Synthesis of constrained L-phenylalanine derivatives incorporating a benzazepinone or an azepinone ring as VCAM/VLA-4 antagonists

Article abstract of DOI:10.1016/S0040-4039(01)01926-8

Novel constrained L-phenylalanine derivatives incorporating a benzazepinone or an azepinone ring were synthesized in 13 and 8 steps, respectively, employing a key base-catalyzed intramolecular cyclization reaction. The product, 2, was comparable in potenc

Full text of DOI:10.1016/S0040-4039(01)01926-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8757–8760
Synthesis of constrained -phenylalanine derivatives incorporating
L
a benzazepinone or an azepinone ring as
VCAM/VLA-4 antagonists
Achyutharao Sidduri,* Jian Ping Lou, Robert Campbell, Karen Rowan and Jefferson W. Tilley
Roche Research Center, Hoffmann-La Roche Inc., Nutley, NJ 07110, USA
Received 4 September 2001; accepted 4 October 2001
Abstract—Novel constrained L-phenylalanine derivatives incorporating a benzazepinone or an azepinone ring were synthesized in
13 and 8 steps, respectively, employing a key base-catalyzed intramolecular cyclization reaction. The product, 2, was comparable
in potency in a VCAM/VLA-4 ELISA assay to the corresponding unconstrained analog 1 suggesting that cyclization favored the
bioactive conformation. However, compound 4 was 100-fold less potent than the unconstrained analog 3. © 2001 Elsevier Science
Ltd. All rights reserved.
We have been interested for some time in developing
orally active VCAM/VLA-4 antagonists for the treat-
ment of inflammatory diseases and have previously
reported the identification of a series of potent N-
acylphenylalanines.1–3 The N-benzoyl and N-
(methoxyethyl)cyclopentanoyl derivatives 1 and 3 are
among the more interesting of these compounds based
on their potency in our ELISA and cell based assays.
We have previously proposed that members of this
class reside in a compact, gauche(−) conformation and
that key features of 1 and 3, including the carboxylic
acid, the N-benzoyl or N-(methoxyethyl)cyclo-
pentanoyl moieties and the phenylalanine aromatic ring
overlap very closely with the corresponding elements in
our NMR model of a cyclic peptide lead VCAM/VLA-
4 antagonist.4
bioactive conformation. This effort lead to our selection
of the benzazepinone 2 and spirocyclic azapinone 4 as
synthetic targets and their preparation as shown in
Schemes 1–3. A comparison of the modeled conforma-
tion of 2 with the putative bioactive conformation of 1
is shown in Fig. 1.5
As shown in Schemes 1 and 2, the synthesis of 2
involved three crucial steps: synthesis of the sterically
hindered benzoic acid 10, coupling of 10 with methyl
4-nitro-
L
-phenylalanine 11, and
a
base-promoted
intramolecular cyclization reaction of the iodide 13 to
form the azepinone ring. The required 2,6-disubstituted
benzoic acid 10 was prepared in six steps starting from
commercially available 2-allyl-6-methyl phenol 5. The
allylic double bond of 5 was functionalized through a
hydroboration–oxidation protocol with 9-BBN in THF
to obtain the dihydroxy compound 6 in 67% yield as a
single regioisomer. The primary hydroxyl group of 6
was protected selectively by treatment with TBDMS-Cl
and imidazole in DMF and the phenol was converted
into the triflate 7 in 94% yield for the two steps.
In order to test this conformational hypothesis and
simultaneously improve the potency and bioavailability
of these lead compounds, we modeled various con-
strained analogues in which the amide NH was incor-
porated into a ring in an effort to mimic the putative
* Corresponding author. Tel.: (973) 235-7143; fax: (973) 235-7122; e-mail: achyutharao.sidduri@roche.com
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01926-8

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A. Sidduri et al. / Tetrahedron Letters 42 (2001) 8757–8760
Although attempted direct formation of a carboxyl
group under a variety of palladium-catalyzed condi-
tions in aqueous DMSO was unsuccessful, car-
bomethoxylation was employed successfully. Thus, the
triflate 7 was treated with palladium acetate in the
presence of 10 mol% of dppp and triethylamine under
50 psi carbon monoxide in DMSO and methanol at
80°C to afford 8 in quantitative yield. Basic hydrolysis
of the ester group of 8 resulted in the formation of
2-(3-hydroxypropyl)-6-methylbenzoic acid 9. During
the hydrolysis, the TBDMS hydroxyl-protecting group
was lost and was then reintroduced using a two-step
protocol. In the first step, 9 was reacted with excess
Figure 1.
OH
OH
OTBDMS
OTf
OTBDMS
CO2Me
OH
a, b
c, d
e
6
5
7
8
OH
OTBDMS
CO2H
g, h
CO2H
f
9
10
Scheme 1. Reagents and conditions: (a) 9-BBN, THF, reflux, 24 h; (b) NaOH, H2O2, 0–40°C, 4 h, 67%; (c) TBDMSCl, imidazole,
DMF, rt, 5 h, 96%; (d) Tf2O, DMAP, CH2Cl2, −70°C to rt, 3 h, 93%; (e) Pd(OAc)2, dppp, CO, DMSO, MeOH, NEt3, 80°C, 4
h, 99%; (f) LiOH, MeOH, H2O, reflux, 15 h, 99%; (g) TBDMSCl, imidazole, DMF, rt, 15 h; (h) K2CO3, MeOH, rt, 20 h, 83%.
O2N
O2N
I
OTBDMS
O2N
OTBDMS
HN
b, c
a
OMe
OMe
HN
+
CO2H
OMe
ClH.H2N
O
O
O
O
O
10
11
12
13
Cl
Cl
R
H
N
H
N
Cl
Cl
O
O
g
d
f
OMe
N
OMe
OH
N
N
O
O
O
O
O
O
16
2
14, R = NO2
15, R = NH2
e
Scheme 2. Reagents and conditions: (a) HBTU, DIPEA, DMF, rt, 15 h, 28%; (b) TBAF, THF, 0°C to rt, 3 h, 62%; (c) PPh3,
imidazole, I2, CH2Cl2, rt, 15 h, 70%; (d) KOBut, THF, −70°C to rt, 24 h, 68%; (e) Zn dust, NH4Cl, MeOH, H2O, reflux, 30 min,
90%; (f) 2,6-dichlorobenzoyl chloride, DIPEA, CH2Cl2, rt, 15 h, 67%; (g) NaOH, EtOH, 40–45°C, 4 h, 95%.

A. Sidduri et al. / Tetrahedron Letters 42 (2001) 8757–8760
8759
O2N
CO2R1
CO2Me
Cl
a
c
d
OMe
HN
Cl
O
O
18, R1 = CH3
17
20
b
19, R1 = H
Cl
O
O2N
O2N
H
N
Cl
e
f, g, h
OMe
OMe
N
HN
OH
N
I
O
O
O
O
O
O
21
4
22
Scheme 3. Reagents and conditions: (a) LDA, THF, −70 to −50°C, 1 h, then 4-chloro-1-bromobutane, −70°C to rt, 15 h, 64%; (b)
NaOH, THF, MeOH, 45–50°C, 4 h, 65%; (c) methyl 4-(nitro)-L-phenylalanine hydrochloride, HBTU, DIPEA, DMF, rt, 15 h,
55%; (d) NaI, acetone, reflux, 15 h, 96%; (e) KOBut, THF, −70°C to rt, 20 h, 49%; (f) Zn dust, NH4Cl, MeOH, H2O, rt, 2 h, 90%;
(g) 2,6-dichlorobenzoyl chloride, DIPEA, CH2Cl2, rt, 15 h, 67%; (h) NaOH, EtOH, 45–50°C, 4 h, 95%.
The synthesis of 4 is outlined in Scheme 3; the main
highlight of the synthesis is the base promoted
intramolecular cyclization of iodide 21 to the spiro-
cyclic azapinone 22. The desired 1-(4-chlorobutyl)-
cyclopentane carboxylic acid 19 was prepared in two
steps starting from commercially available cyclopen-
tanecarboxylic acid methyl ester 17. In the first step, the
anion generated from 17 in the presence of LDA in
THF at −70°C was alkylated with 4-chloro-1-bromo-
butane. The alkylated product 18 was isolated in 64%
yield after distillation under high vacuum. The methyl
ester group of 18 was hydrolyzed by treatment with
sodium hydroxide in THF and methanol at 40–45°C.
The slow reaction and moderate yield of the acid 19
was attributable to the steric environment of the ester
group.
TBDMSCl and imidazole in DMF to obtain the bis-
protected intermediate which, upon exposure to potas-
sium carbonate in methanol gave the protected acid 10
in 83% yield.
The coupling reaction between compound 10 and
methyl 4-nitro- -phenylalanine hydrochloride salt 11
L
was carried out in DMF using HBTU in the presence
of DIPEA to give 12 in low yield (28%). The low yield
is likely due to the steric hindrance caused by the
presence of two ortho substituents in the acid and we
did not attempt to optimize the reaction. To set a stage
for an intramolecular cyclization, the TBDMS group of
12 was removed under normal conditions (TBAF,
THF, rt, 24 h) and the resulting hydroxyl group was
transformed to the corresponding iodide using
triphenylphosphine,
dichloromethane to furnish compound 13 in 70% yield.
iodine
and
imidazole
in
The coupling reaction between the cyclopentyl acid 19
and methyl 4-nitro-L-phenylalanine was carried out in
DMF using HBTU in the presence of DIPEA to give
20 in moderate yield (55%). To set a stage for
intramolecular cyclization, the chloride 20 was con-
verted to the corresponding iodide 21 by treatment with
sodium iodide in acetone at reflux (96%). As expected,
the intramolecular cyclization proceeded cleanly when a
dilute THF solution of 21 was cooled to −78°C, treated
with potassium tert-butoxide in THF and was allowed
to warm slowly to room temperature to afford the
cyclized product 22 in 49% yield. The synthesis was
completed uneventfully under the conditions described
above to afford the target compound 4 in 57% yield for
the three steps.6
Attempts to carry out the crucial intramolecular
cyclization of 13 were unsuccessful using sodium
hydride as a base in THF under variety of conditions
and generally lead to elimination products. However,
when potassium tert-butoxide was added to a dilute
THF solution of 13 at −78°C and the mixture was
allowed to warm slowly to room temperature, the reac-
tion proceeded cleanly to provide a 68% yield of the
cyclized product 14. To complete the synthesis, the
nitro group of 14 was reduced using zinc dust and
ammonium chloride in aqueous methanol at reflux for
30 min to produce the amine 15 in 90% yield. Treat-
ment of 15 with 2,6-dichlorobenzoyl chloride in the
presence of DIPEA in dichloromethane followed by
ester hydrolysis under basic conditions afforded a 95%
yield of the target compound 2.6
Compounds were assayed for VLA-4 antagonist activ-
ity using a solid-phase, dual antibody ELISA in which
VLA-4 derived from Ramos cells was allowed to com-

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A. Sidduri et al. / Tetrahedron Letters 42 (2001) 8757–8760
pete for bound recombinant human VCAM in the
presence of serial dilutions of test compound. VLA-4
bound to VCAM-1 was detected by a complex of
anti-b1 antibody and HRP-conjugated anti-mouse IgG:
chromogenic substrate (K-Blue).3 In this assay system
compounds 1, 2, 3 and 4 had IC50s of 1.2, 8, 0.25, and
25 nM, respectively. Thus, introduction of the ben-
zazepinone ring constraint lead to a highly potent
VCAM/VLA-4 antagonist suggesting that compound 2
can access a bioactive conformation and that the com-
pound–receptor interaction is not dependent on the
presence of the phenylalanine NH. On other hand, the
introduction of the azapinone ring in cyclopentane
series lead to a moderately potent VCAM/VLA-4
antagonist, the constrained molecule was 100-fold less
potent than its conformationally more mobile parent.
Guthrie, R. W.; Huang, T.-N.; Hull, K. G.; Sidduri, A.;
Tilley, J. W. WO 9910313, 1999.
4. Fotouhi, N.; Joshi, P.; Fry, D.; Cook, C.; Tilley, J. W.;
Kaplan, G.; Hanglow, A.; Rowan, K.; Schwinge, V.;
Wolitzky, B. Bioorg. Med. Chem. Lett. 2000, 10, 1171–
1174.
5. Modeling experiments were carried out using the SYBYL
ver. 6.0 modeling package employing the Sybyl force field
including electrostatics. The model for 2 was derived from
a X-ray crystal structure of a closely related compound
which will be described in a forthcoming paper. The
conformational model for 3 was build from 2 by append-
ing the appropriate atoms and carrying out a minimization
with an annealing step.
6. All new compounds reported herein were characterized by
1H NMR and high resolution mass spectroscopy. Data for
compound 2: 1H NMR (400 MHz, DMSO-d6), spectrum
run at room temperature, l 12.84 (br s, 1H), 10.65 (s, 1H),
7.63–7.46 (m, 5H), 7.32–7.09 (m, 3H), 7.04–6.94 (m, 2H),
5.5 (m, 1H), 3.43–3.0 (m, 4H), 2.94–2.5 (m, 2H), 2.34 (s,
3H, ArCH3, belongs to one rotamer), 2.09–1.94 (m, 2H,
ArCH2, belongs to the above rotamer), 1.76 (s, 3H,
ArCH3, belongs to another rotamer), 1.54–1.41 (m, 2H,
ArCH2, belongs to the later rotamer). Spectrum run at
100°C (400 MHz, DMSO-d6), l 10.33 (s, 1H), 7.56 (d, 2H,
J=8.2 Hz), 7.5 (d, 1H, J=6.8 Hz), 7.5 (d, 1H, J=8.8 Hz),
7.45 (dd, 1H, J=6.8, 8.8 Hz), 7.24 (d, 2H, J=8.2 Hz), 7.17
(t, 1H, J=7.3 Hz), 7.05 (d, 1H, J=7.3 Hz), 6.94 (d, 1H,
J=7.3 Hz), 5.3 (m, 1H), 3.32 (dd, 1H, Jgem=14.5, Jvic=5.8
Hz), 3.12 (dd, 1H, Jgem=14.5, Jvic=10.7 Hz), 3.21–2.83
(m, 2H, NCH2), 2.55 (m, 2H, ArCH2), 2.10 (m, 3H,
ArCH3), 1.76 (m, 2H); HRMS (FAB)+ calcd for
C27H24N2O4Cl2 511.1191, found 511.1195 (M+H). Data
for compound 4: 1H NMR (400 MHz, DMSO-d6), spec-
trum run at room temperature, l 12.5 (br s, 1H), 10.64 (s,
1H), 7.6–7.54 (m, 4H), 7.5 (dd, 1H, J=6.8, 8.8 Hz), 7.15
(d, 2H, J=8.2 Hz), 5.0 (m, 1H), 3.25–2.95 (m, 4H), 2.0 (m,
1H), 1.65–1.25 (m, 13H); HRMS (FAB)+ calcd for
C26H28N2O4Cl2 503.1505, found 503.1523 (M+H).
Acknowledgements
The authors are grateful to members of the Physical
Chemistry group in Discovery Chemistry Department
for spectroscopic measurements and interpretations.
References
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