Study of direct macrocycle formation via the cyclisation of propargyl 2-azidobenzoate

Article abstract of DOI:10.1016/j.tetlet.2012.12.076

Macrocycles were synthesised via the cyclisation of propargyl 2-azidobenzoate using 'click' chemistry. An LC/MS-based method for the separation of macrocycles is described. In the present work, the influence of various catalysts and reaction conditions on

Full text of DOI:10.1016/j.tetlet.2012.12.076

Tetrahedron Letters 54 (2013) 1218–1221
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Study of direct macrocycle formation via the cyclisation of propargyl
2-azidobenzoate
a
a
a
a,b,
⇑
ˇ
Ludmila Hradilová , Martin Grepl , Barbora Dvoráková , Pavel Hradil
a
ˇ
Farmak a.s. Na Vlcinci 16/3, 771 17 Olomouc, Czech Republic
b Department of Organic Chemistry, Faculty of Science, University of Palacky, 17. listopadu 12, 771 46 Olomouc, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Macrocycles were synthesised via the cyclisation of propargyl 2-azidobenzoate using ‘click’ chemistry. An
LC/MS-based method for the separation of macrocycles is described. In the present work, the influence of
various catalysts and reaction conditions on the course of the reaction is evaluated. The reaction is very
sensitive to the conditions, and it is demonstrated that it is possible to modify the ratio of macrocycles by
varying the reaction conditions. The cyclisation of propargyl 2-azidobenzoate represents an economical
method for the preparation of the studied macrocycles.
Received 7 October 2012
Revised 28 November 2012
Accepted 18 December 2012
Available online 27 December 2012
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Macrocycles
2-Azidobenzoate
Click reaction
LC/MS
Cyclisation
Macrocyclic molecules are frequently studied due to their un-
ique properties. These molecules are widely occurring and include
porphyrins, cyclopeptides and numerous other molecules. Other
macrocycles have been synthesised in the laboratory, including
cyclophanes, crown ethers and glycophanes. Macrocycles display
remarkable biological activities1,2 or have technical applications.
Macrocyclic compounds with triazole rings have been applied as
optical and electrochemical chemosensors.3
Azidobenzoate 1 is a very reactive starting material that is use-
ful for the preparation of benzoxazepine 7.4,5 The reaction is gen-
eral and can be used for the preparation of derivatives of
compound 7. We demonstrated that compound 1 has limited sta-
bility and, at laboratory temperature, partly dimerises to form
compound 8. This reaction is too slow for the routine production
of this compound. Therefore, macrocycle 2 formed using this meth-
od during the heating of compound 1 (not properly stored) is pro-
duced only as an impurity.5 As macrocyclic compounds of this type
can be used in various technical applications, we have developed a
general procedure for the synthesis of macrocycles of various ring
sizes. The preparation of macrocycles 2, 3 and 4 was described re-
cently,6 and consisted of few steps and was laborious. During our
work on this problem, we found that azidobenzoate 1 affords a
mixture of macrocycles when treated with a copper catalyst. This
method represents a simple and attractive strategy for the prepa-
ration of these interesting molecules (Scheme 1).7
We have developed a simple and effective LC/MS method that
makes it possible to quickly compare the results of different
experiments.8
The structures of the final products were confirmed by HRMS
and, in the case of compounds 2, 3, 4 and 7 by comparison with
a standard. There was also the possibility of formation of linear
oligomers 8, 9 and 10. We proved recently6 that these molecules
were very reactive and they were not stable under these condi-
tions. The retention times and HRMS of standards 8, 9 and 10
(Fig. 1) are given in Supplementary data to verify the separation
possibilities of our analytical method. As we supposed, the linear
molecules were not found in the reaction mixture obtained under
the conditions of the ‘click’ reaction.
An example of a separation is presented in Figure 2. The LC/MS
analysis of the recently prepared standards is presented in Supple-
mentary data. First, we determined the amount of catalyst neces-
sary for a successful and sufficiently fast reaction. We started
with copper sulfate and ascorbic acid, which have been used effec-
tively for the construction of macrocycles in the past.6 This catalyst
however was not effective in a molar amount of approximately 5%.
An acceptable reactivity for compound 1 was obtained with a
molar ratio of catalyst to azide of 0.5–1. Under these conditions,
the major products were macrocycle 2, with a yield of approxi-
mately 50%, and macrocycle 3, with a yield of 31%. In addition,
macrocycles 4 and 5 were formed. The composition of the reaction
mixture did not depend on the quantity of the catalyst. When the
⇑
Corresponding author at: Department of Organic Chemistry, Faculty of Science,
University of Palacky, 17. listopadu 12, 771 46 Olomouc, Czech Republic. Tel.: +420
585634418; fax: +420 585634465.
E-mail address: hradil@orgchem.upol.cz (P. Hradil).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.076

L. Hradilová et al. / Tetrahedron Letters 54 (2013) 1218–1221
1219
O
O
O
O
N
N
O
O
N
N
N
N
O
N
N
N
O
N
N
N
N3
N
O
N
8
O
N
O
O
N
O
N
N
N
N
O
N
N
N
O
N
O
N
O
O
N
O
rt
O
2
N
N
N
N
3
4
O
O
Cu-catalyst
DMF
O
O
N
N3
N
N
1
O
N
O
DMF
N
O
N
O
reflux
O
N
N
N
N
N
O
O
N
N
N
N
O
O
O
O
O
O
O
O
O
N
N
N
N
N
N
N
N
N
N
O
N
N
N
O
N
N
O
O
O
O
O
O
7
N
N
N
N
6
5
N
N
Scheme 1. The mixture of macrocycles formed by the cyclisation of propargyl 2-azidobenzoate.
O
O
O
O
O
N
N
N
N
N
N
N3
O
N3
O
O
N
N
O
O
O
N
O
O
N
N
N3
O
N
N
N
N
O
O
N
N
O
O
N
10
8
9
Figure 1. Structures of open oligomers.
solvent was changed to a mixture of water and N,N-dimethylform-
amide (DMF), the amount of compound 2 increased to 60%. The
influence of temperature on the reaction was unexpected. This
influence was studied using a system with copper sulfate and
ascorbic acid as catalysts. Higher temperatures had no significant
influence on the ratio of the formed products. At a temperature
of À15 °C, the production of higher macrocycles 3 and 4 increased
(Table 1, entries 5–7). The reaction did not proceed at a tempera-
ture of À40 °C. The surprising results were obtained when the cat-
alytic activity of copper(I) halides was tested. If the reaction
proceeded, the same products were formed, with the only differ-
ences in the proportion of macrocycles in the reaction mixture.
The reaction proceeded smoothly if the co-catalyst like N,N-diiso-
propylethylamine (DIPEA) or triethylamine (Et3N) was added to
copper(I) chloride or copper(I) bromide. The reaction did not run
without one of these co-catalysts. Nevertheless, the reaction cata-
lysed solely by copper(I) iodide was fast. The reaction surprisingly
proceeded slower if the mixture of copper(I) iodide and DIPEA was
used. Conversely, the reaction rate was fast as the combination of
copper(I) iodide with Et3N was used and it was unexpectedly more
selective and afforded 80% macrocycle 2 (Table 1, entries 16–23).
An interesting course of the reaction was observed after the
addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) being similar
in all cases. The reaction was very fast, and the starting material
was not observed after 15 min in the reaction mixture. In addition
to DBU, macrocycle 2 and an unknown compound were present in
the reaction mixture. After a few hours of stirring, the main com-
pounds disappeared, and only DBU and an unknown impurity
showing a very small peak were observed in the HPLC chromato-
gram. The weak catalytic activity of copper(I) chloride or bromide
could be explained by their lower solubility in the reaction mixture
compared to copper(I) iodide. After the addition of tertiary amines,
a solution is formed and reaction is very fast. However, at present
we have no explanation for the results of the catalytic activity of
the combination of copper(I) halides with DBU and copper(I) io-
dide with tertiary amines. We are currently working on this
problem.
We also tested the influence of copper powder. At room tem-
perature, the reaction was very slow, and after 48 hours, the yields
of macrocycle 2 and dimer 8 were only 7% and 21%, respectively
(Table 1, entry 25). The rest of the starting material was un-
changed. It can be seen from this result that the catalyst was not
effective. The formation of product 8 by spontaneous dimerisation
of azido derivative 1 was described recently.5 The extent of the
reaction was increased using ultrasound. The influence of ultra-
sound on heterogenous reactions is commonly known. After 4 h,
the starting material was not found in the reaction mixture, and
the formation of a mixture of macrocycles was observed (Table 1,

1220
L. Hradilová et al. / Tetrahedron Letters 54 (2013) 1218–1221
Figure 2. LC/MS method—example of the separation of a reaction mixture.
Table 1
Results of the cyclisation of propargyl 2-azidobenzoate (1) under various conditions
Entry
Starting material
Conditions
Products (%)
Solvent
(mL)
Catalyst
(equiv)
Co-catalyst (equiv)
Temp
(°C)
Time
1
2
3
4
5
6
7
8
1
DMF (0.5)
Reflux
30 min
15 min
15 min
1 h
0
0
0
0
0
0
0
1.8
0
0
0
0
97
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(2)
DMF
(0.5)
DMF
(2)
DMF + H2O
(0.15+0.35)
DMF + H2O
(0.05 + 0.1)
MeCN
(0.5)
Acetone
(0.5)
EtOH
(0.5)
CuSO4
(1)
CuSO4
(1)
CuSO4
(0.5)
CuSO4
(1)
CuSO4
(0.2)
CuSO4
(4)
asc.aca
(2.1)
60
25
52.7
48.4
45
27.6
31.3
31
15.1
12
4.6
8.2
6.8
7.3
8
0
3
asc.aca
(2.1)
0
4
asc.aca
(1.05)
asc.aca
(2.1)
25
17.2
22.9
18.8
15.3
9.1
0
5
À15
À15
À15
À25
À30
25
4 h
29.3
32.5
37.7
36.4
41
36.8
36
3.6
4.6
3
6
asc.aca
(0.42)
asc.aca
(8.4)
30 h
5.5 h
4 h
7
36.3
17
7.6
0
8
CuSO4
(1)
asc.aca
(2.1)
37.4
0
0
9
CuSO4
(1.6)
CuSO4
(0.2)
CuSO4
(0.5)
CuSO4
(1)
CuSO4
(1)
CuSO4
(1)
asc.aca
(3.36)
asc.aca
(0.42)
asc.aca
(1.05)
asc.aca
(2.1)
2 h
38.2
17.8
19.15
0
16.3
10.8
13
4.5
6.2
5.4
0
0
10
11
12
13
14
15
2.5 h
15 min
6 h
0
61.9
59.25
0
3.2
3.2
0
25
0
25
97
0
0
asc.aca
(2.1)
25
6 h
40.7
42.6
30.9
33.9
25.2
18.7
11.4
10.4
5.4
8.8
6.6
4.5
5.2
3.4
1.9
asc.aca
(2.1)
25
6 h
11.8
31.3
THF
(0.5)
CuSO4
(1)
asc.aca
(2.1)
25
6 h

L. Hradilová et al. / Tetrahedron Letters 54 (2013) 1218–1221
1221
Table 1 (continued)
Entry
Starting material
Conditions
Products (%)
Solvent
(mL)
Catalyst
(equiv)
Co-catalyst (equiv)
Temp
(°C)
Time
1
2
3
4
5
6
7
8
16
17
18
19
20
21
22
23
24
25
26
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
DMF
(0.5)
CuI
(0.2)
CuI
(0.2)
CuI
25
25
25
25
25
25
25
25
25
25
25
2 h
4 h
1 h
2 h
2 h
4 h
4 h
7 h
20 h
48 h
4 h
0
66.2
51.5
80.2
52.1
57.5
59.4
22.7
45.1
59
11.1
10
4.5
6.2
4.2
14.1
15.7
12.6
0
0
0
0
0
0
0
0
0
0
0
0
2
0
0
0
DIPEA
(5)
TEA
(5)
DIPEA
(5)
Et3N
(5)
Et3N
(5)
Et3N
(5)
DIPEA
(5)
3.4
1.2
0
0
0
8.2
30.2
22.8
28
0
0
0
(0.2)
CuBr
(0.2)
CuBr
(0.2)
CuCl
(0.5)
CuCl
(0.25)
CuCl
(0.5)
Cu(OAc)2
(0.5)
Cu
1.5
3.8
0
0
0
0
0
0
0
0
0
77.3
34.8
0
0
0
0
0
7.5
21.3
1
6.1
6.7
0
0
0
0
0
0
0
66.7
0
7.5
0
0
21
0
(0.5)
Cu
(0.5)
Ultrasound
45.8
29.9
16.1
6.4
1.8
a
asc.ac.—Ascorbic acid.
2. Tyndall, J. D. A.; Fairlie, D. P. Curr. Med. Chem. 2001, 8, 893–907.
3. Kim, S. H.; Choi, S. H.; Kim, J.; Lee, S. J.; Quang, D. T.; Kim, J. S. Org. Lett. 2010, 12,
560–563.
entry 26). The influence of solvents such as acetonitrile, acetone,
EtOH and THF on the reaction course was also tested. All these sol-
vents were inferior to DMF. The results of these experiments are
summarised in Table 1, entries 12–15.
4. Garanti, L.; Molteni, G.; Zecchi, G. Heterocycles 1994, 38, 291–296.
ˇ
ˇ
5. Hradilová, L.; Grepl, M.; Hlavác, J.; Lycka, A.; Hradil, P. J. Heterocycl. Chem. 2012,
in press.
In conclusion, we have confirmed that the cyclisation of azi-
dobenzoate 1 using click chemistry results in the production of a
mixture of macrocycles. This mixture was successfully analysed.
The reaction was very sensitive to the reaction conditions, and it
was demonstrated that it was possible to modify the ratio of the
macrocycles by varying the reaction conditions. In addition, the re-
sults showed that various copper salts have significantly different
effects on the reaction. This study also revealed the distinct prop-
erties of copper(I) iodide relative to other copper halides, and the
important role of the addition of various tertiary amines was dem-
onstrated. The studied reaction provides an economic method for
the preparation of larger quantities of the studied macrocycles.
Column chromatography is necessary for the preparation of macro-
cycles 3, 4 or higher. Chromatographic separation of macrocycles
was described recently.6
ˇ
ˇ
6. Hradilová, L.; Grepl, M.; Hlavác, J.; Lycka, A.; Hradil, P. Synthesis 2012, 44, 1398–
1404.
7. A solution of propargyl 2-azidobenzoate (1) (25 mg, 0.1205 mmol) in DMF
(0.15 mL) was added to a solution of the catalyst or to a solution of the catalyst
and the co-catalyst in DMF (0.35 mL), and the reaction mixture was stirred at a
given temperature for the appropriate amount of time. Samples (1 drop) were
diluted with H2O (1 mL) and extracted with EtOAc (0.5 mL). Samples were taken
after 15 min and 30 min and then in 1 h intervals. The reaction performed at
temperature À15 °C was conducted in a refrigerator evaporator, and a chest
freezer was used for the reaction performed at À25 °C. These reactions were not
stirred but were shaken during sampling. Long-duration reactions were sampled
only during working hours. The reaction details are summarised in Table 1.
8. LC/MS analyses were performed with a Thermo Exactive instrument (Thermo
Scientific, USA). The chromatographic apparatus consisted of an Accela 1250 LC
pump, an autosampler and a column thermostat. The separation was performed
on a Luna C18, 3
lm, 50 Â 2 mm i.d. column (Phenomenex, USA) using binary
gradient elution. The mobile phase comprising acetonitrile and water with 0.1%
formic acid was mixed from 30 to 50% of acetonitrile over a period of 8 min,
followed by isocratic elution up to 12 min stop time. A 5 min equilibration was
performed before the next injection. The flow rate was kept at 300
the column temperature was 30 °C. Samples were prepared as follows: 200
EtOAc solution was evaporated using a vacuum at laboratory temperature
(5 min), the residue was dissolved in 10 mL of a 9:1 mixture of acetonitrile and
l
L/min, and
L of
l
Acknowledgments
water (1 min sonication), and then 200
mixture of acetonitrile and water were added and mixed as a final dilution
before the injection of 10 L. An exactive high-resolution mass spectrometer
lL of this solution and 800 lL of a 3:7
This project was supported in part by the Ministry of Education,
Youth and Sport of the Czech Republic (grants MSM6198959216
and ME09057) and by FM EEA/Norska (A/CZ0046/1/0022).
l
based on an orbitrap mass analyser was used with Atmospheric Pressure
Chemical Ionisation (APCI). The spectrometer was tuned to obtain the maximum
response for m/z 90–1300. The source parameters were set to the following
values: APCI temperature 400 °C, spray voltage +3.5 kV, transfer capillary
temperature 330 °C and sheath gas/aux gas (nitrogen) flow rates 25/10. The
separated compounds were observed by recording the TIC (total ion current)/
time signal, and the area % was calculated for quantification. The HRMS spectra
of the target peaks allowed the determination of their elemental compositions
due to the high intensities of their protonated molecules. The identification of
the respective structures was performed, and the differences between the
experimental and calculated values were less than 1 ppm.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.12.
076.
References and notes
1. Wessjohann, L. A.; Ruijter, E.; Garcia-Rivera, D.; Brandt, W. Mol. Diversity 2005, 9,
171–186.