Synthesis of 1,2,3-triazole-linked pyrrolobenzodiazepine conjugates employing 'click' chemistry: DNA-binding affinity and anticancer activity

Article abstract of DOI:10.1016/j.bmcl.2007.12.063

1,2,3-Triazole based molecules are useful pharmacophores for several DNA-alkylating and cross-linking agents. A series of A/C8, C/C2 and A/C8-C/C2-linked 1,2,3-triazole-PBD conjugates have been synthesized by employing 'click' chemistry. These molecules h

Full text of DOI:10.1016/j.bmcl.2007.12.063

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1468–1473
Synthesis of 1,2,3-triazole-linked pyrrolobenzodiazepine
conjugates employing ‘click’ chemistry: DNA-binding affinity
and anticancer activity
Ahmed Kamal,a,* N. Shankaraiah,a V. Devaiah,a K. Laxma Reddy,a
Aarti Juvekar,b Subrata Sen,b Nisha Kurianb and Surekha Zingdeb
aBiotransformation Laboratory, Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
bAdvanced Centre for Treatment, Research & Education in Cancer, Navi Mumbai, India
Received 14 September 2007; revised 4 December 2007; accepted 22 December 2007
Available online 28 December 2007
Abstract—1,2,3-Triazole based molecules are useful pharmacophores for several DNA-alkylating and cross-linking agents. A series
of A/C8, C/C2 and A/C8-C/C2-linked 1,2,3-triazole–PBD conjugates have been synthesized by employing ‘click’ chemistry. These
molecules have exhibited promising DNA-binding affinity and anticancer activity in selected human cancer cell lines.
Ó 2007 Elsevier Ltd. All rights reserved.
Triazoles are an important class of heterocycles, which
display an ample spectrum of biological activities and
are widely employed as pharmaceuticals and agrochem-
icals. These molecules have been reported to possess
significant antibacterial, antifungal and antihelmintic
activities.1 They have been regarded as an interesting
unit in terms of biological activity2 and some of them
have also shown significant anticancer activity in many
of the human cell lines. In recent years, alkylating
agents have been extensively studied with regard to
cancer chemotherapy, this has led to the development
of many new and more selective alkylating agents like
some molecules based on triazole moiety as anticancer
drugs.3 The conventional route to triazoles is the Huis-
gen dipolar cycloaddition of alkynes with organic
azides. This procedure involved the 1,3-dipolar cyclo-
addition of azides with organic alkynes possessing an
electron-withdrawing substituent4 to provide 1,2,3-
triazoles5.
immense interest as potential anticancer and gene-tar-
geting agents.6 These include anthramycin (1), tomay-
mycin (2) and DC-81 (3), that are known to
monoalkylate by covalent binding to the N2 of a guan-
ine base in the DNA minor groove through their electro-
philic imine functionality with preference for Pu-G-Pu
sequence (Fig. 1).7 The development of new practical
synthetic strategies has allowed the exploration of sev-
eral analogues based on PBD ring system including
the joining of two PBD units through various spacers
and PBD unit linked to certain known antitumour
agents. Further extensive studies have been carried out
in both the solution8 and solid-phase9 synthesis of
PBDs, and a sound understanding of structure–activity
relationships within the family has been developed.10
In continuation of these efforts we have designed and
synthesized some conjugates of 1,2,3-triazoles tethered
to the PBD moiety employing ‘click’ chemistry.11 The
DNA binding ability and in vitro anticancer activity
for these novel conjugates have also been investigated.
The naturally occurring pyrrolo[2,1-c][1,4]benzodiaze-
pines (PBDs) usually interact covalently with DNA
in a sequence-selective manner and have generated
In view of the biological importance of 1,2,3-triazoles
and PBDs, it was of considerable interest to develop
novel conjugates incorporating both the ring systems,
such a combination could provide two types of DNA-
alkylating moieties in a single molecule. In this context
1,2,3-triazole has been linked to A/C8, C/C2 and A/
C8-C/C2-positions of the PBD moiety. Interestingly, it
has been observed that solubility of 1,2,3-triazole–PBD
Keywords: 1,2,3-Triazoles; Pyrrolo[2,1-c][1,4]benzodiazepines; DNA-
binding affinity; Anticancer activity.
*
Corresponding author. Tel.: +91 40 27193157; fax: +91 40
27193189; e-mail: ahmedkamal@iict.res.in
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.12.063

A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1468–1473
1469
OH
H
OCH3
H
N
HO
H
N
H3C
N
H3CO
N
CONH2
O
O
2
1
N
N
N
10
9
11
N
HO
H3CO2C
N
H
O
n
H
8
A
B11a
1
H3CO2C
C
N
7
H3CO
5
2
N
6
H3CO
4
3
O
O
4a: n = 3
4b: n = 4
4c: n = 5
3
R1
R2
N
H
N
N
N
H3CO2C
N
O
O
H
N
N
n
N
N
H3CO2C
O
N
H3CO
N
N
N
H3CO2C
CO2CH3
5a: R1 = R2 = H
H3CO2C
CO2CH3
6a: n = 3
6b: n = 4
6c: n = 5
5b: R1 = R2 = OCH3
5c: R1 = OBn, R2 = OCH3
Figure 1. Chemical structures of anthramycin (1), tomaymycin (2), DC-81 (3) and 1,2,3-triazole–PBD conjugates (4a–c, 5a–c and 6a–c).
conjugates is enhanced in most of the organic solvents;
this may be attributed to the ester functionalities that
are present in the 1,2,3-triazole moiety.
tion. This process provides the biologically diverse
molecules in a single step. These triazole linked nitro-
thioacetal intermediates 9a–c upon reduction with
SnCl2. 2H2O and deprotection followed by cyclization
with HgCl2/CaCO3 affords the desired A/C8-linked tria-
Synthesis of (2S)-N-[4-(bromoalkoxy)-5-methoxy-2-nitro-
benzoyl]pyrrolidine-2-carboxaldehyde diethylthioacetals
(7a–c) has been accomplished by employing the reported
method.12 These upon azidation with NaN3 followed by
cycloaddition with dimethyl acetylenedicarboxylate pro-
duces the corresponding 9a–c13 through a ‘click’ reac-
14
zole–PBD conjugates (4a–c) as shown in Scheme 1.
Synthesis of (2S,4S)-N-[2-nitrobenzoyl]-4-azidopyrroli-
dine-2-carboxaldehyde diethylthioacetals (10a–c) has
been carried out by the reported method.15 These com-
NO2
NO2
N3
O
n
CH( SEt)2
Br
O
n
CH( SEt)2
i
N
N
H3CO
H3CO
O
O
7a: n = 3
7b: n = 4
7c: n = 5
8a-c
n = 3-5
ii
N
N
N
N
N
N
H3CO2C
NO2
H3CO2C
O
CH( SEt)2
N
O
n
H
iii&iv
n
H3CO2C
H3CO2C
N
H3CO
N
H3CO
O
O
9a-c
4a: n = 3
4b: n = 4
4c: n = 5
n = 3-5
Scheme 1. Reagents and conditions: (i) NaN3 (0.5 M) in DMSO, 80 °C, 6 h, 85–88%; (ii) dimethyl acetylenedicarboxylate, dry benzene, reflux, 6 h,
87–92%; (iii) SnCl2Æ2H2O, MeOH, reflux, 5 h, 75–83%; (iv) HgCl2–CaCO3, CH3CN–H2O (4:1), rt, 12 h, 58–62%.

1470
A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1468–1473
R1
R2
R1
R2
NO2
CH(SEt)2
N3
NO2
CH(SEt)2
i
N
N
N
N
N
O
O
10a: R1 = R2 = H
CO2CH3
H3CO2C
11a-c
10b: R1 = R2 = OCH3
10c: R1 = OBn, R2 = OCH3
ii&iii
R1
R2
N
H
N
N
N
N
O
H3CO2C
5a: R1 = R2 = H
5b: R1 = R2 = OCH3
5c: R1 = OBn, R2 = OCH3
CO2CH3
Scheme 2. Reagents and conditions: (i) dimethylacetylene dicarboxylate, dry benzene, reflux, 6 h, 88–90%; (ii) SnCl2Æ2H2O, MeOH, reflux, 5 h, 80–
85%; (iii) HgCl2–CaCO3, CH3CN–H2O (4:1), rt, 12 h, 58–61%.
NO2
NO2
HO
CH(SEt)2
BnO
CH(SEt)2
i
N
N
N
N
H3CO
H3CO
N
N
N
N
O
O
CO2CH3
CO2CH3
H3CO2C
H3CO2C
11c
12
ii,iii&iv
N
N
H3CO2C
NO2
N
O
n
CH( SEt)2
H3CO2C
N
N
H3CO
N
N
O
13a-c
CO2CH3
H3CO2C
n = 3-5
v&vi
N
N
H3CO2C
N
N
O
H
n
H3CO2C
N
H3CO
N
N
N
O
6a: n = 3
H3CO2C
6b: n = 4
6c: n = 5
CO2CH3
Scheme 3. Reagents and conditions: (i) BF3ÆOEt2, EtSH, CH2Cl2, rt, 12 h, 75%; (ii) dibromoalkane spacers, K2CO3, dry DMF, 12 h, rt, 78–80%; (iii)
NaN3 (0.5 M) in DMSO, 80 °C, 6 h, 89–92%; (iv) dimethyl acetylene dicarboxylate, dry benzene, reflux, 6 h, 88–90%; (v) SnCl2Æ2H2O, MeOH, reflux,
5 h, 78–85%; (vi) HgCl2–CaCO3, CH3CN–H2O (4:1), rt, 12 h, 58–62%.

A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1468–1473
1471
Table 1. Thermal denaturation data for 1,2,3-triazole-linked PBD
conjugates (4a–c, 5a–c and 6a–c) with calf thymus CT-DNA
described in the above procedure to yield the desired
A/C8 and C/C2-linked 1,2,3-triazole–PBD conjugates
6a–c as illustrated in Scheme 3.
Compound
[PBD]/[DNA]
Molar ratioa
DTmb (°C)
After incubation
at 37 °C
18 h
0 h
The DNA-binding ability for these 1,2,3-triazole linked
PBD conjugates has been determined by thermal dena-
turation studies using calf thymus (CT)-DNA. These
studies have been carried out at PBD/DNA molar ratio
of 1:5. Interestingly, most of the compounds (4a, 4c; 5a–
c and 6a, 6c) elevate the helix melting temperature of
CT-DNA in the range of 2.2–5.9 °C after 18 h incuba-
tion at 37 °C. It is observed that in compounds 4a–c
to 6a–c, when the linker spacer comprises of odd num-
ber of carbons the DNA-binding ability is good. How-
ever, the DNA-binding ability is completely absent in
the compounds when there are even number of carbons
in the linker spacer (4b and 6b). Interestingly, further in
compounds 6a and 6c the DNA-binding affinity is sig-
nificant, when the triazole moiety is present at both C8
as well as C2-positions. In case of compounds 5a–c there
is not much difference in their DTm values, wherein the
triazole moiety is linked at C2-position of the PBD ring
without an alkane spacer. Compound 6c has shown the
highest DTm value of 5.9 °C upon 18 h of incubation at
37 °C. Whereas, the naturally occurring DC-81 (3)
exhibits a DTm of 0.7 °C after 18 h of incubation under
similar conditions (Table 1).
4a
4b
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1.5
0.0
2.2
1.0
1.1
0.9
2.6
0.0
3.1
0.3
3.1
0.0
3.6
2.3
2.8
2.2
4.5
0.0
5.9
0.7
4c
5a
5b
5c
6a
6b
6c
DC-81
a For CT-DNA alone at pH 7.00 0.01, Tm = 69.6 °C 0.01 (mean
value from 10 separate determinations), all DTm values are 0.1–
0.2 °C.
b For a 1:5 molar ratio of [PBD]/[DNA], where CT-DNA concentra-
tion = 100 lM and ligand concentration = 20 lM in aqueous sodium
phosphate buffer [10 mM sodium phosphate + 1 mM EDTA, pH
7.00 0.01].
pounds 10a–c upon cycloaddition with dimethyl acety-
lenedicarboxylate afford the corresponding 11a–c. Fur-
ther steps are similar to that which have been
described in the previous method to afford the desired
C/C2-linked triazole–PBD conjugates 5a–c14 as shown
in Scheme 2.
Compounds 4a–c, 5b, 5c and 6a–c have been evaluated
for their in vitro anticancer activity in selected human
cancer cell lines of breast, oral, ovary, colon, lung,
prostate and cervix by using sulforhodamine B (SRB)
method.16,17 The compounds exhibiting GI50 6 10À5 M
(10 lM) are considered to be active on the respective cell
lines. Table 2 reveals that compound 6c exhibits broad-
spectrum activity in the cell lines used. However, com-
pound 6b is found to be inactive and 5a has not been
evaluated. The activity of the compounds ranged from
GI50 0.12 to 30.50 lM. The positive control compound
adriamycin demonstrated highly significant activity with
the GI50 in the range from 0.10 to 7.25 lM and for DC-
81 the GI50 ranged from 0.10 to 2.37 lM. The efficacy of
In continuation of our later efforts to explore the diver-
sity in both A/C8 and C/C2-positions of the PBD ring
system, synthesis of such 1,2,3-triazole–PBD conjugates
(6a–c) has been carried out accordingly. Compound 11c
has been prepared by employing the similar procedure
as described in the previous protocol of Scheme 2. Com-
pound 11c upon debenzylation with BF3 Æ OEt2/EtSH
affords the corresponding debenzylated product 12. This
upon etherification with different dibromoalkanes, azi-
dation followed by cycloaddition with dimethyl acety-
lenedicarboxylate, provides the precursors 13a–c.
Further steps are also similar to that which have been
Table 2. GI50 values (lM) for compounds 4a–c, 5b, 5c and 6a–c in selected human cancer cell lines
Compound
Zr-75-1a
MCF7a
KBb
Guravb
DWDb
A2780c
Colo205d
A549e
PC3f
SiHag
h
4a
4b
2.53
2.12
1.93
2.47
—
30.50
26.60
2.36
7.19
25.10
2.39
25.50
2.24
2.48
0.13
2.09
0.21
2.17
1.09
27.90
2.01
24.70
28.71
6.12
28.70
25.40
29.14
2.23
2.12
2.04
2.15
0.21
2.04
2.17
h
—
4c
5b
26.83
2.39
26.01
2.26
29.71
26.01
0.914
25.81
0.19
h
2.17
27.55
2.12
26.40
h
—
—
h
h
—
h
h
5c
6a
—
—
—
h
h
h
—
h
1.05
h
—
2.42
h
—
—
7.12
h
—
—
h
h
—
h
—
h
—
h
h
6b
6c
—
—
—
—
5.51
1.79
2.37
2.05
0.18
0.17
1.92
0.17
0.17
1.82
0.17
0.16
0.15
0.10
1.49
0.16
0.17
0.14
1.92
0.15
0.11
2.41
7.25
0.16
0.17
1.81
0.20
0.12
0.18
0.17
ADR
DC-81
a Breast cancer.
b Oral cancer.
c Ovary cancer.
d Colon cancer.
e Lung cancer.
f Prostate cancer.
g Cervix cancer.
h GI50 value not attained at the concentrations used in the assay; ADR (adriamycin); DC-81.

1472
A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1468–1473
G.; Laxman, N.; Ramulu, P.; Srinivas, O.; Neelima, K.;
Kondapi, A. K.; Srinu, V. B.; Nagarajaram, H. A. J. Med.
Chem. 2002, 45, 4679; (d) Kamal, A.; Devaiah, V.; Reddy,
K. L.; Kumar, M. S. Bioorg. Med. Chem. 2005, 13, 2021;
(e) Kamal, A.; Reddy, K. L.; Devaiah, V.; Shankaraiah,
N.; Kumar, M. S.; Reddy, G. S. K. Lett. Drug Design
Discov. 2005, 1, 55.
the compounds is as shown here in the descending order
6c > 4b > 6a > 5b > 4c and 4a > 5c. In terms of sensitiv-
ity for the cell lines it is observed that SiHa cell line is the
most sensitive followed by A2780 > DWD > MCF7 >
Colo205 > PC3 > A549 > Gurav and KB > Zr-75-1
(GI50 0.12–30.50 lM). Overall, the in vitro anticancer
activity exhibited by these new 1,2,3-triazole-linked
PBD conjugates is promising.
9. (a) Kamal, A.; Reddy, K. L.; Devaiah, V.; Shankaraiah,
N.; Reddy, D. R. S. Mini-Rev. Med. Chem. 2006, 6, 53; (b)
Kamal, A.; Reddy, G. S. K.; Raghavan, S. Bioorg. Med.
Chem. Lett. 2001, 11, 387; (c) Kamal, A.; Reddy, G. S. K.;
Reddy, K. L. Tetrahedron Lett. 2001, 42, 6969; (d) Kamal,
A.; Reddy, G. S. K.; Reddy, K. L.; Raghavan, S.
Tetrahedron Lett. 2002, 43, 2103; (e) Kamal, A.; Shanka-
raiah, N.; Devaiah, V.; Reddy, K. L. Tetrahedron Lett.
2006, 47, 6553; (f) Kamal, A.; Shankaraiah, N.; Reddy, K.
L.; Devaiah, V. Tetrahedron Lett. 2006, 47, 4253; (g)
Kamal, A.; Reddy, K. L.; Devaiah, V.; Shankaraiah, N.
Synlett 2004, 1841.
10. (a) Gregson, S. J.; Howard, P. W.; Thurston, D. E. Bioorg.
Med. Chem. Lett. 2003, 13, 2277; (b) Kamal, A.; Reddy,
K. L.; Devaiah, V.; Shankaraiah, N. Synlett 2004, 2533;
(c) Kamal, A.; Devaiah, V.; Reddy, K. L.; Shankaraiah,
N. Adv. Synth. Catal. 2006, 348, 249.
In conclusion, some new 1,2,3-triazole-linked pyr-
rolobenzodiazepine conjugates have been synthesized
by utilizing the ‘click’ process (1,3-dipolar cycloaddi-
tion). Some of the synthesized compounds have shown
noticeable DNA-binding affinity and potential antican-
cer activity in selected human cancer cell lines with in-
creased solubility. Further, detailed mechanistic and
molecular modelling studies for these compounds are
in progress.
Acknowledgments
11. (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew.
Chem. 2001, 113, 2056; (b) Kolb, H. C.; Finn, M. G.;
Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004.
12. Kamal, A.; Ramulu, P.; Srinivas, O.; Ramesh, G.; Kumar,
P. P. Bioorg. Med. Chem. Lett. 2004, 14, 4791.
13. Biagi, G.; Giorgi, I.; Livi, O.; Manera, C.; Scartoni, V.;
Betti, L.; Giannaccini, G.; Lucacchini, A. Farmaco II
1999, 54, 615.
We are thankful to the Department of Biotechnology
(BT/PR/7037/Med/14/933/2006), New Delhi, for finan-
cial assistance. Three of the authors N.S., V.D. and
K.L.R. are thankful to CSIR, New Delhi, for the award
of research fellowships.
14. Selected data: compound 4a: 1H NMR (400 MHz, CDCl3):
d 7.60 (d, 1H, J = 4.39 Hz); 7.42 (s, 1H); 6.67 (s, 1H); 4.80–
4.89 (m, 2H); 4.08–4.13 (m, 1H); 3.98–4.03 (m, 1H); 3.94
(s, 3H); 3.89 (s, 3H); 3.88 (s, 3H); 3.75–3.82 (m, 1H); 3.64–
3.69 (m, 1H); 3.51–3.57 (m, 1H); 2.45–2.51 (m, 2H); 2.28–
2.34 (m, 2H); 2.02–2.08 (m, 2H); FABMS: m/z 472
[M++H]; compound 4b: 1H NMR (400 MHz, CDCl3): d
7.60 (d, 1H, J = 4.91 Hz); 7.43 (s, 1H); 6.70 (s, 1H); 4.61–
4.65 (t, 2H, J = 7.37 Hz); 3.97–4.08 (m, 2H); 3.91 (s, 3H);
3.89 (s, 3H); 3.85 (s, 3H); 3.71–3.77 (m, 1H); 3.63–3.67 (m,
1H); 3.46–3.53 (m, 1H); 2.22–2.26 (m, 2H); 2.04–2.11 (m,
2H); 1.95–2.01 (m, 2H); 1.80–1.87 (m, 2H); FABMS: m/z
References and notes
1. (a) Hardman, J.; Limbird, L.; Gilman, A. Goodman and
Gilman’s The Pharmacological Basis of Therapeutics. 9th
ed.; McGraw-Hill: New York, 1996; p. 988; (b) Gennaro,
A. R.; Remington. The Science and Practice of Pharmacy,
Mack Easton, PA, 1995; Vol. II, pp 1327; (c) Richardson,
K.; Whittle, P. J. Eur. Pat. Appl. EP 1984, 115, 416;
Richardson, K.; Whittle, P. J. Chem. Abstr. 1984, 101,
230544; (d) Ammermann, E.; Loecher, F.; Lorenz, G.;
Janseen, B.; Karbach, S.; Meyer, N. Brighton Crop Prot.
Conf. Pests. Dis. 1990, 2, 407; Ammermann, E.; Loecher,
F.; Lorenz, G.; Janseen, B.; Karbach, S.; Meyer, N. Chem.
Abstr. 1991, 114, 223404h; (e) Heindel, N. D.; Reid, J. R.
J. Heterocycl. Chem. 1980, 17, 1087.
1
486 [M++H]; compound 4c: H NMR (400 MHz, CDCl3):
d 7.67 (d, 1H, J = 3.90 Hz); 7.50 (s, 1H); 6.78 (s, 1H); 4.63
(t, 2H, J = 7.01, 7.79 Hz); 4.03–4.12 (m, 2H); 3.99 (s, 3H);
3.97 (s, 3H); 3.93 (s, 3H); 3.78–3.84 (m, 1H); 3.70–3.74 (m,
1H); 3.54–3.60 (m, 1H); 2.30–2.34 (m, 2H); 1.97–2.07 (m,
4H); 1.88–1.93 (m, 2H); 1.50–1.58 (m, 2H); 13C NMR
(50 MHz, CDCl3): d 13.7; 22.3; 23.8; 27.9; 29.2; 31.2; 46.3;
50.1; 52.3; 53.1; 55.8; 68.1; 76.5; 77.4; 110.2; 111.3; 119.8;
129.4; 139.5; 140.3; 147.4; 150.4; 158.6; 160.2; 162.2; 164.3;
FABMS: m/z 500 [M++H]; compound 5a: 1H NMR
(200 MHz, CDCl3): d 7.50–7.57 (m, 4H); 7.61 (d, 1H);
5.80–5.88 (m, 1H); 4.44–4.53 (m, 1H); 4.04 (s, 3H); 4.00 (s,
3H); 3.26–3.31 (m, 1H); 2.99–3.07 (m, 1H); 2.18–2.37 (m,
2H); EIMS: m/z 383 [M+]; compound 5b: 1H NMR
(400 MHz, CDCl3) d 7.97–7.99 (d, 1H, J = 4.39 Hz); 7.51
(s, 1H); 6.85 (s, 1H); 5.63–5.69 (m, 1H); 4.35–4.41 (m, 1H);
4.13–4.17 (m, 1H); 4.03 (s, 3H); 3.99 (s, 3H); 3.96 (s, 3H);
3.94 (s, 3H); 3.84–3.89 (m, 1H); 3.26–3.32 (m, 1H); 3.03–
3.11 (m, 1H); FABMS: m/z 444 [M++H]; compound 5c: 1H
2. (a) Dehne, H. In Methoden der Organischen Chemie
(Houben-Weyl); Schumann, E., Ed.; Thieme: Stuttgart,
1994; Vol. E8d, p 305; (b) Wamhoff, H.. In Comprehensive
Heterocyclic Chemistry; Katritzky, A. R., Rees, C. W.,
Eds.; Pergamon: Oxford, 1984; 5, p 669.
3. De las Heras, F. G.; Alonso, R.; Alonso, G. J. Med.
Chem. 1979, 22, 496.
4. (a) Journet, M.; Cai, D.; Kowal, J. J.; Larsen, R. D.
Tetrahedron Lett. 2001, 42, 9117; (b) Katritzky, A. R.;
Zhang, Y.; Singh, S. Heterocycles 2003, 60, 1225.
5. (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.;
Sharpless, K. B. Angew. Chem. 2002, 114, 2708; (b)
Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
K. B. Angew. Chem., Int. Ed. 2002, 41, 2596.
6. Thurston, D. E. Br. J. Cancer 1990, 80, 65.
7. Thurston, D. E. In Molecular Aspects of Anticancer Drug-
DNA Interactions; Neidle, S., Waring, M. J., Eds.;
Macmillan: London, 1993; p 54.
8. (a) Kamal, A.; Ramesh, G.; Srinivas, O.; Ramulu, P.
Bioorg. Med. Chem. Lett. 2004, 14, 471; (b) Kamal, A.;
Reddy, K. L.; Reddy, G. S. K.; Reddy, B. S. N.
Tetrahedron Lett. 2004, 45, 3499; (c) Kamal, A.; Ramesh,
NMR (400 MHz, CDCl3):
d
7.41–7.43 (d, 1H,
J = 7.01 Hz); 7.24–7.34 (m, 6H); 7.10 (s, 1H); 5.12 (s,
2H); 4.88–4.99 (m, 1H); 4.38–4.42 (m, 1H); 3.95 (s, 3H);
3.89 (s, 3H); 3.85 (s, 3H); 3.12–3.22 (m, 1H); 2.93–2.97 (m,
1H); 2.21–2.31 (m, 1H); 2.12–2.19 (m, 1H); FABMS: m/z
1
520 [M++H]; compound 6a: H NMR (400 MHz, CDCl3):

A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1468–1473
1473
d 7.58 (d, 1H, J = 2.34 Hz); 7.48 (s, 1H); 6.75 (s, 1H); 5.07–
5.26 (m, 1H); 4.80–4.89 (m, 2H); 4.30–4.37 (m, 2H); 3.96
(s, 3H); 3.95 (s, 6H); 3.93 (s, 3H); 3.92 (s, 3H); 3.73–3.75
(m, 1H); 2.66–2.92 (m, 2H); 2.44–2.53 (m, 4H); FABMS:
m/z 655 [M++H]; compound 6b: 1H NMR (200 MHz,
CDCl3): d 7.58 (d, 1H, J = 3.64 Hz); 7.49 (s, 1H); 6.78 (s,
1H); 5.31–38 (m, 1H); 5.11–5.24 (m, 1H); 4.70 (m, 2H);
4.28–4.34 (m, 2H); 3.95 (s, 15H);3.11–3.21 (m, 2H); 2.06–
2.19 (m, 2H); 1.79–1.85 (m, 4H); FABMS: m/z 669
[M++H]; compound 6c: 1H NMR (300 MHz, CDCl3): d
7.60 (d, 1H, J = 4.72 Hz); 7.56 (s, 1H); 6.77 (s, 1H); 5.22
(m, 1H); 5.16–5.26 (m, 1H); 4.71 (m, 2H); 4.31–4.37 (m,
2H); 3.96 (s, 15H); 3.15–3.21 (m, 2H); 2.16–2.21 (m, 2H);
2.02–2.12 (m, 4H); 1.76–1.92 (m, 4H); FABMS: m/z 683
[M++H].
15. Kamal, A.; Ramulu, P.; Srinivas, O.; Ramesh, G. Bioorg.
Med. Chem. Lett. 2003, 13, 3955.
16. Vichai, V.; Kirtikara, K. Nat. Protoc. 2006, 1, 1112.
17. Skehn, P.; Storeng, R.; Scudiero, A.; Monks, J.; McMo-
han, D.; Vistica, D.; Jonathan, T. W.; Bokesch, H.;
Kenney, S.; Boyd, M. R. J. Natl. Cancer Inst. 1990, 82,
1107.