Design of LFA-1 antagonists based on a 2,3-dihydro-1H-pyrrolizin-5(7aH)-one scaffold

Article abstract of DOI:10.1016/j.bmcl.2007.01.036

A new class of lymphocyte function-associated antigen-1 (LFA-1) antagonists is described. Elaboration of the 2,3-dihydro-1H-pyrrolizin-5(7aH)-one scaffold resulted in the synthesis of potent inhibitors of the LFA-1/ICAM-1 interaction. Along with the in vi

Full text of DOI:10.1016/j.bmcl.2007.01.036

Bioorganic & Medicinal Chemistry Letters 17 (2007) 1908–1911
Design of LFA-1 antagonists based on a 2,3-dihydro-1H-
pyrrolizin-5(7aH)-one scaffold
Dharmpal S. Dodd,a,* Steven Sheriff,a ChiehYing J. Chang,a Dawn K. Stetsko,a
Linda M. Phillips,a Yingru Zhang,a Michele Launay,b Dominique Potin,b
Wayne Vaccaro,a Michael A. Poss,a Murray McKinnon,a Joel C. Barrish,a
Suzanne J. Sucharda and T. G. Murali Dhara
aBristol-Myers Squibb Pharmaceutical Research Institute, P.O. Box 4000, Princeton, NJ 08543-4000, USA
bCerep, 19 avenue du Quebec, 91951 Courtaboeuf cedex, France
Received 15 December 2006; accepted 10 January 2007
Available online 24 January 2007
Abstract—A new class of lymphocyte function-associated antigen-1 (LFA-1) antagonists is described. Elaboration of the 2,3-dihy-
dro-1H-pyrrolizin-5(7aH)-one scaffold resulted in the synthesis of potent inhibitors of the LFA-1/ICAM-1 interaction. Along with
the in vitro activity, we present the X-ray crystal structure of the complex of compound 9b, in a novel binding mode to the I-domain
of LFA-1.
Ó 2007 Elsevier Ltd. All rights reserved.
The cell adhesion molecule LFA-1 (lymphocyte function-
associated antigen-1), also known as CD11a/CD18, is
known to play a crucial role in many immunological func-
tions including antigen presentation, leukocyte traffick-
ing, B- and T-cell activation. LFA-1 belongs to the b2
integrin family and is found on all leukocytes.1 The
ligands for LFA-1, the ICAMs 1–3 (intercellular adhe-
sion molecules), are expressed on both leukocytes and
the endothelium. In particular, the interaction of LFA-
1 with ICAM-1 results in leukocyte adhesion to the endo-
thelium and facilitates their extravasation to tissue
sites.2,3 The LFA-1/ICAM interaction has been proposed
to be an attractive therapeutic target in the treatment of
several inflammatory disorders such as psoriasis and
transplant rejection.4,5 The recent approval of efalizumab
(RaptivaÒ), a fully humanized anti-LFA-1 antibody for
the treatment of moderate to severe psoriasis, provides
excellent validation of the target.6
have disclosed several diverse chemical series of com-
pounds.7,8 Some representative structures are shown in
Figure 1. NMR and X-ray co-crystallographic studies
have established that many of these inhibitors bind to
the IDAS site (I-domain allosteric site) of the I-domain
located on the CD11a unit.7–9 The X-ray structure of
our clinical candidate BMS-5871017a in complex with
the I-domain of LFA-1 shows the following: (i) the
Br
S
O
Cl
N
O
Cl
Cl
O
HO
N
N
NMe
Me
Me
N
Cl
CN
O
O
BMS-587101
BIRT-377
Br
In the search for small molecules that would interfere
with the LFA-1/ICAM-1 interaction, we and others
Me Me
NO2
S
Ac
O
Cl
N
N
N
N
O
(1)
O
Cl
Keywords: LFA-1 antagonists; 2,3-Dihydro-1H-pyrrolizin-5-(7aH)-one;
LFA-1/ICAM interaction.
A-286982
*
Corresponding author. Tel.: +1 609 252 5273; e-mail: dharmpal.
dodd@bms.com
Figure 1. Representative LFA-1 antagonists reported in the literature.
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.01.036

D. S. Dodd et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1908–1911
1909
p-cyanophenyl aromatic ring forms a favorable edge-to-
NC
NC
face p–p interaction with the 3,5-dichlorophenyl ring;
(ii) the urea carbonyl is hydrogen bonded to the amino
acid residues Lys-NH2 305, Lys-NH2 287, and Glu-
COOH 284 via a water molecule. Unlike the urea car-
bonyl, the amide carbonyl does not appear to make
any discernible contacts with the protein.
O
a
b,c
Cl
MeO2C
Boc
MeO2C
Boc
MeO
N
N
N
3
4
5
O
Cl
CN
f
As a continuation of our work7b on the modification of
the reported bicyclic[5.5]hydantoin core 1,8c we herein
describe exploration of the 2,3-dihydro-1H-pyrrolizin-
5(7aH)-one core 2 as a suitable surrogate in the design
of LFA-1 inhibitors. Replacement of the N-6 nitrogen
of the urea with a carbon atom and introduction of
unsaturation in ring A provide structural rigidity similar
to the bicyclic[5.5]hydantoin class of compounds, while
retaining the all important carbonyl (C-5) as well as
the 3,5-dichlorophenyl and benzyl groups necessary for
high affinity. In addition, the pyrrolizinone core allows
for elaboration at the C-7 position, thus providing an
understanding of the SAR at this position (Fig. 2).
CN
Cl
Cl
OH
Cl
Cl
OTf
d
N
N
O
6
O
8
g or h
e
CN
CN
Cl
Cl
R
O
Cl MeO
N
N
7
Cl
O
9: R= CN
10: R=Me
The synthesis of the 2,3-dihydro-1H-pyrrolizin-5(7aH)-
ones is outlined in Schemes 1 and 2. In our initial ap-
proach (Scheme 1), homochiral N-boc-L-proline methyl
ester (3) was elaborated to give 7-hydroxy pyrrolizin-5-
one 6 in reasonable overall yield. Compound 6 served
as the key intermediate for the synthesis of compounds
7, 9, and 10. Unfortunately, C–C coupling of the O-tri-
flate 8 was not general and failed with copper mediated
alkyl/aryl Grignards or lithium reagents. This necessitat-
ed the development of a more general approach utilizing
3 as the starting material (Scheme 2). The new sequence
(Scheme 2) was used to prepare compounds 15–18.
Scheme 1. Reagents and conditions: (a) i—LiHMDS, THF, À78 °C,
ii—4-cyanobenzyl chloride, THF, 30%; (b) 4 N HCl-dioxane; (c) (3,5-
dichlorophenyl)acetic acid, EDCI, DIPEA, DMF, 95%; (d) LiHMDS,
THF, 90%; (e) (MeO)3PO, K2CO3, reflux, 10 h, 30%; (f) Tf2O, DIPEA,
DCM, À10 °C, 60%; (g) Pd(PPh3)4, Zn(CN)2, DIPEA, DMF, 195 °C,
10 min microwave, 75%; (h) (CH3)2(CN)CuLi, THF, À78 °C, 15%.
Br
Br
O
MeO2C
Boc
a
b
MeO2C
Boc
O
N
Boc
N
N
N
3
11
12
Inhibition of the LFA-1/ICAM binding interaction
using the HeLa/HSB assay7b for a limited number of
racemic 2,3-dihydro-1H-pyrrolizin-5(7aH)-ones is shown
in Table 1. The data clearly suggest that the 2,3-dihy-
dro-1H-pyrrolizin-5(7aH)-one core is an excellent surro-
gate for the bicyclic[5.5]hydantoin system. The enol 6
was essentially inactive, as was the bulky 3-pyrazolyl
derivative 17. Compound 7 with a H-bond accepting
methoxy group at C-7 displayed sub-micromolar poten-
cy in the HeLa/HSB assay. The potency increase was
Br
Br
O
O
R
d,e
Br
c
R
Boc
N
N
14
Cl
O
13
Cl
CN
Cl
R
O
Cl
Cl
R
O
f
g
N
significantly more pronounced for compounds
9
N
15: R=Et
16: R=c-Pr
17: R=3-Pyrazolyl
Cl
(R = CN) and 18 (R = Et), which displayed IC50s of
40 and 52 nM, respectively, in the HeLa/HSB binding
assay. The C-7 SAR for the cyano analog (9) and the
ethyl analog (18) was intriguing considering the hydro-
philic and hydrophobic nature of these two substituents.
These data suggested that either (i) the C-7 position of
18: R=Et
Scheme 2. Reagents and conditions: (a) i—LiHMDS, THF, À78 °C,
ii—4-bromobenzyl bromide, THF, 85%; (b) LiN(OMe)Me, THF,
À10 °C; (c) RMgBr or lithium pyrazol-1-id-3-yllithium, THF rt; (d)
4 N HCl-dioxane; (e) (3,5-dichlorophenyl)acetic acid, EDCI, DIPEA,
DMF, 90–95%; (f) 5% KOH in MeOH, reflux (60–80%); (g) Pd(PPh3)4,
Zn(CN)2, DMF, 200 °C, 20 min microwave (80%).
X
Br
O
Cl
Cl
R
7
Cl
Cl
7a
7a
1
1
7
A
B
A
O5
B
N
the 2,3-dihydro-1H-pyrrolizin-5(7aH)-one core tolerates
a variety of lipophilic and hydrophobic substituents or
(ii) the binding mode for these compounds with the I-
domain of LFA-1 may be different. To further under-
stand this discrepancy in the SAR, the cyano analog
(9) was co-crystallized with the I-domain of LFA-1.
N
N
O5
2
1
Figure 2. Bicyclic[5.5]hydantoin core (1) and 2,3-dihydro-1H-pyrroli-
zin-5(7aH)-one core (2).

1910
D. S. Dodd et al. / Bioorg. Med. Chem. Lett. 17 (2007) 1908–1911
Table 1. In vitro activity of racemic 2,3-dihydro-1H-pyrrolizin-
5(7aH)-ones
O
N
O
Cl
Cl
Cl
Cl
N
N
S
HeLa/HSB bindinga
(IC50 or % inhibition)
R
Compound
R
X
BIRT-377
26 nMb
N
6
7
OH
CN
CN
CN
CN
Br
27% at 1 lM
430 nM
40 nM
CN
CN
OCH3
CN
CH3
9a: HeLa/HSB
binding IC50=1.4 μM
9b: HeLa/HSB
binding IC50=15 nM
9
10
15
16
17
18
660 nM
106 nM
Et
Figure 4. Absolute stereochemistry of 9a and 9b.
Cyclopropyl
3-Pyrazolyl
Et
Br
Br
200 nM
30% at 10 lM
52 nM
CN
a Values are averages of at least two determinations.
b Kd value in an LFA-1/ICAM-1 binding assay as reported in the
literature.9
group at C-6 is not positioned as deeply in the hydro-
phobic pocket between a-helices 1 and 7 and b-strands
1, 3, and 4 as was found for BMS-587101.
Compound 9 was separated into its individual enantio-
mers 9a and 9b employing a Chiralpak AD column
and the compounds tested for LFA-1/ICAM binding
inhibition in the HeLa/HSB assay. As expected, there
was a marked difference in the inhibition potential of
these enantiomers. The faster eluting enantiomer, 9a,
was much less active and displayed an IC50 of 1.4 lM,
while the slower eluting enantiomer, 9b, had an IC50
of 15 nM (Fig. 4). The absolute stereochemistry for 9b
was established as S at the 7a position employing vibra-
tional circular dichroism (VCD) spectroscopic analy-
sis.12 This result is consistent with the absolute
stereochemistry of the compound in the X-ray co-crystal
structure (vide supra).
When racemic 9 was co-crystallized with the I-domain of
LFA-1, a single enantiomer (S) was found bound to the
I-domain. Figure 3 shows the X-ray co-crystal structure
˚
of this enantiomer (designated 9b) determined at 1.75 A
resolution.10 The X-ray co-crystal structure revealed a
novel binding mode for compound 9b when compared
to the hydantoin class of LFA-1 antagonists such as
BMS-587101 and BIRT-377, where the absolute stereo-
chemistry at the equivalent of the C-7a center is (R)9.
This ‘flipped’ binding mode for compound 9b is a direct
result of the vinyl cyano group at C-7a forming a hydro-
gen bonding network through a water molecule to Lys-
NH2 287 and Lys-NH2 305 and Glu-COOH 284, unlike
BMS-587101 where the urea carbonyl, which corre-
sponds to the C-5 carbonyl of 9b, forms the same hydro-
gen bonding network. In addition, the Tyr-OH 166,
which is too far from the C-2 carbonyl of BMS-
587101 to form a H-bond, has now moved in the direc-
tion of the carbonyl of 9b, and is capable of forming a
direct H-bond. Interestingly, the 3,5-dichlorophenyl
The accommodation of 9b in a binding mode opposite
to that of the hydantoin class of LFA-1 antagonists is
evidence of the flexibility of the I-domain of the LFA-
1 protein. Since only 9 was subjected to X-ray crystallo-
graphic analysis, we cannot be certain that this opposite
‘flipped’ binding mode is an inherent property of the 2,3-
dihydro-1H-pyrrolizin-5(7aH)-one core or just unique
to the C-7 cyano substituted compounds in this series.
Racemic 18, containing an ethyl group at C-7, was equi-
potent to 9 (IC50 = 52 nM) in the HeLa/HSB binding as-
say. The lack of hydrogen-bonding ability of the ethyl
group does not preclude the possibility that this com-
pound may be binding in the ‘traditional’ mode similar
to the hydantoin class of LFA-1 antagonists.
In conclusion, we have designed a novel class of LFA-1/
ICAM antagonists based on the 2,3-dihydro-1H-pyrr-
olizin-5(7aH)-one scaffold. X-ray crystal structural anal-
ysis of our most active compound 9b revealed a ‘flipped’
binding mode to the I-domain of the LFA-1 protein.
The stereochemistry of 9b was found to be 7a(S), which
is opposite to that found in the hydantoin class of LFA-
1 antagonists.
Acknowledgments
Figure 3. X-ray structure of 9b complexed with the I-domain of LFA-1.
Also shown is the initial electron density (magenta: 2Fo-Fc 1) prior to
fitting compound 9b. The water molecule is shown as a red sphere and
hydrogen bonds are shown as small cyan spheres. Figure created using
PyMol.11
The authors thank Deepa Calambur for providing the
LFA-1 protein for X-ray crystallographic studies and
Jack Z. Gougoutas for helpful discussions regarding
the VCD spectra for compound 9b.

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