Solid-phase synthesis of new pyrrolobenzodiazepine-chalcone conjugates: DNA-binding affinity and anticancer activity

Article abstract of DOI:10.1016/j.bmcl.2008.02.047

A new class of C8-linked pyrrolo[2,1-c][1,4]benzodiazepine-chalcone conjugates have been prepared by employing a solid-phase synthetic protocol. In this strategy an intramolecular aza-Wittig reductive cyclization approach has been utilized. Interestingly, some of these molecules have shown enhanced DNA-binding affinity and promising anticancer activity on a large number of human cancer cell lines.

Full text of DOI:10.1016/j.bmcl.2008.02.047

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 2434–2439
Solid-phase synthesis of new pyrrolobenzodiazepine–chalcone
conjugates: DNA-binding affinity and anticancer activity
Ahmed Kamal,* N. Shankaraiah, S. Prabhakar, Ch. Ratna Reddy,
N. Markandeya, K. Laxma Reddy and V. Devaiah
Chemical Biology Laboratory, Division of Organic Chemistry, Indian Institute of Chemical Technology,
Hyderabad, Andra Pradesh 500 007, India
Received 18 January 2008; revised 15 February 2008; accepted 18 February 2008
Available online 5 March 2008
Abstract—A new class of C8-linked pyrrolo[2,1-c][1,4]benzodiazepine–chalcone conjugates have been prepared by employing a
solid-phase synthetic protocol. In this strategy an intramolecular aza-Wittig reductive cyclization approach has been utilized. Inter-
estingly, some of these molecules have shown enhanced DNA-binding affinity and promising anticancer activity on a large number
of human cancer cell lines.
ꢀ 2008 Elsevier Ltd. All rights reserved.
Solid-phase combinatorial synthesis has become an
important tool in many areas of research; including
chemical biology and drug discovery,1 as the demand
for libraries of small organic molecules continue to
grow. The development of practical synthetic strategies
for building libraries of drug-like molecules is an impor-
tant aspect for medicinal chemists to enable the cellular
functions and to explore new drug candidates.
tophenones makes them an attractive pharmacophoric
scaffold.
The pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) are a
group of potent, naturally occurring, antitumour antibi-
otics produced by various Streptomycies species.4 These
compounds bind selectively in the minor groove of
DNA, forming a covalent aminal bond between the elec-
trophilic C11-position of the PBD and the nucleophilic
N2-amino group of a guanine base,5 resulting in their
biological activity. A number of naturally occurring syn-
thetic compounds based on this PBD ring system, such
as anthramycin, chicamycin, abbeymycin, DC-81 (2)
and its dimers,6 have shown varying degrees of DNA-
binding affinity and anticancer activity. Recently, we
have investigated mixed imine-amide PBD dimers7 as
efficient DNA-binding agents with significant anticancer
activity in a number of human cancer cell lines and also
developed new hybrids of PBD monomers.8 A number
of PBD conjugates have been designed and synthesized
in this laboratory, that have exhibited significant cyto-
toxic property with increased DNA-binding ability.9
Further, a variety of PBD based molecules have been
synthesized by employing aza-Wittig reductive cycliza-
tion strategy in solution-phase10 as well as on solid-
phase.11 In continuation of these efforts towards the syn-
thesis of structurally modified PBD hybrids and also
development of new solid-phase synthetic strategies,12
we herein report the preparation of a new class of C8-
linked PBD–chalcone conjugates (Fig. 1). Moreover,
During the course of our efforts directed towards the
development of new anticancer agents as well as in the
combinatorial diversification, we were interested in the
development of solid-phase synthetic strategy for some
new molecules based on PBD linked-chalcone conju-
gates as potential anticancer agents. Chalcones have re-
ceived significant attention for their antitumour
properties over the last few years, particularly in view
of their similar mode of action to the structurally related
combretastatin (1).2 The anticancer activity of certain
chalcones, known to result in binding to tubulin and
prevent it from polymerizing microtubules,3 is the target
for a number of clinical useful anticancer compounds
including natural products like paclitaxel, vincristine
and colchicine. Furthermore, the ease of preparation
of chalcones from substituted benzaldehydes and ace-
Keywords: Pyrrolo[2,1-c][1,4]benzodiazepines; Chalcones; DNA-bind-
ing affinity; Anticancer activity; Aza-Wittig reductive cyclization.
*
Corresponding author. Tel.: +91 40 27193157; fax: +91 40
27193189; e-mail: ahmedkamal@iict.res.in
0960-894X/$ - see front matter ꢀ 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.02.047

A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2434–2439
2435
for larger scale reactions. Typically 12 equivalents of
both aldehyde 8 (vanillin) and NaOMe (0.5 M in
MeOH) was added to resin 7 (preswelled in an equal vol-
ume of THF) to afford the required resin-bound enone
(9) as shown in Scheme 1. This reaction was monitored
by FT-IR, which shows a strong peak of the carbonyl
10
MeO
MeO
9
A
6
11
N
HO
H
8
B11a
1
C
N
4
7
H3CO
5
2
OMe
1
OH
3
O
2
OMe
absorption at 1747 cmÀ1
.
N
OH
O
O
H
n
The starting materials 12a–c were prepared from methyl
2-azido-4-hydroxy-5-methoxybenzoate (10) which was
accomplished by employing the reported method.14
Etherification of these compounds was performed by
using a variety of dibromoalkanes to provide the bro-
mo-substituted azidoesters (11a–c). These upon hydroly-
sis in the presence of 2 N LiOH followed by coupling
with proline methylester by employing Et3N afford
12a–c in good yield ranging from 78% to 83% as shown
in Scheme 2.
N
N
OCH3 H3CO
O
O
3a; n = 1
3b; n = 2
3c; n = 3
O
H
H
N
OH
O
O
O
n
OCH3 H3CO
O
4a; n = 1
4b; n = 2
4c; n = 3
The PBD–chalcone conjugates (3a–c and 4a–c) were
prepared by employing the resin-bound compound 9
and bromo-substituted azidobenzoyl proline methylest-
ers (12a–c), which were then coupled in the presence
of K2CO3 to afford the corresponding solid-supported
compounds 13a–c as indicated by IR spectra that shows
a strong azide stretching at 2110 cmÀ1. These resin-
bound ester functionalities (13a–c) were reduced selec-
tively with DIBAL-H followed by aza-Wittig reductive
cyclization (Staudinger reaction) with triphenyl phos-
phine (TPP) to produce 14a–c. This step is also con-
firmed by IR analysis wherein the azido peak is
absent. Finally, the resin-bound compounds 14a–c were
cleaved by employing TFA/CH2Cl2 (1:1) to afford the
crude products. This procedure was further repeated
once again to ensure the complete cleavage of the prod-
uct from the resin, which was later purified by column
chromatography using (silica gel, 100–200 mesh) ethyl
acetate/methanol (9:1) to give 3a–c15 (yields 65–78%).
The resin-bound chalcone–PBD dilactams (4a–c) have
also been prepared by employing a similar procedure.
The reduction of resin-bound azido group 15a–c was
accomplished by employing TPP and followed by cleav-
Figure 1. Chemical structures of biologically important combretasta-
tin (1), DC-81 (2) and PBD–chalcone conjugates (3a–c and 4a–c).
these molecules have been evaluated for their DNA-
binding ability and a representative compound has been
investigated for its potential as an anticancer agent.13
The synthetic pathway started from commercially avail-
able 2-hydroxyacetophenone (5), which was coupled to
commercially available Merrifield resin (6, 2.00 mmol/
g) with NaH to give resin-bound 2-hydroxyacetophe-
none (7) in excellent yield. This reaction was confirmed
by IR spectrum, which showed the keto group absorp-
tion at 1739 cmÀ1. Further, in attempts to condense
the resin-bound 2-hydroxyacetophenone 7 with alde-
hyde, NaOMe (as a 0.5 M solution in MeOH) was found
to be the base of choice, the co-solvent with suitable re-
sin swelling properties is necessary to ensure complete
conversion to the enone product. Toluene as well as
THF gave satisfactory results for this reaction; however,
THF is better with respect to consistency of the results
OH
Cl
OHC
OCH3
O
O
OH
O
O
OH
CH3
8
6
ii
i
CH3
OCH3
O
5
7
9
Scheme 1. Reagents and conditions: (i) NaH, dry DMF, 50–60 ꢁC, 48 h; (ii) NaOMe/THF/MeOH, rt, 4 days.
N3
HO
N3
O
Br
O
N3
O
Br
O
COOCH3
ii, iii
i
n
n
OCH3
OCH3
N
H3CO
H3CO
H3CO
O
11a-c; n = 1-3
12a-c; n = 1-3
10
Scheme 2. Reagents and conditions: (i) K2CO3, dibromoalkanes, DMF, rt, 12 h; (ii) LiOH (2 N), THF/MeOH/H2O (3:1:1), rt, 2 h; (iii) (a) (COCl)2,
dry CH2Cl2, 1–3 drops DMF, 0 ꢁC–rt, 2 h; (b) Et3N, L-proline methylester hydrochloride, THF, 0 ꢁC–rt, 2 h.

2436
A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2434–2439
O
O
OH
N3
COOCH3
Br
O
n
+
N
H3CO
12a-c; n = 1-3
OCH3
O
9
i
N3
O
O
O
COOCH3
n
N
OCH3 H3CO
O
O
13a-c; n = 1-3
ii, iii
iii
N
O
O
O
O
H
n
N
OCH3 H3CO
O
14a-c; n = 1-3
O
H
H
N
O
O
O
O
n
iv
N
OCH3 H3CO
O
15a-c; n = 1-3
N
OH
O
O
H
n
N
OCH3 H3CO
iv
O
O
3a; n = 1
3b; n = 2
3c; n = 3
O
H
N
OH
O
O
O
H
n
N
OCH3 H3CO
O
4a; n = 1
4b; n = 2
4c; n = 3
Scheme 3. Reagents and conditions: (i) K2CO3, dry DMF, 50 ꢁC, 48 h; (ii) DIBAL-H, dry CH2Cl2, À78 ꢁC, 2 h; (iii) TPP, dry toluene, rt, 16 h; (iv)
TFA/CH2Cl2 (1:1), rt, 2 h, 65–78%.
age with TFA/CH2Cl2 (1:1) to give 4a–c15 as shown in
Scheme 3.
Compound 3b elevates the helix melting temperature
of CT-DNA by 1.7 ꢁC at 0 h; however, there is not much
difference in the melting temperatures after incubation
for 18 h (2.1 ꢁC). Whereas for compound 3c the DTm
is 3.1 ꢁC at 0 h while the helix melting temperature in-
creases to 5.0 ꢁC after incubation for 18 h at 37 ꢁC. In
the same experiment the naturally occurring DC-81 ele-
vates the helix melting temperature of CT-DNA by
0.7 ꢁC after incubation for 18 h. It is interesting to ob-
serve that for both the PBD conjugates 3a and 3c the
DTm values are significant when the length of alkyl
spacer is odd numbered, probably as they have a better
fit in the minor groove of DNA.
The DNA-binding ability of these C8-linked chalcone–
PBD conjugates was examined by thermal denaturation
studies using calf thymus (CT) DNA. These studies
show the melting stabilization (DTm) for the CT-DNA
duplex at pH 7.0, incubated at 37 ꢁC, where PBD/
DNA molar ratio is 1:5. In this assay, the helix melting
temperature changes (DTm) for each compound were
studied after 0 and 18 h of incubation at 37 ꢁC. Interest-
ingly, in this study PBD–chalcone conjugates com-
pounds 3a–c have shown melting temperature
increases from 1.7 to 8.1 ꢁC (Table 1). The DTm of com-
pound 3a is 5.0 ꢁC at 0 h, while the melting temperature
increases to 8.1 ꢁC upon incubation for 18 h at 37 ꢁC.
Similarly, non-covalent DNA-interactive chalcone–
PBD dilactam conjugates were evaluated for their

A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2434–2439
2437
Table 2. In vitro anticancer activity of compound 3a on 60 human
cancer cell lines
Table 1. Thermal denaturation data for C8-linked PBD–chalcone
conjugates (3a–c and 4a–c) with calf thymus (CT) DNA
b
Cancer panel/cell
line
GI50 (lM) Cancer
panel/cell line
GI50 (lM)
Compound
[PBD]/[DNA]
molar ratioa
DTm (ꢁC) after
incubation at
37 ꢁC for
Leukaemia
CCRF-CEM
HL-60(TB)
K-562
Ovarian
0.021
0.017
0.026
0.017
0.025
0.012
IGROV1
OVCAR-3
OVCAR-4
OVCAR-5
OVCAR-8
SK-OV-3
0.018
0.021
0.061
0.025
0.036
0.406
0 h
18 h
3a
3b
1:5
1:5
1:5
1:5
1:5
1:5
1:5
5.0
1.7
3.1
2.0
1.0
7.1
0.3
8.1
2.1
5.0
3.5
1.9
9.0
0.7
MOLT-4
RPMI-8226
SR
3c
4a
4b
4c
Non-small cell lung
A549/ATCC
EKVX
Renal
DC-81
0.031
0.204
0.039
0.044
0.138
0.039
0.307
0.034
0.026
786-0
0.025
0.047
0.025
0.02
ACHN
CAKI-1
RXF 393
SN12C
TK-10
UO-31
a For CT-DNA alone at pH 7.00 0.01, Tm = 69.6 ꢁC 0.01 (mean
value from 10 separate determinations), all Tm values are 0.05–
0.1 ꢁC.
HOP-62
HOP-92
NCI-H226
NCI-H23
0.028
0.041
0.105
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].
NCI-H322M
NCI-H460
NCI-H522
Prostate
DU-145
0.025
DNA-binding affinity by thermal denaturation studies
using a similar procedure. These conjugates 4a–c also
exhibited significant helix melting temperature values
ranging from 1.0 to 9.0 ꢁC. Compounds 4a and 4b have
shown noticeable DNA-binding affinity of 2.0 and
1.0 ꢁC with calf thymus CT-DNA and there is not
much difference in their helix melting temperatures
upon incubation at 37 ꢁC for 18 h (3.5 and 1.9 ꢁC).
However, DTm of compound 4c is 7.1 ꢁC at 0 h while
the melting temperature increases to 9.0 ꢁC upon incu-
bation for 18 h (Table 1). It is interesting to observe
that compound 4c showed enhanced DNA-binding
ability compared to chalcone–PBD imine conjugates
(3a–c).
Colon
Breast
MCF7
COLO 205
HCC-2998
HCT-116
HCT-15
HT29
0.027
0.138
0.033
0.247
0.030
0.018
0.019
0.032
NCI/ADR-RES 0.308
231/ATCC
HS 578T
0.026
0.023
0.019
0.017
0.029
0.016
MDA-MB-435
BT-549
T-47D
KM12
SW-620
MDA-MB-468
CNS
SF-268
SF-539
SNB-19
SNB-75
U251
0.024
0.028
0.029
0.036
0.022
Compound 3a as a representative member of this class
that exhibited significant DNA-binding ability has been
evaluated for its in vitro activity against the standard
sixty human tumour cell lines, derived from nine can-
cer types (leukaemia, non-small cell lung, colon,
CNS, melanoma, ovarian, renal, prostate and breast
cancer). This compound shows good anticancer po-
tency in a wide spectrum of cell lines causing 50% cell
growth inhibition (GI50), and the values range from
0.01 to 0.40 lM as seen from Table 2. Moreover, it
exhibits less than 30 nM potency as seen from the
GI50 values in almost 32 cancer cell lines. Further the
cytotoxic potency (LC50) is significant in some selected
human cancer cell lines ranging from 0.07 to 0.98 lM.
The in vitro cytotoxicity (LC50) for the naturally occur-
ring DC-81 is 0.38 and 0.33 lM in L1210 and PC6 can-
cer cell lines, respectively.16 It is interesting to note that
the mean graph midpoint values of compound 3a are
log10 GI50 (À7.43), log10 TGI (À6.51) and log10 LC50
(À5.18).
Melanoma
LOX IMVI
MALME-3M
M14
0.018
0.037
0.046
0.067
0.020
0.024
0.112
0.021
SK-MEL-2
SK-MEL-28
SK-MEL-5
UACC-257
UACC-62
aza-Wittig reductive cyclization process have been uti-
lized. These chalcone–PBD conjugates have shown
noticeable DNA-binding affinity and potential antican-
cer activity in a number of human cancer cell lines for
a representative compound of this class.
Acknowledgments
In conclusion, the synthesis of biologically important
DNA-interactive C8-linked pyrrolo[2,1-c][1,4]benzodi-
azepine–chalcone conjugates has been demonstrated
for the first time. In this process solid-supported syn-
thetic method, aldol condensation and intramolecular
We thank National Cancer Institute (NCI), Maryland,
for providing data on anticancer assay in human cancer
cell lines and the authors N.S., S.P.R., Ch.R.R.,
N.M.K., K.L.R. and V.D. are grateful to CSIR, New
Delhi, for the award of Research fellowships.

2438
A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2434–2439
same temperature for 48 h. Further, the reaction mixture
References and notes
was cooled to room temperature, quenched with ice-cold
water, and then filtered, washed with DMF (3· 10 mL),
H2O (3· 10 mL), CH3OH (3· 10 mL), CH2Cl2 (3· 10 mL)
and then dried under vacuum to give resin-bound 2-
hydroxyacetophenone (7). Later a solution of NaOMe in
1. (a) Ley, S. V.; Baxendale, I. R. Nat. Rev. Drug. Disc. 2002,
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MeOH (31.30 mL of
a 0.5 M solution in MeOH,
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D. S.; Garciakendall, D. Experientia 1989, 45, 209.
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15.6 mmol) was added to a mixture of resin-bound 2-
hydroxyacetophenone (7, 2.34 g) and 4-hydroxy-3-
methoxybenzaldehyde (8, 2.63 g, 17.38 mmol) in anhy-
drous THF (30 mL). Then the flask was capped and
placed on an orbital shaker for 4 days. The reaction
mixture was filtered and washed sequentially with the
following solvents THF (3· 40 mL), MeOH (3· 40 mL),
THF (3· 40 mL), MeOH (3· 40 mL) and finally washed
with THF (3· 40 mL). After which the resin was dried on
a Buchner funnel to give 9. Later to a suspension of resin
(9, 0.886 g, 1.7 mmol/g) in dry DMF (15 mL), K2CO3
(1.17 g, 8.5 mmol) and 12a (1.49 g, 3.4 mmol) were added
and the reaction mixture was stirred at 50 ꢁC for 48 h to
afford 13a. Then the reaction mixture was brought to
room temperature and filtered, washed sequentially with
the following solvents DMF (3· 15 mL), DMF/water (8:2,
3· 15 mL), DMF (3· 15 mL), THF (3· 15 mL) and finally
washed with CH2Cl2 (3· 15 mL) then dried. Further, to a
suspension of resin (13a, 0.440 g, 0.80 mmol) in CH2Cl2
(10 mL) was added DIBAL-H (2.28 mL of a 1 M solution
in hexane, 2.28 mmol) drop wise at À78 ꢁC under nitrogen
atmosphere, and the reaction mixture was stirred at the
same temperature for 2 h. Then the reaction was quenched
by the addition of 10 mL of 0.5% HCl, filtered off and
rinsed with hexane, water, THF, CH2Cl2 and dried in
vacuo for 2 h. The derivatized resin (0.382 g, 0.57 mmol)
was taken in round bottom flask, dry toluene (20 mL) and
TPP (0.760 g, 2.85 mmol) was added and allowed to stir
for 5 h at room temperature to afford reductive cyclized
resin-bound product 14a. This resin was filtered and
washed with toluene (2· 25 mL), CH2Cl2 (2· 25 mL),
ether (2· 25 mL) and then dried. Finally, the resin (14a,
0.345 g) was cleaved by employing TFA/CH2Cl2 (1:1,
10 mL) to afford the crude product 3a. This procedure was
repeated once again to ensure the complete cleavage of the
product from the resin. The filtrate was saturated with
aqueous NaHCO3 solution, and then extracted with ethyl
acetate; the organic layer was separated, dried over
Na2SO4 and evaporated in vacuo to afford the crude
product. This upon purification by column chromatogra-
phy using (silica gel, 100–200 mesh) ethyl acetate/metha-
7. Kamal, A.; Ramesh, G.; Laxman, N.; Ramulu, P.;
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Med. Chem. Lett. 2008, 18, 1468; (c) Kamal, A.; Shanka-
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1
nol (9:1) as eluent gave compound (3a, 25 mg, 68%). H
NMR (400 MHz, CDCl3): d 12.94 (s, 1H); 7.92–7.95 (d,
1H, J = 7.79 Hz); 7.65–7.67 (t, 1H, J = 3.12, 3.90 Hz);
7.47–7.54 (m, 2H); 7.16 (s, 1H); 6.93–7.07 (m, 4H); 6.86 (s,
1H); 5.39–5.44 (dd, 1H, J = 3.12, 10.13 Hz); 4.20–4.34 (m,
4H); 3.93 (s, 3H); 3.88 (s, 3H); 3.53–3.84 (m, 2H); 2.28–
2.45 (m, 4H); 2.02–2.08 (m, 2H); 1.71 (m, 2H); 13C NMR
(50 MHz, CDCl3): d 14.0; 22.5; 24.1; 29.3; 31.5; 44.5; 46.6;
53.6; 56.0; 65.3; 79.4; 112.8; 117.8; 118.5; 121.5; 123.3;
126.9; 127.7; 129.5; 131.5; 136.1; 140.7; 145.6; 147.8; 149.7;
150.7; 151.2; 161.5; 162.3; 164.5; 192.0; 193.5; FABMS:
m/z 557 [M++H]; HRMS calcd for C32H32N2O7 556.3768,
found 556.3772.
12. (a) Kamal, A.; Laxman, N.; Ramesh, G.; Neelima, K.;
Kondapi, A. K. Chem. Commun. 2001, 437; (b) Kamal, A.;
Reddy, K. L.; Devaiah, V.; Shankaraiah, N. Synlett 2004,
2533; (c) Kamal, A.; Devaiah, V.; Reddy, K. L.; Shanka-
raiah, N. Adv. Synth. Catal. 2006, 348, 249.
13. Kamal, A.; Prasad, B. R.; Reddy, A. M. USA and PCT
Patent Appl. filed.
1
Compound 3b: H NMR (400 MHz, CDCl3): d 12.94 (s,
1H); 7.91–7.96 (m, 3H); 7.66–7.67 (d, 1H, J = 4.30 Hz);
7.47–7.54 (m, 2H); 6.91–7.07 (m, 5H); 6.82 (s, 1H); 5.40–
5.44 (dd, 1H, J = 2.15, 10.77 Hz); 4.09–4.22 (m, 4H); 3.93
(s, 3H); 3.89 (s, 3H); 3.55–3.85 (m, 2H); 2.30–2.36 (m,
2H); 2.01–2.12 (m, 6H); FABMS: m/z 571 [M++H].
HRMS calcd for C33H34N2O7 570.4033, found
570.4028.
14. Kamal, A.; Shankaraiah, N.; Devaiah, V.; Reddy, K. L.
Tetrahedron Lett. 2006, 47, 9025.
15. Typical procedure for compound 3a: To a suspension of
Merrifield resin (6) (2000 mg, 2.00 mmol/g, 1% cross-
linked) in DMF (30 mL), 2-hydroxyacetophenone (5,
554 mg, 4.0 mmol) was added followed by NaH (276 mg,
6.0 mmol) at 0 ꢁC, then heated to 50 ꢁC and stirred at the

A. Kamal et al. / Bioorg. Med. Chem. Lett. 18 (2008) 2434–2439
2439
1
1
Compound 3c: H NMR (400 MHz, CDCl3): d 12.81 (s,
Compound 4b: H NMR (400 MHz, CDCl3): d 12.82 (s,
1H); 8.88 (br s, 1H); 7.78–7.92 (m, 3H); 7.38–7.56 (m,
2H); 7.19–7.25 (m, 1H); 6.88–7.10 (m, 4H); 6.56 (s, 1H);
5.31–5.35 (dd, 1H, J = 3.12, 10.26 Hz); 4.11–4.22 (m,
4H); 3.96 (s, 3H); 3.89 (s, 3H); 3.51–3.80 (m, 2H); 2.21–
2.36 (m, 2H); 1.84–2.12 (m, 6H); FABMS: m/z 587
[M++H]; HRMS calcd for C33H34N2O8 586.4027, found
586.4039.
1H); 7.81–7.90 (m, 3H); 7.60–7.61 (d, 1H, J = 3.90 Hz);
7.42–7.49 (m, 2H); 6.83–7.04 (m, 5H); 6.72 (s, 1H); 5.35–
5.39 (dd, 1H, J = 2.34, 10.13 Hz); 4.02–4.12 (m, 4H); 3.92
(s, 3H); 3.91 (s, 3H); 3.52–3.85 (m, 2H); 2.27–2.34 (m,
2H); 1.89–2.08 (m, 6H); 1.66–1.75 (m, 2H); FABMS: m/z
585 [M++H]; HRMS calcd for C34H36N2O7 584.4293,
found 584.4298.
Compound 4a was prepared according to the procedure
described in the earlier preparation of 3a. The resin-bound
dilactam (15a, 0.325 g) was added by employing TFA/
CH2Cl2 (1:1, 10 mL) to afford the product (4a, 20 mg, 75%).
1H NMR (400 MHz, CDCl3): d 12.79 (s, 1H); 8.74 (br s,
1H); 7.77–7.90 (m, 3H); 7.36–7.50 (m, 2H); 7.15–7.19 (m,
1H); 6.85–7.04 (m, 4H); 6.54 (s, 1H); 5.33–5.38 (dd, 1H,
J = 3.02, 10.57 Hz); 4.10–4.20 (m, 4H); 3.92 (s, 3H); 3.87 (s,
3H); 3.50–3.75 (m, 2H); 2.20–2.33 (m, 2H); 1.85–2.02 (m,
4H); FABMS: m/z 573 [M++H]; HRMS calcd for
C32H32N2O8 572.3762, found 572.3768.
Compound 4c: 1H NMR (400 MHz, CDCl3): d 12.98
(s, 1H); 8.91 (br s, 1H); 7.85–7.98 (m, 2H); 7.41–7.59
(m, 3H); 6.91–7.12 (m, 5H); 6.42 (s, 1H); 5.40–5.45 (dd,
1H, J = 3.11, 10.68 Hz); 4.09–4.19 (m, 4H); 3.96 (s,
3H); 3.88 (s, 3H); 3.55–3.79 (m, 2H); 2.21–2.25 (m,
4H); 1.95–2.12 (m, 6H); 1.65–1.75 (m, 2H); FABMS:
m/z 601 [M++H]. HRMS calcd for C34H36N2O8
600.4292, found 600.4298.
16. Bose, D. S.; Thompson, A. S.; Smellie, M.; Berardini, M.
D.; Hartley, J. A.; Jenkins, T. C.; Neidle, S.; Thurston, D.
E. J. Chem. Soc. Chem. Commun. 1992, 9, 1518.