
3-fold improvement in potency (2b, Ki = 290 nM,
Fig. 1).7
Further analysis of this X-ray structure (Fig. 2; PDB
numbers: 1MUE and 1ZGV for 1 and 2a, respectively)
has indicated that the majority of heteroatoms within
the triazolopyrimidine ring system are not engaged in
specific, constructive interactions with the enzyme. As
a result, an alternative and structurally more simple core
was sought in which a modular synthesis (vide infra)
would facilitate optimization of this series. A basic crite-
rion in this analysis was the preservation of crucial

J. Z. Deng et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4411–4416
Table 1. SAR of P1–P2 linker and P1 phenyl ring
4413
N
N
triazole =
N
N
N
N
O
X
H
F
F
N
N
N
N
tetrazole =
R1
R2
R2
Compound
X
R1
IIa Ki (nM)
2xaPTT (lM)
3
4
NH
NHCH2
CH2
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
Cl
670
150
—
—
5
25
—
—
6
CH2CH2
CH2
CH2
16,000
3.4
2.1
7
>8.2
3.2
5.2
8
Tetrazole
Triazole
Triazole
9
10
CH2
CH2
H
Cl
10.0
0.08
0.81
Table 2. SAR of P2 oxazolopyridines
R3
N
N
N
N
triazole =
N
N
N
O
H
F
F
N
O n
N
N
tetrazole =
N
R1
R2
Compound
n
R1
R2
R3
IIa Ki (nM)
2xaPTT (lM)
Xa Ki (lM)
11
12
13
14
15
16
17
18
19
0
0
0
0
1
1
1
1
1
H
Triazole
Triazole
Triazole
H
H
16
3.0
47
0.77
0.75
>20
3
Cl
Cl
Cl
Cl
Cl
H
H
0.31
0.04
7.8
15
0.40
0.41
Me
Me
H
>8.2
H
>8.2
3.0
H
Me
H
2.3
9.4
1.5
2.0
6
14
Triazole
Triazole
Tetrazole
1.4
H
Me
H
0.49
0.38
7.2
2.7
H
both enzyme and functional potency (15–19, Table 2).3
For instance, triazole analog 17 containing the N-oxide
was 2-fold more potent in the clotting assay (2xaPTT)
than its pyridine version 11. Reintroduction of the inser-
tion loop substituent (i.e., 6-methyl group) affords a 6-
fold potency improvement (13–14, 16, and 18). Notably,
this series of thrombin inhibitors exhibits an excellent
selectivity profile against the homologous proteases
trypsin (>20,000-fold) and factor Xa (>200-fold).
protease selectivity versus trypsin, chymotrypsin, acti-
vated protein C, kallikrein, and plasmin (>32 lM,
>9000-fold); tPA selectivity was somewhat diminished
(2.9 lM, 880-fold). The chlorotriazole P1 was also com-
patible with the P3 piperidine, generating the potent
dual IIa/Xa inhibitor 23 with an extremely low 2xaPTT
value.
The geminal fluorines of 2,2-difluoro-2-(2-pyridyl)ethyl
P3 have been proposed to exert a potency enhancing
effect via inductive reinforcement of the p–p interaction
between the P3 pyridine ring and Trp-215 of the en-
zyme.3 Since the aromatic ring (and the potential for
p–p interactions) has been removed from the piperidine
series, deletion of the fluorines would be expected to
have minimal perturbation on thrombin binding.
Indeed, this modification did not significantly impact
thrombin or Xa activity (24 vs. 22). Resolution of the
racemic piperidine revealed the differential binding of
each enantiomer (25 vs. 26), with the same antipode
being more active for both IIa and Xa. The combina-
tion of the racemic piperidine P3 and the potent
Further investigation into the P3 SAR (Table 3) has re-
vealed that reduction of the pyridine to a piperidine, a
modification intended to attenuate plasma protein bind-
ing, afforded an improvement in thrombin potency (21
vs. 11). Additionally, incorporation of this 2,2-difluoro-
2-(2-piperidyl)ethyl P3 ligand unexpectedly conferred
significantly greater factor Xa activity to this series (21:
Ki (Xa) = 51 nM vs. 11: Ki (Xa) = 47,000 nM). As a re-
sult of the increased factor Xa binding and enhanced
polarity of the piperidine, these derivatives exhibit a
large improvement in functional potency. Significantly,
introduction of this fXa activity did not compromise

4414
J. Z. Deng et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4411–4416
Table 3. SAR of P3 piperidinesa
N
N
N
N
triazole =
N
N
R3
N
O
H
X
X
N
N
N
tetrazole =
N
R1
R2
Compound
X
R1
R2
R3
IIa Ki (nM)
Xa Ki (nM)
tryps Ki (lM)
tPA Ki (nM)
2xaPTT (lM)
20
21
22
23
24
25b
26b
27
28
29
30
31
32
33
F
Cl
H
H
H
H
H
H
H
H
H
H
Bn
3.3
4.6
12.0
50.5
11.0
1.2
44
86
2900
840
1400
240
2000
2100
—
1.6
F
Triazole
Tetrazole
Triazole
Tetrazole
H
0.36
0.18
0.11
0.11
0.29
—
F
H
1.2
17
F
Cl
H
0.06
1.7
1.5
23
H
H
H
H
H
H
H
H
H
H
7.0
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1.7
12.0
460
3.9
16
H
100
0.04
0.14
0.07
0.16
0.25
0.52
4.6
342
1.6
1.5
1.3
0.70
1.6
2.9
6.9
Triazole
Triazole
Triazole
Triazole
Triazole
Triazole
Triazole
140
100
67
0.07
0.13
0.15
0.08
0.06
0.44
1.74
3.7
2.2
CH2CO2Et
CH2CO2H
CH2CH2OH
CH2CHF2
CH2CF3
4.2
2.4
110
96
4.7
290
3500
54
a Racemic, except where noted.
b Enantiomers.
chlorotriazole P1 produced 27, a 40 pM thrombin
inhibitor that demonstrated excellent in vitro anticoag-
ulant potency (2xaPTT = 70 nM).
Me
H
C
N
N
H
N
N
H
N
H
O
O
Solution of the X-ray structure of 21 bound to thrombin
revealed a series of weak hydrogen bonding interactions
(Fig. 3). The exocyclic NH and oxazole ring N are en-
gaged in hydrogen bonds with Gly-216 (dN–O = 3.3 A,
˚
dN–N = 3.6 A). The P3 piperidinium nitrogen appears
H
F
F
N
N
N
˚
N
Cl
N
34
35
to be solvent-exposed and engaged in forming an intra-
molecular hydrogen bond with the aminopyridine ring
nitrogen (dN–N = 3.5 A). Support for the critical role
that this interaction serves is provided by 34 (Chart 1),
in which replacement of the pyridine nitrogen with a
Ki(IIa) = 1000 nM
Ki(Xa) = 60000 nM
Ki(IIa) = 3.1 nM
Ki(Xa) = 400 nM
11
˚
Chart 1.
carbon reduces IIa and Xa activity by 218- and 1200-
fold, respectively. In line with the solvent-exposed nat-
ure of the piperidine nitrogen is its tolerance to varied
substitution (e.g., alkyl, carboxyalkyl, and fluoroalkyl;
28–33, Table 3). The lack of a specific role for the fluo-
rines adjacent to the P3 piperidine is also apparent from
this X-ray structure. The P1 aryl ring reaches deep into
the lipophilic S1 pocket and the triazole is coordinated
˚
to an ordered water molecule (dN–O = 2.9 A) and
additionally engaged in a donor-atom–p interaction
with the electron-rich disulfide bridge at the subsite en-
trance.9 The catalytic Ser-195 lies relatively close
˚
(dC–O = 3.0 A) to the polarized methylene bridging the
oxazole to the P1 aryl group and appears to be engaged
in a non-classical hydrogen bonding interaction.6
Based upon the structure bound to thrombin, dual
inhibitor 21 was modeled into the human factor Xa ac-
tive site (Fig. 3; PDB code: 1ZGI).12 The most signifi-
cant difference between the IIa and Xa active sites is
the composition of the distal hydrophobic (S3/S4) and
proximal (S2) binding pockets.2 In thrombin, Ile-174
Figure 3. Overlay of 21 bound in the fIIa and fXa (yellow) active sites.
Key amino acids of the fXa active site are depicted in magenta.

J. Z. Deng et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4411–4416
4415
terminates the distal pocket, while the aromatic amino
N
N
a
b
acid Phe-174 performs this role in fXa. In S2, there exist
more dramatic differences: thrombin possesses a well-de-
fined hydrophobic binding cleft (insertion loop) defined
by the side chains of Leu-99, Tyr-60A, and Trp-60D; in
comparison, Leu-99 is replaced by a more sterically
demanding Tyr-99 in fXa, severely occluding this bind-
ing pocket. These differences in the enzyme structure
cause the oxazolopyridine ring of 21 in the Xa structure
to tilt away from the Tyr-99 (allowed to do so owing to
lack of Trp-60D). This change could account for the loss
of fXa binding affinity observed upon exchange of the
P3 piperidine for a pyridine, as this shift of the bicyclic
core would prevent the more rigid P3 pyridine from fully
extending into the S3 subsite. Consistent with this
restricted fit in the Xa S2 binding pocket is the 30-fold
loss in potency (vs. 20), due to the steric interaction with
Tyr-99, upon introduction of a methyl group on the
pyridine ring in 35 (Chart 1).
HO
OH
Cl
Cl
NO2
NO2
37
36
c, d
N
+
Cl
OH
N
NH2
NO2
38
BOC
39
Cl
N
O
N
e
+
HO
N
Boc
40
N
OH
H
NH2
N
N
N
41
N
N
H
N
H
O
N
N
OH
Based upon its high level of in vitro anticoagulant
potency, the dual IIa/Xa inhibitor 27 was selected for
in vivo characterization. Complete efficacy was observed
in the rat FeCl3 arterial thrombosis model (0/6 occlu-
sions)13 at low plasma levels (final plasma concentra-
tion = 121 nM), which significantly elevated the
thrombin time (4.65-fold elevation) while producing a
moderate effect upon the aPTT (ca. 1.25-fold elevation).
Upon i.v. dosing of 27 (0.65 mpk) to dogs, a long plas-
N
H
Boc
HN
O
f, g
N
Cl
Cl
N
N
42
N
N
N
27
Scheme 1. Reagents and conditions: (a) POCl3, Bn(Et)3NCl, CH3CN,
reflux; (b) CsOAc, DMF, 80 °C; (c) (iPr)2NEt, EtOH, reflux; (d) 45 psi
H2, 10% Pd/C, EtOAc; (e) EDC, HOAt, (iPr)2NEt, DMF; (f) Ph3P,
i-PrO2CN@NCOiPr, DCM; (g) TFA/DCM.
ma
Vd = 7.2 L/kg) was observed; unfortunately, this analog
half-life
(t1/2 = 4.2 h;
Clp = 26.6 mL/min/kg;
exhibited very low levels of oral bioavailability (Fig. 4).
Deprotection of the Boc group of the cyclized intermedi-
ate completed the synthesis of inhibitor 27.
Dog crossover PK for 27
10
Synthesis of P1 2-triazolyl-5-chlorophenyl acetic acid 41
was achieved, as shown in Scheme 2. Displacement of
the 2-chloro group of 2,5-dichlorobenzonitrile 43 with
1,2,4-triazole, followed by acid hydrolysis, afforded ben-
zoic acid 44. Reduction to the benzyl alcohol and treat-
ment with thionyl bromide afforded benzyl bromide 45.
Bromide 45 was displaced by sodium cyanide and the
Average
Average
1
0.1
0.01
0
1
2
3
4
5
6
Time (Hours)
N
Cl
N
CN
c, d
Figure 4.
N
O
a, b
OH
Synthesis of a representative P2 oxazolopyridine inhibi-
tor 27 is shown in Scheme 1.14 Conversion of 3-nitro-
pyridine-2,4-diol 36 to its 2,4-dichloro analog 37 was
completed with phosphorus oxychloride. A cesium ace-
tate-mediated reaction, which allowed selective substitu-
tion of 4-chloro group of 37 with a hydroxyl group,
afforded key intermediate 38. Displacement of the
remaining 2-chloro group with P3 amine 39, followed
by hydrogenation of the nitro group, resulted in the
intermediate 40. Aminopyridinol 40 was coupled to P1
2-triazolyl-5-chlorophenyl acetic acid 41 to give interme-
diate 42. Formation of the central P2 oxazolopyridine
ring was accomplished by a selective intramolecular
cyclization under Mitsunobu reaction conditions.15
Cl
43
Cl
44
N
N
N
N
N
N
N
N
N
f
e
OH
Br
CN
O
Cl
Cl
Cl
41
46
45
Scheme 2. Reagents and conditions: (a) 1,2,4-triazole, Cs2CO3,
(nBu4)NI, DMF, 90 °C; (b) concd HCl, reflux ; (c) BOP, (iPr)2NEt,
NaBH4; (d) SOBr2, DCM; (e) NaCN, dioxane/H2O, reflux; (f) concd
HCl/AcOH, reflux.

4416
J. Z. Deng et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4411–4416
7. The inhibition constants (Ki) versus thrombin (factor IIa)
and factor Xa for each compound and the concentration
needed to double the activated partial thromboplastin
time (2xaPTT) in human plasma (if Ki < 15 nM) were
determined: Lewis, S. D.; Ng, A. S.; Lyle, E. A.; Mellott,
M. J.; Appelby, S. D.; Brady, S. F.; Stauffer, K. S.; Sisko,
J. T.; Mao, S.-S.; Veber, D. F.; Nutt, R. F.; Lynch, J. J.;
Cook, J. J.; Gardell, S. J.; Shafer, J. A. Thromb. Haemost.
1995, 74, 1107.
resulting arylacetonitrile 46 was hydrolyzed to give the
desired phenyl acetic acid 41.
Thrombin-inhibitor X-ray crystal structures, in combi-
nation with the installation of binding elements
optimized within the pyrazinone series of thrombin
inhibitors, were utilized to transform a weak triazolo-
pyrimidine lead into a series of potent oxazolopyridines.
A modification intended to attenuate plasma protein
binding (i.e., conversion of the P3 pyridine to a piperi-
dine) conferred significant factor Xa activity to this
series. Ultimately, these dual thrombin/factor Xa inhib-
itors demonstrated excellent in vitro and in vivo antico-
agulant efficacy.
8. The composition of the oxazole ring is also important as
the isomer i incurs a 180-fold and the imidzaole ii a 12-fold
potency loss versus 5.
N
N
H
N
N
H
N
NH
F
F
F
F
O
N
Cl
Cl
References and notes
1. Diener, H. C. Cerebrovasc. Dis. 2004, 17, 16; Gustafsson,
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M., Jr.; Detwiler, T. J., Jr.; Lynch, J. J., Jr.; Yan, Y.;
Chen, Z.; Kuo, L.; Gardell, S. J.; Shafer, J. A.; Vacca, J. P.
J. Med. Chem. 2003, 46, 461.
i
ii
Ki = 290 nM
Ki = 4500 nM
9. Young, M. B.; Barrow, J. C.; Glass, K. L.; Lundell, G. F.;
Newton, C. L.; Pellicore, J. M.; Rittle, K. E.; Selnick, H.
G.; Stauffer, K. J.; Vacca, J. P.; Williams, P. D.; Bohn, D.;
Clayton, F. C.; Cook, J. J.; Krueger, J. A.; Kuo, L. C.;
Lewis, S. D.; Lucas, B. J.; McMasters, D. R.; Miller-Stein,
C.; Pietrak, B. L.; Wallace, A. A.; White, R. B.; Wong, B.;
Yan, Y.; Nantermet, P. G. J. Med. Chem. 2004, 47, 2995.
10. Tucker, T. J.; Lumma, W. C.; Lewis, S. D.; Gardell, S. J.;
Lucas, B. J.; Baskin, E. P.; Woltmann, R.; Lynch, J. J.;
Lyle, E. A.; Appleby, S. D.; Chen, I-W.; Dancheck, K. B.;
Vacca, J. P. J. Med. Chem. 1997, 40, 1565.
11. The conformation of the piperidine nitrogen cannot be
unequivocally assigned based upon electron density gen-
erated in this X-ray experiment. The other conformation is
possible in that it could benefit from a cation–p interac-
tion; however, this scenario is unlikely due to the
argument outlined in the text.
12. To reproduce the geometries observed in the 21-IIa crystal
structure for the interactions between the P2 NH and
Gly216, and between the triazole and C220, the P1 group
was fixed in the S1 pocket by holding the carbon atom
para to the triazole fixed, and the exocyclic N–O(Gly216)
˚
distance was held at 3.3 0.1 A.
4. Williams, P.D.; Coburn, C.; Burgey, C.S.; Morrissette,
M.M. WO 02/064211
5. Fairlie, D. P.; Tyndall, J. D. A.; Reid, R. C.; Wong, A. K.;
Abbenante, G.; Scanlon, M. J.; March, D. R.; Bergman,
D. A.; Chai, C. L. L.; Burkett, B. A. J. Med. Chem. 2000,
43, 1271.
13. Reported as the number of carotid artery vessels
(n = 6) occluding after a 10 lg/kg/min 180 min infusion
(FeCl3 arterial injury is initiated after 120 min, fol-
lowed by a 60 min observation period). Kurz, M. D.;
Main, B. W.; Sandusky, G. E. Thromb. Res. 1990, 60,
269.
6. Steiner, T. Angew. Chem. Int. Ed. 2002, 41, 48; Jeffrey, G.
A. An Introduction to Hydrogen Bonding; Oxford Univer-
sity Press: Oxford, 1997.
14. Burgey, C.S.; Deng, Z.J. US 2003/0225131 A1.
15. Deluca, M. R.; Kerwin, S. M. Tetrahedron 1997,
53, 454.