Bioorganic & Medicinal Chemistry Letters 10 (2000) 1253±1256
Synthesis and Activity Studies of Conformationally Restricted
ꢀ-Ketoamide Factor Xa Inhibitors
Joseph Cacciola,* John M. Fevig, Pieter F. W. Stouteny, Richard S. Alexander,
Robert M. Knabb and Ruth R. Wexler
DuPont Pharmaceuticals Company, PO Box 80500, Wilmington, DE 19880-0500, USA
Received 24 February 2000; accepted 29 March 2000
AbstractÐConformationally restricted borolysine compounds containing a 2-(2-cyanophenylthio) benzoyl in the P3 position
unexpectedly led to enhanced factor Xa inhibition. In an eort to improve both the potency and selectivity of this series by
extending into the S0 domain, we have replaced the boronic acid with a-ketoamides, utilizing a novel process that was developed in
our labs. # 2000 DuPont Pharmaceuticals Company. Published by Elsevier Science Ltd. All rights reserved.
Factor Xa (fXa) plays a crucial role in the coagulation
cascade by occupying the juncture of the intrinsic and
extrinsic clotting pathways. The physiological role of fXa
is the proteolytic cleavage of prothrombin to thrombin,
which once generated can proceed to cleave ®brinogen to
®brin. Fibrin can then proceed to crosslink with acti-
vated platelets to form a ®brin blood clot. Imbalances in
the coagulation cascade can lead to excessive thrombotic
conditions such as pulmonary embolism, myocardial
infarction, deep vein thrombosis and stroke. Current
antithrombotic therapies all suer from drawbacks
associated with inconvenient dosing regimens, the need
for careful patient monitoring, or undesirable side
eects. Due to the central role of fXa in the coagulation
cascade, its inhibition is an attractive method of thrombus
prevention. During the screening of a series of con-
formationally restricted boropeptide thrombin inhibi-
tors,1 it was observed that placement of a nitrile moiety in
the 20 position (compound 3) of the diaryl thioether
unexpectedly led to enhanced fXa inhibition while low-
ering the corresponding thrombin activity (Table 1).
both greater potency and selectivity into this series, it was
decided to retain the 2-cyanophenylthio P3 fragment
while replacing the boronic acid with lysine a-ketoamides
to extend the inhibitors into the S0 site. This method of
extending into the S0 site has proven to be an eective
means of producing potent a-ketoamides at both Merck3
and Corvas.4 This substitution could be readily accom-
plished by utilizing a methodology developed recently in
our laboratories.5 This method, shown in Scheme 1,6
involves the use of azide ion to undergo a regioselective
C3 epoxy amide opening to give the azido alcohol 10,
which can then be rapidly converted to the desired a-
ketoamide.
Initially, simple alkyl and aryl substituted a-ketoamides
were prepared to probe the S0 site. As can be seen in
Table 2, an aryl substituent not only imparts greater
activity toward fXa, but also elicits a ꢀ10-fold increase
in thrombin anity. The crystal structure of 18 in the
thrombin active site was determined to guide our eorts
deeper into the S0 pocket.7
In order to determine the role of the nitrile in in¯uen-
cing the fXa/thrombin activity, a crystal structure of 3
in the thrombin active site was analyzed. Unfortunately,
due to disorder in the P3 diaryl region, the crystal
structure failed to yield any information that could be
used to improve the fXa anity. In an eort to engineer
Clear density is seen for most of the inhibitor, however,
a portion of the P2-P3 part of the inhibitor is character-
ized by poor electron density (Fig. 1). The phenyl ring of
the inhibitor that resides in the S0 pocket is well ordered
and is observed packing against the main chain residue of
Gly-193. Continuous electron density is observed between
the side chain of Ser-195 and the a-carbonyl carbon of the
inhibitor (1.3 A), the corresponding oxygen also forms a
hydrogen bond with His-57 (2.6 A) . The oxygen of the
phenyl amide occupies the oxyanion hole and forms
hydrogen bonds with Ser-195 (2.9 A), Asp-194 (3.0 A),
*Corresponding author. Tel.: +1-302-695-2301; fax: +1-302-695-
1502; e-mail: joseph.cacciola@dupontpharma.com
yCurrent address: Pharmacia & Upjohn, Viale Pasteur 10, 20014 Ner-
viano (Mi), Italy.
0960-894X/00/$ - see front matter # 2000 DuPont Pharmaceuticals Company.
Published by Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00215-8