Design, synthesis, and evaluation of a novel class of 2,3-disubstituted- tetrahydro-β-carboline derivatives

Article abstract of DOI:10.1016/j.bmcl.2012.04.020

Several novel tetrahydro-β-carboline derivatives with amino acid residues at the 2-position and a glucosamine group at the 3-position of the tetrahydro-β-carboline nucleus were synthesized from a readily available starting material, tryptophane, and were evaluated for their anti-inflammatory activity in the present study. Our results showed that all of the derivatives tested exhibited a significant inhibition of xylene-induced inflammation in mice.

Full text of DOI:10.1016/j.bmcl.2012.04.020

Bioorganic & Medicinal Chemistry Letters 22 (2012) 3718–3722
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis, and evaluation of a novel class of 2,3-disubstituted-
tetrahydro-b-carboline derivatives
⇑
Li Zeng, Jianwei Zhang
College of Pharmaceutical Sciences, Capital Medical University, Beijing 100069, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several novel tetrahydro-b-carboline derivatives with amino acid residues at the 2-position and a gluco-
samine group at the 3-position of the tetrahydro-b-carboline nucleus were synthesized from a readily
available starting material, tryptophane, and were evaluated for their anti-inflammatory activity in the
present study. Our results showed that all of the derivatives tested exhibited a significant inhibition of
xylene-induced inflammation in mice.
Received 21 February 2012
Revised 20 March 2012
Accepted 4 April 2012
Available online 11 April 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Amino sugar
Tetrahydro-b-carboline
Glucosamine
Inflammation
Polycyclic indolic compounds have attracted a great deal of
interest amongst synthetic chemists over the years, partially be-
cause of their diverse biological activities demonstrated.1 In addi-
tion, the relatively rigid structures associated with polycyclic
indolic compounds are expected to show a substantial selectivity
in their interactions with enzymes or receptors. Compounds bearing
this ring system have displayed a range of biological properties
including antimalarial, antitumor, anti-HIV, anticonvulsive, hyp-
notic and anxiolytic, antiviral, and antimicrobial activities2–8 as well
as inhibition of topoisomerase-II9 and cGMP-dependent pro-
cesses.9,10 Antiplasmodial activity has also been reported.11 The
b-carboline alkaloids are a large group of synthetic and naturally
occurring compounds.12–18 Recently these compounds attracted a
considerable attention due to their widespread potent biological
and pharmaceutical properties.19,9,20–22 The structure of tetrahy-
dro-b-carboline seems essential for many biologically important in-
dole alkaloids23 and represents an important lead structure for the
development of novel pharmacological agents. Previous studies re-
vealed that the oral bioavailability of tetrahydro-b-carbolines was
relatively low due to their poor water solubility and high lipophilic-
ity. Their poor bioavailability results in a decrease in effectiveness. It
is known that many drug candidates fail to exert therapeutic poten-
tial due to their bioavailability. In order to overcome this problem, a
series of novel tetrahydro-b-carboline derivatives with increased
hydrophilicity were designed and synthesized.
compounds by simple chemical modifications of naturally occur-
ring lead compounds. In order to search for better anti-inflamma-
tion agents, tetrahydro-b-carboline was used as the basic system
and a series of 2,3-disubstituted-tetrahydro-b-carboline deriva-
tives were designed and synthesized.
It has been reported that some orally administered peptide ana-
logues could be effectively absorbed through the intestinal peptide
transport system. The various side chains of different amino acids
allow the addition of amino acids to carboline to manipulate the
pharmacokinetics profiles of the compounds. In addition, various
amino acids can be introduced to enhance solubility. Therefore,
tetrahydro-b-carboline-amino acid derivatives were designed and
synthesized. The amino acid residues were introduced to the 2-po-
sition of the tetrahydro-b-carboline nucleus.
Glucosamine (GlcN), an amino monosaccharide, is the most
common amino sugar and is generally found as an N-acetylated
and b-linked glycoside. It is found in connective tissues and gastro-
intestinal mucosal membranes. However, the ability to synthesize
GlcN in the body declines with age. This, in turn, incapacitates the
generation of proteoglycan, which is known to result in senile
osteoarthritis (OA).24 Therefore, GlcN salts (sulfate and chloride)
are thought to be beneficial in promoting the repair of damaged
cartilage. Since the first publication of W. Bohne in 1969 showing
that GlcN can be used as a single pharmacological agent to relieve
the symptoms of OA, GlcN has received a great deal of attention
from the public as a potential treatment for OA.25 Furthermore,
GlcN possesses antioxidant activities due to its pronounced reduc-
ing power, superoxide/hydroxyl-radical scavenging ability. It has
been reported that GlcN possesses a unique anti-inflammatory
One of the successful and effective approaches in the search for
new bioactive agents from natural products is to synthesize novel
⇑
Corresponding author. Tel.: +86 10 8391 1522; fax: +86 10 8391 1533.
E-mail address: jwzhang2006@163.com (J. Zhang).
activities and inhibits IL-1b- and TNF-a-induced NO production
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.04.020

L. Zeng, J. Zhang / Bioorg. Med. Chem. Lett. 22 (2012) 3718–3722
3719
O
O
O
I
OH
II
OH
OH
Boc
NH2
NH
N
N
H
N
H
N
H
3
2
1
OAc
O
III
AcO
AcO
OAc
NH2
4
AcO
AcO
O
AcO
AcO
O
AcO
AcO
O
OAc
OAc
OAc
O
O
O
IV
V
N
N
N
H
H
H
N
N
OAc
NH
OAc
OAc
N
H
N
H
N
H
AA Boc
Boc
7a-b
5
6
Scheme 1. Synthesis of tetrahydro-b-carboline derivatives, Compounds 7a and 7b. Reagents and conditions: (I) formaldehyde and sulfuric acid; (II) Boc2O and triethylamine;
(III) 2-amino-1,3,4,6-tetra-O-acetyl-2-deoxy-b-D-glucosamine, DCC, HOBt, and NMM; (IV) hydrogen chloride in ethyl acetate (4 mol/L); (V) Boc-L-Ala-OH or Boc-L-Gly-OH,
NMM, DCC, HOBt.
HO
HO
O
HO
HO
O
OH
OH
OH
O
O
O
HO
HO
OH
OH
O
NH2
N
N
H
H
N
HO
I
II
HO
NH
N
N
H
N
H
N
H
Boc
Boc
3
8
9
Scheme 2. Synthesis of tetrahydro-b-carboline derivative, Compound 9. Reagents and conditions: (I) glucosamine, DCC and NMM; (II) CF3COOH.
O
O
O
OH
II
I
OBzl
OBzl
Ala Boc
N
NH
NH
N
H
N
H
N
H
11
III
10
2
HO
HO
O
OH
O
O
IV
OH
OH
Ala Boc
N
H
N
N
OH
O
N
H
N
Ala Boc
HO
HO
OH
H
13
12
NH
2
V
HO
HO
OH
O
O
N
H
N
OH
N
H
Ala
14
Scheme 3. Synthesis of tetrahydro-b carboline derivative, Compound 14. Reagents and conditions: (I) Polyphosphoric acid, benzyl alcohol/80 °C; (II) Boc-L-Ala-OH, NMM,
DCC and HOBt; (III) Pd/C, H2; (IV) glucosamine, DCC and NMM; (V) hydrogen chloride in ethyl acetate (4 mol/L).
in normal human articular chondrocytes. It is also beneficial for
inflammatory bowel diseases.26
There is an increased interest in the use of hybrid molecules for
drug discovery against a multitude of disease indications.27–31
Amide bond is very stable at pH 7.4 and in culture medium. Amides
are widely found in natural products, pharmaceuticals, and poly-
mers. They can be easily prepared by coupling reactions between
carboxylic acids and amines.32–36 The use of glycosyl amides in
the tetrahydro-b-carboline derivatives designed was based upon
a number of considerations: (1) GlcN possesses certain biological

3720
L. Zeng, J. Zhang / Bioorg. Med. Chem. Lett. 22 (2012) 3718–3722
Table 1
Structures of tetrahydro-b-carboline derivatives
No
3
Chemical structure
No
Chemical structure
O
AcO
AcO
O
OAc
OH
N
O
5
N
H
Boc
N
H
N
OAc
N
H
Boc
AcO
AcO
AcO
O
AcO
O
OAc
OAc
6
O
7a
O
N
H
N
H
NH
N
N
OAc
OAc
N
H
N
H
Ala Boc
AcO
HO
HO
AcO
O
OAc
OH
O
O
7b
O
8
N
H
N
H
N
HO
OAc
N
H
N
H
Gly Boc
Boc
HO
HO
O
O
OH
O
OBzl
NH
9
10
12
14
N
H
N
H
NH HO
N
H
O
O
OBzl
OH
11
13
N
N
N
N
N
H
N
H
Ala Boc
Ala Boc
HO
HO
O
HO
HO
O
OH
OH
O
O
N
H
N
H
OH
OH
N
H
N
H
Ala Boc
Ala
activities; (2) N-acetyl GlcN has several potential advantages over
GlcN as a potential anti-inflammatory agent. In addition, it is ex-
pected to have an impact on the solubility of tetrahydro-b-carbo-
line derivatives. Therefore, tetrahydro-b-carboline derivatives
bearing various amino acid residues at the 2-position and GlcN
group at the 3-position were designed, synthesized and evaluated.
The synthetic routes for the preparation of tetrahydro-b-carbo-
line derivatives from tryptophane (Compound 1) as the starting
material are presented in Schemes 1–3. As shown in Scheme 1, tet-
rahydro-b-carboline-3-carboxylic acid (Compound 2) was obtained
via the Pictet-Spengler cyclization37 of tryptophan with formalde-
hyde under acidic conditions in almost quantitative yield.
Subsequent acylation of Compound 2 with (Boc)2O resulted in
N-Boc-tetrahydro-b-carboline-3-carboxylic acid (Compound 3)37
in a 76% yield. Compound 3 and 2-amino-1,3,4,6-tetra-O-acetyl-
52% yield. Subsequently, removal of the Boc group with hydrogen
chloride in ethyl acetate led to N-terminal free intermediates
(Compound 6) which was reacted with either Boc protected gly-
cine or alanine using the standard peptide coupling method
(DDC, HOBt, and N-methylmolpholine) to give mimeticpeptide
(Comopunds 7a and 7b) as single b-anomers in 36% and 58% yield,
respectively. Removal of acetyl groups from 7a or 7b using K2CO3
and CH3ONa in methanol, respectively, were failed. The attempt
to remove acetyl groups resulted in undesired Boc-Ala-carboline-
3-carboxylic acid (Compound 12) (Scheme 3).
As shown in Scheme 2, an attempt was made to synthesize tet-
rahydro-b-carboline derivatives using unprotected GlcN. GlcN salt
(chloride) was coupled with Compound 3 to give Compound 8 as a
single a-anomer in a 30% yield, which can be attributed to the ano-
meric effect. Subsequently, removal of the Boc group with trifluo-
roacetic acid in dichloromethane resulted in Compound 9.
Unfortunately, the coupling reaction of Compound 9 with Boc-pro-
tected amino acids, Ala and Gly, yielded only an intractable
mixture of several products. Based on the results from Schemes 1
and 2, Scheme 3 was designed. In the presence of polyphosphoric
acid and benzyl alcohol, Compound 2 was converted to carbo-
line-3-carboxylic acid benzyl ester (Compound 10).38 Boc-pro-
2-deoxy-b-D-glucosamine were reacted using the standard peptide
coupling method (dicyclohexylcarbodiimide (DDC), 1-hydroxyben-
zotriazole (HOBt), and N-methylmolpholine) in solution. HOBt was
used to activate Boc-protected tetrahydro-b-carboline-3-carbox-
ylic acid and DCC was used as coupling reagent in tetrahydrofuran
(THF). The products obtained were purified by silica gel column
chromatography to afford only one b-anomer (Compound 5) in a

L. Zeng, J. Zhang / Bioorg. Med. Chem. Lett. 22 (2012) 3718–3722
3721
Table 2
tected Ala was coupled with Compound 10 to afford protected
mimeticpeptide Compound 1138 in a 77% yield. Removal of the
benzyl group from Compound 11 was carried out by palladium cat-
alyzed hydrogenolysis to give the corresponding C-terminal free
compound, Boc-Ala-carboline-3-carboxylic acid (Compound
1238), in almost quantitative yield. Subsequent coupling of GlcN
salt (chloride) and Compound 12 provided Compound 13 as a mix-
Anti-inflammatory activities of tetrahydro-b-carboline derivatives determined using
an xylene-induced ear edema model
Compound tested
Edema weight (X SD mg)
Inhibition ratio (%)
CMC
4
5
7a
7b
13
14
Aspirin
6.5 0.27a
3.3 0.28a
2.7 0.27a
1.7 0.17a
4.1 0.31a
4.3 0.35b
3.4 0.21b
0.91 0.071a
68.3
71.2
82.0
58.6
51.9
66.0
89.5
ture of anomers (61% yield,
HPLC on a Waters 2695 equipped with an XTerra RP C18 column
m particle size; Ireland). The mobile phase
a:b 1.2:1) which were separated by
(4.6 ꢀ 250 mm ID, 5
l
was a gradient of methanol/water (40–50%) over 20 min. Elution
of two anomers was monitored by a UV spectrometer at 210 nm.
HPLC-MS was used to identify the anomers. The respective reten-
tion times were 8.7 and 12.2 min, respectively. A mixture of ano-
mers (Compound 13) were obtained because the hydroxy group
Dose of tetrahydro-b-carboline derivatives = 10
lmol/kg, dose of aspirin = 100 mg/
kg; n = 12.
a
Compared to control, P <0.01.
Compared to control, P <0.05.
b
at the anomeric carbon as
a-anomer in the molecule is closer to
the Boc-protected alanine residue at the 2-position of the tetrahy-
dro-b-carboline nucleus leading to steric interaction. Finally, Com-
suggesting that the tetrahydro-b-carboline structure had a some
contribution to the activity. This research provided useful informa-
tion for the further design of novel potent anti-inflammatory
agents.
In summary, tetrahydro-b-carboline derivatives containing
amino acid residues at the position-2 and glucosamine group at
the position-3 were prepared and evaluated for their anti-inflam-
matory effect. The tetrahydro-b-carboline nucleus had a definite
contribution to the activity. Among the derivatives tested, Com-
pound 7a exhibited the most potent anti-inflammatory activity.
pound 14 was obtained as a mixture of anomers (90% yield,
a:b
3.3:1) by removal of the Boc group with hydrogen chloride in ethyl
acetate. The structures of tetrahydro-b-carboline derivatives are
shown in Table 1.
Compounds 4, 5, 7a, 7b, 13, 14 were screened for their anti-
inflammatory activity by using a xylene-induced ear edema model
assay.39 The compounds 13 and 14 used in the anti-inflammatory
activity are a mixture of anomers. The animal protocol was ap-
proved by the Committee on Animal Care and Usage (Capital Med-
ical University). Male ICR mice (body weight, 18–20 g) were used.
The animals were maintained on a 12/12 h light/dark cycle at con-
stant temperature and humidity, and provided with free access to
food and water in the home cage. They were allowed to acclimate
to their new surroundings for 1 day before experiment. Mice were
divided into eight groups of twelve. Mice administered orally with
0.5% CMC were used as the negative control group, and mice
administered orally with Aspirin (at a dosage of 100 mg/kg) in
CMC were used as positive control. The derivatives tested were
prepared as fine homogenized suspensions in 0.5% CMC at the con-
centration of 1 mM and were administered orally to the animals at
Acknowledgements
This work was supported by National Natural Scientific Founda-
tion of China (20642004) and Educational Council Foundation of
Beijing (KM200710025012).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.04.020.
a dosage of 10 lmol/kg body weight 30 min before xylene was ap-
plied to both the anterior and posterior surfaces of the right ear.
The left ear was considered as control. Two hours after xylene
treatment, all mice were sacrificed by diethyl ether anesthesia
and both ears were removed and weighed. The increase in weight
caused by xylene was measured through subtracting the weight of
the untreated left ear section from that of the treated right ear sec-
tion. The inhibition ratio (%) in describing the anti-inflammatory
effects was determined:
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