Himbacine derived thrombin receptor (PAR-1) antagonists: Structure-activity relationship of the lactone ring

Article abstract of DOI:10.1016/j.bmcl.2006.06.042

The structure-activity relationship (SAR) of the lactone ring of himbacine derived thrombin receptor (PAR-1) antagoinsts (e.g., 2-5) is described. The effect of the lactone carbonyl group on binding to PAR-1 is dependent on the substitution pattern of the

Full text of DOI:10.1016/j.bmcl.2006.06.042

Bioorganic & Medicinal Chemistry Letters 16 (2006) 4969–4972
Himbacine derived thrombin receptor (PAR-1) antagonists:
Structure–activity relationship of the lactone ring
Yan Xia,a,* Samuel Chackalamannil,a Tze-Ming Chan,a Michael Czarniecki,a
a
a
a
a
a
´
Darıo Doller, Keith Eagen, William J. Greenlee, Hsingan Tsai, Yuguang Wang,
Ho-Sam Ahn,a George C. Boykowa and Andrew T. McPhailb
aSchering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, NJ 07033, USA
bDuke University, Durham, NC, USA
Received 19 May 2006; revised 8 June 2006; accepted 12 June 2006
Available online 7 July 2006
Abstract—The structure–activity relationship (SAR) of the lactone ring of himbacine derived thrombin receptor (PAR-1) antago-
insts (e.g., 2–5) is described. The effect of the lactone carbonyl group on binding to PAR-1 is dependent on the substitution pattern
of the pyridine ring. A stereoselective intramolecular Michael addition reaction to the vinyl pyridine group was observed for these
pyridine analogs of himbacine in basic conditions at elevated temperature.
Ó 2006 Elsevier Ltd. All rights reserved.
Thrombin plays essential roles in hemostasis and wound
healing.1 While it stops bleeding through clot formation
in the coagulation cascade, overactivity under patholog-
ical conditions often results in arterial or venous throm-
bosis which will lead to cardiovascular events such as
myocardial infarction or stroke. Various agents that
attenuate the activity of thrombin have been used as
antithrombotic drugs, including indirect or direct
thrombin inhibitors (e.g., heparin or hirudin). However,
their wider use has been tempered by a lack of oral activ-
ity and bleeding side effects.2 Thrombin plays two roles
in thrombus formation: the first is the enzymatic cleav-
age of soluble fibrinogen to form fibrin which cross links
to form the matrix of the clot. Another is the activation
of platelet aggregation through thrombin receptors on
the platelets.3 Fibrin and aggregated platelets together
comprise the insoluble thrombus. Thrombin is the most
potent activator of platelet aggregation, although other
activators such as ADP and thromboxane A2 also con-
tribute. The cloning and characterization of the throm-
bin receptor or protease-activated receptor-1 (PAR-1)
in the early 1990s presented an attractive approach of
using small molecule PAR-1 antagonist as a novel orally
active antithrombotic agent.4 Because thrombin is the
most potent activator of platelet aggregation, and be-
cause a PAR-1 antagonist would selectively block the
action of thrombin on platelet aggregation without
affecting formation of fibrin or platelet aggregation by
other activators, a PAR-1 antagonist might achieve
good antithrombotic effects with less bleeding side
effects.5–7
O
O
O
H
H
H
H
H
H
O
O
O
3
H
H
H
H
H
H
Me
Me
Me
( )-1, R = Me
IC50 = 300 nM
( )-3, R = H
IC50 = 60 nM
N
6
N
R
N
5
R
IC50 = 11 nM
5
( )-2, R = Et
IC50 = 85 nM
( )-4, R = OMe
IC50 = 15 nM
CF3
We recently described a novel series of small molecule
PAR-1 antagonists derived from the natural product
himbacine (2–5).8,9 These compounds have a vinyl pyri-
dine appendage attached to the central ring of the tricy-
clic lactone core instead of the vinyl piperidine structure
as in himbacine. We have discovered that the pyridine
ring of the lead compound 1 is an important structural
motif for PAR-1 binding. Optimization of structure–
activity relationships (SARs) at the 5- or 6- positions
Keywords: Thrombin receptor antagonist; PAR-1 Antagonist; Himba-
cine; Structure–activity relationship.
*
Corresponding author. Tel.: +1 908 740 3510; fax: +1 908 740
7164; e-mail: yan.xia@spcorp.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.06.042

4970
Y. Xia et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4969–4972
of the pyridine ring led to potent PAR-1 antagonists
such as compounds 4 and 5. Additionally, the 5-phenyl
substituted pyridine compounds such as 5 displayed
good oral activity.8 In addition to the pyridine ring,
the lactone ring also caught our attention as a potential
structural element that can affect PAR-1 binding.10 We
were interested in the effects of substitutions at the C-3
position of the lactone ring and the importance of the
lactone functional group. Here we describe the results
of our studies which help to define the pharmacophore
for PAR-1 binding for the himbacine derived PAR-1
antagonists. During the SAR studies, we discovered an
interesting stereospecific intramolecular Michael addi-
tion reaction for these pyridine analogs of himbacine.
with 2-chloroquinoline followed by separation gave the
trans products 9a–d.
The synthetic transformations of the lactone ring of the
lead compounds 2–58,9 are exemplified in Scheme 2. Dii-
sobutylaluminum hydride reduction of compound 4
gave the lactol 4a12 which was either converted to the
2-methoxytetrahydrofuran derivative 4c13 with boron
trifluoride etherate and methanol or to the tetrahydrofu-
ran derivative 4d with boron trifluoride etherate and tri-
ethyl silane. Alternatively, the lactone ring of 4 was
reduced with lithium aluminum hydride to give diol
4b. Other lactone compounds such as 2, 3, and 5 under-
went similar transformations to give the derivatives
shown in Table 2. We then attempted to open up the lac-
tone ring to obtain a hydroxy acid derivative. The lac-
tone ring is quite stable to acidic conditions.14 In basic
conditions (10% KOH in dioxane) at room temperature,
the lactone compounds were converted to a highly polar
intermediate product first but upon neutralization and
work up were recovered unchanged. In more vigorous
basic conditions (10% KOH in dioxane at reflux), how-
ever, they were converted in good yield to products
which are consistent by spectroscopic analysis with a
The synthesis of compounds with different substitutions
on the lactone C-3 carbon is shown in Scheme 1. The
tricyclic alkyne intermediates 7a–d were synthesized
from commercially available alkynyl alcohols 6a–d as
described previously.11 The trimethylsilyl group of
7a–d was cleaved and the resulting alkyne was heated
with tributyltin hydride and AIBN to give the vinyltin
intermediates 8a–d which contain >85% of the trans
isomer. Palladium catalyzed coupling reactions of 8a–d
O
H
O
O
H
H
H
H
a, b
c
HO
R1
ref. 11
O
3
O
O
R1
R1
R2
R1
R2
R2
R2
H
H
H
H
9a, R1 = H, R2 = Me
9b, R1 = H, R2 = Et
9c, R1 = R2 = H
6a-d
SnBu3
N
TMS
9d, R1 = R2 = Me
8a-d
(>85% trans)
7a-d
Scheme 1. Reagents and conditions: (a) K2CO3, MeOH, rt, 80%; (b) Bu3SnH, AIBN, toluene, 120 °C, 80%; (c) Pd(PPh3)4, 2-chloroquinoline,
toluene, 100 °C, 80%.
HO
O
R
HO
H
H
H
H
H
H
H
H
H
H
H
H
c or d
a
b
O
HO
Me
4
Me
Me
N
N
N
4b
4c R = OMe
or
4a
4d R = H
OMe
OMe
OMe
H
e
3
g, h
;
H
OMe
N
N
H
H
H
H
H
O
H
H
O
O
H
4h
3e, R = OH
3g, R = NHBn
f
R
Scheme 2. Reagents and conditions: (a) DIBAL, toluene, À78 °C, 1.5 h, 95%; (b) LAH, THF, rt, 0.5 h, 96%; (c) BF3ÆOEt2, MeOH, 0 °C to rt, 3 h,
60%; (d) BF3ÆOEt2, Et3SiH, À78 °C to rt, 3 h, 75%; (e) 10% KOH–dioxane (1:1), reflux, 20 h, 85%; (f) BnNH2, EDCI, HOBt, DMF, 16 h, 46%; (g)
i—diethyl (isocyanomethyl)phosphonate, n-BuLi, ether, À78 to 0 °C; ii—HCl (concd); (h) diethyl (6-methoxy-quinolin-2-ylmethyl)phosphonate
(Ref. 9), n-BuLi, THF, 0 °C to rt.

Y. Xia et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4969–4972
4971
Table 2.
tetrahydrofuran structure such as 3e (Scheme 2). Appar-
ently the lactone was hydrolyzed under the basic condi-
tions and the intermediate hydroxyl acid underwent an
intramolecular Michael addition of the secondary
hydroxyl group to the vinyl pyridine to form the tetrahy-
drofuran ring. The acid 3e was further derivatized with
benzylamine to give the benzyl amide 3g. An X-ray anal-
ysis of 3g established the stereochemistry around the tet-
rahydrofuran ring.15 The cyclization appears to be
stereoselective since only one diastereoisomer was ob-
served for the product. Attempts to selectively protect
the primary hydroxyl group of the diols such as 4b also
resulted in formation of tetrahydrofuran products from
intramolecular Michael addition of the secondary
hydroxyl group to the vinyl pyridine group. Finally,
an analog 4h of compound 4 where the lactone ring
was removed was prepared by a two-step sequence from
the commercially available trans-1-decalone: a one car-
bon homologation by the procedure of Moskal and
Van Leusen16 and Wadsworth–Emmons–Horner olefin-
ation of the resulting aldehyde using diethyl (6-methoxy-
quinolin-2-ylmethyl)phosphonate.9
Het:
O
O A
Me
H
H
H
N
N
N
N
H
Et
OMe
MeO-Q
CF3
Het
Q
6-Et-Py
5-Ar-Py
O
HO
MeO
HO
A-Ring:
O A
O A
O A
O A
HO A
Me
Me
Me
Me
Me
Lactone
Lactol
Diol
MeO-THF
THF
Compound
A-ring
Het
IC50 (nM)
( )-2
( )-3
( )-4
5
Lactone
Lactone
Lactone
Lactone
Lactol
Lactol
Lactol
Diol
6-Et-Py
Q
85
60
MeO-Q
5-Ar-Py
6-Et-Py
Q
15
11
( )-2a
( )-3a
( )-4a
( )-2b
( )-4b
5b
3500
2000
62
The PAR-1 binding activities of the compounds were
measured by the previously reported in vitro assay using
purified human platelet membrane.17 The IC50 values
for the compounds that have different substitutions at
the lactone C-3 position are shown in Table 1.18 Chang-
ing the methyl group to an ethyl group (9a and 9b) de-
creased the binding affinity by 10-fold. Removing the
C-3 methyl group (9c) or having an additional methyl
group (9d) at the C-3 carbon did not affect the bind-
ing (9c, 9d ꢀ 9a). Since no apparent improvement in
binding was observed we decided to use the C-3 mono-
methyl group in our subsequent SAR studies due to the
ease in stereoselective synthesis from chiral starting
materials (both enantiomers of 6a are commercially
available).
MeO-Q
6-Et-Py
MeO-Q
5-Ar-Py
Q
3750
74
Diol
Diol
18
( )-3c
( )-4c
( )-3d
( )-4d
5d
MeO–THF
MeO–THF
THF
7000
217
MeO-Q
Q
8000
623
THF
THF
MeO-Q
5-Ar-Py
426
( )-4h
3e
3g
7000
5000
Inactive
oxytetrahydrofurans 3c and 4c), or removal of the
lactone carbonyl group to give the tetrahydrofuran
derivatives (3d–5d) gave weaker binding at the receptor.
However, the degree of reduction in binding affinity
upon conversion of the lactones to other functional
groups is dependent on whether the lactones have the
optimal pyridine moiety for PAR-1 binding. With less
than optimal pyridine moiety this reduction is more pro-
nounced than compounds with optimal pyridine moiety.
For example, in the less than optimal 6-ethylpyridine
series, the lactol 2a and the diol 2b are about 40-fold less
potent than the lactone 2, whereas in the 6-methoxy-
quinoline series where the lactone 4 has a higher affinity
for the PAR-1 receptor, the lactol 4a and the diol 4b are
only about 4- to 5-fold less potent than the lactone 4. In
the 5-(3-trifluoromethyl-phenyl)-pyridine series where
we have discovered some of our most potent PAR-1
antagonists, the diol 5b is almost as potent as the lactone
5. Compounds that retain a H-bond acceptor property
of the lactone carbonyl oxygen such as the lactols
(2a–4a), diols (2b, 4b, and 5b), and lactol ethers (2-meth-
oxytetrahydrofurans 3c and 4c) gave better binding.
Compounds that lack the H-bond acceptor property
such as the tetrahydrofuran derivatives 3d–5d
gave much weaker binding (ca. 40-fold or more) even
in the 5-(3-trifluoromethyl-phenyl)-pyridine series. The
The PAR-1 binding activities of the lactone carbonyl
modified compounds are listed in Table 2. The lactone
moiety is the most potent group for PAR-1 binding.
Conversions from the lactones (2–5) to the lactols
(2a–4a), diols (2b, 4b, and 5b), or lactol ethers (2-meth-
Table 1.
O
H
O
3
R1
R2
H
H
9a, R1 = H, R2 = Me
9b, R1 = H, R2 = Et
9c, R1 = R2 = H
N
9d, R1 = R2 = Me
Compound
R1
R2
IC50 (nM)
( )-3
60
150
( )-9a
( )-9b
( )-9c
( )-9d
H
Me
Et
H
1500
200
H
Me
H
Me
150

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Y. Xia et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4969–4972
6. Chackalamannil, S.; Ahn, H.-S.; Xia, Y.; Doller, D.;
Foster, C. Curr. Med. Chem. Cardiovasc. Hematol. Agents
2003, 1, 37.
7. Scarborough, R. M.; Pandey, A.; Zhang, X. Ann. Rep.
Med. Chem. 2005, 40, 85.
8. Chackalamannil, S.; Xia, Y.; Greenlee, W. J.; Clasby, M.;
Doller, D.; Tsai, H.; Asberom, T.; Czarniecki, M.; Ahn,
H.-S.; Boykow, G.; Foster, C.; Agans-Fantuzzi, J.; Bry-
ant, M.; Lau, J.; Chintala, M. J. Med. Chem. 2005, 48,
5884.
9. Clasby, M. C.; Chackalamannil, S.; Czarniecki, M.;
Doller, D.; Eagen, K.; Greenlee, W. J.; Lin, Y.; Tsai, H.;
Xia, Y.; Ahn, H.-S.; Agans-Fantuzzi, J.; Boykow, G.;
Chintala, M.; Foster, C.; Bryant, M.; Lau, J. Bioorg. Med.
Chem. Lett. 2006, 16, 1544.
10. For lactone ring SAR of himbacine analogs at the
muscarinic M2 receptor, see: Malaska, M. J.; Fauq, A.
H.; Kozikowski, A. P.; Aagaard, P. J.; McKinney, M.
Bioorg. Med. Chem. Lett. 1995, 5, 61.
11. Chackalamannil, S.; Davies, R. J.; Wang, Y.; Asberom,
T.; Doller, D.; Wong, J.; Leone, D.; McPhail; Andrew, T.
J. Org. Chem. 1999, 64, 1932.
12. Compound 4a was isolated as a single diastereomer. The
stereochemistry of the hydroxyl group was assigned based
on the small coupling constant for the anomeric proton (J
<2 Hz). This result is in agreement with the result
observed by Prof. Kozikowski and co-workers.10
13. Compound 4c was isolated as a single diastereomer. The
stereochemistry of the methoxy group was assigned based
on the small coupling constant for the anomeric proton (J
<2 Hz).
compound 4h that lacks the lactone ring altogether
decreased the binding by two orders of magnitude com-
paring with compound 4. The intramolecular Michael
addition products (3e and 3g) are significantly less active
as shown in Table 2.
In summary, the SAR of the lactone ring for PAR-1
binding for the himbacine derived PAR-1 antagonists
was evaluated. The C-3 monomethyl group is the pre-
ferred group for stereoselective synthesis of these him-
bacine derived PAR-1 antagonists. The lactone ring is
found to be an important structural motif for PAR-1
binding. Changes to the lactone functional group tend
to give significant loss of binding affinity. This is likely
due to the hydrogen-bond acceptor property of the
lactone carbonyl group. However, the contribution to-
ward binding affinity of the lactone group is less than
that of the pyridine group. Changes to the lactone are
more tolerated for the optimal pyridine groups such as
the 6-methoxyquinoline or the 5-(3-trifluoromethylphe-
nyl)-pyridine group. This SAR information is useful in
designing new PAR-1 antagonists derived from himba-
cine. A stereoselective intramolecular Michael addition
reaction was observed for these pyridine analogs of
himbacine in basic conditions at elevated tempera-
tures. The resulting tricyclic rings have highly defined
conformation and may serve as template for new lead
discoveries. Additional SAR results for the himbacine
derived PAR-1 antagonists will be published in due
course.
14. Conditions examined: reflux with 6 N HCl in dioxane;
reflux in 30% H2SO4; reflux with 33% HBr in AcOH;
reflux with BBr3 in 1,2-dichloroethane.
15. CCDC 608102 contains the supplementary crystallograph-
ic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting
The Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336033.
16. Moskal, J.; Van Leusen, A. M. Recueil des Travaux
Chimiques des Pays-Bas 1987, 106, 137.
17. Ahn, H.-S.; Foster, C.; Boykow, G.; Arik, L.; Smith-
Torhan, A.; Hesk, D.; Chatterjee, M. Mol. Pharmacol.
1997, 51, 350. A modification of the assay was described in
Supporting information in Ref. 8. Assays were carried out
in duplicate. Compounds of high interest (IC50 < 100 nM)
were assayed multiple times (n P 5, SD 20%)..
18. Compound 3 was included for reference. From our
unpublished results the SAR in the unsaturated tricyclic
lactone series generally parallels that in the saturated series
with ca. 2-fold decreases in binding potency.
References and notes
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2. Bisacchi, 6th ed.. In Burger’s Medicinal Chemistry and
Drug Discovery; Abraham, D. J., Ed.; John Wiley and
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3. Coughlin, S. R. Trends Cardiovasc. Med. 1994, 4, 77.
4. Vu, T.-K. H.; Hung, D. T.; Wheaton, V. I.; Coughlin, S.
R. Cell 1991, 64, 1057.
5. Maryanoff, B. E.; Zhang, H.-C.; Andrade-Gordon, P.;
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