Synthesis and evaluation of 4-alkoxy-[1′-cyclobutyl-spiro(3,4- dihydrobenzopyran-2,4′-piperidine)] analogues as histamine-3 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2011.11.038

A novel class of 4-alkoxy-[1′-cyclobutyl-spiro(3,4-dihydrobenzopyran- 2,4′-piperidine)] analogues were designed and synthesized as H ₃R antagonists. Structure-activity relationship identified sulfone 27 with excellent H₃R affinities

Full text of DOI:10.1016/j.bmcl.2011.11.038

Bioorganic & Medicinal Chemistry Letters 22 (2012) 186–189
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and evaluation of
4-alkoxy-[10-cyclobutyl-spiro(3,4-dihydrobenzopyran-2,40-piperidine)]
analogues as histamine-3 receptor antagonists
⇑
Nadine C. Becknell , Reddeppa Reddy Dandu, Jacquelyn A. Lyons, Lisa D. Aimone, Rita Raddatz,
Robert L. Hudkins
Discovery Research, Cephalon, Inc., 145 Brandywine Parkway, West Chester, PA 19380, USA
a r t i c l e i n f o
a b s t r a c t
A novel class of 4-alkoxy-[10-cyclobutyl-spiro(3,4-dihydrobenzopyran-2,40-piperidine)] analogues were
designed and synthesized as H3R antagonists. Structure–activity relationship identified sulfone 27 with
excellent H3R affinities in both humans and rats, and acceptable pharmacokinetic properties. Further,
compound 28 achieved single digit nanomolar H3R affinities in both species with minimum hERG
activity.
Article history:
Received 1 November 2011
Revised 8 November 2011
Accepted 9 November 2011
Available online 16 November 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Histamine H3
H3R antagonists
CEP-26401
Spiro-dihydrobenzopyran
Sulfone
Histamine-3 receptors (H3Rs) are predominantly located in the
central nervous system (CNS) and function as presynaptic
autoreceptors regulating histamine release and as presynaptic
heteroreceptors regulating the release of multiple neuro-
transmitters including acetylcholine, dopamine, norepinephrine,
and serotonin.1–10 Thus, H3R antagonists have potential use for
the treatment of a variety of central nervous system (CNS) diseases
such as sleep disorders, cognitive disorders, attention-deficit hyper-
activity disorder (ADHD) and Alzheimer’s disease (AD).3–9 H3Rs
have attracted a lot of attention from the pharmaceutical industry
in the development of H3R ligands for the treatment of CNS dis-
orders.3–9 We identified a novel class of pyridazin-3-one H3R anta-
gonists/inverse agonists with exceptional drug-like properties, and
in vivo profiles.11,12 6-{4-[3-(R)-2-Methylpyrrolidin-1-yl)propoxy]-
phenyl}-2H-pyridazin-3-one 1 (Irdabisant; CEP-26401) ( Fig. 1)
was selected as a clinical candidate and recently completed phase
I.12 During our H3 discovery project research, we actively pursued
a variety of structural core modifications.13,14 One strategy was to
evaluate the constrained spiro-3,4-dihydrobenzopyran-2,40-piperi-
dine fragment as an amine replacement and optimize the
western portion of the core. This letter describes the synthesis of
4-alkoxy-[10-cyclobutyl-spiro(3,4-dihydrobenzopyran-2,40-piperi-
dine)] ether analogues 2 and evaluation of their H3R binding SAR,
pharmacokinetics (PK), selectivity and drug-like properties.15
The synthesis of compounds 8–12 is shown in Scheme 1.
Cyclocondensation of 2,5-dihydroxyacetonphenone with 4-
3
oxo-piperidine-1-carboxylic acid t-butyl ester provided compound
4 in a good yield.16 Reduction of ketone 4 and simultaneous
elimination of the Boc group afforded compound 5.16,17 Reductive
amination of 5 with excess cyclobutanone gave the key phenol
intermediate 6. Alkylation of phenol 6 with various electrophiles,
such as (2-bromo-ethoxymethyl)-benzene resulted in compound
7. Debenzylation of 7 gave primary alcohol 8. Similarly, reaction
of 6 with propylene oxide, (R)-2-ethyl-oxirane, (S)-2-ethyl-oxirane,
and 1,2-epoxy-2-methylpropane provided compounds 9–12.18
The synthesis of ethers 13–23 was accomplished by O-alkylation
of phenol 6 with the corresponding alkyl and heterocyclic halides or
mesylate intermediates (Scheme 2). The alkyl and cyclic mesylates
were synthesized from the corresponding alcohol precursors and
methanesulfonyl chloride. Oxidation of sulfur compounds 14, 15,
22, and 23 was accomplished with oxone or hydrogen peroxide to
afford the sulfone targets 24–28 in good yields.
H
O
N
N
O
R
Me
O
N
O
N
2
1
(Irdabisant; CEP-26401)
⇑
Corresponding author. Tel.: +1 610 738 6240.
E-mail address: nbecknell@cephalon.com (N.C. Becknell).
Figure 1.
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.11.038

N. C. Becknell et al. / Bioorg. Med. Chem. Lett. 22 (2012) 186–189
187
O
O
HO
HO
b
c
HO
a
O
O
N
O
NH
OH
4
5
3
O
HO
O
O
d
Ph
O
e
HO
O
O
O
N
N
N
6
7
8 (92%)
f
OH
OH
OH
O
O
O
O
O
O
N
N
N
10 (58%)
11 (59%)
9 (26%)
OH
O
O
N
12 (79%)
Scheme 1. Reagents and conditions: (a) 4-oxo-piperidine-1-carboxylic acid t-butyl ester, pyrrolidine, MeOH, reflux, 18 h, 87%; (b) NaBH4, MeOH, 0 °C, 1 h; Et3SiH, TFA,
CH2Cl2, rt, 24 h, 95%; (c) cyclobutanone, NaCNBH3, AcOH, THF, rt, 5 h, 74%; (d) (2-bromo-ethoxymethyl)-benzene, NaH, DMF, 65 °C, 4 h, 67%; (e) H2, Pd/C, MeOH, cat. HCl,
MeOH, 45 psi, 4 h, 92%; (f) propylene oxide/(R)-2-ethyl-oxirane/(S)-2-ethyl-oxirane/1,2-epoxy-2-methylpropane, 1 N NaOH, rt, 12 h.
O
HO
R
a
O
O
N
N
6
13-23
O
O
O
O
O
O
S
O
S
14
S
b or c
O
O
15
22
23
O
O
O
N
N
N
24 (68%)
26 (79%)
25 (80%)
O
S
O
O
O
O
S
O
O
N
O
27 (83%)
N
28 (27%)
Scheme 2. Reagents and conditions: (a) ROMs/RBr/RCl, NaH, DMF, 85 °C, 1–18 h, 20–70%; (b) oxone, MeOH, rt, 4–24 h; (c) H2O2, AcOH, rt, 6 h (for 27).
The structure–activity relationships (SARs) (Table 1) revealed
that the acyclic alkyl alcohols, ethers, and thioethers, in general
had favorable human H3R binding affinities with Ki values less than
50 nM. The exception was the primary alcohol 8 (hH3R Ki = 126 nM).
To a lesser degree the rat H3R affinity was weaker (Ki >50 nM) with
the exclusion of 25 (rH3R Ki = 41 nM). The order of hH3R binding
affinity in the alcohol series was tertiary > secondary ꢀ primary
(12 > 9 ꢀ 8). In the case of the secondary alcohols, little difference
was observed in the H3R affinity between butan-2-ol isomers 10
and 11 and propan-2-ol 9. Converting primary alcohol 8 to
methylether 13 improved H3R affinity by threefold. Replacing
methoxy group with methylthio group (13 to 14) showed little affect
on the H3R affinity, and further increasing the alkyl chain from
methylsulfanyl-ethoxy 14 (hH3R Ki = 38 nM) to propoxy 15 (hH3R
Ki = 15 nM) improved hH3R affinity by about twofold. The effect of
the sulfur oxidation state on H3R affinity was also investigated
where the corresponding sulfone showed comparable affinity with
the thioether (compare 14 with 24, and 15 with 25).
Next, the SAR was investigated with cyclic-ether and cyclic
thioether analogues for comparison with their acyclic counterparts.
In general, the cyclic derivatives showed improved H3R affinity
(Table 2). S-Tetrahydrofuran (THF) isomer 17 was about threefold
more potent than methoxyethyloxy 13, and the eudismic ratio
for S-17 was about 2 and 3 for human and rat H3Rs compared to
R-16. A number of changes in the cyclic series were tolerated, such
as changing the THF to a tetrahydrothiophene (18) ring, expanding
the THF to tetrahydropyran 21 (THP) and tetrahydrothiopyran 22,
or insertion of a methylene linker (19, 20, and 23) between the
spiro-ether core and the cyclic group. In the five-member ring
sulfur series oxidation of tetrahydrothiophene 18 to sulfone 26
showed little affect on H3R affinity. However in the six-membered
tetrahydrothiopyran series sulfur oxidation produced the most
potent compounds in the series. The hH3R Ki values were 7 nM
for 27 and 4 nM for methylene spaced 28.
Based on these results compounds 25, 27, and 28 were further
profiled in the discovery flow. Compounds 25, 27, and 28 had

188
N. C. Becknell et al. / Bioorg. Med. Chem. Lett. 22 (2012) 186–189
Table 1
Table 3
H3R binding data for sprio-benzopyran-piperidines
Rat pharmacokinetics for 25, 27, and 28
O
R
25
27
28
iv (1 mg/kg)
t1/2 (h)
Vd (L/kg)
O
N
0.9 0.2
1.2 0.1
16
1.7 0.1
2.8 0.5
19
1.9 0.5
3.6 0.8
23
CL (mL/min/kg)
1
3
5
Compd
R
hH3R Kia (nM)
rH3R Kia (nM)
po (5 mg/kg)
AUC0–t (ng h/mL)
Cmax (ng/mL)
%F
586 89
142 26
1791 220
460 108
713 33
8
9
126
25
368
99
HO
157
22
5
2
13
2
48
6
OH
B/Pa
0.73 0.01
1.0 0.1
0.73 0.05
OH
a
Based on the 6 h time point after 5 mg/kg po dosing.
R-10
S-11
15
26
86
OH
112
excellent selectivity over hH1R, hH2R, and hH4R subtypes with Ki
values >10 M, and showed acceptable metabolic stability in liver
microsomes from human, mouse, rat, and monkey (t1/2 >40 min).
Acyclic sulfone 25 had IC50 values >30 M for inhibition of cyto-
OH
l
12
13
18
42
74
91
l
O
chrome P450 enzymes (CYPs) CYP1A2, 2C9, 2C19, 2D6, and 3A4,
indicating low potential for drug-drug interactions. Compared to
25, cyclic sulfones 27 and 28 showed moderate 2D6 activity
14
15
38
15
86
64
S
S
(27 = 6.6 lM, 28 = 11 lM). Based on the metabolic stability profiles
S
S
24
25
39
10
157
41
25, 27, and 28 were evaluated for pharmacokinetic (PK) properties
in the rat (Table 3). Acyclic sulfone 25 had acceptable iv PK but
showed low oral bioavailability and low brain to plasma ratio.
Compounds 27 and 28 showed acceptable PK profiles with longer
iv half-lives, comparable clearance, and higher volume of distribu-
tion. Overall 27 had high oral exposure based on Cmax, AUC0–t and
oral bioavailability. Further profiling 27 and 28 hERG activity (MDS
Pharma Services PatchExpress) showed a difference, the methylene
O
O
O
O
The assay-to-assay variation was typically within 2.5-fold.
a
Ki values are an average of 2 or more determinations.
spaced linker compound 28 had a hERG IC50 >30
27, which was 7 M.
lM, compared to
Table 2
l
H3R binding data for sprio-benzopyran-piperidine
Compound 27 was further screened against a panel of 65 GPCRs,
ion channels and enzymes (MDS Pharma Services, LeadProfile
screen). The targets identified from the broad selectivity screen
O
R
O
N
with activity >50% inhibition at 10
tinic acetylcholine receptor (nACh, 63%), alpha adrenergic subtypes
2C ( 2C, 78%) and 1D ( 1D, 50%). Compound 27 had moderate bind-
lM concentration were nico-
a
a
Compd
R
hH3R Kia (nM)
rH3R Kia (nM)
ing to plasma protein from humans (77%) and rats (73%) and it
showed an acceptable 27% free fraction in the rat brain homoge-
nate, comparable to the free fraction in rat plasma. Compound 27
was tested for proof-of-concept in the rats in the dipsogenia
assay10,19 and produced significant 61% inhibition of RAMH-
induced drinking at the highest dose of 3 mg/kg po.
R-16
27
119
O
O
S
S-17
15
17
39
42
18
In summary, a new series of constrained 4-alkoxy-[10-cyclo-
butyl-spiro(3,4-dihydrobenzopyran-2,40-piperidine)] ether ana-
logues were designed and evaluated as H3R antagonists. In general
the series had high H3R affinity in humans with hH3R Ki <50 nM in
both alkyl and cyclic ethers series. The cyclic-tetrahydrothiopyran
sulfones 27 and 28 achieved single digit nanomolar hH3R binding
affinity and the methylene spaced analogue 28 had weak hERG
O
19
20
18
16
49
50
O
21
15
42
O
(IC50 >30 lM) activity. Proof-of-concept in vivo functional activity
in the rat dipsogenia model using 27 showed a significant 61%
inhibition of RAMH-induced drinking at 3 mg/kg po. Further
optimization and evaluation of the spiro series will be reported in
due course.
22
23
26
23
12
23
62
48
44
S
S
O
O
S
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