AlCl3-NaI assisted cleavage of polymer-bound esters with concomitant amine coupling and azido-reductive cyclization: Synthesis of pyrrolobenzodiazepine derivatives

Article abstract of DOI:10.1016/j.tetlet.2013.06.033

A practical one-pot solid-phase parallel diversity-oriented synthesis (DOS) strategy has been successfully applied for the construction of scaffolds embedded with privileged pyrrolo [2,1-c][1,4]benzodiazepines (PBDs) and their dilactams. The synthetic approach involves AlCl₃-NaI assisted cleavage of resin-bound ester, with amine coupling and tandem azido-reductive cyclization as the key step. The maximum skeletal diversity has been expanded through the introduction of different linkers at A-C8-position of PBD with various substituents from a single key intermediate. Gratifyingly, the final compounds have been purified by the solid-supported liquid-liquid extraction (SLE) technique. Interestingly, some of these molecules have shown enhanced DNA-binding affinity in comparison with naturally occurring DC-81. This novel approach is acquiescent for the construction of new pharmaceutical drug candidates for the evaluation of biological profile in drug discovery.

Full text of DOI:10.1016/j.tetlet.2013.06.033

Tetrahedron Letters 54 (2013) 4435–4441
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
AlCl3–NaI assisted cleavage of polymer-bound esters with
concomitant amine coupling and azido-reductive cyclization:
synthesis of pyrrolobenzodiazepine derivatives
a
b
a
a
Ahmed Kamal a, , S. Prabhakar , Nagula Shankaraiah , Nagula Markandeya , P. Venkat Reddy ,
⇑
Vunnam Srinivasulu a, Manda Sathish a
a Medicinal Chemistry & Pharmacology, CSIR–Indian Institute of Chemical Technology, Hyderabad 500007, India
b Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), Hyderabad 500037, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 February 2013
Revised 7 June 2013
Accepted 10 June 2013
Available online 15 June 2013
A practical one-pot solid-phase parallel diversity-oriented synthesis (DOS) strategy has been successfully
applied for the construction of scaffolds embedded with privileged pyrrolo [2,1-c][1,4]benzodiazepines
(PBDs) and their dilactams. The synthetic approach involves AlCl3–NaI assisted cleavage of resin-bound
ester, with amine coupling and tandem azido-reductive cyclization as the key step. The maximum skel-
etal diversity has been expanded through the introduction of different linkers at A-C8-position of PBD
with various substituents from a single key intermediate. Gratifyingly, the final compounds have been
purified by the solid-supported liquid–liquid extraction (SLE) technique. Interestingly, some of these
molecules have shown enhanced DNA-binding affinity in comparison with naturally occurring DC-81.
This novel approach is acquiescent for the construction of new pharmaceutical drug candidates for the
evaluation of biological profile in drug discovery.
Keywords:
Pyrrolobenzodiazepines
Solid-phase synthesis
Combinatorial chemistry
AlCl3–NaI
Ó 2013 Elsevier Ltd. All rights reserved.
Azido-reductive cyclization
SLE method
10
N
OH
Combinatorial chemistry plays a key role in drug discovery for
the lead identification of new chemical entities (NCEs) containing
heterocyclic scaffold, which continues to be an essential compo-
nent in exploring the biological activity.1 The solid-phase organic
synthesis (SPOS) has emerged as a powerful tool in the production
of small molecules intending to exploit combinatorial chemistry
towards the potential drug candidates.2 The core components of
chemical biology are the identification of novel small-molecule
modulators as perturbing agents in biological systems and the
application of these small bioactive molecules to pinpoint on the
development of specific DNA-interactive agents.3 In this scenario,
there is a great demand for drug-like small molecules with wider
biological applications in identifying specific small molecule mod-
ulators. To address this issue, the organic chemistry community
has investigated diversity-oriented synthesis (DOS) as a new strat-
egy to efficiently populate the chemical space with skeletally di-
verse molecules through complexity generating reactions.4
9
OCH3
H
11
H
N
HO
8
7
H
C
H3CO
11a
1
A
6
B
N
4
H3CO
2
N
5
O
DC-81
CONH2
3
O
anthramycin
O
O
H
N
O
R1
O
R1
O
N
H
H
N
R2
N
n
n
R2
N
N
H3CO
H3CO
n = 2,3
n = 2,3
X
O
O
1a p
2a−g
−
X = H, OMs, OTs
Figure 1. A library of pyrrolobenzodiazepine derivatives (1a–p and 2a–g) and their
natural products DC-81 and anthramycin.
low molecular weight antitumor antibiotics. A good example is
the PBD ring system, which represents a currently well-known
privileged structural motif observed in various natural products,
such as DC-81 and anthramycin (Fig. 1), derived from various
Streptomyces species.5,6 The mechanism of action of these PBDs
has been extensively investigated and their interaction with
duplex DNA has been rationalized.7 PBD-5,11-diones have also
been employed as interesting intermediates in the synthesis of
naturally occurring and synthetically modified PBD imines, such
Of particular importance for the lead identification is the
incorporation of novel heterocyclic cores into library design and
production. Recently, there has been an increasing interest in the
synthesis of DNA sequence selective binding agents, particularly
⇑
Corresponding author. Tel.: +91 40 27193157; fax: +91 40 27193189.
E-mail address: ahmedkamal@iict.res.in (A. Kamal).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.033

4436
A. Kamal et al. / Tetrahedron Letters 54 (2013) 4435–4441
Table 1
Synthesis of a library of substituted pyrrolo [2,1-c][1,4]benzodiazepine-5,11-diones (1a–p)
Entry
Product
Yieldsa (%)
56, 25b
HRMS (calcd)
518.2267
HRMS (found)
518.2257
H
N
O
H
N
O
H
1a
O
N
H3CO
H3CO
H3CO
O
H
N
O
H
N
O
H
1b
1c
60
50
534.2216
534.2179
534.2182
534.2185
OCH3
OCH3
O
O
N
H3CO
O
H
N
O
H
N
O
H
H3CO
N
H3CO
O
CH3
N
H
N
O
H
O
MeO
1d
1e
1f
50
54
52
518.2267
456.2110
474.2004
518.2263
456.2101
474.1980
O
N
H3CO
O
O
H
O
N
H3CO
O
H
N
H
N
H3CO
O
O
H
N
O
O
H
N
H
N
H3CO
O
H3CO
O
H
O
H
N
O
N
H
1g
60
504.2110
504.2104
N
H3CO
O
O
H
N
O
O
H3CO
H
N
1h
1i
50
62
454.2210
548.2372
454.2202
548.2364
N
O
O
H
N
O
O
H
N
H
N
H3CO
OCH3
H3CO
O
O
H
N
O
O
H
N
H
OCH3
1j
58
55
718.2410
548.2372
718.2390
548.2376
N
H3CO
H3CO
H3CO
OTs
O
O
H
O
N
O
H
N
H
1k
N
H3CO
OCH3
O

A. Kamal et al. / Tetrahedron Letters 54 (2013) 4435–4441
4437
Table 1 (continued)
Entry
Product
Yieldsa (%)
HRMS (calcd)
642.2097
HRMS (found)
O
H
N
O
O
H
N
H
1l
62
642.2080
N
H3CO
H3C
OCH3
H3CO
OMs
O
H
N
O
H
N
O
H
1m
1n
1o
53
57
54
60
488.2155
494.1453
496.1654
505.1693
488.2153
494.1451
496.1658
505.1691
O
N
MeO
CH3
O
H
O
H
N
N
O
H
O
N
Cl
MeO
O
H
O
H
H
N
N
O
O
N
F
MeO
F
O
H
N
O
H
N
O
H
1p
O
N
O2N
MeO
O
a
Yields were obtained based on preparative thin layer chromatography.
AlCl3 alone used as reagent.
b
as tomaymycin and chicamycin.8 Moreover, this tricyclic ring
system has been used for a number of pharmaceutical applica-
tions.9–12 Our research group has been extensively involved in
the development of new solid-phase13 and solution-phase14 syn-
thetic strategies, and thus includes the library generation tech-
niques. Earlier, we have developed the solid-phase combinatorial
approaches for the construction of privileged pyrrolo [2,1-
c][1,4]benzodiazepines (PBDs) and imidazo-pyridines.15 In contin-
uation of the efforts, we report herein the use of DOS strategy and
the solid-phase parallel synthetic approach to construct a library of
natural product-like small molecules that contain PBD with maxi-
mum skeletal diversity. In this context, we have developed and fo-
cused our attention on the generation of DNA-interactive PBDs
(1a–p and 2a–g, Fig. 1) as an attractive core structure that we
envisaged and derivatized to produce a DNA-binding targeted
compound library.
In the present investigation, we have extensively demonstrated
the AlCl3–NaI assisted cleavage of resin-bound esters with amine
coupling and tandem azido-reductive cyclization approach in a
one-pot manner. Over the past few years, the aluminium based
catalysts or reagents have been found to be more efficient in the
pharmaceutical applications. Anhydrous AlCl3 is a typical strong
Lewis acid. Recently, the aluminium containing reagents or cata-
lysts have been reported for the aromatic azido reductions in com-
bination with other metals like Fe, Ni and Zn.14d,16 In this protocol,
we have employed the AlCl3 and NaI reagent system for aromatic
azido reduction apart from resin-bound ester cleavage and amine
coupling. Similarly, Morphy and co-workers17 have examined a
variety of reagents for the conversion of esters to amides,18–22
among them A1Cl3 was selected as the best reagent of choice for
this transformation. Moreover, we have explored a direct reaction
and release cleavage concept using Wang resin as linker. Also we
employed a solid-supported liquid–liquid extraction (SLE)23 tech-
nique that is more effective in the removal of excess amine and
excess reagent (AlCl3–NaI) residues from the final products in a
high-throughput format. More precisely, our core aim is to display
the diversity at A-C8/C-C2 and C10–C11-position (imine/amide) of
the PBD ring system, by changing the chain length of resin-bound
esters with a variety of primary or secondary amines, resulting in a
generation of a library of new PBDs.
A number of methodologies have been developed for the
solution-phase synthesis of tricyclic PBD-5,11-diones employing
different types of approaches ranging from deprotective to reduc-
tive cyclizations.24 However, there are very few reports on the so-
lid-phase synthesis of these biologically important compounds.16
In the literature, not much attention has been given to PBD-5,11-
diones with diversity at the A-C8/C-C2-position.
Our synthetic strategy started from the preparation of methyl
2-azido-4-hydroxy-5-methoxybenzoate (5),26 which could be
envisaged directly from the reported method.13c Next, the Wang
resin (3) was coupled to the bromo acids 4a,b with DIC and DMAP.
This chemical transformation was confirmed by the IR stretching
vibration of resin-bound ester at 1745 cmÀ1. Etherification was
successfully performed with intermediate 5 and resin-bound
bromo ester using K2CO3 as base to provide the desired polymer-
supported precursors 6a,b. This was indicated by IR spectra that
show a strong azide stretching at 2110 cmÀ1. Hydrolysis of poly-
mer-bound aromatic methyl esters by careful addition of 1 N
NaOH,13d,25 was carried out and this was subsequently coupled
with substituted L-proline esters 7a–c in the presence of EDCI
and HOBt to provide the key resin-bound intermediates 8a–f. This
step was confirmed by IR spectra, as the new amide bond stretch-
ing peak appeared at 1635 cmÀ1. After establishment of the key
intermediates 8a–f, we attempted a Lewis acid-assisted one-pot
model experiments with AlCl3 alone and in combination with
NaI. It has been shown that the later reagent system was effective
and useful (Table 1, entry 1). This reaction was carried out at room
temperature by using CH2Cl2 as the solvent. Herein, we employed a
set of amine building blocks 9a–o for the library generation as
shown in Fig. 2.

4438
A. Kamal et al. / Tetrahedron Letters 54 (2013) 4435–4441
complex reaction products with p-amino acetoxy (9n) and p-amino
NH2
OCH3
NH2
H3CO
NH2
OCH3
acetanilide (9o) as substrates by employing AlCl3–NaI reagent
system. Next, the removal of excess amine was accomplished by
using SLE with diatomaceous earth as the support. In this respect,
we have utilized Varian’s Hydromatrix (diatomaceous earth) for
the amine extraction. The choice of aqueous buffer for extraction
depended on the physical properties of the amine to be extracted.
This extraction was found to be an effective method for the
removal of water-soluble impurities in a high throughput format.
In general, 2 N aqueous hydrochloric acid was employed for hydro-
phobic amines, and water was the preferred media for removal of
hydrophilic amines. The organic material passed through the
Hydromatrix support into a collection plate below, while the
amine salts were retained by the solid matrix, resulting in the
effective removal of the amine impurities. The removal of the
amine depends upon its C log P value.23f Further, it has been ob-
served from the SLE method that the purity of the corresponding
product depends on its solubility in respective solvent used for
the purification and also depends on their substitution pattern at
the C8/C2-position of the PBDs.
In continuation of these efforts, we turned our attention for the
construction of the core DNA-interactive PBD-imines by changing
the functionality at C10–C11-position of the central ring system.
The resin-bound 2-azido-5-methoxybenzoyl proline esters (8a–f)
were selectively reduced to their corresponding resin-bound 2-azi-
do-5-methoxybenzoyl prolinaldehyde (10a,b) with DIBAL-H13b
which was confirmed by IR spectra and a strong aldehyde stretch-
ing vibration was observed at 1744 cmÀ1 and carbonyl hydrogen
(O@C–H) stretching was also observed at 2758 cmÀ1. Similarly,
the resin-bound aldehydes 10a,b were treated with excess amount
of amines 9b–g in the presence of AlCl3–NaI to afford the desired fi-
nal products 2a–g (Scheme 1) in 52–66% yields as shown in Table 2.
Next, the DNA-binding ability of substituted PBD-5,11-diones
1a–p and their imines 2a–g was examined by thermal denaturation
H3CO
H3CO
OCH3
9c
9a
9b
NH2
NH
CH3
NH2
NH2
H3CO
OCH3
OCH3
9e
9g
9f
NH2
9d
H3C
NH2
NH
NH2
H3CO
Cl
CH3
9j
9h
9i
9k
9o
NH2
NH2
AcO
NH2
AcHN
NH2
F
O2N
F
9l
9m
9n
Figure 2. A set of amine building blocks 9a–o (R1 and R2) for the library generation.
Finally, the resin-bound 2-azido-5-methoxybenzoyl proline
esters (8a–f) were treated with AlCl3–NaI followed by the addition
of excess amines 9a–o, and thus purified by employing the SLE
technique to afford the desired C8-linked PBD-5,11-diones 1a–p
in 50–62% yields as shown in Table 1 (Scheme 1).27 We have inves-
tigated the functional group scope of this method by using a vari-
ety of substituted amino building blocks 9a–o. It was found that
this method is tolerable to many functionalities such as chloro,
fluoro, methyl, methoxy and nitro groups including some sensitive
functional groups such as mesyl and tosyl (1j and 1l, Table 1).
However, the reaction did not proceed well and obtained the
O
O
Br
n = 2; 4a
4b
HO
i)
n
n = 3;
O
N3
O
O
n
DIC, DMAP, CH2Cl2, rt, 48 h
OH
OCH3
ii) K2CO3, DMF, 50 oC, 48 h
H3CO
HO
N3
O
3
n = 2,3; 6a,b
OCH3
H3CO
OCH3
O
5
i) 1N NaOH, 1,4-dioxane
80 oC, 12 h
ii) EDCI/HOBt, CH2Cl2
HN
X
7a
X = H;
rt, 12 h
7b
7c
X = OMs;
X = OTs;
O
O
HNR1R2
O
O
H
9a o
( − )
OCH3
X
R1
O
N3
N
O
AlCl3-NaI/CH2Cl2
rt, 6 h
H
O
N
R2
n
n
amine extraction
N
N
H3CO
H3CO
(SLE method)
X
O
O
n = 2, X = H; 8a
n = 2,3; 1a−p
n = 2, X = OMs; 8b
n = 2, X = OTs; 8c
8d
n = 3, X = H;
8e
n = 3, X = OMs;
n = 3, X = OTs;
8f
DIBAL-H, CH2Cl2
−78 oC, 2 h
O
R1
O
n
9b g
( − )
O
HNR1R2
N
H
N
R2
N3
AlCl3-NaI/CH2Cl2
rt, 6 h
O
CHO
O
n
N
H3CO
amine extraction
(SLE method)
N
H3CO
O
n = 2,3; 2a−g
X
O
n = 2,3; 10a,b
X = H
Scheme 1. Solid-phase synthetic strategy for the generation of a library of substituted PBD-5,11-diones (1a–p) and their imines (2a–g) via azido-reductive cyclization
approach employing AlCl3–NaI.

A. Kamal et al. / Tetrahedron Letters 54 (2013) 4435–4441
4439
Table 2
Synthesis of a library of substituted pyrrolo [2,1-c][1,4]benzodiazepines (2a–g)
Entry
Product
Yieldsa (%)
HRMS (calcd)
496.244
HRMS (found)
H3CO
H
N
N
O
H
2a
55
496.2459
496.2460
496.2465
OCH3
OCH3
O
O
N
H3CO
O
H
N
N
O
H
H3CO
2b
2c
55
54
496.2454
496.2457
N
H3CO
O
H3CO
H3CO
H
N
N
O
H
H
O
N
N
H3CO
O
CH3
N
N
O
H3CO
2d
2e
62
55
480.2498
436.2224
480.2517
436.2236
O
H3CO
O
O
N
O
H
N
H
N
H3CO
O
H3CO
O
N
O
H
N
2f
66
52
466.2346
510.2604
466.2341
510.2590
H
N
H3CO
O
O
N
O
H
N
H
2g
N
H3CO
OCH3
H3CO
O
a
Yields were obtained based on preparative thin layer chromatography.
studies using calfthymus (CT) DNA. These studies show the melting
stabilization ( Tm) 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 ( Tm) for each compound were stud-
ied at 0 and after 18 h of incubation at 37 °C. In this study, the first
series of non-covalent DNA-interactive PBD dilactams 1b, 1e–g and
1i–o have shown helix melting temperature ranging from 0.5 to
derivative DTm values are significant by changing the alkyl chain
length, probably as they have a better fit in the minor groove of du-
plex DNA.
D
D
In conclusion, we have developed a convenient solid-phase
protocol for the synthesis of C8-linked PBD-5,11-diones and their
PBD-imines employing azido-reductive cyclization approach by
using AlCl3 in combination with NaI. These PBD-5,11-diones and
their imines have shown potential DNA-binding affinity. In this
protocol, the key step is the resin cleavage, amine coupling with
concomitant azido reductive-cyclization followed by the SLE puri-
fication technique to provide the scaffolds in a one-pot manner.
The products have been obtained in modest to good yields and pur-
ity. This strategy provides an efficient way to access PBDs that are
of pharmaceutical interest. More importantly, this type of strate-
gies enables rapid validation of synthetic sequences and building
blocks for library synthesis, which often represents the bottleneck
in the process of generating new chemical libraries.
1.8 °C (Table 3). Interestingly, the
shown significant melting temperature (
comparison with DC-81 ( Tm 0.7 °C, Table 3) for 18 h incubation
D
Tm of compound 1j and 1o have
D
Tm 1.7 °C and 1.8 °C) in
D
at 37 °C. However, some of the compounds (1a, 1c, 1d, 1h and 1p)
have not shown any elevated temperature even at 0 h and 18 h.
Similarly, the DNA interactive PBD imines 2a–g were evaluated
for their DNA-binding affinity by thermal denaturation studies.
Interestingly, the
DTm of compound 2a have shown remarkable
DNA-binding affinity at 2.9 °C at 0 h, while the melting temperature
increases to 4.2 °C up on incubation for 18 h at 37 °C. Next, the
compounds 2c and 2e were also shown significant helix melting
temperature ranging from 2.1 to 3.7 °C. In the same experiment
the naturally occurring DC-81 elevates the helix melting tempera-
ture of CT-DNA by 0.7 °C after incubation for 18 h. It is interesting
to observe that some of these PBD dilactams and their imine
Acknowledgements
The authors S.P., N.M.K., P.V.R., V.S. and M.S. are thankful to
CSIR, New Delhi, for providing the fellowships.

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A. Kamal et al. / Tetrahedron Letters 54 (2013) 4435–4441
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Compounds (PBD) [PBD]/[DNA] molar ratioa
D
Tm (°C) After incubation
b
at 37 °C for
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2008, 49, 1465–1468; (g) Kamal, A.; Devaiah, V.; Shankaraiah, N.; Reddy, K. L.
Synlett 2006, 2609–2612; (h) Kamal, A.; Devaiah, V.; Reddy, K. L.; Shankaraiah,
N. Adv. Synth. Catal. 2006, 348, 249–254; (i) Kamal, A.; Reddy, K. L.; Devaiah, V.;
Shankaraiah, N. Synlett 2004, 2533–2536.
1a
1b
1c
1d
1e
1f
1g
1h
1i
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
—
—
0.9
—
0.5
—
—
—
0.7
1.0
0.9
—
0.9
1.2
1.0
0.7
0.6
1.0
1.3
—
2.9
2.2
2.8
0.5
2.1
2.3
1.9
0.3
0.9
1.1
1.3
—
1.1
1.7
1.1
0.9
0.6
1.1
1.7
—
4.2
2.9
3.7
0.7
3.1
2.7
2.2
0.7
1j
1k
1l
1m
1n
1o
1p
2a
2b
2c
2d
2e
2f
2g
DC-81
15. (a) Kamal, A.; Reddy, K. L.; Devaiah, V.; Shankaraiah, N.; Reddy, G. S. K.;
Raghavan, S. J. Comb. Chem. 2007, 9, 29–42; (b) Kamal, A.; Devaiah, V.; Reddy, K.
L.; Rajendar.; Shetti, R. V. C. R. N. C.; Shankaraiah, N. J. Comb. Chem. 2007, 9,
267–274.
16. (a) Chen, J. G.; Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Am. Chem. Soc. 1987,
109, 1726–1729; (b) Chen, J. G.; Basu, P.; NG, L.; Yates, J. T., Jr. Surf. Sci. 1988,
194, 397–418; (c) Li, C. B.; Zheng, P. W.; Zhao, Z. X.; Zhang, W. Q.; Li, M. B.;
Yang, Q. C.; Cui, Y.; Xu, Y. L. Chin. Chem. Lett. 2003, 14, 773–775; (d) de
Carvalho, M.; Sorrilha, A. E. P. M.; Rodrigues, J. A. R. J. Braz. Chem. Soc. 1999, 10,
415–420; (e) Zheng, P.-W.; Duan, X.-M.; Wang, W.; Wang, X.-J.; Guo, Y.-P.; Tu,
Q.-D. Chin. J. Chem. 2006, 24, 825–826; (f) Kamal, A.; Reddy, K. L.; Devaiah, V.;
Reddy, G. S. K. Tetrahedron Lett. 2003, 44, 4741–4745.
a
For CT–DNA alone at pH 7.00 0.01, Tm = 69.6 °C 0.01 (mean value from 8
separate determinations), all Tm values are 0.05 – 0.15 °C.
b
For a 1:5 M ratio of [PBD]/[DNA], where CT–DNA concentration = 100 lM and
ligand concentration = 20 lM in aqueous sodium phosphate buffer [10 mM sodium
phosphate + 1 mM EDTA, pH 7.00 0.01].
c
Not shown any DNA-binding affinity.
Supplementary data
17. Barn, D. R.; Morphy, J. R.; Rees, D. C. Tetrahedron Lett. 1996, 37, 3213–3216.
18. Yazawa, H.; Tanaka, K.; Kariyone, K. Tetrahedron Lett. 1974, 15, 3995–3996.
19. Yamamoto, Y.; Yatagai, H.; Maruyama, K. J. Chem. Soc., Chem. Commun. 1980,
16, 835–836.
Supplementary data associated with this article can be found, in
theonlineversion, at http://dx.doi.org/10.1016/j.tetlet.2013.06.033.
20. Gless, R. D. Synth. Commun. 1986, 16, 633–638.
References and notes
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Srivastava, H. K.; Sastry, G. N. Tetrahedron 2010, 66, 5498–5506; (b) Thurston,
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W.; Thurston, D. E. Tetrahedron Lett. 2000, 41, 6171–6174.
26. Diazotization reaction followed by reaction with sodium azide was handled
with care for the preparation of methyl 2-azido-4-hydroxy-5-
methoxybenzoate (5) by wearing safety glasses, face mask, and gloves, and
reaction is performed in a well equipped fume hood (see detailed safety
precautions of handling organic azides and their toxicity in the supporting
information).
1. (a) Dolle, R. E.; Bourdonnec, B. L.; Worm, K.; Morales, G. A.; Thomas, C. J.; Zhang,
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High Throughput Screening 2007, 10, 735–750; (c) Kennedy, J. P.; Williams, L.;
Bridges, T. M.; Daniels, R. N.; Weaver, D.; Lindsley, C. W. J. Comb. Chem. 2008,
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Ottenheijm, H. C. J.; Rees, D. C. Tetrahedron 1998, 54, 15385–15443.
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Kook, A.; Hurley, L. H.; Barkley, M. D.; Remers, W. J. Med. Chem. 1988, 31, 583–
590; (c) Wang, J. J.; Hill, G. C.; Hurley, L. H. J. Med. Chem. 1992, 35, 2995–3002;
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27. In a typical reaction, the Wang resin (3, 2.0 g, 1.2 mmol/g, 100–200 mesh and
1% DVB) was swollen in CH2Cl2 (20 mL). 5-Bromopentanoic acid (4, 2.15 g,
12.0 mmol) and DIC (1.51 g, 12.0 mmol) were dissolved in the minimum
volume of CH2Cl2/DMF (1:1) required for complete dissolution. The activated
scaffold solution was added to the resin, followed by the addition of slurry of

A. Kamal et al. / Tetrahedron Letters 54 (2013) 4435–4441
4441
DMAP (4 mg, 10 mol %) in CH2Cl2 (0.5 mL). The reaction vessel was shaken at
room temperature for 48 h. The resin was washed with CH2Cl2 (2 Â 25 mL),
DMF (2 Â 25 mL), MeOH (2 Â 25 mL) again followed by DMF (2 Â 25 mL),
CH2Cl2 (3 Â 25 mL), and then further dried in vacuo overnight to afford the
resin-bound 5-bromopentanoic acid in good yield (2.73 g, 81%). Next, to the
resin-bound 5-bromopentanoic acid (2.70 g, 3.9 mmol), swelled in DMF
(10 mL), K2CO3 (2.13 g, 15.6 mmol) was added at ambient temperature, and
the reaction suspension was stirred for another 30 min. Later, methyl 2-azido-
4-hydroxy-5-methoxybenzoate (5, 1.73 g, 7.8 mmol) was added to the resin.
The reaction suspension was stirred at 50 °C for 48 h. The solid-support was
washed with water (3 Â 20 mL), CH2Cl2 (2 Â 20 mL), MeOH (3 Â 15 mL), and
then dried in vacuo to afford the resin-bound precursor 6a (3.07 g, 75%). To a
suspension of this resin-bound ester in 1,4-dioxane (10 mL) was added 1 N
NaOH solution (2.5 mL) and the reaction mixture was heated at 80 °C for 12 h.
On cooling, the resin was filtered and rinsed with water (2 Â 15 mL), water/
dioxane (1:9, 2 Â 15 mL), MeOH (2 Â 15 mL), CH2Cl2 (2 Â 15 mL), Et2O
(2 Â 15 mL) and dried in vacuo to afford the resin-bound acid. Next, to the
resin-bound 2-azido-4-(5-ethoxy-5-oxopentyloxy)-benzoic acid (2.81 g,
2.6 mmol) swelled in CH2Cl2 (10 mL), EDCI (0.99 g, 5.2 mmol), HOBt (0.71 g,
filtered and washed with H2O (3 Â 10 mL), CH2Cl2 (2 Â 10 mL), MeOH
(3 Â 10 mL) and Et2O (3 Â 10 mL) to afford the resin bound methyl 5-(5-
azido-4-(2-formylpyrrolidine-1-carbonyl)-2-methoxyphenoxy) pentanoate 8a
in good yield (2.89 g, 71%). To a suspension of this resin (0.110 g, 1.2 mmol) in
CH2Cl2 (5 mL), AlCl3 (0.79 g, 6 mmol), NaI (0.22 g, 2 mmol) and 2-(4-
methoxyphenyl)ethanamine (9a, 0.35 mL, 2.4 mmol) were added at room
temperature and stirred for 6 h. Aqueous 1 M potassium carbonate solution
(2 mL) was added to the reaction mixture followed by excess of NaI, quenched
with saturated sodium thiosulfate (Na2S2O3), and then resin was separated by
simple filtration and washed with CH2Cl2 (10 mL). The removal of excess
amine impurities from the final resin cleaved crude product was achieved by
solid-supported liquid–liquid extraction (SLE) with a fritted vessel previously
packed with ‘Varian’s Hydromatrix’. The crude compound 1a which contains
excess of amine was passed through the Hydromatrix support into a collection
plate below, while the amine salts were retained by the solid matrix, resulting
in the effective removal of the amine impurities. This filtrate and washings
were evaporated to dryness under reduced pressure. Finally, it was further
purified by the preparative thin layer chromatography by using ethyl
acetate:methanol (98:2) as eluent to afford the corresponding compound 1a
in high purity (brown solid, 0.013 g, 56%).
5.2 mmol) and
L-proline methyl ester (7a, 0.84 g, 6.5 mmol) were added. This
reaction mixture was stirred for 12 h at room temperature, then resin was