Structure-activity relationships of 2-substituted 5,7-Diarylcyclopenteno[1,2-b]pyridine-6-carboxylic acids as a novel class of endothelin receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(02)00663-7

Synthesis and structure-activity relationships of 2-substituted-5,7-diarylcyclopenteno[1,2-b]pyridine-6-carboxylic acids, a novel class of endothelin receptor antagonists, were described. Derivatization of a lead structure 1 (IC₅₀=2.4 nM, 170-fold selectivity) by incorporating a substituent such as an alkyl, alkoxy, alkylthio, or alkylamino group into the 2-position of the cyclopenteno[1,2-b]pyridine skeleton was achieved via the key intermediate 8. Introduction of an alkyl group led to the identification of potent ET_A/ET_B mixed receptor antagonists, a butyl (2d: IC₅₀=0.21 nM, 52-fold selectivity) and an isobutyl (2f: IC₅₀=0.32 nM, 26-fold selectivity) analogue. In contrast, installment of a primary amino group resulted in ET_A selective antagonists, a propylamino 2p (IC₅₀=0.12 nM, 520-fold selectivity) and an isopropylamino 2q (IC₅₀=0.10 nM, 420-fold selectivity) analogue. These results suggested that a substituent at the 2-position of the 5,7-diarylcyclopenteno[1,2-b]pyridine-6-carboxylic acids played a key role in the binding affinity for both ET_A and ET_B receptors.

Full text of DOI:10.1016/S0960-894X(02)00663-7

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3041–3045
Structure–Activity Relationships of 2-Substituted
5,7-Diarylcyclopenteno[1,2-b]pyridine-6-carboxylic Acids as a
Novel Class of Endothelin Receptor Antagonists
Kenji Niiyama,* Hirobumi Takahashi, Toshio Nagase, Hisaki Kojima, Yuka Amano,
Kasumi Katsuki, Takeru Yamakawa, Satoshi Ozaki, Masaki Ihara, Mitsuo Yano,
Takahiro Fukuroda, Masaru Nishikibe and Kiyofumi Ishikawa
Tsukuba Research Institute, Banyu Pharmaceutical Co. Ltd., 3 Okubo, Tsukuba, Ibaraki 300-2611, Japan
Received29 May 2002; accepted9 August 2002
Abstract—Synthesis and structure–activity relationships of 2-substituted-5,7-diarylcyclopenteno[1,2-b]pyridine-6-carboxylic acids, a
novel class of endothelin receptor antagonists, were described. Derivatization of a lead structure 1 (IC50=2.4 nM, 170-foldselec-
tivity) by incorporating a substituent such as an alkyl, alkoxy, alkylthio, or alkylamino group into the 2-position of the cyclo-
penteno[1,2-b]pyridine skeleton was achieved via the key intermediate 8. Introduction of an alkyl group led to the identification of
potent ETA/ETB mixedreceptor antagonists, a butyl ( 2d: IC50=0.21 nM, 52-foldselectivity) andan isobutyl ( 2f: IC50=0.32 nM, 26-
foldselectivity) analogue. In contrast, installment of a primary amino group resultedin ET
selective antagonists, a propylamino
A
2p (IC50=0.12 nM, 520-foldselectivity) andan isopropylamino 2q (IC50=0.10 nM, 420-foldselectivity) analogue. These results
suggestedthat a substituent at the 2-position of the 5,7-diarylcyclopenteno[1,2- b]pyridine-6-carboxylic acids played a key role in the
binding affinity for both ETA andET receptors.
B
# 2002 Elsevier Science Ltd. All rights reserved.
Introduction
clinical potential in the endothelin-mediated disorders
2
that are mentionedabove.
Endothelin-1 (ET-1),1 andits closely relatedisopeptides
(ET-2, ET-3) were identified as potent vasoconstrictor
peptides consisting of 21 amino acids. The endothelins
exert their diverse biological actions through distinct cell
surface G-protein coupledreceptors (GPCR) named
ETA andET B. The ET-1 selective ETA receptor subtype
mediates vasoconstriction and vascular smooth muscle
proliferation. The isopeptide non-selective ETB receptor
subtype can mediate either vasodilation or vasocon-
striction, depending on the tissue type. The diversity of
physiological effects elicitedby the endothelins has been
implicatedin the pathogenesis of a variety of disease
states such as hypertension, renal failure, cerebral
vasospasm, pulmonary hypertension, andcongestive
heart failure. Elevatedlevels of endothelins have been
observedin many of these idsease states. Therefore,
endothelin receptor antagonists are expected to have
A number of reports exist in the literature describing
non-peptide endothelin antagonists including the ETA
4
selective,3 ETB selective,5 andET A/ETB mixedagents
known to date. It should be noted that these antagonists
coincidentally possess a structural feature with an acidic
moiety positionedbetween the two aromatic rings. We
4b
hadan interest in the structure of SB-209670 because
of its high structural rigidity as well as its high binding
affinity for ET receptors. Our exploration for new
pharmacophores basedon this structure ledto the
identification of a potent non-peptide endothelin recep-
tor antagonist, 5,7-diarylcyclopenteno[1,2-b]pyridine-6-
carboxylic acid 1 shown in Figure 1.6 A preliminary
account of this work has been previously presented, and
herein we describe the versatile synthetic method of 2-
substituted cyclopentenopyridine derivatives and their
structure–activity relationships (SARs) on the in vitro
binding affinity for ETA andET receptors andselec-
B
tivity for ETA over ETB receptors.
*Corresponding author. Tel.: +81-298-77-2220; fax: +81-298-77-
2027; e-mail: niiymakj@banyu.co.jp
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00663-7

3042
K. Niiyama et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3041–3045
yield. The anhydride 5 was reactedwith a 4-methoxy-
phenyl Grignardreagent to provide a mixture of regio-
isomers 6a and 6b (6:1) in 96% yield. Transformation of
6a to an enone 7 was achievedin 45% yieldby the fol-
lowing reaction steps:7 (1) conversion of the carboxylic
acid to the corresponding imidazolide; (2) condensation
of the imidazolide with the enolate of tert-butyl acetate
followed by cyclization; and (3) dehydration assisted by
silica gel. Addition of a 3,4-methylenedioxyphenyl
Grignardreagent to the enone 7 gave an allyl alcohol in
96% yield. Protection of the tertiary alcohol as a 2-(tri-
metylsilyl)ethoxymethyl (SEM) ether followedby oxi-
dation of the phenyl sulfide to the corresponding
sulfone with m-CPBA afforded the key intermediate 8 in
91% yield.
Figure 1. New class of non-peptide endothelin receptor antagonist.
Results and Discussion
Chemistry
In the conventional method,7 a substituent on the pyri-
dine ring had to be installed during an early stage of the
synthesis, which preventedus from efficiently synthesiz-
ing a number of compounds to elucidate the SARs of
this series. Therefore, we hadto develop a robust syn-
thetic methodfor substituted5,i7a-rdylcyclo-
penteno[1,2-b]pyridine-6-carboxylic acid derivatives. It
is known that phenylsulfonyl group at the 2- and4-
position of pyridine is substituted with alkylmetal
reagents to provide 2- and 4-alkylpyridines, respec-
tively.8 We appliedit to the cyclopenteno[1,2- b]pyridine
system andinvestigatedthe synthesis of 2-substituted
analogues. After extensive investigation, we developed a
versatile methodto synthesize a variety of 2-substituted
cyclopenteno[1,2-b]pyridines via substitution reaction of
a intermediate 8 with various nucleophiles. The syn-
thetic procedure is shown in Scheme 1.
The phenylsulfonyl group was substitutedfor an alkyl
group with an alkyllitium at À78 ꢀC to give the adduct 9
in 50–60% yield.8 A similar substitution reaction was
achievedby treatment with a metal reagent such as
lithium alkylamide, potassium alkoxide, or potassium
alkylmercaptide to produce an alkylamino, alkoxy, or
alkylthio derivative, respectively. After deprotection of
the SEM ether under an acidic condition (HCl/MeOH),
the allyl alcohol system was reduced with zinc in the
presence of hydrogen chloride, which afforded the
desired cis–cis isomer as a major product.7 Epimeriza-
tion andhydrolysis of the ester moiety were simulta-
neously accomplishedunder basic conditions (NaOH/
dioxane) to give the final compound 2. This method
enabledus to efficiently synthesize a variety of com-
pounds for investigation of SARs of this class of com-
pounds.
Synthesis of the key intermediate 8 was startedfrom
dimethyl 2,3-pyridinedicarboxylate 3, which was con-
verted to 2-phenylthiopyridine-5,6-dicarboxylic anhy-
dride 5 by a five-step reaction sequence in 38% overall
Biological properties
Compounds that were synthesized via the above-descri-
bedmethodwere evaluatedin the binding assay (inhib-
Scheme 1. Synthesis of 2-substituted5,7-diarylcyclopenteno[1,2- b]pyridine-6-carboxylic acids. Reagents and conditions: (a) (1) H2O2, Na2WO4,
AcOH, 80 ꢀC, 94%; (2) POCl3, 130 ꢀC, 55%; (b) (1) PhSH, K2CO3, DMF, 80 ꢀC, 80%; (2) NaOH, MeOH, 60 ꢀC, quant; (3) Ac2O, 110 ꢀC, 91%; (c)
(1) 4-methoxyphenylmagnesium bromide, THF, À78 ꢀC, 96%; (d) (1) CDI, DMF; (2) LDA, CH3CO2t-Bu, THF, À78 ꢀC; (3) SiO2, CH2Cl2, 45%; (e)
(1) 3,4-methylenedioxyphenylmagnesium bromide, THF, À78 ꢀC, 96%; (2) SEMCl, EtN(i-Pr)2, CH2Cl2, 40 ꢀC, 93%; (3) mCPBA, CHCl3, 98%; (f)
RLi, THF, À78 ꢀC; (g) (1) HCl–MeOH, 0 ꢀC; (2) Zn, HCl, THF–EtOH, 0 ꢀC; (3) NaOH, dioxane–H2O, 120 ꢀC.

K. Niiyama et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3041–3045
3043
itory activity against 125I-labledET-1 binding to both
2d with oxygen atom(s) gave interesting results. A 3-
hydroxypropyl (2j), an ethoxymethyl (2l), or a propoxy
(2m) analogue showedsub-nano molar binding affinity
to ETA receptors, while a 2-carboxyethyl analogue 2k
showed a drastic decrease in potency. These results
suggest that a hydroxy and an alkoxy group at the 2-
position are tolerated, while an acidic functional group
is not tolerated. On the other hand, incorporation of an
oxygen atom into the butyl group of 2d, in particular at
the b- and d-position, resulted in a drastic reduction of
the binding affinity to ETB receptors, which ledto
identification of potent and ETA selective antagonists
such as 2j (IC50=0.55 nM, 350-foldselectivity) and 2l
(IC50=0.51 nM, 270-foldselectivity). Introduction of a
human ETA andET receptors),9 andthe data were
B
comparedwith those of the leadcompound 1. Table 1
shows the binding affinities of 2-alkylsubstituted ana-
logues (2a–2i). With respect to the binding affinity to
ETA receptors, incorporation of a methyl (2a) or ethyl
group (2b) afforded approximately a 2-fold reduction in
potency. On the other hand, incorporation of a propyl
(2c) or a butyl group (2d) resultedin approximately a
10-foldincrease in potency. Introduction of a longer
straight alkyl chain, pentyl group (2e), brought a con-
siderable decrease in the binding affinity in comparison
with that of 2c or 2d. 3-Butenyl analogue 2g showed
comparable binding affinity to that of the butyl ana-
logue 2d. Among branchedalkyl analogues, a isobutyl
analogue 2f hadthe affinity between the propyl ( 2c) and
the butyl (2d) analogue, anda cyclopropyl analogue 2h
hadthe affinity between the ethyl ( 2b) andthe propyl
(2c) analogue. These results suggest that the binding
affinity to ETA receptors is affectedby the length of the
alkyl chain, anda 4-carbon unit such as a butyl or 3-
butenyl group is optimal. In regardto the binding affi-
nity to ETB receptors, the incorporation of a alkyl
group enhancedthe binding affinity, andthe potency
increasedin proportion to the length of the alkyl chain,
which indicates that 2-alkyl analogues are less ETA
selective than 1. Introduction of phenyl group (2i) resul-
ted in a significant decrease in binding affinity to ETA
sulfur atom resultedin 40-foldedcrease in the ET
binding affinity and 5-fold decrease in the ETB binding
A
affinity in comparison with that of 2d, which ledto a
ETA/ETB mixedantagonist 2n (7-foldselectivity)
though the binding affinity to ETA receptors was lower
than that of 1.
The effects of introducing a nitrogen atom into the butyl
group of 2d on the binding affinity to ETA andET
B
receptors were also investigated. A propylamino analo-
gue 2p showed2-foldhigher ET
binding affinity than
A
2d, while an ethylaminomethyl analogue 2o was 33-fold
less active against ETA receptors and85-foldless active
against ETB receptors. These results suggest that incor-
poration of a nitrogen atom into the a-position of the
butyl group is crucial to the binding affinities. On the
other hand, ETB binding affinity of 2p decreased 6-fold
in comparison with that of 2d, which indicates that 2p is
the most ETA selective antagonist with more than 500-
receptors, andincreasedbinding affinity to ET
recep-
B
tors. As a result, incorporation of alkyl groups at the 2-
position ledto identification of highly potent ET A/ETB
mixedreceptor antagonists such as 2d (IC50=0.21 nM,
52-foldselectivity) and
selectivity).
2f (IC50=0.32 nM, 26-fold
foldselectivity for ET
over ETB receptors. An iso-
A
propylamino analogue 2q showedsimilar binding affi-
nities andselectivity to 2p. It is interesting to note that
N-methylation of 2q resultedin a 32-foldreduction in
the ETA binding affinity and a 3-fold increase in ETB
The binding affinities of analogues with a heteroatom-
containing substituent (2j–2s) are summarizedin Table
2. Replacement of a carbon atom in the butyl group of
Table 1. In vitro potency of 2-alkylsubstitutedderivatives
CompdR
IC
50 (nM) (n)
Selectivity
ETB/ETA
ETA
ETB
1
H
Methyl
Ethyl
Propyl
Butyl
Pentyl
Isobutyl
3-Butenyl
Cyclopropyl
Phenyl
2.4Æ0.5 (11)
5.7 (1)
4.5 (1)
0.37Æ0.01 (2)
0.21Æ0.02 (5)
1.9Æ0.5 (3)
0.32Æ0.04 (3)
0.20Æ0.06 (3)
1.6Æ0.2 (2)
15 (1)
410Æ110 (3)
100 (1)
87 (1)
30Æ1 (2)
11Æ2 (5)
9.1Æ1.4 (3)
8.4Æ1.0 (3)
14Æ1 (2)
63Æ9 (2)
130 (1)
170
18
19
81
52
4.8
26
70
39
8.7
2a
2b
2c
2d
2e
2f
2g
2h
2i

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K. Niiyama et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3041–3045
Table 2. In vitro potency of derivatives with heteroatom-containing substituent at the 2-position
CompdR
IC
50 (nM) (n)
Selectivity
ETB/ETA
ETA
ETB
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
3-Hydroxypropyl
2-Carboxyethyl
Ethoxymethyl
Propoxy
Propylthio
Ethylaminomethyl
Propylamino
0.55Æ0.06 (2)
150Æ21 (2)
0.51Æ0.03 (2)
0.92Æ0.17 (6)
8.2 (1)
7.0Æ0.6 (2)
0.12Æ0.02 (2)
0.10Æ0.03 (3)
3.2 (1)
190Æ51 (2)
530Æ150 (2)
140Æ16 (2)
65Æ7 (6)
53 (1)
940Æ540 (2)
63Æ0.3 (2)
42Æ19 (3)
14 (1)
350
3.5
270
71
6.5
130
520
420
4.4
9.5
Isopropylamino
N-Methylisopropylamino
1-Pyrrolidinyl
2.2Æ0.5 (2)
21Æ3 (2)
binding affinity compared with that of 2q, which indi-
cates that 2r is the least selective antagonist with 4-fold
selectivity. 2r is considered to be an ideal ETA/ETB
Acknowledgements
We are grateful to Ms. Regina Stamatis, Merck & Co.,
for critical reading of this manuscript. We also thank
Mr. Shinnosuke Abe for analytical support.
mixedantagonist though its binidng affinity to ET
A
receptors was comparable to that of 1. A pyrrolidinyl
analogue 2s also showedan ET A/ETB mixedantag-
onistic character. These results suggest that the a-
branch of the substituent at the 2-position also playeda
key role in the ETA binding affinity. As a result, incor-
poration of a nitrogen atom into the butyl group ledto
the identification of highly potent ETA selective
antagonists such as 2p (IC50=0.12 nM, 520-foldselec-
tivity) and 2q (IC50=0.10 nM, 420-foldselectivity) and
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dine-6-carboxylic acids. Through the derivatization via
the method, we found that an alkyl substituent such as a
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