Synthesis and cytotoxicity of novel 2,2'-Bis- and 2,2',2''-Tris-indolyl- methanes-based bengacarboline analogs

Article abstract of DOI:10.1007/s12272-012-0601-1

Tungstosilicic acid hydrate was employed as an efficient catalyst for the synthesis of bisindolylmethanes 4 using the Friedel-Crafts reaction of N-sulfonyl tryptamine 5 with various aromatic aldehydes, except 3-formylindole. In the excluding case, tris-indolylmethane 7 was formed via a sequential addition-elimination-addition process. The bioactivity test revealed that the phenolic hydroxyl group plays an important role in cytotoxicity; it demonstrated that ortho- and para-hydroxy bis-indolylmethane (BIM) analogs (4b and 4d) displayed cytotoxic potency toward HepG2 (human hepatocellular liver carcinoma cell line) and MOLT-3 (human lymphoblastic leukemia cell line) cancer cell lines. Significantly, both analogs showed slightly higher inhibitory efficacy than the control drug, etoposide, in HepG2 cells, and the analog 4d exhibited the most potent activity against MOLT-3 cell lines, with an IC₅₀ value of 1.62 μg/mL.

Full text of DOI:10.1007/s12272-012-0601-1

Arch Pharm Res Vol 35, No 6, 949-954, 2012
DOI 10.1007/s12272-012-0601-1
Synthesis and Cytotoxicity of Novel 2,2'-Bis- and 2,2',2''-Tris-indolyl-
methanes-Based Bengacarboline Analogs
Ratchanok Pingaew1, Supaluk Prachayasittikul1,2, Somsak Ruchirawat3, and Virapong Prachayasittikul4
1Department of Chemistry, Faculty of Science, Srinakharinwirot University, Bangkok 10110, Thailand, 2Center for Innovation
Development and Technology Transfer, Faculty of Medical Technology, Mahidol University, Bangkok 10700, Thailand,
3Chulabhorn Research Institute and Chulabhorn Graduate Institute, Bangkok 10210, Thailand, and 4Department of Clinical
Microbiology and Applied Technology, Faculty of Medical Technology, Mahidol University, Bangkok 10700, Thailand
(Received July 16, 2011/Revised October 18, 2011/Accepted November 18, 2011)
Tungstosilicic acid hydrate was employed as an efficient catalyst for the synthesis of bis-
indolylmethanes
4
using the Friedel-Crafts reaction of
N
-sulfonyl tryptamine
5
with various
was
aromatic aldehydes, except 3-formylindole. In the excluding case, tris-indolylmethane
7
formed via a sequential addition-elimination-addition process. The bioactivity test revealed
that the phenolic hydroxyl group plays an important role in cytotoxicity; it demonstrated that
ortho- and para-hydroxy bis-indolylmethane (BIM) analogs (4b and 4d) displayed cytotoxic
potency toward HepG2 (human hepatocellular liver carcinoma cell line) and MOLT-3 (human
lymphoblastic leukemia cell line) cancer cell lines. Significantly, both analogs showed slightly
higher inhibitory efficacy than the control drug, etoposide, in HepG2 cells, and the analog 4d
exhibited the most potent activity against MOLT-3 cell lines, with an IC50 value of 1.62 µg/mL.
Key words: Bis-indolylmethane, Tris-indolylmethane, Bengacarboline, Heteropoly acid, Cyto-
toxicity
INTRODUCTION
An indole moiety is present in a variety of biologically
active natural products and synthetic compounds. Its
pharmacological properties include cytotoxic, antiviral,
antimicrobial, antiparasitic, anti-inflammatory, anti-
serotonin, Ca-releasing, and calmodulin-antagonistic
activities (Gul and Hamann, 2005; Kochanowska-
Karamyan and Hamann, 2010; Ishikura et al., 2010;
Ruiz-Sanchis et al., 2011). Additionally, general bis-
Fig. 1. Structure of bengacarboline.
and tris-indolylmethanes (BIMs and TIMs) structures as an inhibitor of topoisomerase II at 32
have also been found, and they possess numerous et al., 1997). The synthesis of bengacarboline
activities, especially anticancer potency (Shiri et al., derivatives (
and ) were reported (Fig. 2) (Pouilhès
2010). Among the indole family, bengacarboline
et al., 2003, 2008); in those studies, the analogs ( and
(Fig. 1), isolated from the Fijian ascidian Didemnum ) were shown to be more potent cytotoxic agents than
sp., displayed in vitro cytotoxicity, with a mean IC50 of the parent compound (Pouilhès et al., 2008). The
µ
M (Foderato
1
and its
2
3
1
2
3
0.9
µ
g/mL, on a wide range of tumor cell lines. It acted cytotoxic effect of the analogs was demonstrated by
the ability to induce an accumulation of cells in the S
phase of DNA synthesis (Pouilhès et al., 2008). Based
on the basic skeleton, the promising lead molecules
have inspired us to examine a related structure
Correspondence to: Ratchanok Pingaew, Department of Chem-
istry, Faculty of Science, Srinakharinwirot University, Bangkok
10110, Thailand
Tel: 662-649-5000 ext 8253, Fax: 662-259-2097
E-mail: ratchanok@swu.ac.th
required for medicinal application. Accordingly, BIM
4
(Fig. 2) is a compound of choice due to the valuable
949

950
R. Pingaew et al.
and 13C- nuclear magnetic resonance (NMR) spectra
were recorded on a Bruker AVANCE 300 NMR spec-
1
trometer (operating at 300 MHz for H and 75 MHz
for 13C). Mass spectra were recorded on a Bruker Dal-
tonics (microTOF).
Synthesis of 4-nitro-N-(2-(1H-indol-3-yl)ethyl)-
benzenesulfonamide (5)
A solution of tryptamine (1.6022 g, 10 mmol) in di-
chloromethane (50 mL) was added dropwise to a stirred
mixture of 4-nitrobenzenesulfonyl chloride (2.2162 g,
10 mmol) and sodium carbonate (1.4839 g, 14 mmol)
in dichloromethane (20 mL). The reaction mixture was
stirred at room temperature overnight, and distilled
water (20 mL) was added. The organic phase was
separated, and the aqueous phase was extracted with
dichloromethane (2
× 30 mL). The organic extracts
were combined and washed with water (30 mL). The
organic layer was dried over anhydrous sodium sulfate,
filtered, and evaporated to dryness under reduced
pressure. The crude product was further purified by
Fig. 2. Structure of bengacarboline analogs.
bioactivity of the BIM family and its comparable column chromatography on silica gel using 1:9 acetone-
structure with the bengacarboline analogs (
2
and
3
). hexane to obtain the pure product.
The general strategy for BIM synthesis is an acid-
Brown solid; yield 85%; mp 134-135oC; IR (UATR)
promoted Friedel-Crafts reaction of indole with alde- cm−1: 3411, 3307, 1607, 1526, 1347, 1159. 1H-NMR (300
hyde (Shiri et al., 2010). A wide variety advances in MHz, DMSO-d6
reaction conditions using various catalysts have been 3.11 (q, = 6.8 Hz, 2H, CH2NH), 6.92 (t,
published (Shiri et al., 2010). In this regard, hetero- 1H, C5-Ar ), 7.00 (t, = 7.7 Hz, 1H, C6-Ar
polyacids (HPAs) have gained considerable attention 1H, C2-Ar ), 7.25 (d, = 8.0 Hz, 1H, C7-Ar
in organic synthesis as versatile catalysts with envir- (d, = 7.7 Hz, 1H, C4-Ar ), 7.91 (d, = 8.7 Hz, 2H,
onmentally benign properties (Timofeeva, 2003; Ueda C2'-Ar
and C6'-Ar ), 8.12 (t, = 5.4 Hz, 1H, N SO2),
and Kotsuki, 2008). They are normally stronger than 8.25 (d,
= 8.7 Hz, 2H, C3'-Ar and C5'-Ar ), 10.79
conventional Lewis acid (e.g., AlCl3) and mineral acids (s, 1H, N
); 13C-NMR (75 MHz, DMSO-d6
25.8, 43.8,
(e.g., H2SO4 and HCl). Moreover, several advantages 111.1, 111.8, 118.4, 118.7, 121.3, 123.6, 124.7, 127.3,
)
δ
2.79 (t,
J
= 7.1 Hz, 2H, ArCH2),
= 7.4 Hz,
), 7.09 (s,
), 7.36
J
J
H
H
H
J
J
H
J
H
J
H
H
J
H
J
H
H
H
) δ
of HPA include its commercial availability, ease of 128.2, 136.6, 146.6, 149.6. TOF-MS
handing, and extremely low toxicity (Timofeeva, 2003; (Calcd for C16H16N3O4S: 346.0856).
Ueda and Kotsuki, 2008). Application of the catalysts
m/z: 346.0869
for the synthesis of BIMs has been documented
(Murugan et al., 2005; Sivaprasad et al., 2006; Azizi et
al., 2007; Heravi et al., 2008). In this article, a novel
General procedure for the synthesis of Bis-
and Tris-indolylmethanes (4a-g, 7)
A mixture of
N-sulfonyltryptamine 5 (0.5 mmol),
series of BIMs
sized using tungstosilicic acid hydrate (H4O40SiW12
as a catalyst, and their cytotoxicities were evaluated.
4-related bengacarboline were synthe- aldehyde 6 (0.3 mmol), and H4O40SiW12Þaq (20 mol%)
·
aq) in acetonitrile (5 mL) was stirred at room temperature
for 2-6 h. After complete conversion, as indicated by
TLC, the reaction mixture was concentrated in a vac-
uum and purified by silica gel column chromatography
using 1:9 acetone-hexane to obtain the pure product.
MATERIALS AND METHODS
All reagents were used as purchased from commercial
suppliers without further purification. Melting points
Bis-indolylmethane 4a
were determined using a Griffin melting point appar- Prepared according to the general procedure to give
atus and were uncorrected. Column chromatography the title compound as a light brown solid in 70% yield;
was carried out using silica gel 60 (70-230 mesh ASTM). mp 179-180oC; IR (UATR) cm−1: 3393, 3323, 1606, 1527,
1
Analytical thin-layer chromatography (TLC) was per- 1348, 1161. H-NMR (300 MHz, acetone-d6
)
δ
2.80-
1
formed with silica gel 60 F254 aluminum sheets. H- 3.10 (m, 8H, 4
×CH2), 6.18 (s, 1H, CHPh), 6.80-7.31 (m,

Novel 2,2'-Bis- and 2,2',2''-Tris-indolylmethanes
951
13H, Ar
7.80 (d,
H
J
, 2
= 8.7 Hz, 4H, 2
C3'-Ar
). 13C-NMR (75 MHz, acetone-d6
×
N
H
SO2), 7.45 (d,
J
= 7.1 Hz, 2H, Ar
, 2 C6'-Ar
), 9.68 (s, 2H, 156.3, 156.4. TOF-MS
25.0, 40.4, C39H34N6NaO9S2: 817.1721).
H
), 108.2, 110.0, 111.3, 115.5, 118.0, 119.0, 121.1, 124.1,
×
C2'-Ar
, 2
H
×
H
), 8.16 128.0, 128.5, 129.6, 131.6, 135.8, 135.9, 146.3, 149.7,
(d,
J
= 8.7 Hz, 4H, 2
×
H
×C5'-Ar
H
)
m/z: 817.1713 (Calcd for
2×NH
δ
43.5, 43.6, 108.6, 111.2, 111.3, 118.0, 119.0, 121.2, 124.1,
127.0, 128.0, 128.5, 128.7, 135.2, 135.3, 135.9, 136.0, Bis-indolylmethane 4e
141.0, 146.4, 149.7. TOF-MS
C39H35N6O8S2: 779.1952).
m
/
z
: 779.1944 (Calcd for Prepared according to the general procedure to give
the title compound as a light brown solid in 52% yield;
mp 139-140oC; IR (UATR) cm−1: 3387, 3317, 1607, 1527,
1
Bis-indolylmethane 4b
1348, 1160. H-NMR (300 MHz, acetone-d6
)
δ
2.90-
OCH3), 6.14
= 8.2 and 1.9 Hz, 1H, Ar ),
SO2), 7.21 (d, = 7.1 Hz,
), 7.79 (d,
), 8.18 (d, = 8.7
), 9.71 (s, 2H, 2 ).
25.1, 40.3, 43.5, 48.9,
= 8.8 55.1, 108.2, 111.3, 111.8, 112.5, 117.9, 119.0, 120.6,
), 9.69 121.0, 124.1, 127.9, 128.5, 133.4, 135.8, 135.9, 146.3,
24.8, 148.5, 149.5, 149.7. TOF-MS : 861.1949 (Calcd for
Prepared according to the general procedure to give 3.10 (m, 8H, 4
the title compound as an orange solid in 63% yield; mp (s, 1H, C ), 6.72 (dd,
210-211oC; IR (UATR) cm−1: 3400, 3300, 1607, 1527, 6.84-7.06 (m, 8H, Ar
1348, 1159. 1H-NMR (300 MHz, acetone-d6
2.80-3.10 2H, Ar ), 7.45 (d,
= 5.5 Hz, 8.9 Hz, 4H, 2 C2'-Ar
), 6.91-7.19 Hz, 4H, 2 C3'-Ar , 2
), 7.74 (d,
13C-NMR (75 MHz, acetone-d6
), 8.10 (d,
), 8.97 (s, 1H, O
). 13C-NMR (75 MHz, acetone-d6
×C
H2), 3.68, 3.80 (2s, 6H, 2
×
H
J
H
H
, 2
×
N
H
J
)
J
δ
H
J
= 7.1 Hz, 2H, Ar
, 2 C6'-Ar
C5'-Ar
H
J =
(m, 8H, 4
2H, 2
(m, 9H, Ar
= 8.8 Hz, 4H, 2
Hz, 4H, 2 C3'-Ar
(s, 2H, 2
×
C
H2), 6.35 (s, 1H, C
H
), 6.68 (t,
×
H
×
H
J
×N
H
SO2), 6.78 (t,
), 7.39 (d,
C2'-Ar
, 2 C5'-Ar
J
= 7.3 Hz, 1H, Ar
H
H
×
H
×
H
) δ
×NH
H
J
= 7.2 Hz, 2H, Ar
J
×
H
, 2 C6'-Ar
×
H
J
H
δ
×
H
×
H
×N
H
)
m/z
34.9, 43.3, 107.9, 111.2, 115.6, 117.9, 118.9, 120.1, 120.9, C41H38N6NaO10S2: 861.1983).
123.9, 127.5, 127.8, 128.4, 128.5, 129.7, 135.5, 135.8,
146.3, 149.6, 154.2. TOF-MS
C39H34N6NaO9S2: 817.1721).
m/z: 817.1711 (Calcd for Bis-indolylmethane 4f
Prepared according to the general procedure to give
the title compound as a red solid in 57% yield; mp
186-187oC; IR (UATR) cm−1: 3386, 3316, 1607, 1527,
Bis-indolylmethane 4c
Prepared according to the general procedure to give 1348, 1159. 1H-NMR (300 MHz, acetone-d6
)
J
δ
2.90-3.10
= 8.2 Hz,
the title compound as a pale orange solid in 38% yield; (m, 8H, 4
×C
H2), 6.13 (s, 1H, C
H
), 6.64 (d,
mp 169-170oC; IR (UATR) cm−1: 3387, 3317, 1607, 1527, 1H, Ar
H
), 6.75 (d,
, 2
= 7.6 Hz, 2H, Ar
, 2 C6'-Ar ), 8.18 (d,
C5'-Ar ), 9.66 (s, 2H, 2
25.1, 40.3, 43.5, 55.4, 108.2, 111.3,
NH). 13C-NMR (75 MHz, acetone- 112.3, 115.2, 117.9, 119.0, 121.0, 124.1, 127.9, 128.6,
25.9, 40.5, 44.4, 55.5, 109.1, 112.1, 114.8, 118.8, 132.2, 135.9, 145.7, 146.4, 147.7, 149.7. TOF-MS
J
= 8.2 Hz, 1H, Ar
H
), 6.80-7.07 (m,
),
= 8.6 Hz, 4H,
1
1348, 1160. H-NMR (300 MHz, acetone-d6
)
δ
2.80- 7H, Ar
H
J
×NHSO2), 7.21 (d,
J = 7.6 Hz, 2H, ArH
3.05 (m, 8H, 4
), 6.72-7.19 (m, 12H, Ar
7.7 Hz, 2H, Ar ), 7.76 (d,
C6'-Ar ), 8.13 (d, = 8.8 Hz, 4H, 2
Ar ), 9.61 (s, 2H, 2
d6
×
C
H2), 3.73 (s, 3H, OCH3), 6.08 (s, 1H, 7.45 (d,
H
), 7.80 (d,
J
C
H
H
J
, 2
= 8.8 Hz, 4H, 2
C3'-Ar
×N
H
SO2), 7.41 (d,
C2'-Ar
, 2
J
H
=
,
2
×
C2'-Ar
H
×
H
J
= 8.6 Hz, 4H, 2×C3'-
H
×
Ar , 2
H
×
H
)
×NH
). 13C-NMR (75
2×
H
J
×
×
H
×
C5'- MHz, acetone-d6
δ
H
)
δ
m/z:
119.8, 121.9, 124.9, 128.8, 129.4, 130.4, 133.6, 136.6, 825.2023 (Calcd for C40H37N6O10S2: 825.2007).
136.8, 147.1, 150.5, 159.6. TOF-MS m/z: 831.1855
(Calcd for C40H36N6NaO9S2: 831.1877).
Bis-indolylmethane 4g
Prepared according to the general procedure to give
the title compound as a red solid in 54% yield; mp
Bis-indolylmethane 4d
Prepared according to the general procedure to give 172-173oC; IR (UATR) cm−1: 3385, 3298, 1607, 1528,
1
the title compound as a red solid in 48% yield; mp 1348, 1160. H-NMR (300 MHz, acetone-d6
)
δ
2.85-
OCH3), 6.13 (s,
, C6''-Ar ), 6.81-7.48
), 7.79 (d, = 8.7 Hz, 4H,
), 8.18 (d, = 8.7 Hz, 4H, 2C3'-
), 9.72 (s, 2H, 2
). 13C-NMR (75
= 8.7 Hz, 4H, MHz, acetone-d6 25.1, 40.8, 43.4, 55.8, 106.3, 108.2,
= 8.7 Hz, 4H, 2C3'- 111.3, 117.9, 118.9, 121.0, 124.1, 127.9, 128.6, 131.3,
), 9.62 (s, 2H, 2 ). 135.1, 135.9, 146.3, 148.1, 149.7. TOF-MS : 877.1918
25.1, 39.7, 43.4, 43.5, (Calcd for C41H38N6NaO11S2: 877.1932).
121-122oC; IR (UATR) cm−1: 3401, 3313, 1608, 1528, 3.05 (m, 8H, 4
×C
H2), 3.70 (s, 6H, 2
×
1
1348, 1160. H-NMR (300 MHz, acetone-d6
)
δ
J
2.85- 1H, C
= 8.4 (m, 11H, Ar
SO2), 2C2'-Ar , 2C6'-Ar
= 7.3 Hz, 2H, Ar ), Ar , 2C5'-Ar
H
Ar), 6.66 (s, 2H, C2''-Ar
H
H
3.10 (m, 8H, 4
Hz, 2H, C3''-Ar
6.94-7.07 (m, 6H, Ar
×
C
H2), 6.08 (s, 1H, C
H
), 6.75 (d,
H
, 2 SO2, O
×
N
H
H
J
H, C5''-Ar
H
), 6.84 (br t, 2H, 2
×NH
H
H
J
H), 7.20 (d,
J
H
H
H
×NH
7.44 (d,
2C2'-Ar
J
H
= 7.7 Hz, 2H, Ar
, 2C6'-Ar ), 8.16 (d,
), 8.51 (s, 1H, O
13C-NMR (75 MHz, acetone-d6
H), 7.78 (d,
J
) δ
H
J
H
δ
ArH, 2C5'-Ar
H
×NH
m/z
)

952
R. Pingaew et al.
Table I. Reactiona of indole
tungstosilicic acid hydrate
5
with aldehydes
6 using
Tris-indolylmethane 7
Prepared according to the general procedure to give
the title compound as a light brown solid in 31% yield;
Entry
Aldehyde (
6)
Product Yield (%)
mp 212-213oC; IR (UATR) cm−1: 3383, 3314, 1607, 1526,
1
2
3
4
5
6
7
8b
C6H5CHO
4a
4b
4c
4d
4e
70
63
42
48
52
57
54
31
1
1347, 1159. H-NMR (300 MHz, acetone-d6
)
δ
2.85-
), 6.80 (br t, 3H,
), 7.18 (d, = 7.7
= 7.2 Hz, 3H, Ar ), 7.70 (d,
, 3 C6'-Ar ), 8.08 (d,
C5'-Ar ), 9.85 (s, 2H, 3
13C-NMR (75 MHz, acetone-d6)
25.2, 33.5, 43.3, 108.8,
111.5, 118.2, 119.2, 121.3, 124.1, 127.9, 128.7, 133.7,
136.0, 146.2, 149.7. TOF-MS : 1068.2061 (Calcd for
2-OH-C6H4CHO
4-OMe-C6H4CHO
4-OH-C6H4CHO
3,4-(OMe)2-C6H3CHO
3-OMe,4-OH-C6H3CHO
3.10 (m, 12H, 6
SO2), 6.99-7.09 (m, 6H, Ar
Hz, 3H, Ar ), 7.49 (d,
= 8.8 Hz, 6H, 3 C2'-Ar
Hz, 6H, 3 C3'-Ar , 3
×CH2), 6.56 (s, 1H, CH
3×NH
H
J
H
J
H
J
= 8.8
×
H
×
×
H
J
4f
4g
×
H
H
δ
×NH).
3,5-(OMe)2,4-OH-C6H2CHO
3-CHO-indole
7
(not 4h)
a20 mol% H4O40SiW12
·
aq, CH3CN, rt 2-6 h; b20 mol% H4O40SiW12
m/z
·
aq, CH3CN, 60oC 9 h
C49H43N9NaO2S3: 1068.2086).
Cytotoxic assay
The cytotoxic assay was performed as previously
described (Tengchaisri et al., 1998). Briefly, cell lines
suspended in RPMI 1640 containing 10% fetal bovine
serum (FBS) were seeded at 1 × 104 cells (100
µL) per
well in 96-well plates and incubated in a humidified
atmosphere, 95% air, 5% CO2 at 37ºC. After 24 h, addi-
tional medium (100
and vehicle was added to a final concentration of 50
g/mL, 0.2% dimethyl sulfoxide (DMSO), and further
µL) containing the test compound
µ
incubated for 3 days. After that, the cells were fixed
with EtOH-H2O (95:5, v/v), stained with crystal violet
solution, and lysed with a solution of 0.1 N HCl in
Scheme 1. Synthesis of BIMs 4.
MeOH, after which absorbance was measured at 550 similar conditions and furnished BIMs 4, except 4h, in
nm. The number of surviving cells was determined moderate yield. To the best of our knowledge, these
from the absorbance. Etoposide and doxorubicin were BIMs have not been reported. Surprisingly, treatment
used as the reference drugs.
RESULTS AND DISCUSSION
Chemistry
of indole
methane
5
7
with 3-formylindole provided tris(2-indolyl)
instead of the expected BIM 4h. The reac-
tion required elevated temperature (60ºC) to consume
within 9 h. However, the reaction was not completed
at room temperature, even when it was prolonged to
5
1
N
-sulfonyltryptamine
of tryptamine with -nitrobenzenesulfonyl chloride in methine proton as a singlet at 6.56 ppm and a triple
the presence of Na2CO3. The alkylation of -sulfonyl- set of symmetrical -sulfonyltryptamine moieties.
tryptamine with benzaldehyde was performed using However, the formation of the TIM system using 3-
5 was prepared by treatment 48 h. The H-NMR spectra of TIM 7 illustrated a
p
N
N
5
20 mol% catalyst at room temperature. Double nucleo- formylindole has also been observed (Chakrabarty and
philic attack of the indole carbon on the electrophilic Sarkar, 2002; Hazra et al., 2008). Mechanistically, as
center of benzaldehyde gave the anticipated BIM 4a shown in Scheme 2, the formation of indoleninium in-
1
in 70% yield within 2 h (Table I; entry 1). The H- termediate III may be considered to take place through
NMR spectra of BIM 4a was characterized by the rearrangement and elimination of water. Subsequently,
presence of a methine proton as a singlet at 6.18 ppm nucleophilic addition of indole
5 on intermediate III
and a typically symmetrical signal of two -sulfonyl- led to BIM (Biswas and Jackson, 1969; Chakrabarty
N
4
tryptamine moieties. The structure of BIM 4a was and Sarkar, 2002; Chakrabarty et al., 2002). In the case
further confirmed by 13C-NMR, 2D-NMR, infrared (IR), of using 3-formylindole (R=indole), the formation of TIM
and high-resolution mass spectrometry (HRMS). The
success of forming 4a prompted us to extend the indole, followed by re-addition of the
reaction of indole with a range of other aldehydes amine (Chakrabarty and Sarkar, 2002).
as shown in Table I. The reaction was performed under
7
was proposed to proceed via a loss of a molecule of
N-sulfonyltrypt-
5
6,
5

Novel 2,2'-Bis- and 2,2',2''-Tris-indolylmethanes
953
Scheme 2. Proposed reaction mechanism of
4
and 7 formations.
the control drug, etoposide, against HepG2 cells. In
conclusion, we have achieved an application of tung-
Cytotoxic activity
Cytotoxicity of the synthesized BIMs
4
and TIM 7
were assayed against HuCCA-1 (human cholangiocar- stosilicic acid hydrate as an influential catalyst for the
cinoma cancer cells), HepG2 (human hepatocellular synthesis of BIMs
liver carcinoma cell line), A549 (human lung carcinoma yield. We found that BIMs
4
and TIM
7
in moderate to good
were obtained when
4
cell line) and MOLT-3 (human lymphoblastic leukemia benzaldehydes were employed. However, in the case of
cell line), as summarized in Table II. Unfortunately, 3-formylindole, TIM was formed via the cascade of
most synthesized BIMs, 4a 4c 4e-4g, and TIM were an addition-elimination-addition process. It is note-
not active toward any the tested cell lines. However, worthy that the ortho-hydroxy (4b) and para-hydroxy
mono hydroxy analogs (4b and 4d) displayed cytotoxic 4d) groups of the phenolic moiety play an important
7
,
,
7
(
potency against HepG2 and MOLT-3 cells. More role in their cytotoxic efficacy toward HepG2 and
importantly, the hydroxy group at either the ortho or MOLT-3 cell lines. Significantly, the method offers
para position on the aromatic ring (R of 4) played a several attractive advantages, including the use of a
significant role in the cytotoxic effect. It has been readily available and non-corrosive catalyst as well as
noted that the inhibitory potency of the para hydroxy a simple process for the synthesis of these biologically
analog (4d) was completely lost when the steric active compounds.
crowding of methoxy group appeared at the o-position
of the hydroxy group, as presented in compounds 4f
and 4g. It is conceivable that the compounds require
H-bonding (from phenolic OH) and hydrophobic
ACKNOWLEDGEMENTS
We gratefully acknowledge the support of the re-
binding of the aromatic ring for their cytotoxicities. search grants of Srinakharinwirot University (B.E.
Significantly, the mono hydroxy analogs (4b and 4d 2552). This project is supported by the office of the
)
displayed cytotoxicity with slightly lower IC50 than Higher Education Commission and Mahidol University
under the National Research Universities Initiative.
R.P. sincerely thanks the Science and Technology Re-
search Grant of the Thailand Toray Science Founda-
tion for its financial support.
Table II. Cytotoxic activity of compounds 4b and 4d
against four cancer cell lines
Cytotoxic activity (IC50,
µg/mL)
compound
4c 4d-4g
HuCCA-1 HepG2 A549 MOLT-3
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4a
4b
4d
,
,
,
7
>50
>50
>50
NDa
0.43
>50
17.67
17.50
18.20
0.55
>50
>50
>50
NDa
0.38
>50
3.95
1.62
0.019
NDa
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