Step-economic and cost effective synthesis of coumarin based blue emitting fluorescent dyes

Article abstract of DOI:10.1016/j.tetlet.2014.06.071

Cost effective and green protocols for the synthesis of two new series of coumarin based blue light emitting fluorophores named as 'Beta Fluors' and 'Alpha Fluors' are described. The coumarin alkylamide based Beta Fluors are developed using a one-step multi-component process in the presence of phenyl boronic acid as an efficient green catalyst. The Alpha Fluors are structured with coumarin-triazole-carboxamide peptidomimetics and their synthesis involves the 'click with MCR' concept. The new fluorophores gave high Stoke's shift values for the emission wavelengths and their structural features are promising for further fine tuning to obtain preferred emission maxima.

Full text of DOI:10.1016/j.tetlet.2014.06.071

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Step-economic and cost effective synthesis of coumarin based blue
emitting fluorescent dyes
⇑
T. V. Soumya, P. Thasnim, D. Bahulayan
Department of Chemistry, University of Calicut, Malappuram 673635, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Cost effective and green protocols for the synthesis of two new series of coumarin based blue light emit-
ting fluorophores named as ‘Beta Fluors’ and ‘Alpha Fluors’ are described. The coumarin alkylamide based
Beta Fluors are developed using a one-step multi-component process in the presence of phenyl boronic
acid as an efficient green catalyst. The Alpha Fluors are structured with coumarin–triazole–carboxamide
peptidomimetics and their synthesis involves the ‘click with MCR’ concept. The new fluorophores gave
high Stoke’s shift values for the emission wavelengths and their structural features are promising for
further fine tuning to obtain preferred emission maxima.
Received 22 May 2014
Revised 14 June 2014
Accepted 17 June 2014
Available online xxxx
Keywords:
Coumarin
Multi-component reactions
Fluorophores
Ó 2014 Elsevier Ltd. All rights reserved.
Peptidomimetics
Click chemistry
In the context of developing reactions that use catalysts based
on inexpensive and non-toxic materials, boronic acids are attract-
ing increased attention of both material and medicinal chemists.1,2
Boronic acids are reported to be useful for catalyzing reactions
such as direct activation of alcohols and carboxylic acids,3 esterifi-
cations and amidations,4 imine hydrolysis,5 epoxide opening,6
Biginelli reaction,7a cycloadditions,7b,7c aldol condensation,8 Friedel
Crafts alkylations,9 Nazarov cyclization’s, etc.10 In spite of all these
available examples, convenient methodologies based on BAC in
terms of operational simplicity and economy for the development
of advanced functional molecules are still in demand.
Fluorogenic probes are one of such advanced functional mole-
cules capable of interacting with biological targets in vivo or
in vitro to accomplish the identification of targets or analytes.11
The interaction with targets causes changes in the spectroscopic
properties of the probes and these changes can be used for decod-
ing the information about the targets.11 Fluorogenic probes are
usually made by linking a signaling entity (which undergoes spec-
troscopic changes during interaction with target) with a labeling
moiety (which enables reaction with the target) with or without
the aid of a spacer.11,12 Several fluorogenic probes are now avail-
able based on the use of fluorochromes such as coumarin, rhoda-
mine, cyanine, naphthalene, quinoline, squaraine, xanthene, etc.11b
Among the various fluorogenic probes, the coumarin based
ones are more prominent and the commercial versions of such
fluorophores include Alexa FluorÒ dyes, AMCA, (Fig. 1) and DyLight
Fluors.13,13a,13b All these dyes are derivatives of aminomethyl-
coumarin carboxylic acids and most of them are substituted with
electron attracting and electron repelling groups at their 3 and or
7 positions.
For organic fluorophores, it is necessary that, the absorbing part
of the molecule must be structurally rigid. The structural rigidity is
usually maintained by making the fluorophoric core as a substi-
tuted fused heterocyclic system11b or placing an exocyclic func-
tional group that can impart rigidity to the whole system, for
example, the exocyclic amide functionality in AMCA-X SE (Fig. 1).
The synthesis of fused heterocycle based fluorophores requires
multistep protocols leading to the escalation of cost of production
and high market price. Similarly, the current versions of exocyclic
rigid functionality fluorophores suffer the drawbacks such as auto-
fluorescence, insufficient brightness for the emission wavelengths,
low cell permeability and are used only with highly abundant
targets.
Compared to the fused ring systems, the exocyclic rigid func-
tionality based fluorophores are relatively cheap and researchers
O
H2N
-O3S
O
O
H2N
O
O
O
O
N
OSu
O
N
H
5
O
O
Alexa Fluor(R) 350
AMCA-X SE
⇑
Corresponding author. Tel.: +91 9995538062; fax: +91 494 2400269.
E-mail address: bahulayan@yahoo.com (D. Bahulayan).
Figure 1. Representative examples of commercially available coumarin based blue
emitting fluorescent dyes.
http://dx.doi.org/10.1016/j.tetlet.2014.06.071
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Soumya, T. V.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.071

2
T. V. Soumya et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 1
are now giving lots of attention to the development of their newer
and cheaper versions. Among the various bioactive rigid function-
alities, amide group or its surrogates are highly useful as privileged
scaffolds for imparting stability, rigidity, and red shift in the
emission maxima of small molecular probes. The recent activities
of our research group are focused on the development of bioactive
linear and cyclic peptidomimetics based on the fusion of amide
groups or its surrogates on carbonyl compounds.14 As part of our
continuing interest in this area, here we report the synthesis of
two new series of blue light emitting fluorogenic probes based
on the fusion of amide and or its isosteres based peptidomimetics
on coumarin core. The representative synthesis of fluorophore 1 is
given in Scheme 1.
Results of the optimization studies for the synthesis of 1a using catalysts 1A and 1B
HO OH
HO
OH
B
B
F
F
1B
1A
Entry
Catalyst
Loading (mol %)
T °C
Time (h)
Yield (%)
1
2
3
4
5
6
7
1A
1A
1A
1A
1A
1B
1B
5
rt
rt
rt
rt
70
rt
70
4
4
4
4
4
4
4
76
78
79
94
76
95
83
10
15
20
20
20
20
As shown in Scheme 1, compound 1 is prepared by condensing
3-acetyl coumarin with an aromatic aldehyde, acetyl chloride, and
acetonitrile in the presence of 20 mol % of phenyl boronic acid at
room temperature.15 The aqueous work-up of the reaction mixture
afforded near quantitative amount of the coumarin acetamide 1a
in high purity. Optimization reactions for the synthesis of 1a were
carried out for finding out the amount of catalyst and temperature
requirements. The efficiency of phenyl boronic acid (1A) and
2,6-difluoro phenyl boronic acid (1B) for catalyzing this reaction
at room temperature and at the boiling point of acetonitrile was
studied. As given in Table 1, the room temperature reactions with
20 mol % of both the catalysts afforded maximum amount of
products and the performance of both the catalysts was found to
be almost equal. Since, 1A is cheaper than 1B, we used 1A for
further studies.
Table 2
List of Beta Fluors and their absorption/emission maxima values
O
O
O
O
Cl
1a
94%
O
HN
HN
1b
99%
Abs/Em (nm)
303/428
O
HN
Abs/Em (nm)
345/436
O
O
O
O
Cl
O
O
F
1c
O
96%
Abs/Em (nm)
1d
O
HN
91%
Abs/Em (nm)
350/426
O
305/386
O
Following this protocol, we have synthesized 10 coumarin alkyl
amides with various substitution patterns at the alkyl amide part
(Table 2). The products with an electron withdrawing group at
the acetamide phenyl ring gave better yield, compared to the prod-
ucts with electron donating groups at the same phenyl. A marginal
decrease in yield was observed in the case of 1e which was formed
in 52% yield. In this case, we have isolated a side product from the
O
O
O
OAc
O
O
O
1e
HN
52%
Abs/Em (nm)
346/426.5
1f
78 %
Abs/Em (nm)
304/431
O
HN
O
O
O
O
O
OH
O
O
reaction mixture (
a-b-unsaturated ketone, 26%) formed via an
aldol type reaction.
1g
72%
O
HN
1h
94%
The fluorophoric properties of all the compounds were studied
by measuring the absorption and emission spectra in dichloro-
methane from 0 to 10 pH. As a representative example, the normal-
ized absorption and emission spectra of 1a recorded in
dichloromethane at neutral pH are given in Figure 2. Compound
1a showed an absorption maxima centered at 345 nm and an emis-
sion maxima centered at 436 nm with high Stoke’s shift values.
These values were found to be stable to the changes in pH from
0 to 10. Fluorophores with high Stoke’s shift values (the distance
between the excitation maxima and emission maxima) are highly
useful for bio-imaging applications, because, when using such
compounds, there is no possibility of overlapping the excitation
wavelengths with the emission wavelengths and therefore it is
very easy to detect the fluorescence emission from biological tar-
gets.16 All the compounds 1a–i gave fluorescence emission at the
blue emitting region with high Stoke’s shift values and remain
intact to changes in pH.
Abs/Em (nm)
304/422
O
O
HN
HN
Br
O
Abs/Em (nm)
346/436
NO2
O
O
O
Br
O
91%
O
HN
1i
1j
O
98%
Abs/Em (nm)
303/382
O
Abs/Em (nm)
305/393.5
O
1a(Absmax 345nm)
1a(Emmax 436nm)
1.0
0.8
0.6
0.4
0.2
The substituent effects on the absorption/emission properties of
1a-j did not follow any pattern. The molecules 1b, 1c, 1f, 1g, and 1j
300
400
500
600
Wavelength
3-keto coumarin
O
CH3
O
O
O
O
O
O
Figure 2. The normalized absorption and emission spectra of blue emitting ‘Beta
Fluor’ 1a in dichloromethane at neutral pH.
Cl
20 mol%
+
boronic acid
OHC
HN
CH2CN
X
X
Stirr, rt., 4 hrs
showed absorption maxima at 303–305 nm region and 1a, 1d, 1e,
and 1h showed the same at 345–350 region. In the emission part,
the molecules 1a, 1b, 1d, and 1e–h showed emission maxima at
422–436 region and 1c, 1i, and 1j showed the same at 382–393
region. Since the alkyl amide part of 1a–j are b-amido ketones
1
O
1a
, X = H
94%
alkyl amide
Scheme 1. Synthesis of fluorogenic coumarin alkyl amides based on a one pot four
component reaction.
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T. V. Soumya et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Cl
+
H
O
NaN3
O
DCM
K2CO3
HO
O
N
O
Cl
N
H
N
+
DMA
N3
N
H
3
R.T, 3 days
O
CN
O
2
R.T, 4 hrs
H2N
CuSO4.5H2O,
Sod.ascorbate
DMSO/ t-BuOH/H2O
O
O
O
O
O
N
N
N
H
N
O
N
O
O
O
4
Spacer
O
5
Scheme 2. Synthesis of ‘Alpha Fluors’ 5 based on azide–alkyne [3+2] cycloaddition between propargyl esters of coumarin and a-(azido) acylamino acetamides.
Table 3
List of Alpha Fluors prepared and their absorption/emission values
F
O
O
O
Cl
O
O
O
O
N
N
N
N
N
N
H
H
N
N
O
O
N
N
Abs/Em(nm)
294/423
O
O
5a
O
Abs/Em(nm)
294/412.5
5b
Br
Br
N
O
O
O
N
N
N
H
Br
O
O
N
O
O
N
N
N
H
N
N
O
5c
O
O
Abs/Em(nm)
292/410
Abs/Em(nm)
295/412
O
O
5d
F
F
H
N
O
O
O
N
N
N
O
O
O
O
N
N
N
N
H
N
O
O
5e
O
N
Abs/Em(nm)
294/415
5f
O
O
Abs/Em(nm)
294/411
F
Br
Cl
O
O
O
O
O
N
N
N
H
O
N
N
N
H
N
N
O
N
O
N
O
O
O
Cl
5h
O
5g
Abs/Em(nm)
294/410
Abs/Em(nm)
294/414
Cl
Br
O
O
O
O
O
O
O
N
N
N
H
N
N
N
N
H
N
O
N
O
N
O
Br
O
O
5j
5i
Abs/Em(nm)
294/412
Abs/Em(nm)
293/410
Br
O
O
O
N
N
N
H
Br
O
O
N
O
N
N
N
O
H
N
N
O
N
O
O
5l
Abs/Em(nm)
294/414
O
O
5k
Abs/Em(nm)
294/413
O
O
O
O
N
N
N
H
N
O
O
O
O
O
N
N
N
H
N
N
O
Abs/Em(nm)
289/380
O
N
5n
5m
O
Abs/Em(nm)
288/377
Br
and are considered as b-amino acid derivatives, we decided to
name our molecules as ‘Beta Fluors’. During the bio-profiling
experiments, these new fluorophores are expected to cleave at
the alkyl amide bond to release highly fluorescent b-amino couma-
rin moieties.17
Encouraged by the fluorescence properties obtained for the cou-
marin alkyl amides 1a–j, we decided to study the fluorophoric
properties of the peptidomimetic versions of the structure 1. In
one of our previous communications,18 we reported the synthesis
of such peptidomimetics using click chemistry19 as a ligation tool
Please cite this article in press as: Soumya, T. V.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.071

4
T. V. Soumya et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5a(Absmax 294nm)
5a(Emmax 423nm)
5m(Absmax 288nm)
5m(Emmax 377nm)
established the usefulness of phenyl boronic acid as an effective
green catalyst. For the three-step synthesis of the second category
fluorophores named as ‘Alpha Fluors’, we effectively linked the con-
cept of click chemistry with multi-component reaction strategies.
The final molecules are featured with the presence of enough num-
ber of diversity points useful for substitution with electron with-
drawing or donating groups for fine tuning the emission
properties. We hope that the low cost and step-economic probes
reported in this Letter will be useful for academic and industrial
R&D researchers who are engaged in bio-imaging and cell tracking
studies.
1.0
0.8
0.6
0.4
0.2
300
400
500
Wavelength
Figure 3. The normalized absorption and emission spectra of Alpha Fluor 5a and
5m in dichloromethane at neutral pH.
Supplementary data
Supplementary data (copies of 1H NMR, 13C NMR and FT-IR and
MS of selected Alpha Fluors and Beta Fluors) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.06.071.
for connecting multicomponent reaction products. Following our
reported methodology, we have analyzed 14 such coumarin pepti-
domimetics with a general structure 5 and their representative
synthetic route is given in Scheme 2.
References and notes
As shown in Scheme 2, the
a-(azido) acylamino acetamides 3
were synthesized by following the classical four-component Ugi
reaction20 between chloroacetic acid, alkyl/aryl amines, substi-
tuted aromatic aldehydes, and t-Butyl isocyanaide. The chloro
derivatives 2 thus obtained were converted to the corresponding
azides 3 by base catalyzed reaction with sodium azide.21 The
azides were then clicked with propargyl coumarin esters 4 under
the conditions given in Scheme 2 to afford the coumarin–tria-
zole–carboxamide conjugates 5.21 These compounds belong to
the category of fluorophores with a spacer (1,2,3 triazole in the
present case) in-between the signaling and labeling entities. Fol-
lowing the same protocol, we synthesized compounds 5a–n and
their structures are given in Table 3.
The absorption and emission spectra of 5a–n were recorded in
dichloromethane at 0–10 pH. For example, 5a when subjected to
excitation at 294 nm showed fluorescence emission in the blue
region with a maxima centered at 423 nm and also with high
Stoke’s shift value (Fig. 3). Similar to Beta Fluors 1a–j, the fluores-
cence properties of 5a–n were also intact with changes in pH.
Compounds 5m and 5n having a naphthyl group in the carboxam-
ide part showed a blue shift of 46 and 49 nms, respectively, in the
emission maxima with respect to the same obtained for 5a and
maintained the Stoke’s shift values (Fig. 3). The presence of the
spacer triazole in 5a–n can impart better stability to the molecules
and also it is possible to drive the enzyme action to the ester or
amide sights to release highly fluorescent coumarin derivatives
during imaging processes. Since compounds 5a–n contain carbox-
1. Hall, D. G. Structure, Properties, and Preparation of Boronic Acid Derivatives, in
Boronic Acids: Preparation and Applications in Organic Synthesis, Medicine
and Materials (Volume 1 and 2) In Hall, D. G., Ed., 2nd ed.; Wiley-VCH Verlag
GmbH
& Co. KGaA: Weinheim, Germany, 2011. http://dx.doi.org/10.1002/
9783527639328.ch1.
2. (a) Candeias, N. R.; Pedro, M. S. D. Cal.; Andre, V.; Duarte, M. T.; Veiros, L. F.;
Pedro, M. P. Gois Tetrahedron 2010, 66, 2736–2745; (b) Yang, W.; Gao, X.;
Wang, B.; Weinheim: Wiley-VCH. 2005; pp 481–512.; (c) Herradura, P. S.;
Pendola, K. A.; Guy, R. K. Org. Lett. 2000, 2, 2019–2022; (d) Chan, D. M. T.;
Monaco, K. L.; Li, R.; Bonne, D.; Clark, C. G.; Lam, P. Y. S. Tetrahedron Lett. 2003,
44, 3863–3865; (e) Lam, P. Y. S.; Vincent, G.; Bonne, D.; Clark, C. G. Tetrahedron
Lett. 2003, 44, 4927–4931.
3. Georgiou, I.; Ilyashenko, G.; Whiting, A. Acc. Chem. Res. 2009, 42, 756–768.
4. (a) Ishihara, K.; Ohara, S.; Yamamoto, H. J. Org. Chem. 1996, 61, 4196–4197; (b)
Wipf, P.; Wang, X. J. Comb. Chem. 2002, 4, 656–660; (c) Arnold, K.; Davies, B.;
Giles, R. L.; Grosjean, C.; Smith, G. E.; Whiting, A. Adv. Synth. Catal. 2006, 348,
813–820; (d) Maki, T.; Ishihara, K.; Yamamoto, H. Tetrahedron. 2007, 63, 8645–
8657; (e) Al-Zoubi, R.; Marion, O.; Hall, D. G. Angew. Chem. 2008, 120, 2918–
2921. Angew. Chem., Int. Ed. 2008, 47, 2876–2879; (f) Arnold, K.; Batsanov, A. S.;
Davies, B.; Whiting, A. Green Chem. 2008, 10, 124–134; (g) Tommaso, M. Angew.
Chem. 2010, 122, 6992–6995. Angew. Chem., Int. Ed. 2010, 49, 6840–6843.
5. Rao, G.; Philipp, M. J. Org. Chem. 1991, 56, 1505–1512.
6. Hu, X.-D.; Fan, C.-A.; Zhang, F.-M.; Tu, Y.-Q. Angew. Chem. 2004, 116, 1734–
1737. Angew. Chem., Int. Ed. 2004, 43, 1702–1705.
7. (a) Debache, A.; Boumoud, B.; Amimour, M.; Belfaitah, A.; Rhouati, S.; Carboni,
B. Tetrahedron Lett. 2006, 47, 5697–5699; (b) Zheng, H.; Hall, D. G. Tetrahedron
Lett. 2010, 51, 3561–3564; (c) Zheng, H.; McDonald, R.; Hall, D. G. Chem. Eur. J.
2010, 16, 5454–5460.
8. Arnold, K.; Batsanov, A. S.; Davies, B.; Grosjean, C.; Schuetz, T.; Whiting, A.;
Zawatzky, K. Chem. Commun. 2008, 3879–3881.
9. Li, M.; Yang, T.; Dixon, D. J. Chem. Commun. 2010, 46, 2191–2193.
10. (a) McCubbin, J. A.; Hosseini, H.; Krokhin, O. V. J. Org. Chem. 2010, 75, 959–962;
(b) McCubbin, J. A.; Krokhin, O. V. Tetrahedron Lett. 2010, 51, 2447–2449; (c)
Zheng, H.; Lejkowski, M.; Hall, D. G. Chem. Sci. 2011, 2, 1305–1310.
11. (a) Haugland, R. P. The Handbook: A Guide to Fluorescent Probes and Labelling
Technologies, 10th ed.; Molecular Probes: Eugene, OR, 2005; (b) Xiaohua, L.;
Xinghui, G.; Wen, S.; Huimin, M. Chem. Rev. 2014, 114, 590–659; (c) André, N.;
Carsten, S. Angew. Chem., Int. Ed. 2013, 52, 2408–2410.
amide moieties and are considered as derivatives of a-amino acids,
we named our compounds 5a–n as ‘Alpha Fluors’.
The absorption and emission spectra of Beta Fluors 1a–j and
Alpha Fluor 5a–n were compared with the same of the commer-
cially available blue emitting coumarin based fluorescent probe
Alexa Fluor 350 and DyLight⁄350. The fluorescence spectra for
the commercial molecules were obtained from web sources and
preliminary comparisons revealed that, the fluorescence properties
of 1a–j are comparable with Alexa Fluor 350 and the same for 5a–n
are comparable with DyLight⁄350. As pointed out by one of the ref-
erees during the evaluation process of this manuscript, detailed
investigations under identical conditions are required to substanti-
ate this claim and that will be addressed in the full paper version of
this communication.
In conclusion, we have demonstrated a low-cost and green syn-
thesis of two types of fluorogenic probes with blue emission wave-
lengths and comparable performance equivalency with commercial
molecules of the same category. For the one step synthesis of
the first category fluorophores named as ‘Beta Fluors’, we have
12. (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.;
McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515; (b) Shi, W.;
Ma, H. M. Chem. Commun. 2012, 48, 8732.
13. (a) Panchuk-Voloshina, N.; Haugland, R. P.; Bishop-Stewart, J., et al. J.
Histochem. Cytochem. 1999, 47, 1179–1188; (b) Graham, T. D.; Joshua, C. V.;
Kok Hao, C.; Mark, B.; Xiaowei, Z. Nat. Methods 2011, 8, 1027–1040.
14. (a) Shinu, V. S.; Sheeja, B.; Purushothaman, E.; Bahulayan, D. Tetrahedron Lett.
2009, 50, 4838–4842; (b) Shinu, V. S.; Pramitha, P.; Bahulayan, D. Tetrahedron
Lett. 2011, 52, 3110–3150; (c) Bahulayan, D.; Arun, S. Tetrahedron Lett. 2012, 53,
2850–2855; (d) Biny, B.; Bahulayan, D. Tetrahedron Lett. 2014, 55, 227–231.
15. Typical experimental procedure for the synthesis of ‘Beta Fluor’ 1a: A 100 mL
round-bottomed flask was charged with anhydrous acetonitrile (10 mL),
benzaldehyde (0.212 g, 2 mmol), 3-acetyl coumarin (376 mg, 2 mmol), acetyl
chloride (4 mL), and phenyl boronic acid (20 mol %) under constant stirring at
room temperature. The reaction was monitored by TLC and was found to be
complete after 4 h. The mixture was then poured into distilled water and kept
for 1 h. The precipitated colorless solid was collected on a filter, washed with
distilled water (3 Â 25 mL), and dried under vacuum. The vacuum-dried solid
was then washed with anhydrous diethyl ether (2 Â 15 mL) and air dried to
obtain 1a in pure form (630 mg, 94%). FT-IR (KBr): 3318, 2953, 2851, 1747,
Please cite this article in press as: Soumya, T. V.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.071

T. V. Soumya et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5
1681, 1648, 1610, 1560, 1493, 1448, 1369, 1174, 982,831,764 and 543 cmÀ1
.
20 min to form the Schiff-base. To this, one equivalent of tert-butyl isocyanide
(0.83 g, 0.01 mol) and chloroacetic acid (0.95 g, 0.01 mol) was added and
stirred at room temperature for 48 h. After 48 h, the solvent was removed
under vacuum to obtain the crude product. It was washed with petroleum
ether (5 Â 15 mL) to afford the pure carboxamide chloride 2b (876 mg, 92%).
Stage 2: Synthesis of carboxamide azide 2b: One equivalent of 2b (236 mg,
0.05 mol) and sodium azide (1 g) is taken in dimethyl acetamide (6 mL). To this
K2CO3 (1 g) was added and stirred at room temperature for 4 h. After 4 h, the
reaction mixture was diluted with water to obtain 3b as white precipitate. It
was collected and washed repeatedly with water to obtain the pure azide 3b
1H NMR (400 MHz, DMSO-d6): d 8.65 (s, 1H), 8.35–8.33 (d, J = 8 Hz, 1H), 7. 97–
7.95 (d, J = 8 Hz, 1H), 7. 78–7.74 (t, 1H), 7.49–7.41 (m, 2H), 7.37–7.30 (m. 4H),
7.25–7.20 (t, 1H), 5.42–5.36(m, 1H), 3.59–3.53(dd, J = 8 Hz, J = 24 Hz, 1H) 3.45–
3.38(dd, J = 12 Hz and J = 28 Hz, 1H), 1.79 (s, 3H)
16. Jianjun, Q.; Myung-Shin, H.; Yu-Cheng, C.; Ching-Hsuan, T. Bioconjug. Chem.
2011, 22, 1758–1762.
17. Li, J.; Yao, S. Q. Org. Lett. 2009, 11, 405–408.
18. Pramitha, P.; Bahulayan, D. Bioorg. Med. Chem. Lett. 2012, 22, 2598–2603.
19. (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–
2021; (b) Sreeman, K. M.; Finn, M. G. Chem. Soc. Rev. 2010, 39, 1252–1261; (c)
Droumaguet, C. L.; Wang, C.; Wang, Q. Chem. Soc. Rev. 2010, 39, 1233–1239; (d)
El-Sagheer, A. H.; Brown, T. Chem. Soc. Rev. 2010, 39, 1388–1405; (e)
Manzenrieder, F.; Luxenhofer, R.; Retzlaff, M.; Jordan, R.; Finn, M. G. Angew.
Chem., Int. Ed. 2011, 50, 2601–2605; (f) Suzuki, T.; Ota, Y.; Kasuya, Y.; Mutsuga,
M.; Kawamura, Y.; Tsumoto, H.; Nakagawa, H.; Finn, M. G.; Miyata, N. Angew.
Chem., Int. Ed. 2010, 49, 6817–6820; (g) Hong, V.; Presolski, S. I.; Ma, C.; Finn, M.
G. Angew. Chem., Int. Ed. 2009, 48, 9879–9883; (h) Ganesh, V.; Sudhir, S.;
Kundu, T.; Chandrasekharan, S. Chem. Asian J. 2011, 6, 2670–2694; (i) Prakasan,
T.; Dariusz, M.; Krzysztof, J. Chem. Rev. 2013, 113, 4905–4979; (j) Liang, X. L.;
Yongjun, L.; Yuliang, L. Asian J. Org. Chem. 2014, 3, 582–602.
(219 mg, 91%). Compound 3b: FT-IR, KBr, cmax: 3392.2, 3343.9, 2965.0, 2928.4,
2103.9, 1686.4, 1655.0, 1538.9, 1485.9, 1391.4, 1257.4, 1014.4, 743.4; 1H NMR
(400 MHz, DMSO-d6): d 8.09 (s, 1H), 7.37–7.35 (d, J = 7.6 Hz, 2H), 6.94–6.93 (d,
J = 7.6 Hz, 2H), 7.20–7.172 (m, 2H), 7.10–7.07 (m, 2H), 6.61 (s, 1H), 3.8–3.53
(m, 2H), 1.26 (s, 9H). Stage 3: Synthesis of Alpha Fluor 5b: One equivalent of
propargyl coumarin ester 4 (114 mg, 0.05 mol) and 239 mg (0.05 mol) of the
azide 3b were dissolved in minimum amount of DMSO. To this, 2 ml of t-BuOH,
1 mL of water, CuSO4Á5H2O (300 mg), and sodium ascorbate (300 mg) are
added and stirred in room temperature for 20 h. After 20 h, the mixture was
poured into cold water to obtain the crude precipitate of 5b. It was then
repeatedly washed with water and finally with a small amount of diethyl ether
20. (a) Ugi, I. Angew. Chem., Int. Ed. Engl. 1962, 1, 8–21; (b) Ugi, I.; Domling, A.;
Gruber, B.; Almstetter, M. Croat. Chem. Acta 1997, 70, 631–647; (c) Ugi, I.;
Domling, A.; Horl, W. Endeavour 1994, 18, 115–122; (d) Domling, A. Chem. Rev.
2006, 106, 17–89.
21. Typical Experimental Procedure for the three stage synthesis of Alpha Fluor 5b:
Stage 1: Synthesis of carboxamide azide 3b: An equi-molar amount of 2-chloro
benzaldehyde (700 mg, 0.005 mol) and 4-bromoaniline (851 mg, 0.005 mol) is
dissolved in dichloromethane (8 mL) and stirred at room temperature for
to obtain the white powder of 5b (338 mg, 96%). FT-IR, KBr, cmax: 3312, 2970,
2933, 2107, 1773, 1706, 1671, 1610, 1565, 1484., 1391, 1206, 764; 1H NMR
(400 MHz, DMSO-d6): d 8.74 (s, 1H), 8.13 (S, 1H), 8.01 (s, 1H), 7.92–7.94 (d,
J = 8.0 Hz 2H),7.73–7.70 (m, 1H), 7.53–7.52 (d, J = 7.6 Hz, 1H), 7.42–7.36 (m,
3H), 7.11–7.08 (m, 3H), 6.92–6.90 (d, J = 7.2 Hz, 2H), 6.26 (s, 1H), 5.29 (s, 2H),
5.16–4.34 (m, 2H), 1.24 (s, 9H); MS: m/z Calcd for C33H29N5O6BrCl, 706.9;
Found, 706.0.
Please cite this article in press as: Soumya, T. V.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.071