Synthesis of bivalent ligands of β-carboline-3-carboxylates via a palladium-catalyzed homocoupling process

Article abstract of DOI:10.1016/j.tetlet.2005.07.022

The first synthesis of bivalent ligands of β-carboline-3-carboxylates has been achieved from β-carboline-3-carboxylate tert-butyl ester (βCCt) via Sonogashira and palladium-catalyzed homocoupling processes. The Boc protected intermediate, an iodo-β-carboline-3-carboxylate, was employed to provide a general entry into a series of bivalent ligands structurally similar to βCCt.

Full text of DOI:10.1016/j.tetlet.2005.07.022

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6363–6368
Synthesis of bivalent ligands of b-carboline-3-carboxylates via
a palladium-catalyzed homocoupling process
Wenyuan Yin, P. V. V. Srirama Sarma, Jun Ma, Dongmei Han, Jessica L. Chen and
James M. Cook*
Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA
Received 9 June 2005; revised 6 July 2005; accepted 6 July 2005
Abstract—The first synthesis of bivalent ligands of b-carboline-3-carboxylates has been achieved from b-carboline-3-carboxylate
tert-butyl ester (bCCt) via Sonogashira and palladium-catalyzed homocoupling processes. The Boc protected intermediate, an
iodo-b-carboline-3-carboxylate, was employed to provide a general entry into a series of bivalent ligands structurally similar to
bCCt.
Ó 2005 Elsevier Ltd. All rights reserved.
‘‘The neurotransmitter known as GABA (c-amino buty-
ric acid) is the brainÕs principal calming influence’’, as
indicated by Wickelgren in 1982.1 Conventional benzo-
diazepines (BZs), clinically used for the treatment of
anxiety and insomnia, bind to GABAA (type A) recep-
tors, through which they exert their effects by allosteri-
cally modulating GABA receptors (GABAA/BzR).2–4
A number of compounds chemically different from the
benzodiazepines bind to this receptor with high affinity
comparable to that of the pharmacologically and clini-
cally active benzodiazepines;5 such is the case with
b-carboline-3-carboxylates.6–9 The most a1 GABAA/BzR
subtype selective ligand reported to date,10 b-carbo-
line-3-carboxylate-tert-butyl ester (bCCt 1), has been
employed by several research groups to investigate the
role of different GABAA receptor subtypes in mediating
the subjective effects of BZs,11–20 as well as an orally
active agent, effective in the reduction of ethanol self-
administration in alcohol preferring rats and high alco-
hol drinking rats.11,12,18 The a1 preferring antagonist
bCCt 1 is effective in these studies and may provide an
alternative mechanism for the treatment of human
alcoholics.11,18
provide a bivalent analogue (P–X–P), that may exhibit
enhanced selectivity and potency relative to its monova-
lent ligand (P–X) when a suitable ÔXÕ is employed.21–23
Such was the case with the design and synthesis of the
first bivalent a5 subtype selective GABAA/BzR antago-
nist.24 In addition, a series of ÔP–XÕ analogues, 6-substi-
tuted-b-carboline-3-carboxylates have been synthesized
and bind more potently in vitro to the a1b3c2 BzR
subtype.25,26 These ligands have been modeled in the
GABAA/BzR pharmacophore model,27 and the 6-
substitutents ÔXÕ align well in the LDi region (a region
in the pharmacophore adjacent to the extracelluar
domain of the receptor, important to a1 selectivity).27
A ÔP–XÕ analogue, 6-trimethylsilanylethynyl-bCCt 3
has been synthesized and found in vitro to prefer the
a1 subtype (Table 1). Consequently, the synthesis of
bivalent analogues ÔP–X–PÕ 4 and 5 were designed,
based on the affinity of 6-trimethylsilanylethynyl-bCCt
3 (Fig. 1). Outlined below is the synthesis of a series of
bivalent analogues of b-carboline-3-carboxylates via a
palladium-catalyzed homocoupling process through
the common intermediate, iodo-bCCt 2. These agents
will provide useful tools with which to study alcohol
self-administration as well as provide potential clinical
agents to treat human alcoholics.11,18
Several reports have appeared wherein the active phar-
macophore of an analogue (P) has been linked to a sec-
ond molecule of itself through a connecting unit (X) to
The overall synthetic strategy is shown in Schemes 1 and
2. The efficient synthesis of the key intermediate, 6-iodo-
bCCt 2, was carried out in a modified route from
commercially available D,L-tryptophan 6: the tetra-
hydro-b-carboline 7 was prepared by condensation of
Keywords: b-Carbolines; bCCt; Bivalent ligands; Homocoupling.
*
Corresponding author. Tel.: +1 414 229 5856; fax: +1 414 229
5530; e-mail: capncook@uwm.edu
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.022

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W. Yin et al. / Tetrahedron Letters 46 (2005) 6363–6368
Table 1. In vitro binding affinity of beta-carbolines and bivalent ligand
4 at GABAA/BzR sub-types (values reported are in nM)
verted into the ethyl ester 8 on stirring with 2 equiv of
concentrated sulfuric acid in ethanol in 85% yield. This
tetrahydro ester was then subjected to oxidation with
active MnO2 to provide bCCE 9, according to the proce-
dure of Agarwal, as modified by White.31 The bCCE
obtained in this fashion was transformed into the acid
10 by hydrolysis and converted into bCCt 1 by the
CDI method, as modified by Ma.32,33 The b-carboline-
3-carboxylic acid 10 was converted into the imidazole
derivative on stirring with 1,1-carbonyl-dimidazole in
DMF.34 This intermediate 11 was then treated with dry
tert-butanol in the presence of DBU at 80 °C to provide
bCCt 1 in 70% yield. The bCCt 1 was then treated with
I2/CF3COOAg35 in chloroform to provide 6-iodo-bCCt
2 in 85% yield. The synthesis of bCCt 1 was accom-
plished (from bCCE 9) in two steps in 63% overall yield.
axb3c2
x
a1
0.72
14.4
6.8
30
a2
15
44.9
30
a3
18.9
123
36
a4
a5
110.8
65.3
108
>300
a6
1
2
3
4
>3000
>4000
2000
>4000
>4000
1000
124
100
>300
>4000
Ki values represent the mean of two determinations which differed by
less than 10%. Data were generated using LtkÀ cell membranes
expressing human axb3c2 receptors. 1.8 nM[3H]Ro15-1788 and 8 nM
[3H]Ro 15-4513 (for cells expressing a4b3c2 and a6b3c2) were used as
radioligands.
tryptophan 6 with formaldehyde via a Pictet–Spengler
reaction,8,28,29 according to the method of Snyder.30
The addition of ethanolic hydrogen chloride solution
to provide the ethyl ester was modified for large-scale
preparation. The tetrahydro acid 7 (in hand) was con-
This is a much-improved route over previous meth-
ods.10,36 The successful improvement centered on the
activation of the carboxylic acid group with CDI and
O
O
O
TMS
I
O
O
O
N
N
N
N
H
N
H
N
H
1
2
3
iodo-βcct
βcct
6-trimethylsilanylethynyl-βcct
O
O
O
N
O
O
O
O
O
N
N
N
N
N
N
N
H
H
H
H
5
4
Figure 1.
O
CO2
CO2H
C2H5OH/2 eq H2SO4
37 % aq CH2O
O
NH3
NH
NH
37 o C
95 %
reflux
85 %
N
H
N
N
H
6
H
7
8
500 g/12 L
1 kg/22 L
O
CO2H
NaOH/H2O
reflux, 90 %
CDI/DMF
rt, 2h
MnO2/benzene
O
N
N
N
H
10
reflux
80 %
N
H
9
O
O
O
N
I
CF3COOAg/I2
O
t-BuOH, DBU
O
N
N
80 o- 85 o C,
18h, 70 %
CHCl3, reflux,
85 %
N
H
N
N
N
N
H
H
2
1
11
Scheme 1.

W. Yin et al. / Tetrahedron Letters 46 (2005) 6363–6368
6365
Si
O
O
O
I
TMS
H
1.2 eq (Boc)2O
CH2Cl2, DMAP
O
I
O
O
N
N
N
Boc
N
N
Boc
13
PdCl2(PPh3)2/CuI
THF/Et3N
N
H
2
12
TBAF/H2O
O
O
O
H
PdCl2(PPh3)2, CuI
THF/Et3N
O
O
O
N
N
N
N
Boc
N
Boc
N
Boc
14
15
cumene,
1) PdCl2(PPh3)2, CuI
BrCH2CO2Et/THF,(iPr)2NEt
2) cumene,
O
O
O
O
O
O
O
O
N
N
N
N
N
N
N
N
H
H
H
H
4
5
Scheme 2.
the addition of a large excess of dry tert-butyl alcohol.
The steps for the synthesis of bivalent bCCt ligands 4
and 5 are illustrated in Scheme 2.
thermal conditions at high dilution. This constituted a
very efficient synthesis of bivalent ligand 5; the bisacetyl-
ene 16 had been obtained in 60% yield. This was supe-
rior to the oxidative coupling and homocoupling
reactions attempted, to date, on this system.38,45–47
In order to efficiently effect a palladium mediated Sono-
gashira process at position-6 of b-carbolines (a reactive
electron-rich indole heterocycle), protection/deactiva-
tion of the indole Na–H group was necessary. The
iodo-bCCt 2 was protected with the Boc group at the
N (1) position to afford 12 under standard conditions.
A Sonogashira coupling process37 was then employed
to couple the Boc protected iodo-bCCt 12 with trimeth-
ylsilylacetylene in the presence of bis(triphenylphos-
phine) palladium(II) chloride to provide the
trimethylsilylacetylenyl-b-carboline 13 in 93% yield. In
the presence of tetrabutylammonium fluoride, the silyl
function was removed to provide the 6-ethynyl-b-carbo-
line-3-carboxylic acid tert-butyl ester 14 in 95% yield. A
Sonogashira process was then employed to couple 14
with iodo-bCCt 2 to afford the rigid two carbon linked
bivalent ligand 15 of bCCt. The Boc protecting group
was then removed thermally by heating in cumene at
high dilution and 4 was formed in 95% yield. Attempts
to form bivalent ligand 4 by the reaction of iodo-bCCt
2 and trimethyl-silyl-acetylenyl b-carboline 1338–40 or
by reaction of iodo-bCCt 2 and bistributylstannyl-acet-
ylene41 were not successful. The newly modified methods
for the Sonogashira coupling process38,42 on Boc pro-
tected iodo-bCCt 12 were also attempted, but the yields
were very low. Therefore, the three step (coupling–desi-
lylation–coupling) as the source of the acetylenic bridge
was the most successful, to date.43 The potential a1b3c2
selective ligand 5 can be synthesized from the Boc pro-
tected 6-ethynyl-bCCt 14 by following the homocou-
pling method reported by Zhang and co-workers,44
which is the first example applied to b-carbolines. The
removal of the Boc group was executed again under
Simple catalytic hydrogenation (Scheme 3) of bivalent
ligands 15 and 16, followed by removal of the Boc group
under thermal conditions provided the flexible alkyl
linked bivalent ligands 17 and 18, respectively.
Bivalent ligands similar to the b-carboline-3-carboxylate
systems described above are shown in Figure 2.
The use of the Boc protecting group in Schemes 2 and 3
increased the solubility of the starting coupling reactants
(Boc protected 6-iodo-bCCt 12 and Boc protected 6-
ethynyl-bCCt 14) to facilitate the above processes. Con-
sequently, palladium-catalyzed coupling processes were
successfully employed on these indoles in good yields
via different iodo-b-carboline-3-carboxylates and the
reactions took place at room temperature, as well.
Encouraged by the success of the CDI method in
Scheme 1, the bivalent ligands 4 and 5 were transformed
into the key intermediates 23 and 24 employed to exe-
cute a general synthesis of bivalent ligands of different
b-carboline-3-carboxylates (see 19–22). As shown in
Scheme 4, bivalent ligands 4 and 5 were treated with
TFA. The acids 23 and 24 have been converted into var-
ious bivalent ligands (see 19–22) following the above
CDI-mediated method. Further work is underway to
extend this approach to other bivalent ligands, which
have chemical structures similar to that of bCCt 1. This
would also provide a route for the preparation of flexi-
ble bivalent ligands with alkyl linkers through catalytic
hydrogenation.

6366
W. Yin et al. / Tetrahedron Letters 46 (2005) 6363–6368
O
O
O
O
O
O
O
O
N
N
N
N
N
N
N
N
Boc
Boc
Boc
Boc
15
16
1) 40psi H2, CH2Cl2/EtOH(Pd/C)
2) xylene/reflux
1) 40psi H2, CH2Cl2/EtOH(Pd/C)
2) xylene/reflux
O
O
O
O
O
O
O
O
N
N
N
N
N
N
N
H
H
H
N
H
18
17
Scheme 3.
R1O
N
OR1
N
R2O
OR2
N
O
O
O
O
N
N
N
N
N
H
H
H
H
4
R1=C(CH3)3
5
R2=C(CH3)3
19 R1=CH2CH3
21 R1=CH2CF3
20 R2=CH2CH3
22 R2=CH2CF3
Figure 2.
O
O
HO
N
OH
O
O
O
O
N
N
N
TFA
N
N
N
N
H
H
H
H
n
n
4 & 5 (n=1,2)
23 & 24
19-22
Scheme 4.
In addition, these rigidly linked linear bivalent ligands 4
and 5 fit the BzR/GABA(A) pharmacophore/receptor
model (Fig. 3) as indicated in a previous study,25,48
and may provide the desired a1 selectivity through spe-
cific occupation of the LDi region of the pharmaco-
phore/receptor model.26,48
designed to bind to a1 BzR. This work will provide en-
try into many bivalent ligands to complete an SAR in
search of a1 subtype selective antagonists. Conse-
quently, bivalent ligands 5, 17, 18, 19–22 have been sent
out for biological screening, and further investigation of
the binding affinity of this series of bivalent analogues is
underway.
The in vitro binding data on bivalent ligand 449 (see
Table 1) indicated it did bind to BzR receptors with
some a1 subtype selectivity. This is the first example of
a bivalent ligand, which contains a b-carboline sub-
structure, which has been demonstrated to potently bind
to BzR. It clearly illustrates that bivalent ligands can be
In summary, a successful route to iodo-bCCt 2 has been
developed. With this improved process in hand, build-
ing blocks for the synthesis of bivalent ligands from b-
carboline-3-carboxylates linked at position-6 by an
acetylenic or butadinyl group are now available by

W. Yin et al. / Tetrahedron Letters 46 (2005) 6363–6368
6367
Figure 3. Bivalent bCCt ligands with acetylenic linker (bivalent ligand 4, left) and butyldinyl linker (bivalent ligand 5, right) in the BzR
pharmacophore/receptor model (included volumes).26,48
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Acknowledgements
This work was supported by grants from NIMH &
NIAAA. We thank Dr. John Atack and Dr. Ruth
McKernan (MSD) for the data in Table 1.
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49. (4): 1,2-Bis(9H-b-carboline-3-carboxylic acid tert-butyl
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1327, 1162, 738, 450 cmÀ1; 1H NMR (300 MHz, DMSO) d
1.62 (s, 9H), 7.70–7.80 (m, 2H), 8.7 (s, 1H), 8.94 (s, 1H),
8.99 (s, 1H), 12.25 (s, 1H); 13C NMR (75 MHz, DMSO) d
28.3, 80.8, 89.2, 113.3, 114.6, 117.9, 121.5, 125.8, 127.5,
131.9, 134.2 138.0, 138.6, 140.8, 164.9; MS(FAB)
559(M++1 41). (5): 1,4-Bis(9H-b-carboline-3-carboxylic
acid tert-butyl ester) buta-1,3-diyne: mp >350 °C (dec.) IR
(KBr) 3424, 1708, 1627, 1466, 1369, 1302, 1251, 1154,
1
1107, 1025, 846, 645 cmÀ1; H NMR (300 MHz, DMSO)
d 1.63 (s, 9H), 7.60–7.69 (m, 2H), 8.40 (s, 1H), 8.96 (s, 1H),
9.04 (s, 1H), 12.5 (s, 1H); 13C NMR (75 MHz, DMSO) d
28.3, 78.8, 82.5, 112.9, 118.1, 121.0, 122.1, 127.3, 128.1,
129.5, 131.5, 134.8, 137.1, 141.1, 163, 131.9, 134.2, 138.0,
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