Carbazolothiophene-2-carboxylic acid derivatives as endothelin receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2003.12.067

The SmI₂-promoted three-component coupling reaction of thiophene-2-carboxylate, indole-2-carbaldehyde and acetophenone provides an expedient route to a series of tetracyclic carbazolothiophene compounds bearing the indole and thiophene rings. A

Full text of DOI:10.1016/j.bmcl.2003.12.067

Bioorganic & Medicinal Chemistry Letters 14 (2004) 1129–1132
Carbazolothiophene-2-carboxylic acid derivatives as endothelin
receptor antagonists
Govindarajulu Babu,a Hui-Ming Yu,b,c Shyh-Ming Yanga and Jim-Min Fanga,d,
*
aDepartment of Chemistry, National Taiwan University, Taipei, 106, Taiwan
bInstitute of Biochemistry, Academia Sinica, Taipei, 115, Taiwan
cDepartment of Chemistry, Faculty of Science, Chiang Ma University, Thailand
dGenomic Research Center, Academia Sinica, Taipei, 115, Taiwan
Received 14 November 2003; revised 17 December 2003; accepted 18 December 2003
Abstract—The SmI2-promoted three-component coupling reaction of thiophene-2-carboxylate, indole-2-carbaldehyde and aceto-
phenone provides an expedient route to a series of tetracyclic carbazolothiophene compounds bearing the indole and thiophene
rings. Among these samples, 9-benzyl-4-methyl-4-(4-hydroxyphenyl)-10-oxo-4,10-dihydrocarbazolo[2,3-b]thiophene-2-carboxylic
acid (18) shows the most potent inhibition against the endothelin-1 induced increase of intracellular calcium ion concentration.
# 2003 Elsevier Ltd. All rights reserved.
Human endothelin-1 (ET-1) is a 21 amino acid peptide
that exhibits a potent vasoconstrictor activity, con-
ceivably through its selective interaction with specific
receptor subtypes (ETA, ETB and ETC).1 ET-1 contains
six highly conserved amino acid residues (His16-Trp21)
at the C-terminus, and this hydrophobic C-terminal
hexapeptide alone shows some affinity for ETA receptor.
Several antagonists including BQ123 [cyclo(l-Leu-d-
Val-l-Pro-d-Asp-d-Trp)]2 are designed on the basis of
this peptide structures that incorporate indole moieties.
Some non-peptide endothelin antagonists also consist of
indole scaffolds such as the indole-2-carboxylic acids
PD1594333 and SB2095984 (Fig. 1). On the other hand,
the molecular modeling indicates that an indan deriv-
ative SB2096705 possesses two phenyl substituents to
mimic the amino acid residues of Try-13 and Phe-14 in
ET-1. The two carboxylic groups in SB209670 also
mimic the Asp-18 residue and the C-terminus of ET-1,
which ligate Zn2+ ion on binding with endothelin
receptor.1 We speculated that a new class of carbazo-
lothiophene derivatives (e.g., 7–22) bearing appropriate
substituents might serve as endothelin receptor antago-
nists. Indeed, 5-benzyloxy-3-isopropoxy-benzothiophene-
2-carboxylic acid3,6 has been utilized as a lead com-
pound for development of endothelin antagonists. A
thiophene-3-sulfonamide TBC112517 is also known as
an ETA-selective antagonist, in which the sulfonamide
moiety is considered an isostere of carboxylic acid. We
are thus interested in applying our established method
of three-component coupling reactions of thiophene-
2-carboxylate8 to synthesize carbazolothiophene-2-
Keywords: Coupling reactions; Endothelin; Samarium diiodide; Thio-
phene; Indole.
* Corresponding author. Tel.: +1-8862-23637812; fax: +1-8862-
23636359; e-mail: jmfang@ntu.edu.tw
Figure 1. Some representative endothelin receptor antagonists con-
structed by the indole, indan and thiophene scaffolds.
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.12.067

1130
G. Babu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1129–1132
carboxylate derivatives 7–22, and examined their antag-
onism against the binding of ET-1 with ETA receptor.
corresponding ketone, an acid-catalyzed dehydration to
eliminate a water molecule, and an oxidative aromati-
zation by using Pd(OAc)2 to afford the thiophene pro-
duct 5. The subsequent intramolecular Friedel–Crafts
alkylation thus furnished the tetracyclic skeleton, giving
carbazolothiophene-2-carboxylate 6 as a pivotal com-
pound for the synthesis of other derivatives 7–10.
Saponification of 6 afforded acid 7, whereas demethyl-
ation of 6 with BBr3 gave phenol 8. Alkylation of phenol
8 with methyl 2-bromoacetate or 4-bromobutanenitrile,
followed by hydrolysis in alkaline conditions, gave dia-
cids 9 and 10 in high yields.
By the promotion of samarium diiodide, the three-
component coupling reaction of methyl thiophene-2-
carboxylate (1) with N-methylindole-2-carbaldehyde (2)
and 4-methoxyacetophenone (3) occurred smoothly to
afford a 77% yield of 4 (Scheme 1).9,10 This one-pot
operation presumably proceeded by an initial coupling
of ester 1 with aldehyde 2 to give a dienolate inter-
mediate A,8,11 which was then trapped by ketone 3.
Although diol 4 existed as a mixture of diastereomers,
the subsequent oxidation and dehydration would yield
a single product. Conversion of 4 to 5 was achieved by a
three-step sequence: an oxidation with DDQ to give the
A series of carbazolothiophenes 11–22 were similarly
prepared, initially by the SmI2-promoted three-compo-
nent coupling reactions with appropriate partner sub-
strates. For example, the SmI2-promoted coupling
product of thiophene ester 1, indole aldehyde 2 and 3-
methoxyacetophenone was further elaborated, accord-
ing to the procedure similar to that shown in Scheme 1,
to give compounds 11–13 bearing MeO, CH2CO2H or
(CH2)3CO2H substituents on the meta positions of the
phenyl rings as the ‘meta-analogues’ of 7, 9 and 10.
Compounds 14–19 with the (substituted)benzyl groups
on the nitrogen atoms were also prepared in high yields
by elaboration of the coupling products obtained from
ester 1, 1-benzylindole-2-carbaldehydes and appropriate
ketones. Compounds 20–22 are analogues of 17–19
having the substituents switched over. It was noted that
the benzyl protons in compounds 14–22 occurred at low
fields (ca. d 6.1) presumably due to the deshielding effect
of the adjacent carbonyl group.
The interaction of ET-1 with endothelin receptor is
known to trigger an increase of intracellular Ca2+ con-
centration ([Ca2+]i) as the consequence of multistep
biological events initiated by G-protein.12 To evaluate
the potency of an endothelin receptor antagonist, one
can measure its inhibitory ability against the ET-1
Scheme 1. Reagents and conditions: (i) 1 (1 mmol), 2 (1 mmol), SmI2
(3.6 mmol), THF, HMPA, 0 ꢀC to rt, 1.5 h; then 3 (1.2 mmol), 0 ꢀC to
rt, 10 h; 77% yield of 4. (ii) DDQ, PhH, rt, 4 h; 88%. (iii) cat. p-TsOH,
PhH, reflux, 10 h; 95%. (iv) Pd(OAc)2 (5 equiv), K2CO3, CH3CN, rt,
12 h; 93%. (v) p-TsOH, CH2Cl2, rt, 2 h; 98%. (vi) aq NaOH (0.5%),
THF, 0 ꢀC to rt, 2.5 h; 99%. (vii) BBr3, CH2Cl2, À78 ꢀC; 96%. (viii)
BrCH2CO2Me, K2CO3, CH3CN, 80 ꢀC, 15 h; aq NaOH (0.5%), THF,
0 ꢀC to rt, 2.5 h; 95% yield of 9. (ix) Br(CH2)3CN, K2CO3, CH3CN,
80 ꢀC, 48 h; aq NaOH (40%), MeOH, reflux, 1 h; 84% yield of 10.

G. Babu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1129–1132
1131
induced [Ca2+]i change. According to the reported
experimental protocol,13 Chinese hamster ovary (CHO-
K1) cells were transfected with the rat ETA-expression
plasmid DNA using lipofectin reagent (Life Technolo-
gies Inc., USA). The ETA overexpression CHO-K1 cells
were prior incubated with calcium chelating agent fura-
2 applied as its penta(acetoxymethyl) ester,14 and then
treated with ET-1 in 10À7 M. The [Ca2+]i increase was
monitored at 510-nm fluorescence emission by a ratio-
metric method using dual excitations at 340 and 380 nm
wavelengths.13 This increment of functional assay was
taken as the standard value (100%) to assess the inhi-
bitory potency of compounds 7–22 against the ET-1
binding with receptor. On addition of the test sample 7
(10À6 M) along with ET-1 (10À7 M), only 30Æ5%
increment of [Ca2+]i was observed (a mean value of
three measurements), equivalent to ꢁ70% inhibition.
By comparisons with the known ETA antagonists, 10À6
M of SB209670 completely inhibited the ET-1 induced
[Ca2+]i, whereas BQ123 showed ꢁ60% inhibition
under our assay conditions. Accordingly, compounds 9,
10, 12, 13 and 18 showed high inhibition (>75%) at
10À6 M. Compounds 7, 11, 15 and 19 showed medium
inhibition (50–70%), whereas compounds 14, 16, 17 and
20À22 showed low inhibition. Among our examined
samples, compound 18 appeared to be the best ET-1
antagonist with IC50 ꢁ10 nM. In comparison,
SB209670 is an even more potent antagonist showing
ꢁ85% inhibition at 10 nM.
2. Ishikawa, K.; Fukami, T.; Nagase, T.; Fujita, K.;
Hayama, T.; Niyama, K.; Mase, T.; Ihara, M.; Yano, M.
J. Med. Chem. 1992, 35, 2139.
3. Bunker, A. M.; Edmunds, J. J.; Berryman, K. A.; Walker,
D. M.; Flynn, M. A.; Welch, K. M.; Doherty, A. M.
Bioorg. Med. Chem. Lett. 1996, 6, 1367.
4. Elliott, J. D.; Leber, J. D. PCT Int. Appl. 1994, 48 pp WO
9414434 A1 19940707.
5. (a) Elliott, J. D.; Lago, M. A.; Cousins, R. D.; Gao, A.;
Leber, J. D.; Erhard, K. F.; Nambi, P.; Elshourbagy,
N. A.; Kumar, C.; Lee, J. A.; Bean, J. W.; DeBrosse,
C. W.; Eggleston, D. S.; Brooks, D. P.; Feuerstein, G.;
Ruffolo, R. R.; Weinstock, J.; Gleason, J. G.; Peishoff,
C. E.; Ohlstein, E. H. J. Med. Chem. 1994, 37, 1553. (b)
Clark, W. M.; Tickner-Eldridge, A. M.; Huang, G. K.;
Pridgen, L. N.; Olsen, M. A.; Mills, R. J.; Lantos, I.;
Baine, N. H. J. Am. Chem. Soc. 1998, 120, 4550.
6. Bunker, A. M.; Edmunds, J. J.; Berryman, K. A.; Walker,
D. M.; Flynn, M. A.; Welch, K. M.; Doherty, A. M.
Bioorg. Med. Chem. Lett. 1996, 6, 1061.
7. (a) Wu, C.; Chan, M. F.; Stavros, F.; Raju, B.; Okun, I.;
Mong, S.; Keller, K. M.; Brock, T.; Kogan, T. P.; Dixon,
R. A. F. J. Med. Chem. 1997, 40, 1690. (b) Wu, C.;
Decker, E. R.; Holland, G. W.; Brown, P. M.; Stavros,
F. D.; Brock, T. A.; Dixon, R. A. F. Drugs of Today 2001,
37, 441.
8. (a) Yang, S.-M.; Fang, J.-M. Tetrahedron Lett. 1997, 38,
1589. (b) Yang, S.-M.; Shie, J.-J.; Fang, J.-M.; Nandy,
S. K.; Chang, H.-Y.; Lu, S.-H.; Wang, G. J. Org. Chem.
2002, 67, 5208.
9. Representative procedure for the SmI2 promoted three-
component coupling reactions: Under an atmosphere of
argon, a deep blue SmI2 solution (0.1 M) was prepared by
treatment of Sm (661 mg, 4.4 mmol) with 1,2-diio-
doethane (1.01 g, 3.6 mmol) in HMPA10 (2.8 mL, 16
mmol) and anhydrous THF (32 mL) for 1.5 h at room
temperature. To the SmI2 solution (cooled in an ice bath)
were added a THF solution (3 mL) of methyl thiophene-
2-carboxylate (142 mg, 1 mmol) and N-methylindole-2-
carbaldehyde (159 mg, 1 mmol). The reaction mixture was
stirred at 0 ꢀC for 45 min, and then at room temperature
(27 ꢀC) for 45 min. A THF solution (2 mL) of 4-methoxy-
acetophenone (180 mg, 1.2 mmol) was added at 0 ꢀC, and
the mixture was stirred at 0–27 ꢀC for additional 10 h. The
reaction was quenched by addition of saturated aqueous
NH4Cl solution (0.1 mL). The mixture was passed
through a short silica gel column by rinse with EtOAc/
hexane (1:1). The filtrate was concentrated, and chroma-
tographed on a silica gel column by elution with EtOAc/
hexane (3:7) to give the desired three-component coupling
product 4 (349 mg, 77%) as a mixture of isomers as
In summary, a series of tetracyclic compounds 7–22
bearing the indole and thiophene rings were prepared in
an expedient fashion. The functional assay indicated
that one of these samples (compound 18) can serve as a
lead compound for future exploration of potent endo-
thelin receptor antagonists. The structure–activity rela-
tionship also awaits further investigation.
Acknowledgements
The authors like to thank Prof. Wan-Wan Lin
(Department of Pharmacology, National Taiwan Uni-
versity) and Sheau-Huei Chueh (Department of Bio-
chemistry, National Defence University) for instructing
us [Ca2+
] assay. We also thank National Science
i
Council for financial support.
1
shown by the H NMR analysis. Compounds 5–22 were
fully characterized by spectroscopic methods (IR, MS,
HRMS, 1H and 13C NMR) and elemental analyses. Some
pertinent data are listed. 5: 1H NMR (300 MHz, CDCl3) d
7.83 (s, 1H), 7.59 (d, 1H, J=8.0 Hz), 7.35 (t, 1H, J=8.0
Hz), 7.24 (d, 1H, J=8.0 Hz), 7.11 (t, 1H, J=8.0 Hz), 6.99
(s, 1H), 6.82 (d, 2H, J=8.5 Hz), 6.59 (d, 2H, J=8.5 Hz),
5.34 (s, 1H), 5.33 (s, 1H), 3.92 (s, 3H), 3.70 (s, 3H), 3.57 (s,
3H). 6: mp 113–114 ꢀC; 1H NMR (400 MHz, CDCl3) d
7.54 (s, 1H), 7.42 (m, 2H), 7.25 (m, 3H), 7.01 (m, 1H),
6.79 (dd, 2H, J=6.8, 2.0 Hz), 4.28 (s, 3H), 3.86 (s, 3H),
3.74 (s, 3H), 2.09 (s, 3H). 7: mp >300 ꢀC. 8: mp 251–
252 ꢀC. 9: mp 211–212 ꢀC. 10: mp 261–262 ꢀC. 11: mp
241–242 ꢀC. 12: mp 259–260 ꢀC. 13: mp 271–272 ꢀC. 14:
mp 263–264 ꢀC. 15: mp 162–163 ꢀC. 16: mp 149–150 ꢀC.
References and notes
1. (a) For biological role of endothelins, see: Yanagisawa,
M.; Kurihara, H.; Kimura, S.; Tomobe, Y.; Kobayashi,
M.; Mitsui, Y.; Yazaki, Y.; Goto, K.; Masaki, T. Nature
1988, 332, 311. (b) Doherty, A. M. J. Med. Chem. 1992,
35, 1493. (c) Schiffrin, E. L.; Touyz, R. M. J. Cardiovasc.
Pharmacol. 1998, 32 (Suppl. 3), 2. (d) For review of
endothelin antagonists, see: Elliott, J. D.; Lago, M. A.;
Peishoff, C. E. Endothelin Receptors: from the Gene to the
Human; Ruffolo, R. R., Ed.; CRC: Boca Raton, Florida,
1995; p 79. (e) Doherty, A. M. Drug Discovery Today
1996, 1, 60. (f) Webb, M. L.; Meek, T. D. Med. Res. Rev.
1997, 17, 17. (g) Liu, G. Annu. Rep. Med. Chem. 2000, 35,
73.
17: mp 286–288 ꢀC. 18: mp 307–308 ꢀC; 1H
N MR
(400 MHz, CD3COCD3) d 7.61 (s, 1H), 7.58 (dd, 1H,
J=8.4, 1.2 Hz), 7.35–7.18 (m, 9H), 7.02 (dt, 1H, J=7.6,

1132
G. Babu et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1129–1132
0.8 Hz), 6.78 (td, 2H, J=8.8, 3.2 Hz), 6.16 (d, 1H, J=16.0 12. Sakurai, T.; Yanagisawa, M.; Masaki, T. Trends Phar-
Hz), 6.11 (d, 1H, J=16.0 Hz), 2.19 (s, 3H). 19: mp 244–
245 ꢀC. 20: mp 217–218 ꢀC. 21: mp 291–292 ꢀC. 22: mp
150–151 ꢀC.
macol. Sci. 1992, 13, 103.
13. (a) Arai, H.; Hori, H.; Aramori, I.; Ohkubo, H.; Naka-
nishi, S. Nature 1990, 348, 730. (b) Sakurai, T.; Yanagi-
sawa, M.; Takuwa, Y.; Miyazaki, H.; Kimura, S.; Goto,
K.; Masaki, T. Nature 1990, 348, 732. (c) Sakamoto, A.;
Yanagisawa, M.; Tsujimoto, G.; Nakao, K.; Toyooka, T.;
Masaki, T. Biochem. Biophys. Res. Commun. 1994, 200,
679.
14. (a) Grynkiewicz, G.; Poenie, M.; Tsien, R. Y. J. Biol.
Chem. 1985, 260, 3440. (b) Lin, W.-W.; Chuang, D. M.
Mol. Pharmacol. 1993, 44, 158.
10. (a) For use of hexamethylphosphoramide (HMPA) as a
general additive to samarium diiodide in reaction accel-
eration and regiochemical control, see: Inanaga, J.; Ishi-
kawa, M.; Yamaguchi, H. Chem. Lett. 1987, 1485. (b)
Shiue, J.-S.; Lin, C.-C.; Fang, J.-M. Tetrahedron Lett.
1993, 34, 335.
11. Nakayama, J.; Sugino, M.; Ishii, A.; Hoshino, M. Chem.
Lett. 1992, 703.