6,7-Dihydro-5H-pyrrolo[1,2-a] imidazoles as potent and selective α1A adrenoceptor partial agonists

Article abstract of DOI:10.1016/j.bmcl.2009.03.166

Novel pyrroloimidazoles have been identified as potent partial agonists of the α_1A adrenergic receptor, with good selectivity over the α_1B, α_1D and α_2A receptor subtypes. Pyrimidine 19 possessed attractive CNS d

Full text of DOI:10.1016/j.bmcl.2009.03.166

Bioorganic & Medicinal Chemistry Letters 19 (2009) 3113–3117
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
6,7-Dihydro-5H-pyrrolo[1,2-a] imidazoles as potent and selective a1A
adrenoceptor partial agonists
*
Lee R. Roberts , Paul V. Fish, R. Ian Storer, Gavin A. Whitlock
Department of Chemistry, Pfizer Global Research and Development, Sandwich Labs, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel pyrroloimidazoles have been identified as potent partial agonists of the a1A adrenergic receptor,
with good selectivity over the a1B, a1D and a2A receptor subtypes. Pyrimidine 19 possessed attractive
CNS drug-like properties with good membrane permeability and no evidence for P-gp mediated efflux.
Received 27 February 2009
Revised 27 March 2009
Accepted 29 March 2009
Available online 18 April 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Alpha1a adrenoceptor
Partial agonist
Pyrroloimidazole
CNS penetration
Selectivity
Recent reports described a1A selective partial agonists having
potential clinical utility in the treatment of stress urinary inconti-
nence (SUI).1 As part of a multi-series approach, we sought to dis-
cover templates which could deliver potent and selective a1A
partial agonist activity.2–4 Biology studies indicated a1A partial
agonists impart their in vivo efficacy in models of SUI via a cen-
trally mediated pathway, and partial agonism of the a1A receptor
may also drive SUI efficacy over undesired cardiovascular effects.5
We therefore required preferred compounds which were partial
agonists (Emax < 60%) coupled with physicochemical properties
which would allow good blood brain barrier (BBB) penetration
(low TPSA, low molecular weight, few hydrogen bonding groups),
and ideally improved metabolic stability when compared to com-
pounds described in earlier publications, exemplified by 1. Perme-
ability and efflux in the MDCK mdr-1 cell line was used as a
predictor of likely P-gp mediated efflux, and also as an in vitro pre-
dictor of BBB penetration.6 Design of an alternative series led to the
discovery of 2 (Fig. 1) which was more stable to metabolic activa-
tion than 1 but had sub-optimal pharmacology with respect to po-
tency and intrinsic efficacy (Emax).4 We now wish to report the
results of follow-on work in this Letter to address these two issues.
Our medicinal chemistry strategy concentrated on the place-
ment of substituents on the phenyl ring to increase potency and
lower Emax, whilst retaining metabolic stability. A range of pyrrol-
oimidazoles were synthesized to assess the effects of functional
group changes. The core pyrroloimidazoles were efficiently synthe-
HN
N
N
CN
N
Cl
O
2
1
α
1A EC50 27nM (58%)
α1A EC50 51nM (74%)
Selective over α1B, α1D, α2A
HLM Clint 30μl/min/kg
MDCK mdr-1 AB/BA 40/36
Good BBB penetration
Selective over α1B, α1D, α2A
HLM Clint <7 μl/min/kg
MDCK mdr-1 AB/BA 35/41
Figure 1. Structures, pharmacology and ADME properties of imidazole a1A partial
agonists 1 and 2.
sized according to the general scheme outlined in Scheme 1. The
commercially available bromobenzaldehyde underwent a Wittig
reaction to give the cinnamic acid 3. Conjugate addition of nitro-
methane and reduction of the nitro group to the amine with tin
chloride followed by cyclisation gave the lactam 4.7 The lactam
was converted to the imidate ester using Meervein’s reagent (trim-
ethyltetrafluoroborate) which in turn was displaced with amino-
ethyldiethylacetal and cyclised under acidic conditions to give
the pyrroloimidazole 5 in good yield. The aryl bromides were easily
separated at this point on a chiral preparative HPLC column. Each
enantiomer was reacted with a series of boronic acids, stannanes
and N-linked heterocycles to give the final products 7–27 except
compound 17 which was synthesized via a Heck reaction followed
by hydrogenation.8 Test compounds were assessed in vitro for their
functional activity against human a1A as well as other alpha sub-
* Corresponding author. Tel.: +44 1304 641466; fax: +44 1304 651987.
E-mail address: lee.roberts@pfizer.com (L.R. Roberts).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.03.166

3114
L. R. Roberts et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3113–3117
O2N
O
O
Br
O
Br
Br
(b)
(a)
OEt
H
OEt
H/F
H/F
H/F
3
H
N
N
N
2
R
3
N
Br
Br
N
(c)
(d)
(e)
O
4
H/F
6
5
H/F
H/F
7-26
4
5
Scheme 1. Reagents and conditions: (a) Ph3P = CHCO2Et, toluene, reflux 2 h; (b) MeNO2, DBU, 0 °C to rt; (c) (i) SnCl2, EtOAc, rt; (ii) EtOH, reflux overnight; (d) (i) Me3OBF4,
CH2Cl2, rt; (ii) H2NCH2CH(OEt)2, HCl, EtOH, rt; (iii) HCl/dioxane, H2O, 100 °C; (e) either (i) RSnBu3, PdCl2(Ph3P)2 or (ii) RB(OH)2, Pd or (iii) Pyrazole, CuI (20 mol %),
Phenanthroline (20 mol %), Cs2CO3, MeCN, microwave or (iv) Pd(OAc)2, P(o-tol)3, Et3N, MeCN, 160 °C, microwave followed by H2 (60psi), 10% Pd/C, EtOH, 60 °C.
Table 1
In vitro functional a1A agonist activity, in vitro ADME data and physicohemical properties for compounds 1, 2, 7–27
N
N
1, 2, 7-27
R
Compd
R
a1A EC50
a1A Emax
(%)
a
(nM)
1B EC50 (Emax
)
a
(nM)
1D EC50 (Emax
)
a2A Ki
(nM)
HLM Clint
min/mg
l
l/
MDCK/mdr-1 P-gp (AB/BA
TPSA
Log D7.4
0
(nM)a,b
Papp ꢀ 10ꢁ6 cmsꢁ1
)
ÅA2
1
2
—
31
60
>10,000
>7000
>1700
30
40/36
38
2.6
1.3
CN
51
74
>10,000
>10,000
>10,000
>4500
<7
35/41
42
18
S
7
8
9
12
0.4
2
83
98
91
729 (51%)
213
NT
<8
12
NT
1.7
1.2
2.2
N
N
N
>6500
>1400
391 (75%)
146 (49%)
>3800
805
NT
NT
44
43
O
O
N
N
10
11
12
13
92
62
83
90
68
>10,000
>3570
1020
1750
991
NT
18
8
NT
43
58
31
31
2.2
2.3
2.1
2.1
S
N
3
NT
NT
NT
1
760 (54%)
>10,000
119 (71%)
656 (48%)
44/47
N
81
112
17
NT

L. R. Roberts et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3113–3117
3115
Table 1 (continued)
Compd
R
a1A EC50
a1A Emax
(%)
a
(nM)
1B EC50 (Emax
)
a
(nM)
1D EC50 (Emax
)
a2A Ki
(nM)
HLM Clint
min/mg
l
l/
MDCK/mdr-1 P-gp (AB/BA
TPSA
Log D7.4
0
(nM)a,b
Papp ꢀ 10ꢁ6 cmsꢁ1
)
ÅA2
N N
14
9
85
80
59
64
NT
NT
380
1100
114
280
<7
18
22
<8
NT
36
2.0
1.4
2.5
1.2
N
N
15
16
17
5
>10,000
>10,000
>10,000
>10,000
>10,000
>10,000
NT
36
64
72
H
N
MeSO2
4
11/46
H
N
SO2
12
NT
O
N
H
18
19
57
>10,000
>10,000
202
10
16/42
47
2.5
N
N
19
20
19
77
65
>10,000
>10,000
>3400
>2600
>2370
>2780
<8
<8
NT
44
44
1.3
1.3
F
N
N
1
34/39
F
N
N
21
22
62
33
59
43
>10,000
>10,000
>10,000
>10,000
>3310
>2160
<8
13
NT
NT
44
44
1.3
2.4
F
N
O
F
N
O
23
24
3
70
54
990 (41%)
>10,000
276 (49%)
276 (51%)
469
16
<8
NT
NT
44
47
2.3
2.2
F
N
NH
15
1470
F
(continued on next page)

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L. R. Roberts et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3113–3117
Table 1 (continued)
Compd
R
a1A EC50
a1A Emax
(%)
a
(nM)
1B EC50 (Emax
)
a
(nM)
1D EC50 (Emax
)
a2A Ki
(nM)
HLM Clint
min/mg
l
l/
MDCK/mdr-1 P-gp (AB/BA
TPSA
Log D7.4
0
(nM)a,b
Papp ꢀ 10ꢁ6 cmsꢁ1
)
ÅA2
N
NH
25
3
70
>10,000
NT
NT
10
NT
47
2.1
F
N
N
26
27
5
80
56
>10,000
>10,000
>10,000
>4100
NT
9
NT
NT
36
36
1.7
1.8
F
N
N
27
>3230
12
F
NT denotes not tested.
a
Values are geometric means of at least two experiments.
See Ref. 8 for description of assay conditions.
b
types, in human liver microsomes (HLM), MDCK mdr-1 and Log
D7.4 assays (Table 1).9
agonism against a1B
tent compound was the pyrimidine 8, which was very potent
against a1A had some a1D activity but weak against 2A. This
should be contrasted to the 4-pyridine 13 which had moderate po-
tency against a1A and as an antagonist against 2A. The N-linked
pyrazole 15 was very potent and full 1A agonist and was selective
across the range of subtypes.
In an effort to lower the Emax from substitution at the 2-position
alone, compounds such as sulfonamides 16 and 17 as well as the
amide 18 were synthesized. They retained potency, had a more
desirable Emax of 59%, 64% and 57%, respectively and were selective
, a1D and antagonism against a2A. The most po-
Absolute stereochemistry was determined on one of the enanti-
omers of the simple bromopyrroloimidazole (6) which showed it to
have R stereochemistry (Fig. 2).10 Where this intermediate has
been used, the absolute stereochemistry of the final compounds
derived from this and its opposite enantiomer are shown. Exam-
ples 19–27, although single enantiomers, are of unknown absolute
stereochemistry. Unlike the previous indane imidazole series, all of
the pyrroloimidazoles were separated into their constitutive enan-
tiomers before screening as each enantiomer was active against
a1A and had a different selectivity profile as illustrated by com-
pounds 9 and 10, whereby 10 is significantly weaker against a1A
but has similar a2A Ki0s.
In the preceding paper, we showed that substitution on the 2-
position of the phenyl ring was optimal to retain potency. Replace-
ment of the nitrile in compound 2 with a range of simple heterocy-
cles 8–15 was then investigated, with the aim of maintaining the
Log D7.4 in a similar range to 2 (1.0–2.0) with the simple phenyl
7 shown in comparison. The heterocycles were all potent and very
full agonists (except compound 10), showing varying degrees of
a
a
a
a
over the other a1 subtypes. Unfortunately, 16 and 18 had asymme-
try in the MDCK mdr-1 P-gp assay and likely BBB impairment.
Compounds 16 and 17 had fairly high TPSA values, but the amide
18 had a surprisingly high AB/BA ratio for a TPSA of 47. These three
compounds also had quite pronounced a2A antagonist activity (a2A
Ki 114 nM for 16, 280 nM for 17 and 202 nM for 18).
The results above indicated that balancing the desired pharma-
cological and ADME characteristics through modification of just
the 2-position was unlikely to be successful. Moving the heterocy-
cle to the 3- or 4-positions of the phenyl ring was highly detrimen-
tal to a1A potency (data not shown).
The strategy then focused on retaining the 2-heterocycle and
introducing small groups around the ring in an effort to reduce
Emax and gain more selectivity. Using the pyrimidine 8 as a starting
place, a fluoro group (chosen to minimise increases in Log D) was
moved around the phenyl ring. Compounds 19, 20 and 21 all mod-
ulated Emax to some degree with different drop-offs in
a1A potency
and on the whole the selectivity against the other subtypes was
a
improved. On balance 20 was the compound of most interest being
selective over other a1 and a2 subtypes, it also had excellent HLM
stability and good flux in the MDCK assay.
Placement of a fluoro group at the 4-position consistently low-
ered the Emax when compared across a range of heterocycles in the
2-position and brought in selectivity, exemplified by 22, 24 and 27.
Compound 22 was very interesting as it retained a good level of po-
tency but was very partial, having an Emax of 43%, whilst being very
selective against all other
a subtypes tested. 27 had reasonable
HLM stability and with a low TPSA was expected to have good
CNS penetration.
The pyrimidine 20 had a good balance of pharmacology and
in vitro ADME properties, and was assessed further. Compound
20 had no activity against the hERG channel (up to 22 lM), and
Figure 2. X-ray of the R enantiomer of bromophenylpyrroloimidazole 6.

L. R. Roberts et al. / Bioorg. Med. Chem. Lett. 19 (2009) 3113–3117
3117
5. Conlon, K.; Christy, C.; Whitlock, G. A.; Roberts, L. R.; Stobie, A.; Thurlow, R. J.;
McMurray, G. JPET, submitted for publication.
6. Madin-Darby canine kidney (MDCK) cell line expressing the P-glycoprotein
no Ki’s < 3
against a panel of 150 receptors, ion channels and enzymes at Cer-
ep. It showed very weak CYP inhibition, <10% at 3 M against
CYP1A2, 2C9, 2D6 and 3A4. Additionally, 20 had excellent flux
when assessed in MDCK mdr-1 cells (A–B 34, B–A 39) which was
indicative of excellent permeability combined with no evidence
of P-pg mediated efflux which should be predictive of good BBB
penetration.
In summary, this novel series of pyrroloimidazoles has been
identified as potent and selective partial a1A agonists with poten-
tially good CNS penetration in an inherently more ADME favour-
able template over the original indane imidazoles.
lM were found when the compound was screened
l
transporter (P-gp). Flux across cells was measured at 10
lM
substrate
concentrations.
Figures
quoted
correspond
to
the
flux rates
(Papp ꢀ 10ꢁ6 cm sꢁ1) for apical to basolateral (AB) and basolateral to apical
(BA) directions. See reference: Mahar Doan, K. M.; Humphreys, J. E.; Webster, L.
O.; Wring, S. A.; Shampine, L. J.; Serabjit-Singh, C. J.; Adkison, K. K.; Polli, J. W. J.
Pharm. Exp. Ther. 2002, 303, 1029.
7. Caution should be exercised in the handling of nitro compounds. DSC analysis
was determined on all alkyl nitro analogues.
8. Typical experimental procedure (compound 8): To a solution of aryl bromide 5
(50 mg, 0.19 mmol) in MeCN (2 ml) was added Pd(PPh3)2Cl2 (6.7 mg,
0.0095 mmol) followed by 5-pyrimidineboronic acid (47 mg, 0.38 mmol) and
finally aq 2 M Na2CO3 (285 ll). The mixture was heated at 160 °C in the
microwave for 10 min. The reaction was partitioned between DCM (2 ml) and
water (2 ml). The separated aqueous layer was washed with DCM (2 ml) and
the combined organics dried (MgSO4). The crude product was
chromatographed on silica gel using DCM/MeOH (0–5%) as eluant to give
38 mg (76%) of 8. 1H NMR (400 MHz, CDCl3) d 9.23 (s, 1H), 8.70 (s, 2H), 7.45
(ddd, J = 8, 8, 1.6 Hz, 1H), 7.40–7.35 (m, 2H), 7.23 (dd, J = 8, 2 Hz, 1H), 7.08 (d,
J = 1.6 Hz, 1H), 6.85 (d, J = 1.6 Hz, 1H), 4.20 (dd, J = 10.2, 7.8 Hz, 1H), 4.13–4.05
(m, 1H), 4.00 (dd, J = 10.2, 6.6 Hz, 1H), 3.25 (dd, J = 16, 8.6 Hz, 1H) and 3.02 (dd,
J = 16, 6.6 Hz, 1H). ESCI-MS: m/z: [M+H]+ 263 (100%).
Acknowledgements
We would like to thank Lucy Rogers and Linda Kitching for
screening data; Simon Wheeler, Alan Jessiman, Adam Stennett, Mi-
chael Ralph, Helen Mason, Katherine England, Debbie Lovering and
Edward Pegden for compound synthesis and Cheryl Doherty for X-
ray analysis. We also thank Satish Dayal for ADME assessment of
compounds 7–27.
9. Assay conditions: The Roche clinical agent Ro 115–1240 had an Emax of 60% in
our hands, which was in line with published data, see Blue, D. R.; Daniels, D. V.;
Gever, J., R.; Jett, M. F.; O’Yang, C.; Tang, H. M.; Williams, T. J.; Ford, A. P. D. W.
BJU Int. 2004, 93, 162. Ro 115-1240 was reported not to cause cardiovascular
effects at the dose tested in the clinic, see Ref. 1 Human a1A (clone 54), a1B
(SNB0000700, clone 11) and a1D (SNB0000706, clone 23) were expressed in
Chinese Hamster Ovary cells. Receptor activation was determined via calcium
mobilisation through the Gq pathway using calcium-sensitive fluorescent dye
(Molecular Devices, part number R8033), measured by a Fluorescent Light
Imaging Plate Reader (FLIPr). Eleven point concentration response curves were
calculated, with Emax calculated as a per relative to 10 lM phenylephrine
response.
10. CCDC 724363 contains the supplementary crystallographic data for compound
6: This data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
References and notes
1. Musselman, D. M.; Ford, A. P. D. W.; Gennevois, D. J.; Harbison, M. L.; Laurent, A.
L.; Mokatrin, A. S.; Stoltz, R. R.; Blue, D. R. BJU Int. 2004, 93, 78.
2. Whitlock, G. A.; Conlon, K.; McMurray, G.; Roberts, L. R.; Stobie, A.; Thurlow, R.
J. Bioorg. Med. Chem. Lett. 2008, 18, 2930.
3. Roberts, L. R.; Bryans, J.; Conlon, K.; McMurray, G.; Stobie, A.; Whitlock, G. A.
Bioorg. Med. Chem. Lett. 2008, 18, 6437.
4. Whitlock, G. A.; Brennan, P. E.; Roberts, L. R.; Stobie, A. Bioorg. Med. Chem. Lett.
2009, 19, 3118.