Synthesis and SAR of analogs of the M1 allosteric agonist TBPB. Part II: Amides, sulfonamides and ureas-The effect of capping the distal basic piperidine nitrogen

Article abstract of DOI:10.1016/j.bmcl.2008.09.032

This letter describes the further synthesis and SAR, developed through an iterative analog library approach, of analogs of the highly selective M1 allosteric agonist TBPB by deletion of the distal basic piperidine nitrogen by the formation of amides, sulf

Full text of DOI:10.1016/j.bmcl.2008.09.032

Bioorganic & Medicinal Chemistry Letters 18 (2008) 5443–5447
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and SAR of analogs of the M1 allosteric agonist TBPB. Part II:
Amides, sulfonamides and ureas—The effect of capping the distal basic
piperidine nitrogen
Nicole R. Miller a, , R. Nathan Daniels b, , Thomas M. Bridges a,c, Ashley E. Brady a,c
,
P. Jeffrey Conn a,c, Craig W. Lindsley a,b,c,
*
a Department of Pharmacology, Vanderbilt University Medical Center, Nashville, TN 37232, USA
b Department of Chemistry, Vanderbilt University, Nashville, TN 37232, USA
c Vanderbilt Program in Drug Discovery, Vanderbilt Institute of Chemical Biology, Nashville, TN 37232, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
This letter describes the further synthesis and SAR, developed through an iterative analog library
approach, of analogs of the highly selective M1 allosteric agonist TBPB by deletion of the distal basic
piperidine nitrogen by the formation of amides, sulfonamides and ureas. Despite the large change in basi-
city and topology, M1 selectivity was maintained.
Received 28 August 2008
Revised 5 September 2008
Accepted 8 September 2008
Available online 11 September 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
TBPB
M1
Allosteric agonist
The muscarinic acetylcholine receptors (mAChRs) are members
of the GPCR family A that mediate the metabotropic actions of the
neurotransmitter acetylcholine (ACh). Five distinct subtypes of
mAChRs (M1–M5) have been cloned and sequenced.1–3 mAChR-
regulated cholinergic signaling plays a critical role in a wide vari-
ety of CNS and peripheral functions including memory and atten-
tion mechanisms, motor control, nociception, regulation of sleep
wake cycles, cardiovascular function, renal and gastrointestinal
function and many others.4–6 As a result, agents that can selec-
tively modulate the activity of specific mAChRs have therapeutic
potential in multiple pathological states, including Alzheimer’s dis-
ease and schizophrenia.1–6 However, due to high sequence conser-
vation within the orthosteric binding site of the five mAChR
subtypes, it has been historically difficult to develop subtype selec-
tive ligands, though accounts of an ectopic agonist, such as AC-42,
and brucine, a weak M1 positive allosteric modulator have been re-
ported.1–8
Ab production.9 However, TBPB 1 was an un-optimized screening
lead with antagonist activity at D2 (IC50 = 5.1 M), and despite an
18F]-fallypride micro-PET study that confirmed the antipsychotic
activity observed with 1 was the result of selective M1 activation
and not due to inhibition of D2, we hoped to diminish D2 activity
through a lead optimization campaign.9,10 This effort initially fo-
cused on alternative benzyl moieties and afforded TBPB analogs,
such as 2 which were equivalent to TBPB in terms of M1 efficacy
and mAChR selectivity, but possessed no D2 inhibitory activity.10
l
[
100
M1
M2
M3
M4
M5
80
60
40
20
0
N
R
1, TBPB
N
2
We recently reported on TBPB 1, a potent, centrally active and
highly selective M1 allosteric agonist (Fig. 1), which displayed ro-
bust efficacy in several preclinical antipsychotic models as well
as significant effects on the processing of amyloid precursor pro-
tein (APP) towards the non-amyloidogenic pathway and decreased
N
O
N
H
-11 -10 -9
-8
-7
-6
-5
-4
1, R = CH3
1, R = CF3
Log [Cmpd] (M)
Figure 1. Concentration response curves for TBPB (1) and 2 at M1–M5 and 2 at M1.
Data represent the mean + SEM of three independent determinations. TBPB (1) M1
EC50 = 289 nM, 82% CCh Max, 2 M1 EC50 = 334 nM, 80% CCh Max; 1, TBPB at M2–M5
* Corresponding author. Tel.: +1 615 322 8700; fax: +1 615 343 6532.
E-mail address: craig.lindsley@vanderbilt.edu (C.W. Lindsley).
These authors contributed equally to this work.
EC50 > 50 lM.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.09.032

5444
N. R. Miller et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5443–5447
O
O
O
N
Cl, H
(b)
(a)
N
R1
N
N
N
H
HN
O
O
R1
3
4
5
Scheme 1. Reagents and condition: (a) R1COCl, PS-DIEA, 85–88%; (b) MP-B(OAc)3H, 4-(2-keto-1-benzoimidazolinyl)piperidine or 5-chloro-1-(4-piperidinyl)-2-benzimi-
dazolone, DCM, rt, 24 h, 85–95%. All compounds purified by mass-directed HPLC to analytical purity (>98%).
In the initial lead optimization campaign, we prepared and
evaluated one congener wherein the 2-methylbenzyl moiety in 1
Table 1
Functional activity of TBPB amide analogs 5
was replaced with an ethyl carbamate. Surprisingly, this analog
lost all selectivity for M1 and degenerated into a potent, but pan-
mAChR agonist (M1 EC50 = 2.1 nM (82.6% CCh Max), M2
EC50 = 11.9 nM (80.6% CCh Max), M3 EC50 = 113 nM (24.1% CCh
Max), M4 EC50 = 9.1 nM (96.9% CCh Max), M5 EC50 = 34.3 nM
(50.8% CCh Max). Despite this finding, we decided to pursue amide
congers, as SAR for allosteric ligands can often be very ‘flat’ and
slight structural changes can have radically different pharmacolog-
ical profiles.10–13
Compound
R1
R2
M1 EC50
(l
M)
% CCh Maxa
a
5a
5b
5c
5d
5e
5f
2-CH3
2-CH3
3-CH3
3-CH3
2-CF3
2-CF3
3-Cl
H
Cl
H
Cl
H
Cl
H
0.88
2.71
0.51
2.11
0.91
11.1
0.93
7.11
60
56
53
50
32
24
21
21
5g
5h
3-Cl
Cl
a
EC50s and % CCh maximum (measures activation of M1 relative to 100%
response of CCh) are the mean of at least three determinations. All analogs in this
table are selective for M1 (no activation of M2–M5 at conc. up to 30 M).
l
Other analogs displayed improved M1 efficacy, but at the cost of
enhanced inhibition of D2. In this letter, we describe the synthesis,
SAR and pharmacological profile of new TBPB analogs in which the
benzyl moiety is replaced with either an amide, sulfonamide or
urea linkage to determine the effects on mAChR activation of cap-
ping the distal nitrogen atom and altering the basicity and topol-
ogy of TBPB.
Figure 2. Ten micromolar compound screen for M1 activation. Data represent the
mean SEM of three independent determinations with TBPB (1), ACh and CCh as
Positivie controls. Boxed panels show the sulfonyl chlorides 7a–7k and isocyanates
8a–8r employed in the library (Scheme 2).
Scheme 2. Reagents and condition: (a) H2, Pd/C, rt, 92%; (b) R1SO2Cl (7a–k) or
R1NCO (8a–r), DCM, PS-DIEA, 70–90%. All compounds purified by mass-directed
HPLC to analytical purity (>98%).

N. R. Miller et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5443–5447
5445
Amide analogs of TBPB 5 were prepared by a reductive amina-
100
80
M1
tion sequence employing functionalized acyl piperdiones 4 and
either 4-(2-keto-1- benzoimidazolinyl)piperidine or the 5-chloro
conger and polymer-supported triacetoxyborohydride (Scheme
1). In short order, a 24-member library of amide analogs 5 was pre-
pared and evaluated against M1–M5. Once again, SAR for this ser-
ies was ‘flat’ with only 30% of the analogs affording M1 activation
(Table 1). However, unlike the pan-mAChR carbamate agonist, all
of the active amide congers 5a–5h in Table 1 were selective for
M2
M3
M4
M5
60
40
20
0
M1 (i.e., no activation of M2–M5 at concentrations up to 30 lM).
—9
—8
—7
—6
—5
—4
The SAR for this series was quite different from the benzyl amine
series represented by 1 and 2, wherein substitution was only toler-
ated in the 2-position of the benzyl ring.10 For the amide series,
methyl 5c or chloro 5g groups in the 3-position afforded M1 ago-
nist activity with submicromolar EC50s (510 nM and 930 nM,
respectively). Uniformly, the chlorine in the 5-position of the benz-
imidazolone ring diminished M1 efficacy, in contrast to the benzyl
amine series.10 Moreover, all of the analogs in Table 1 were found
to be partial allosteric agonists (CCh Max < 60%), and none pos-
log [9c] (M)
100
80
M1
M2
M3
M4
M5
60
40
20
0
sessed significant inhibition of D2 (<10% at 10 lM). These data sug-
gest the distal basic nitrogen is not required for M1 activation or
selectivity versus M2–M5.
—9
—8
—7
—6
—5
—4
These data prompted the investigation of other capping agents,
such as sulfonamides and ureas, to introduce additional proton
acceptors or proton donor/acceptors, respectively. In this instance,
we took advantage of our large supply of 1 and removed the benzyl
moiety by hydrogenation and then treated the resulting piperidine 6
with a diverse set of 11 sulfonyl chlorides 7 or 18 isocyanates 8 to
provide the corresponding 29-member library of sulfonamides 9
and ureas 10, respectively (Scheme 2). The library was triaged
log [10l] (M)
Figure 3. Concentration response curves for 9c and 10i at M1–M5. Data represent
the mean SEM of three independent determinations. 9c M1 EC50 = 1.4 M, 82%
CCh Max, 10i M1 EC50 = 6.5 M, 85% CCh Max; M2–M5 EC50 > 30 M.
l
l
l
binding site. Full concentration–response curves were then gener-
ated for 9c and 10i at M1, along with M2–M5 selectivity assays
(Fig. 3). Gratifyingly, these two hits provided complete selectivity
for M1 versus M2–M5 with micromolar EC50s for M1 activation
employing a 10 lM single point M1 activation screen in triplicate,
relative to ACh, CCh and TBPB controls (Fig. 2).9,10 This effort identi-
fied only two compounds, one sulfonamide 9c and one urea 10i, that
activated M1, highlighting again the ‘flat’ SAR for this M1 allosteric
(9c, M1 EC50 = 1.4 0.007
l
M, 82% CCh Max and 10i, M1
H
N
N
O
O
S
O
X
O
N
O S
O
O
N
H
X
N
(a)
13
N
(b)
N
N
H
HN
O
3
11
14
H
N
N
O
N
O
X
O
N
O
N
H
X
N
H
(c)
13
N
SCF3
(b)
N
N
H
HN
O
NH
O
F3CS
3
12
15
Scheme 3. Reagents and condition: (a) PhSO2Cl, PS-DIEA, 88%; (b) MP-B(OAc)3H, 5-halo-1-(4-piperidinyl)-2-benzimidazolones 13, DCM, rt, 24 h, 85–95%; (c) m-SCF3NCO, PS-
DIEA, 80%. All compounds purified by mass-directed HPLC to analytical purity (>98%).

5446
N. R. Miller et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5443–5447
Table 2
reductive amination reaction with a series of 4-, 5- or 6-halogen
Functional activity of TBPB amide analogs 14
functionalized piperidine benzimidazolones 13 to produce analogs
14 and 15, respectively. The SAR was ‘flat’, with all analogs 14
being of comparable M1 potency (Table 2), while most analogs
15 were inactive (Table 3). For both series, the degree of D2 inhibi-
tion varied widely (D2 IC50 = 1.9 lM to >10 lM), but the % CCh Max
was >70% except 14a. As shown in Table 2, neither the location of
fluorine incorporation nor which halogen (F, Cl or Br) was
appended in the 5-position of the benzimidazolone scaffold, the
M1 EC50s and selectivity versus M2–M5 were comparable. Of ana-
logs 14, only the 6-F congener 14c was a weaker partial agonist at
M1, with the other retaining a higher degree of partial agonism
akin to the benzyl amine series and TBPB 1. We found that the var-
iability of D2 inhibition was quite surprising, with halogens in the
5-position, 14b, 14d and 14e, affording micromolar levels of D2
a
a
Compound
X
M1 EC50
(l
M)
% CCh Maxa
D2 IC50 (lM)
14a
14b
14c
14d
14e
4-F
5-F
6-F
5-Cl
5-Br
2.6
2.3
3.3
4.1
4.4
49
82
76
82
81
>10
3.6
>10
2.3
2.7
inhibition (IC50s of 3.6, 2.3 and 2.7
fluorines in the 4- or 6-position, as in 14a and 14c, did not inhibit
D2 at concentrations up to 10 M, suggesting the 5-position of
lM, respectively). In contrast,
a
l
EC50s, % CCh maximum (measures M1 activation relative to 100% CCh response)
and IC50s are the mean of at least three independent determinations. All analogs in
this table are selective for M1 (>30 lM vs M2–M5).
analogs 14 is a molecular switch to dial in D2 inhibitory activity.
This was an important finding as a dual M1 allosteric agonist/D2
antagonist may prove to be an excellent profile for a novel schizo-
phrenia treatment.
Activity for the urea congeners 15 was disappointing. Only two
analogs, 15a and 15c, had any significant efficacy at M1 (15a,
Table 3
Functional activity of TBPB amide analogs 15
EC50 = 9.6 lM, 72% CCh Max.; 15c, EC50 = 3.6 lM, 90% CCh Max).
Interestingly, 15c, with the 5-Cl benzimidazolone moiety, was
more potent than the parent 10i, equally efficacious and possessed
no D2 inhibitory activity, whereas 15a was a ꢀ1
lM D2 antagonist.
Based on these data that allosteric M1 activation proceeds in the
absence of the distal piperidine nitrogen, future exploration will
center on diminishing basicity through the introduction of b-flu-
oroamine moieties to provide congeners 16 and 17 of TBPB (Fig. 4).
In summary, we have identified three novel series of alloste-
ric partial agonists with high selectivity for M1 versus M2–M5
based on the TBPB scaffold, in which the distal piperidine nitro-
gen has been capped to form amides, sulfonamides or ureas. SAR
was ‘flat’ within these series, with subtle changes resulting in
loss of M1 potency, significant decreases in the degree of partial
agonism or significant increases/decreases in D2 inhibition. As
with other allosteric ligands, these data suggest that the alloste-
ric binding site for TBPB and related analogs is shallow. Impor-
tantly, SAR did not track between these capped series, or with
respect to the original TBPB benzyl amine series which further
exemplifies the challenges in the development of allosteric
ligands for GPCRs. However, we were able to demonstrate that
the distal basic amine in TBPB is not required for binding and
activation of M1 at the TBPB allosteric site, and within the sul-
fonamide series 14, the 5-position of the benzimidazolone is a
molecular switch for engendering D2 inhibitory activity. Further
a
a
Compound
X
M1 EC50
(l
M)
% CCh Maxa
D2 IC50 (lM)
15a
15b
15c
4-F
6-F
5-Cl
>10
9.6
3.6
46
72
90
1.1
1.9
>10
a
EC50s, % CCh maximum (measures M1 activation relative to 100% CCh response)
and IC50s are the mean of at least three independent determinations. All analogs in
this table are selective for M1 (>30 lM vs M2–M5).
EC50 = 6.5 0.3
l
M, 85% CCh Max) and neither analog possessed sig-
nificant inhibition of D2 (ꢀ10% at 10
l
M). Unlike the weaker partial
agonist amide series 5, both 9c and 10i are more efficacious M1
agonists, which justified further optimization of these hits.
As shown in Scheme 3, analogs of 9c and 10i were easily pre-
pared in two steps by reacting piperidone 3 with either 7c or 8i
to afford 11 or 12, respectively. Then, 11 and 12 underwent a
R = COR
SO2R
R
N
N
CH3
CONHR
H, (S)-F, (R)-F
H, (S)-F, (R)-F
N
N
H, (S)-F, (R)-F
H, (S)-F, (R)-F
N
N
O
N
O
N
H
H
F, Cl, Br
F, Cl, Br
16
17
Figure 4. Future TBPB analogs 16 and 17: introducing b-fluoroamines to diminish basicity.

N. R. Miller et al. / Bioorg. Med. Chem. Lett. 18 (2008) 5443–5447
5447
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Acknowledgments
The authors thank the NIH and NIMH for support of our
programs (1RO1MH082867-01). The authors specifically acknowl-
edge the support of the Alzheimer’s Association (IIRG-07-57131).
T.M.B. acknowledges an ITTD (T90-DA022873) pre-doctoral train-
ing grant. N.R.M. acknowledges Ion Channel and Transporter Biol-
ogy (T32-NS07491) pre-doctoral training grant, and A.E.B. is
supported by
MH079678-01).
a National Research Service Award (1FM32
11. O’Brien, J. A.; Lemaire, W.; Chen, T.-B.; Chang, R. S. L.; Jacobson, M. A.; Ha, S. N.;
Lindsley, C. W.; Sur, C.; Pettibone, D. J.; Conn, J.; Wiliams, D. L. Mol. Pharmacol.
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