Tryptamine-based human β3-adrenergic receptor agonists. Part 2: SAR of the methylene derivatives

Article abstract of DOI:10.1016/j.bmcl.2004.09.054

In an effort to reduce the cost of production, we conducted further optimization of tryptamine-based β₃-adrenergic receptor (AR) agonists. Optimization of the left-hand side aryl moiety led to the identification of a series of potent β₃

Full text of DOI:10.1016/j.bmcl.2004.09.054

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5963–5966
Tryptamine-based human b3-adrenergic receptor agonists.
Part 2: SAR of the methylene derivatives
Masaaki Sawa,a,* Hirotaka Tateishi,a Kazuhiro Mizuno,a Hiroshi Harada,a Mayumi Oue,b
Hiroshi Tsujiuchi,b Yasuji Furutanib and Shiro Katoa
aChemistry Research Laboratories, Dainippon Pharmaceutical Co., Ltd, 33-94 Enoki-cho, Suita, Osaka 564-0053, Japan
bPharmacology and Microbiology Research Laboratories, Dainippon Pharmaceutical Co., Ltd, 33-94 Enoki-cho,
Suita, Osaka 564-0053, Japan
Received 27 August 2004; revised 14 September 2004; accepted 17 September 2004
Abstract—A series of tryptamine derivatives with modified sulfonamide were designed, synthesized, and evaluated for their ability to
stimulate cAMP accumulation in CHO cells expressing the cloned human b3-adrenergic receptor (AR). For this series of com-
pounds, our objective was to symmetrize the a-position of the tryptamine moiety maintaining its activity and reducing the cost
of production. Compound 11h, having m-aminobenzene, exhibited excellent agonistic activity for b3-AR with excellent subtype
selectivity.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
the cost of production. Therefore, we turned our atten-
tion to compound 2a with just one chiral center as an
analog of compound 2b. Removal of the chiral methyl
group in 2b resulted in remarkable selectivity over the
b1- and b2-ARs, but in a 10-fold decrease of the agonis-
tic activity for b3-AR compared to 2b (Table 1). In this
study, we provide evidence that the replacement of 2-
thiophene moiety in 2a with m-aminobenzene leads to
a potent, highly selective, and cost desirable b3-AR
agonist.
In recent years, life-style related diseases such as nonin-
sulin dependent diabetes mellitus (NIDDM) brought on
by obesity has become a serious problem.1 Elevating the
metabolic rate by activating the human b3-adrenergic
receptor (AR) has attracted much attention as a novel
approach in the treatment of NIDDM and obesity.2
Therefore, the development of potent and selective b3-
AR agonists is an attractive goal for medicinal chem-
ists.3 In particular, subtype selectivity over both b1-
and b2-ARs has been an important issue in developing
b3-AR agonists to avoid the adverse effects observed
with early drug candidates.4
OH
H
Cl
N
N
H
AJ-9677 (1)
O
CO2H
As previously reported, we succeeded in finding some
novel tryptamine-based b3-AR agonists such as com-
pound 1 (AJ-9677)5 and 2b,6 both which exhibited po-
tent agonistic activity with excellent subtype selectivity.
Compound 2b, especially, showed considerable selectiv-
ity over the b1- and b2-ARs (EC50 ratio; 210- and 86-
fold, respectively). However, these compounds have
two chiral centers, which make the synthesis of the
derivatives even more difficult, which would increase
OH
H
H
N
N
S
S
O
O
R
N
H
Me
O
2a: R = H
S
2b: R = Me
O
O
2. Chemistry
Keywords: b3-Adrenergic receptor; Agonist; Tryptamine.
Corresponding author. Tel.: +81 663375904; fax: +81 663387656;
e-mail: masaaki-sawa@dainippon-pharm.co.jp
*
The preparation of a series of tryptamine derivatives
was performed using two methods (methods A or B in
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.09.054

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M. Sawa et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5963–5966
Table 1. Activity of tryptamine-based arylsulfonamides at the cloned
human b-ARs (1)
chloride to afford common intermediate 5. The resulting
bromide 5 was treated with appropriately substituted
tryptamines 7a–d in the presence of diisopropylethylam-
ine and KI, followed by deprotection of TBDMS af-
forded the desired compounds 8a–d.
OH
H
N
H
N
S
S
O
O
R
N
H
R1
The 7-methanesulfonate derivatives 2a,b, and 11a–m
were synthesized in five steps starting from common
intermediate 6, as shown in Scheme 1B. The alcohol 4
was treated with alkaline to provide epoxide 6. The reac-
tion of the resulting epoxide 6 with tryptamine deriva-
tives 9a,b8 produced secondary amines, which were
protected with Boc, and then reduced with Fe. The
resulting anilines 10a,b were treated with an appropriate
sulfonyl chloride to afford sulfonamides. Finally, depro-
tection of the Boc group furnished the desired com-
pounds 2a,b, and 11a–m.9
Compd
R1
R
EC50, nMa (IA, %)b
b3
b1 b2
2a
2b
8a
8b
8c
8d
–OMs
–OMs
–OBn
–OMe
–H
H
1.7 (89)
0.21 (97)
7.6 (81)
13 (87)
21 (73)
25 (77)
(12)c
(23)c
18 (50)
(32)c
Me
H
44 (36)
(10)c
(6)c
H
(16)c
H
(2)c
(3)c
(15)c
–Me
H
(9)c
a Agonistic activity was assessed by measuring cAMP accumulation in
CHO cells expressing human b-ARs.
b Values in parentheses represent the intrinsic activity (IA) given as
percentage of maximal stimulation with isoproterenol.
c EC50 was not determined. Values in parentheses represent % activity
at 1000nM.
3. Biological evaluation
All compounds were tested in vitro for their ability to
stimulate cAMP accumulation in CHO cells expressing
the cloned human b1-, b2-, and b3-ARs,10 and the data
are summarized in Tables 1–3. To identify compounds
with improved agonistic activity for b3-AR, we initially
investigated the effect of the substituents R1 at the 7-
position of the indole ring on the b3-agonistic activity
(Table 1). 7-Benzyloxy derivative 8a and 7-methoxy
derivative 8b showed moderate potency for b3-AR
(EC50 = 7.6 and 13nM, respectively), but were still selec-
tive against b1- and b2-ARs. The fact that replacement
Scheme 1).7 The thiophenesulfonamide analogs 8a–d
were readily available from common intermediate 5
and commercial tryptamine derivatives 7a–d as shown
in Scheme 1A. Commercially available m-nitrophenacyl
bromide 3 was reduced stereoselectively with a borane–
THF complex in the presence of (R)-2-methyl-CBS-
oxazaborolidine to give chiral alcohol 4.6 Protection of
the chiral alcohol 4 with TBDMS, followed by reduction
of the nitro group with Fe provided the corresponding
aniline, which was treated with 2-thiophenesulfonyl
OTBDMS
H
OH
O
N
O
Br
b, c, d
e
a
O2N
Br
O2N
Br
S
S
O
5
3
4
O
O2N
6
(a)
H2N
OH
f, g
H
N
H
N
5
+
S
S
N
H
O
O
R1
N
H
R1
7a-d
8a-d
(b)
OH Boc
N
OH
H2N
H
N
H
N
j, g
h, i, c
R2
O
H2N
6
+
S
R
O
N
H
R
R
N
N
H
OMs
H
OMs
OMs
9a: R = H
10a: R = H
10b: R = Me
2a, b and 11a-m
9b: R = Me
Scheme 1. Reagents: (a) (R)-2-methyl-CBS-oxazaborolidine, BH3, THF; (b) TBDMSCl, iraidazole, DMF; (c) Fe, NH4C1, EtOH, H2O;
(d) 2-thiophenesulfonyl chloride, pyridine, CH2C12; (e) 2N NaOH, THF; (f) i-Pr2NEt, KI, MeCN; (g) 4N HCl–AcOEt; (h) DMF; (i) (Boc)2O,
CHC13; (j) R2–SO2C1, pyridine, CH2Cl2.

M. Sawa et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5963–5966
5965
Table 2. Activity of tryptamine-based arylsulfonamides at the cloned
human b-ARs (2)
Table 4. Binding affinity of tryptamine-based arylsulfonamides at the
cloned human b-ARs
Compd
Binding Ki, nMa
OH
H
N
H
N
R2
b3
b1
b2
S
O
O
2a
2b
33
230
66
223
48
N
H
4.0
OMs
11h
11m
2.6
1.1
89
34
70
20
Compd
R2
EC50, nMa (IA, %)b
b3
b1
b2
a Binding potency is reported as Ki, the binding inhibition constant,
(3)c
(49)c
(24)c
(37)c
(17)c
(18)c
(15)c
determined by inhibition of 125I-iodocyanopindolol binding.
11a
11b
11c
11d
11e
11f
–Et
–CH2CF3
18 (85)
48 (49)
20 (63)
2.8 (82)
6.7 (93)
2.0 (86)
(0)c
(5)c
–CH2CH2CH2Cl
5-Bromothiophene
b3-AR equal to the parent compound 2a
(EC50 = 2.0nM), with high subtype selectivity. These
results suggest that the aromatic function attached to
the sulfonamide group was important for maintaining
b3-agonist activity.
(6)c
4,5-Dichlorothiophene
–Ph
(7)c
(14)c
a See footnote a in Table 1.
b See footnote b in Table 1.
c See footnote c in Table 1.
Next, we examined the effect of substituents at the benz-
ene ring in 11f on the b3-agonistic activity (Table 3). It
was found that the activity of the para-amino analog
11i was weaker by 4-fold (EC50 = 8.4nM) relative to
11f. The ortho-amino analog 11g showed similar potency
at b3-AR (EC50 = 3.0nM) as 11f. However, the shift of
the amino group to the meta-position (11h) dramatically
improved agonistic activity for b3-AR (EC50 = 0.31nM)
with considerable subtype selectivity. Furthermore, it
was noteworthy that the agonistic activity of 11h was al-
most equal to that of compound 11m possessing the chiral
methyl group. Encouraged by these results, we then pre-
pared and tested some meta-substituted benzenesulfona-
mide derivatives with electron-donating or -withdrawing
groups. As can be seen from Table 3, both analogs 11j and
11k containing either a methoxy group (electron-donat-
ing) or a chlorine atom (electron-withdrawing) showed
excellent activity for b3-AR. However, fluorine derivative
11l was somewhat less active for b3-AR than 11f. All of
these results suggest that the enhanced activity with
meta-substitution is not caused by electronic effects, but
rather result from steric effects and the hydrogen-bond
donative property of the NH2 group, which would be
required for activating the receptor.
Table 3. Activity of tryptamine-based arylsulfonamides at the cloned
human b-ARs (3)
OH
H
N
H
N
R3
S
O
O
R
N
H
OMs
Compd
R3
H
R
EC50, nMa (IA, %)b
b3
b1 b2
11f
11g
11h
11i
H
H
H
H
H
H
H
Me
2.0 (91)
3.0 (80)
0.31 (105)
8.4 (85)
0.94 (97)
1.0 (96)
2.4 (97)
0.47 (92)
(13)c
(7)c
(15)c
(9)c
2-NH2
3-NH2
4-NH2
3-OMe
3-Cl
(3)c
(5)c
(6)c
(15)c
(10)c
(13)c
(8)c
11j
(11)c
(11)c
(10)c
11k
11l
3-F
3-NH2
11m
26 (51)
13 (42)
a See footnote a in Table 1.
b See footnote b in Table 1.
c See footnote c in Table 1.
of the methanesulfonate group in 2a with hydrogen
(8c) or a methyl group (8d) resulted in a considerable
decrease in the b3-agonistic activity (EC50 = 21 and
25nM, respectively) suggests the oxygen at the 7-posi-
tion of the indole ring was important for b3-agonistic
activity. These compounds also showed moderate intrin-
sic activity for b3-AR (IA = 73% and 77%, respectively).
Compounds with low cAMP accumulation in b1- and
b2-ARs may have antagonistic profiles, which may cause
unwanted side effects. Thus, compounds 11h and 11m
were subjected to b-ARs binding assays, and the results
are shown in Table 4. In contrast to the parent com-
pound 2a, which showed weak binding affinity to the
Effects of the substituents R2 attached to the sulfon-
amide moiety on the left-hand side phenyl ring were
then examined (Table 2). Replacement of the thiophene
ring with an alkyl chain (compounds 11a–c) resulted in a
major decrease in the potency for b3-AR, while main-
taining the subtype selectivity. Trifluoroethyl derivative
11b showed a substantial loss of intrinsic activity for
b3-AR (IA = 49%). However, extension of the alkyl
chain resulted in a moderate increase in both EC50
and IA (11b vs 11c). Halogen substituents at the thio-
phene ring in 2a (compounds 11d,e) slightly decreased
the agonistic activity for b3-AR (EC50 = 2.8 and
6.7nM, respectively). It is interesting to note that benz-
enesulfonamide 11f showed potent agonistic activity for
b3-AR
(Ki = 33nM),
m-aminobenzenesulfonamide
derivative 11h had a very strong binding constant
(Ki = 2.6nM). Furthermore, 11h shows excellent selec-
tivity over binding to the b1- and b2-ARs (34- and 27-
fold, respectively). These results confirmed that 11h
had high selectivity for b3-AR over b1- and b2-ARs. In
vivo evaluation of these compounds for their ability
to increase metabolic rate is in progress and will be
reported in due course.
Acknowledgements
We would like to thank San Francisco Edit (www.sfedit.
net) for their assistance in editing this manuscript.

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M. Sawa et al. / Bioorg. Med. Chem. Lett. 14 (2004) 5963–5966
procedure similar to the procedure for the preparation
of 9b.
References and notes
9. Analogs 11g–i and 11m were prepared by treatment of the
anilines 10a,b with the corresponding nitrobenzenesulfon-
amide, followed by hydrogenation of the nitro group with
Pd–C. Deprotection of the Boc group afforded the desired
compounds.
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7. All final compounds were characterized using NMR, mass
spectrometry, and HPLC.
10. In a previous report,5 we used CHO cells expressing a high
level of b3-AR as a measurement of compound activity,
that is, the receptor densities were 150,000 receptors/cell
(b3-AR), 12,000 receptors/cell (b1-AR), and 30,000 recep-
tors/cell (b2-AR). To better evaluate the subtype selectiv-
ity, in this study, we used CHO cells expressing a low
density of b3-AR (13,000 receptors/cell), and high densities
of b1- and b2-ARs (320,000 and 600,000 receptors/cell,
respectively). The CHO cells expressing either human b1-,
b2-, or b3-AR were prepared as described by Kato, S.;
Harada, H.; Taoka, I.; Kawashima, H. PCT Patent
Application, WO 2000044721, 2000 and Kato, S.; Harada,
H.; Hirokawa, Y.; Yoshida, N.; Kawashima, H. PCT
Patent Application, WO 9616938, 1996.
8. (R)-a-Methyl-7-methanesulfonyloxytryptamine (R = Me,
9b) was prepared as reported previously.6 7-Methane-
sulfonyloxytryptamine (R = H, 9a) was synthesized from
commercially available 7-benzyloxytryptamine using a