Synthesis and photophysical properties of 2-azolyl-6-piperidinylpurines

Article abstract of DOI:10.1007/s10593-021-02943-1

[Figure not available: see fulltext.] A synthesis of novel fluorescent 2-azolyl-6-piperidinylpurine derivatives was designed. Azolyl substituent at purine C-2 atom was introduced via nucleophilic aromatic substitution or in the case of tetrazolyl and 1,2,3-triazolyl substituents via a ring formation on a preinstalled amine or azide moiety, respectively. The obtained purine intermediates were functionalized at N-9 position using Mitsunobu reaction conditions to achieve amorphous compounds, which form thin-layer films of good quality. The synthesized push-pull systems exhibited fluorescence with emission in range of 360–400 nm and quantum yields up to 66% in CH₂Cl₂ solution and up to 45% in the thin-layer film.

Full text of DOI:10.1007/s10593-021-02943-1

DOI 10.1007/s10593-021-02943-1
Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
Synthesis and photophysical properties
of 2-azolyl-6-piperidinylpurines
Armands Sebris1, Kaspars Traskovskis1,
Irina Novosjolova1*, Māris Turks1*
1 Faculty of Materials Science and Applied Chemistry, Riga Technical University,
3 Paula Valdena St., Riga LV-1048, Latvia;
е-mail: irina.novosjolova@rtu.lv, maris.turks@rtu.lv
Submitted January 15, 2021
Accepted February 24, 2021
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2021, 57(5), 560–567
A synthesis of novel fluorescent 2-azolyl-6-piperidinylpurine derivatives was designed. Azolyl substituent at purine C-2 atom was
introduced via nucleophilic aromatic substitution or in the case of tetrazolyl and 1,2,3-triazolyl substituents via a ring formation on a pre-
installed amine or azide moiety, respectively. The obtained purine intermediates were functionalized at N-9 position using Mitsunobu
reaction conditions to achieve amorphous compounds, which form thin-layer films of good quality. The synthesized push-pull systems
exhibited fluorescence with emission in range of 360–400 nm and quantum yields up to 66% in CH2Cl2 solution and up to 45% in the
thin-layer film.
Keywords: azoles, purines, fluorescence, nucleophilic aromatic substitution, ring formation.
For decades, purine derivatives have been extensively
studied due to their wide spectrum of biological activities.1–4
Purine cycle is known as the most common nitrogen
heterocycle in nature.5,6 Fluorescence properties exhibited
by certain purine derivatives are often used in cell
imaging.7–11 On the contrary, materials science applications
of fluorescent purine derivatives are poorly studied. Lately
Castellano's group has developed organic light emitting
diodes (OLEDs) with purine emitters12,13 and Yang's group
obtained the first thermally activated delayed fluorescence
(TADF) emitter using purine ring as the electron-accepting
group.14
Applications of organic compounds in materials science
are often limited by complicated and expensive processes,
for example, vacuum deposition, which is used for creating
thin substance layers. The alternative is a solution
processing, which requires compounds that form stable
amorphous phase and possess good solubility. These
properties can be achieved by introducing a trityl moiety in
the molecule.15,16
Expanding the previous work of our group,11 we
developed a synthesis of 2-azolyl-6-piperidinylpurine deri-
vatives as push-pull systems with different substituents at
the N-9 atom. While there is a wide range of information
on introduction of various azoles at purine C-6 position,17–21
transformations at purine C-2 position are less common, as
C-6 position is more reactive and preliminary functio-
nalization of C-6 position deactivates substitution at C-2
position.6 The known examples represent pyrazole ring
construction on hydrazine,23 imidazole ring introduction via
an oxidated intermediate of 8-oxoadenine and 8-oxohypo-
xanthine,23 and substituted benzimidazole introduction using
Buchwald–Hartwig cross coupling24,25 or a SNAr reaction.26
The initial synthetic approach to the target compounds
included N9-functionalization using Mitsunobu reaction
between 2,6-dichloropurine (1) and 2-hydroxyethyl 3,3,3-tri-
009-3122/21/57(5)-0560©2021 Springer Science+Business Media, LLC
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Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
Scheme 1. Attempted synthesis of 2-[2-(1H-imidazol-1-yl)-6-(piperidin-1-yl)-9H-purin-9-yl]ethyl 3,3,3-triphenylpropanoates
Cl
HO
N
N
N
N
N
Cl
NH
N
H
N
N
Ph3P, DIAD
N
N
N
N
N
Cl
N
O
O
N
N
N
+
O
THF, 0°C
87%
THF
20°C, 30 min
79%
K3PO4, NMP
160°C, 14 h
N
H
N
Cl
N
O
Cl
N
O
Ph
1
N
5
Ph Ph
OH
O
Detected
by LCMS
2
Ph
Ph
Ph
1. (COCl)2, DMF (cat.)
CH2Cl2, 20°C, 2 h
Ph
3
Ph
O
Ph
Ph
Ph
4
2
Ph
2. HOCH2CH2OH, Et3N
CH2Cl2, 20°C, 12 h
56%
OH
6
phenylpropanoate (2)27 (Scheme 1). The selected side chain
is foreseen to ensure the amorphous properties of otherwise
crystalline heterocycles. Compound 2 containing trityl
group was obtained by esterification of commercially available
or readily prepared28 acid 6. Next, a SNAr reaction of
adduct 3 at C-6 position with piperidine produced inter-
mediate 4 under mild conditions (20°C, 30 min). Then the
next SNAr reaction was tried for the introduction of
imidazole at C-2 position.24 At 160°C in the presence of
K3PO4 as a base, we observed the SNAr process at the C-2
atom. However, these conditions were too harsh, and the
ester moiety was cleaved at the N-9 side chain producing
compound 5 regardless of its relatively high stability which
arises due to sterical hindrance.
To avoid the ester cleavage of substituent at N-9
position we decided to perform firstly the SNAr reaction
with piperidine at C-6 and then with imidazole at C-2
position of N9-unsubstituted purine. Again, the second
SNAr process did not provide the desired product, most
likely due to the presence of a negative charge on the
purine ring arising from its deprotonation.
Then a thermally stable and base-resistant tetrahydro-
pyranyl (THP) protecting group was introduced at N-9
position, and product 729 was obtained in 73% yield starting
from 2,6-dichloropurine (1) (Scheme 2). Compound 7
reacted smoothly with piperidine at C-6 position (20°C,
30 min) providing the key intermediate 830,31 in 95% yield.
The latter underwent SNAr reactions at C-2 position with
azole nucleophiles such as imidazole, 1,2,4-triazole, and
4-phenyl-1,2,3-triazole.
Although relatively harsh reaction conditions have been
applied (K3PO4, N-methylpyrrolidone (NMP), 160°C) the
SNAr substitution process in compound 8 proceeded with
good yields (51–68%). SNAr reaction with 1,2,4-triazole
selectively gave 1H-1,2,4-triazol-1-ylpurine 9b, and no
1,2,4-triazol-4-yl derivative was observed. Reaction with
4-phenyl-1,2,3-triazole yielded a 1:1 mixture of 2-(1H-1,2,3-
triazol-1-yl)purine and 2-(2H-1,2,3-triazol-2-yl)purine deri-
vatives 9c,d with isolated yields 35 and 32%, respectively
(Scheme 2). The identity of compound 9c was confirmed by
an alternative synthetic pathway, in which the 1H-1,2,3-
triazol-1-yl substituent was installed by Cu(I)-catalyzed
Scheme 2. Synthesis of 2-[2-azolyl-6-(piperidin-1-yl)-9H-purin-9-yl]ethyl 3,3,3-triphenylpropanoates 11a,b
561

Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
Scheme 3. Synthesis of 9-alkylated 6-(piperidin-1-yl)-2-(1H-tetrazol-1-yl)-9H-purine derivatives 18a–c
azide-alkyne cycloaddition reaction of azide derivative 13.
The latter was obtained by a SNAr reaction of 2,6-diazido-
purine 12. Toluene as relatively nonpolar solvent and slightly
elevated temperature ensured the C-6 reactivity due to the
prevalence of 6-azido form of the substrate, which can
enter azide-tetrazole tautomeric equilibrium.11,32–35
THP deprotection of compounds 9a,b yielded azolyl-
purines 10a,b, which were further functionalized at N-9
position via Mitsunobu reaction with diisopropyl azodi-
carboxylate (DIAD) to give compounds 11a,b (Scheme 2).
Deprotected compound 10b exhibited extremely poor
solubility in DMF, DMSO, THF, and various alcohols, thus
preventing compound characterization. It was, therefore,
telescoped into the Mitsunobu reaction giving compound
11b.
Attempts to introduce a tetrazole ring at C-2 position of
purine 6 by the SNAr process of 2-chloropurine derivative 8
did not provide the desired product. Since tetrazole is rather
acidic (pKa 4.9), its deprotonated form is a weak nucleo-
phile. Aiming to obtain the desired product we switched to
a ring construction on 2-aminopurine derivative 16
(Scheme 3).36 The latter was synthesized starting from
2,6-dichloropurine (1). Firstly, 2,6-diazidopurine (14)37
was obtained. Subsequent SNAr reaction with piperidine
yielded adduct 15.38 The remaining 2-azido group was
catalytically reduced to amino group. The obtained
2-aminopurine 1639 was used in ring formation with NaN3
and HC(OEt)3 and gave the expected 2-tetrazolylpurine
derivative 17 in 66% yield. Similarly to 9H-purine
derivatives 10a,b, also intermediate 17 underwent selective
alkylation at N-9 atom under Mitsunobu conditions
yielding compounds 18a–c in 63–85% yields (Scheme 3).
Alcohols such as 2-hydroxyethyl 3,3,3-triphenylpropanoate
(2), [3,5-di(9H-carbazol-9-yl)phenyl]methanol (21), and
2-choroethanol were used. The latter provides compound
18c, the chloroethyl substituent of which can serve as a
ꞌꞌsticky endꞌꞌ for further modifications of the side chain.
Reagent 21 for the introduction of the dicarbazolylphenyl
moiety was synthesized from compound 19 using copper-
catalyzed N-arylation to give compound 20,40 followed by
ester reduction with LiAlH4 to compound 21.
Photophysical properties of compounds 11a,b and 18a,b
were investigated in 5·10–5 M CH2Cl2 solution and in the
thin films (Figs. 1, 2 and Table 1). For compounds 11a,b
and 18a, the lowest energy absorption bands correspond to
intramolecular charge transfer transition (ICT) of purine
chromophores indicated by shoulders in the 300–310 nm
range. For compound 18b, the ICT band overlaps with
carbazole absorption as indicated by characteristic maxima
λabs at 323 and 338 nm. The emission maxima λem for
compounds 11a,b and 18a were in the 356–401 nm range,
which corresponds to near UV and purple light. The
increase of number of nitrogen atoms in the azole ring
resulted in a higher electron deficiency, which caused a
bathochromic shift in emission maxima.41 The 3,5-dicarb-
azolylphenyl moiety in compound 18b was intended to
improve hole transfer capabilities for potential use in
OLEDs.42 In the case of purine derivative 18b we observed
562

Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
due to an intermolecular quenching, which is reduced in
solution. Compounds 11b and 18a showed the best
quantum yields in the thin-layer films, 0.45 and 0.42
respectively.
To conclude, we have developed several approaches for
the introduction of various azolyl moieties at C-2 position
of purine ring system containing an electron-donating
piperidinyl substituent at its C-6 position. SNAr reactions
were used for the introduction of imidazolyl, 1H-1,2,4-tri-
azol-1-yl, and 2H-1,2,3-triazol-2-yl substituents at the
purine C-2 position. On the other hand, selective construc-
tion of 1H-1,2,3-triazol-1-yl group on 2-azidopurine
derivative was carried out by Cu(I)-catalyzed azide-alkyne
cycloaddition reaction, and a tetrazolyl substituent was
introduced by three-component assembling on 2-amino-
purine derivative. The developed synthetic pathways
allowed a late stage N9-functionalization of the obtained
heterocyclic assemblies to achieve amorphous states of the
desired push-pull purines for potential materials science
applications in the future. An introduction of 2-chloroethyl
side chain at N-9 position will serve as a functionalization
site for possible chemical modifications in future. The
obtained conjugates exhibited emission in the near UV
region in both solution (up to 66% quantum yield) and in
the thin-layer films (up to 45% quantum yield).
Figure 1. Absorption spectra of 2-azolyl-6-piperidinylpurines
11a,b and 18a,b in 5·10–5 M CH2Cl2 solution.
Experimental
UV-Vis absorption spectra were recorded with a
PerkinElmer Lambda 35 spectrometer. Films for optical
measurements were prepared using spin-coating technique
with a Laurell WS-400B-6NPP/LITE spin-coater on glass
slides, using 30 mg/ml THF solutions. After the coating, all
films were dried in oven at 100°C for 2 h. Emission spectra
and quantum yields for solutions and thin films were
recorded using a QuantaMaster 40 steady state spectro-
fluorometer (Photon Technology International, Inc.) equipped
with a 6 inch integrating sphere by LabSphere, using the
Figure 2. Emission spectra of 2-azolyl-6-piperidinylpurines 11a,b
and 18a,b in 5·10–5 M CH2Cl2 solution.
an emission spectrum, which is a combination of emission
from 6-piperdinyl-2-tetrazolylpurine and 3,5-dicarbazolyl-
phenyl moieties. We explain this by similar lowest energy
singlet levels for carbazole and purine chromophores.
Onset of absorption for both 18a,b which contain the same
purine moiety is at 350 nm. Characteristic maxima of
carbazole emission are also distinguishable in PL band
(362, 378, 397 nm: entry 7, Table 1).
Unlike tetrazole-substituted derivatives 18a,b, com-
pounds 11a,b showed better quantum yield in the thin-layer
film than in the CH2Cl2 solution. This is unusual, since
typically quantum yields in the thin-layer films are lowered
1
software package provided by the manufacturer. H and
13C NMR spectra were recorded on a Bruker 300 (300 and
75 MHz, respectively) spectrometer. Internal standard –
residual non-deuterated solvent peak for 1H nuclei (7.26 ppm
in CDCl3, 2.50 ppm in DMSO-d6) and deuterated solvent
peak for 13C nuclei (77.2 ppm in CDCl3, 39.5 ppm in
DMSO-d6). Nontrivial peak assignments were confirmed with
1
1H–13C HSQC and/or H–13C HMBC spectra. High-resolu-
Table 1. Photophysical properties of 2-azolyl-6-piperidinylpurines 11a,b and 18a,b
Entry
Compound
State
λabs, nm
λem, nm
Emission quantum yield
1
2
3
4
5
6
7
8
CH2Cl2 solution
Thin-layer film
CH2Cl2 solution
Thin-layer film
CH2Cl2 solution
Thin-layer film
CH2Cl2 solution
Thin-layer film
284, 305 (shoulder)
286, 300 (shoulder)
280, 300 (shoulder)
283, 310 (shoulder)
281, 310 (shoulder)
285, 310 (shoulder)
292, 323, 338
356
366
0.01
0.12
0.07
0.45
0.66
0.42
0.51
0.20
11a
11a
11b
11b
18a
18a
18b
18b
373
373
398
401
362, 378, 397
401, 422
295, 326, 340
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Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
tion mass spectra (ESI) were recorded on a Waters Q-TOF
6-(Piperidin-1-yl)-9-(tetrahydro-2H-pyran-2-yl)-
2-(1H-1,2,4-triazol-1-yl)-9H-purine (9b) was prepared
analogously to compound 9a from compound 8 (700 mg,
2.17 mmol, 1.0 equiv), 1,2,4-triazole (225 mg, 3.26 mmol,
1.5 equiv), dry NMP (3.5 ml), anhydrous K3PO4 (1.38 g,
6.52 mmol, 3.0 equiv). Yield 388 mg (51%), colorless
solid. Rf 0.39 (CH2Cl2–MeOH, 20:1). HPLC: tR 5.43 min.
1H NMR spectrum (CDCl3, 50°C), δ, ppm (J, Hz): 1.57‒
2.16 (12H, m, 6CH2); 3.78 (1H, td, 2J = 3J = 11.3, 3J = 2.7)
Micromass spectrometer. Melting points were determined
on a Fisher Digital Melting Point Analyzer Model 355. HPLC
analysis was performed using an Agilent Technologies
1200 Series system equipped with XBridge C18 column,
4.6 × 150 mm, particle size 3.5 μm, with flow rate of 1 ml/min,
using 0.1% aqueous TFA (A) and MeCN (B) for mobile
phase; wavelength of detection was 260 nm; eluent:
gradient 30–95% B in A for 5 min, 95% B for 5 min, 95–
30% B for 2 min. All reactions were followed by TLC on
E. Merck Kieselgel 60 F254 plates with detection by UV
light. Silica gel (60 Å, 40–63 μm, ROCC) was used for
flash chromatography.
2
and 4.12 (1H, d, J = 11.3, CH2O); 4.11‒4.38 (4H, m,
2CH2); 5.82 (1H, dd, 3J = 10.2, 3J = 1.9, NCHO); 7.94 (1H,
s, H-8); 8.09 (1H, s, H triazole); 9.12 (1H, s, H triazole).
13C NMR spectrum (CDCl3, δ, ppm: 22.8; 24.7; 25.0; 26.2;
32.4; 46.5; 68.8; 81.3; 118.8; 136.6; 143.8; 149.4; 150.9;
153.1; 153.8. Found, m/z: 355.1998 [M+H]+. C17H23N8O.
Calculated, m/z: 355.1989.
Commercially available reagents were used as received.
Starting materials 1, 6, and 19 are commercially available.
Compounds 2,27 3,27 7,29 8,30,31 14,37 15,38 and 1639 are
known from literature.
2-(4-Phenyl-1H-1,2,3-triazol-1-yl)-6-(piperidin-1-yl)-
9-(tetrahydro-2H-pyran-2-yl)-9H-purine (9c) and
2-(4-phenyl-2H-1,2,3-triazol-2-yl)-6-(piperidin-1-yl)-9-(tetra-
hydro-2H-pyran-2-yl)-9H-purine (9d), 1:1 mixture, were
prepared analogously to compound 9a from compound 8
(296 mg, 0.92 mmol, 1.0 equiv), 4-phenyl-1H-1,2,3-tri-
azole (200 mg, 1.38 mmol, 1.5 equiv), dry NMP (2 ml),
anhydrous K3PO4 (585 mg, 2.76 mmol, 3.0 equiv). Products
9c,d were separated by silica gel column chromatography
(eluent PhMe in MeCN, gradient 0–10%).
2-[2-Chloro-6-(piperidin-1-yl)-9H-purin-9-yl]ethyl
3,3,3-triphenylpropanoate (4). Piperidine (0.57 ml,
ρ 0.86 g/cm3, 5.79 mmol, 3.0 equiv) was added to a
solution of compound 3 (1.0 g, 1.93 mmol, 1.0 equiv) in
THF (30 ml), and the reaction mixture was stirred at 20°C
for 30 min. Then the reaction mixture was evaporated,
dissolved in CH2Cl2 (30 ml), and washed with aqueous
KH2PO4 solution (3 × 20 ml), saturated NaHCO3 solution
(20 ml), and brine (10 ml). The organic phase was dried
over anhydrous Na2SO4, filtered, and evaporated. Yield
865 mg (79%), colorless solid. Rf 0.32 (CH2Cl2–MeCN,
Compound 9c. Yield 137 mg (35%), colorless solid.
1
Rf 0.28 (CH2Cl2–MeCN, 10:1). HPLC: tR 6.87 min. H NMR
1
20:1). HPLC: tR 8.22 min. H NMR spectrum (CDCl3, 50°C),
spectrum (CDCl3, 50°C), δ, ppm (J, Hz): 1.57‒2.23 (12H,
2
3
3
δ, ppm: 1.63‒1.81 (6H, m, 3CH2); 3.72 (2H, s, CH2); 4.03‒
4.14 (4H, m, 2CH2); 4.10‒4.34 (4H, m, 2CH2); 7.12‒7.31
(15H, m, H Ph); 7.40 (1H, s, H-8 ). 13C NMR spectrum
(CDCl3, 50°C), δ, ppm: 24.8; 26.3; 42.5; 46.4; 46.7; 56.1;
62.3; 118.8; 126.5; 128.0; 129.3; 138.5; 146.5; 152.5; 154.2;
154.3; 170.5. Found, m/z: 566.2328 [M+H]+. C33H33ClN5O2.
Calculated, m/z: 566.2317.
2-(1H-Imidazol-1-yl)-6-(piperidin-1-yl)-9-(tetrahydro-
2H-pyran-2-yl)-9H-purine (9a). Anhydrous K3PO4 (1.38 g,
6.52 mmol, 3.0 equiv) was added to a solution of com-
pound 8 (700 mg, 2.17 mmol, 1.0 equiv), imidazole
(222 mg, 3.26 mmol, 1.5 equiv) in dry NMP (3.5 ml) under
Ar, and the reaction mixture was stirred at 160°C for 14 h.
Then the reaction mixture was poured into H2O (30 ml) and
extracted with PhMe (3 × 20 ml). The organic phase was
washed with brine (3 × 20 ml), H2O (2 × 20 ml), and again
with brine (10 ml), dried over anhydrous Na2SO4, filtered,
and evaporated. Silica gel column chromatography (eluent
CH2Cl2 in MeOH, gradient 0–5%) of the residue provided
product 9a. Yield 523 mg (68%), colorless solid. Rf 0.41
(CH2Cl2–MeOH, 20:1). HPLC: tR 4.75 min. 1H NMR
spectrum (CDCl3, 50°C), δ, ppm (J, Hz): 1.57‒2.15 (12H,
m, 6CH2); 3.82 (1H, td, J = J = 11.2, J = 2.7) and 4.17
2
(1H, d, J = 11.2, CH2O); 4.15‒4.55 (4H, m, 2CH2); 5.85
(1H, dd, J = 10.2, 3J = 2.2, NCHO); 7.35 (1H, t, J = 7.5,
H Ar); 7.45 (2H, t, 3J = 7.6, H Ar); 7.93‒8.01 (3H, m, H Ar,
H-8); 8.69 (1H, s, H triazole). 13C NMR spectrum (CDCl3,
50°C), δ, ppm: 23.0; 24.9; 25.2; 26.3; 32.4; 46.8; 68.9;
81.9; 118.9; 119.3; 126.3; 128.3; 128.9; 130.9; 136.9;
147.5; 149.6; 151.2; 154.1. Found, m/z: 431.2332 [M+H]+.
C23H27N8O. Calculated, m/z: 431.2302.
3
3
Compound 9d. Yield 125 mg (32%), colorless solid.
Rf 0.20 (PhMe–MeCN, 10:1). HPLC: tR 6.78 min. 1H NMR
spectrum (CDCl3, 50°C), δ, ppm (J, Hz): 1.55‒2.11 (11H,
2
m) and 2.15 (1H, d, J = 11.2, 6CH2); 3.83 (1H, td,
2J = 3J = 11.4, 3J = 2.8) and 4.15 (1H, d, 2J = 11.2, CH2O);
4.23‒4.48 (4H, m, 2CH2); 5.93 (1H, dd, 3J = 10.1, 3J = 2.3,
3
4
NCHO); 7.39 (1H, tt, J = 7.5, J = 1.5, H Ar); 7.47 (2H, t,
3J = 7.6, H Ar); 7.93‒8.01 (3H, m, H Ar, H-8 ); 8.15 (1H,
s, H triazole). 13C NMR spectrum (CDCl3, 50°C), δ, ppm:
23.0; 24.9; 25.2; 26.4; 32.6; 46.8; 68.9; 81.5; 119.1; 126.8;
129.0; 129.1; 130.3; 133.9; 136.8; 149.8; 151.1; 151.4; 154.3.
Found, m/z: 431.2275 [M+H]+. C23H27N8O. Calculated, m/z:
431.2302.
2
3
3
m, 6CH2); 3.76 (1H, td, J = J = 11.2, J = 2.8) and 4.15
Alternative method for the synthesis of compound 9c.
Phenylacetylene (251 μl, ρ 0.93 g/cm3, 2.29 mmol, 1.5 equiv)
was added to a solution of compound 13 (500 mg, 1.52 mmol,
1.0 equiv), CuI (58 mg, 0.30 mmol, 0.20 equiv), AcOH
(95 µl, ρ 1.05 g/cm3, 1.67 mmol, 1.1 equiv), and Et3N (230 µl,
ρ 0.73 g/cm3, 1.67 mmol, 1.1 equiv) in CH2Cl2 (15 ml).
The reaction mixture was stirred isolated from daylight for
1 h at 20°C. Then the reaction mixture was poured into H2O
2
(1H, d, J = 11.2, CH2O); 4.10‒4.36 (4H, m, 2CH2); 5.66
(1H, dd, 3J = 9.8, 3J = 2.4, NCHO); 7.09 (1H, s, H imidazole);
7.85 (1H, s, H imidazole); 7.87 (1H, s, H-8); 8.54 (1H, s,
H imidazole). 13C NMR spectrum (CDCl3, 50°C), δ, ppm:
23.0; 24.8; 25.0; 26.2; 31.8; 46.6; 68.9; 81.8; 117.0; 118.2;
129.7; 136.0; 136.3; 149.7; 151.1; 153.8. Found, m/z:
354.2033 [M+H]+. C18H24N7O. Calculated, m/z: 354.2037.
564

Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
(30 ml) and extracted with CH2Cl2 (3 × 20 ml). The combined
procedure for the synthesis of compound 11a. Yield 185 mg
(45%), colorless solid. Rf 0.46 (CH2Cl2–MeOH, 20:1).
HPLC: tR 7.33 min. H NMR spectrum (CDCl3, 50°C),
organic phase was washed with aqueous NaHSO3 (15 ml) and
saturated NaCl (15 ml), dried over anhydrous Na2SO4,
filtered, and evaporated. Silica gel column chromatography
(eluent CH2Cl2 in MeCN, gradient 0–10%) of the residue
provided product 9c as a colorless solid. Yield 506 mg (77%).
2-(1H-Imidazol-1-yl)-6-(piperidin-1-yl)-9H-purine (10a).
p-TsOH·H2O (429 mg, 2.49 mmol, 2.2 equiv) was added to
a solution of compound 9a (400 mg, 1.13 mmol, 1.0 equiv)
in MeOH (12 ml), and the reaction mixture was stirred for
14 h at 60°C. Then the reaction mixture was evaporated,
the residue was suspended in 0.5 M solution of K2CO3 in
H2O–MeOH, 10:1 (20 ml) and filtered off. Recrystalli-
zation of the precipitate from EtOH provided product 10a.
Yield 161 mg (53%), colorless solid, mp 272‒274°C
(decomp., EtOH). Rf 0.23 (CH2Cl2–MeOH, 20:1). HPLC:
tR 3.71 min. 1H NMR spectrum (DMSO-d6, 60°C), δ, ppm:
1.55‒1.80 (6H, m, 3CH2); 4.12‒4.39 (4H, m, 2CH2); 7.05
(1H, s, H imidazole); 7.85 (1H, s, H imidazole); 8.04 (1H,
s, H-8); 8.46 (1H, s, H imidazole); 13.00 (1H, br. s, NH).
13C NMR spectrum (DMSO-d6, 60°C), δ, ppm: 24.2; 25.7;
45.8; 116.8; 117.2; 129.4; 135.5; 138.1; 148.8; 152.0; 153.0.
Found, m/z: 270.1450 [M+H]+. C13H16N7. Calculated, m/z:
270.1462.
2-[2-(1H-Imidazol-1-yl)-6-(piperidin-1-yl)-9H-purin-
9-yl]ethyl 3,3,3-triphenylpropanoate (11a). DIAD (0.09 ml,
ρ 1.03 g/cm3, 0.45 mmol, 1.2 equiv) was added over 15 min
to a solution of compound 9a (100 mg, 0.37 mmol, 1.0 equiv),
Ph3P (117 mg, 0.45 mmol, 1.2 equiv), and 2-hydroxyethyl
3,3,3-triphenylpropanoate (2) (154 mg, 0.45 mmol, 1.2 equiv)
in dry THF (5 ml) at 0°C, followed by stirring for 12 h at
20°C. Then the reaction mixture was evaporated, the
residue was suspended in MeOH (15 ml) and H2O (1 ml)
and cooled at –10°C for 30 min. The formed precipitate
was filtered off and washed with cold MeOH (2 × 5 ml) to
give compound 11a. Yield 173 mg (78%), colorless solid.
Rf 0.39 (CH2Cl2–MeOH, 20:1). HPLC: tR 6.64 min. 1H NMR
spectrum (CDCl3, 50°C), δ, ppm: 1.64‒1.88 (6H, m, 3CH2);
3.71 (2H, s, CH2); 4.04‒4.18 (4H, m, 2CH2); 4.14‒4.42
(4H, m, 2CH2); 7.06‒7.32 (16H, m, H Ph, H imidazole);
7.43 (1H, s, H-8); 7.84 (1H, s, H imidazole); 8.53 (1H, s,
H imidazole). 13C NMR spectrum (CDCl3, 50°C), δ, ppm:
24.8; 26.3; 42.4; 46.2; 46.5; 55.9; 62.1; 117.1; 118.2;
126.5; 128.0; 129.2; 129.9; 136.4; 138.4; 146.3; 149.8;
151.6; 153.9; 170.6. Found, m/z: 598.2930 [M+H]+.
C36H36N7O2. Calculated, m/z: 598.2925.
1
δ, ppm: 1.66‒1.90 (6H, m, 3CH2); 3.75 (2H, s, CH2); 4.10‒
4.24 (4H, m, 2CH2); 4.08‒4.54 (4H, m, 2CH2); 7.08‒7.35
(15H, m, H Ph); 7.50 (1H, s, H-8); 8.10 (1H, s, H triazole);
9.13 (1H, s, H triazole). 13C NMR spectrum (CDCl3, 50°C),
δ, ppm: 24.7; 26.2; 42.4; 46.1; 46.4; 55.9; 62.4; 118.8;
126.4; 128.0; 129.1; 139.0; 143.8; 146.3; 149.4; 151.5; 153.1;
153.8; 170.5. Found, m/z: 599.2879 [M+H]+. C35H35N8O2.
Calculated, m/z: 599.2878.
2,6-Diazido-9-(tetrahydro-2H-pyran-2-yl)-9H-purine
(12). NaN3 (2.38 g, 36.63 mmol, 4.0 equiv) was added to a
solution of compound 727 (2.50 g, 9.16 mmol, 1.0 equiv) in
Me2CO (60 ml), and the reaction mixture was stirred in
dark for 14 h at 50°C. Then the reaction mixture was
evaporated, the residue was suspended in H2O (50 ml),
filtered off, and washed with H2O (3 × 20 ml) to give
compound 12. Yield 2.58 g (98%), colorless solid. Rf 0.26
(CH2Cl2–MeOH, 20:1). HPLC: tR 5.74 min. 1H NMR
spectrum (CDCl3), δ, ppm (J, Hz): 1.58‒2.16 (6H, m,
2
3CH2); 3.75 (1H, td, J = 3J = 11.3, 3J = 2.6) and 4.16 (1H,
2
3
d, J = 11.3, CH2O); 5.68 (1H, dd, J = 10.1, 3J = 2.2,
NCHO); 8.10 (1H, s, H-8 ). 13C NMR spectrum (CDCl3),
δ, ppm: 22.7; 24.9; 31.9; 68.9; 82.1; 121.4; 141.5; 153.2;
153.7; 156.0. Found, m/z: 287.1121 [M+H]+. C10H11N10O.
Calculated, m/z: 287.1112.
2-Azido-6-(piperidin-1-yl)-9-(tetrahydro-2H-pyran-
2-yl)-9H-purine (13). Piperidine (2.59 ml, ρ 0.86 g/cm3,
26.2 mmol, 3.0 equiv) was added to a solution of com-
pound 12 (2.5 g, 8.74 mmol, 1.0 equiv) in PhMe (30 ml).
The reaction mixture was stirred in dark for 1 h at 50°C.
Then the reaction mixture was evaporated, and the residue
was purified by silica gel column chromatography (eluent
CH2Cl2 in MeCN, gradient 0–10%). Yield 1.61 g (56%),
pale-yellow solid. Rf 0.45 (CH2Cl2–MeOH, 20:1). HPLC:
tR 7.20 min. 1H NMR spectrum (CDCl3, 50°C), δ, ppm (J, Hz):
2
3
1.55‒2.11 (12H, m, 6CH2); 3.73 (1H, td, J = J = 11.4,
3J = 2.4) and 4.12 (1H, d, 2J = 11.4, CH2O); 4.09‒4.35 (4H,
3
3
m, 2CH2); 5.64 (1H, dd, J = 10.0, J = 2.0, NCHO); 7.82
(1H, s, H-8). 13C NMR spectrum (CDCl3, 50°C), δ, ppm:
23.0; 24.8; 25.0; 26.2; 32.0; 46.5; 68.9; 81.4; 117.3; 135.5;
151.7; 153.8; 156.2. Found, m/z: 329.1835 [M+H]+.
C15H21N8O. Calculated, m/z: 329.1833.
6-(Piperidin-1-yl)-2-(1H-tetrazol-1-yl)-9H-purine (17).
HC(OEt)3 (240 mg, ρ 0.89 g/cm3, 11.75 mmol, 1.6 equiv) was
added to a solution of compound 16 (1.60 g, 7.34 mmol,
1.0 equiv) and NaN3 (716 mg, 11.02 mmol, 1.5 equiv) in
AcOH (40 ml), and the reaction mixture was stirred for 6 h
at 80°C. Then the reaction mixture was evaporated and
suspended in saturated aqueous NaHCO3 solution (40 ml).
The precipitate was filtered off, washed with saturated
aqueous NaHCO3 solution (2 × 10 ml), H2O (2 × 10 ml),
and dried to provide product 17. Yield 1.31 g (66%),
colorless solid. Rf 0.39 (CH2Cl2–MeOH, 20:1). HPLC:
tR 4.48 min. 1H NMR spectrum (DMSO-d6, 60°C), δ, ppm:
1.54‒1.80 (6H, m, 3CH2); 4.19‒4.39 (4H, m, 2CH2); 8.17
(1H, s, H-8 ); 9.97 (1H, s, H tetrazole). 13C NMR spectrum
(DMSO-d6, 60°C), δ, ppm: 24.1: 25.7; 45.8 (br); 118.3;
2-[6-(Piperidin-1-yl)-2-(1H-1,2,4-triazol-1-yl)-9H-purin-
9-yl]ethyl 3,3,3-triphenylpropanoate (11b). p-TsOH·H2O
(283 mg, 1.49 mmol, 2.2 equiv) was added to a solution of
compound 9b (240 mg, 0.68 mmol, 1.0 equiv) in MeOH
(10 ml), and the reaction mixture was stirred for 14 h at 60°C.
Then the reaction mixture was evaporated and suspended in
0.5 M K2CO3 solution in H2O–MeOH, 10:1 (20 ml), and
filtered. The crude 9H-deprotected compound 10b (100 mg,
0.37 mmol, 1.0 equiv) was used for the synthesis of
compound 11b, using 2-hydroxyethyl 3,3,3-triphenyl-
propanoate (2) (154 mg, 0.45 mmol, 1.2 equiv), Ph3P
(117 mg, 0.45 mmol, 1.2 equiv), DIAD (0.09 ml, ρ 1.03 g/cm3,,
0.45 mmol, 1.2 equiv), and THF (5 ml) according to the
565

Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
139.3; 142.9; 146.8; 151.7; 152.9. Found, m/z: 272.1003
CuI (756 mg, 3.98 mmol, 0.3 equiv), 9H-carbazole (5.54 g,
33.18 mmol, 2.5 equiv), and anhydrous K3PO4 in dry PhMe
(100 ml). The reaction mixture was stirred at 120°C for 48 h.
Then the reaction mixture was centrifuged, the supernatant
was extracted with H2O (3 × 40 ml) and brine (10 ml). The
organic phase was evaporated, and silica gel column
chromatography (eluent hexane in MTBE, gradient 0–
20%), followed by precipitation from MeOH (50 ml) and
washing with cold MeOH (3 × 20 ml), provided product
20. Yield 1.61 g (26%), colorless solid. Rf 0.79 (CH2Cl2).
[M+H]+. C11H14N9. Calculated, m/z: 272.1367.
2-[6-(Piperidin-1-yl)-2-(1H-tetrazol-1-yl)-9H-purin-
9-yl]ethyl 3,3,3-triphenylpropanoate (18a) was prepared
analogously to compound 11a from compound 17 (300 mg,
1.12 mmol, 1.0 equiv), 2-hydroxyethyl 3,3,3-triphenyl-
propanoate (2) (465 mg, 1.34 mmol, 1.2 equiv), Ph3P (352 mg,
1.34 mmol, 1.2 equiv), and DIAD (0.27 ml, ρ 1.03 g/cm3,
1.34 mmol, 1.2 equiv) in THF (10 ml) for 12 h at 20°C.
Yield 570 mg (85%), colorless solid. Rf 0.38 (CH2Cl2–
MeCN, 10:1). HPLC: tR 7.48 min. 1H NMR spectrum (CDCl3,
50°C), δ, ppm: 1.67‒1.89 (6H, m, 3CH2); 3.72 (2H, s,
CH2); 4.10‒4.22 (4H, m, 2CH2); 4.02‒4.62 (4H, m, 2CH2);
7.12‒7.30 (15H, m, H Ph); 7.52 (1H, s, H-8 ); 9.38 (1H, s,
H tetrazole). 13C NMR spectrum (CDCl3, 50°C), δ, ppm:
24.6; 26.2; 42.5; 46.0; 46.4; 55.8; 62.0; 119.3; 126.4; 127.9;
129.0; 139.4; 141.7; 146.2; 147.4; 151.1; 153.7; 170.4.
Found, m/z: 600.2823 [M+H]+. C34H34N9O2. Calculated,
m/z: 600.2830.
9,9'-(5-{[6-(Piperidin-1-yl)-2-(1H-tetrazol-1-yl)-9H-purin-
9-yl]methyl}-1,3-phenylene)bis(9H-carbazole) (18b) was
prepared analogously to compound 11a from compound 17
(118 mg, 0.44 mmol, 1.0 equiv), [3,5-di(9H-carbazol-9-yl)-
phenyl]methanol (21) (212 mg, 0.48 mmol, 1.1 equiv),
Ph3P (138 mg, 0.53 mmol, 1.2 equiv), and DIAD (0.11 ml,
ρ 1.03 g/cm3, 0.53 mmol, 1.2 equiv) in THF (5 ml) for 1 h
at 20°C. Yield 233 mg (77%), colorless solid. Rf 0.75
(CH2Cl2–MeCN, 10:1). HPLC: tR 9.57 min. 1H NMR
spectrum (CDCl3, 50°C), δ, ppm (J, Hz): 1.62‒1.93 (6H,
m, 3CH2); 3.91‒4.67 (4H, m, 2CH2); 5.60 (2H, s, CH2);
1
HPLC: tR 10.26 min. H NMR spectrum (CDCl3), δ, ppm
(J, Hz): 4.00 (3H, s, CH3); 7.34 (4H, t, 3J = 7.5, H Ar); 7.46
3
3
(4H, t, J = 7.5, H Ar); 7.54 (4H, d, J = 7.5, H Ar); 8.04
4
3
(1H, t, J = 1.9, H Ar); 8.17 (4H, d, J = 7.5, H Ar); 8.39
(2H, d, J = 1.9, H Ar). 13C NMR spectrum (CDCl3),
4
δ, ppm: 52.9; 109.7; 120.7; 120.8; 123.9; 126.5; 126.8;
129.4; 134.0; 139.9; 140.5; 168.5. Found, m/z: 467.1772
[M+H]+. C32H23N2O2. Calculated, m/z: 467.1754.
[3,5-Di(9H-carbazol-9-yl)phenyl]methanol (21).44
A solution of LiAlH4 (406 mg, 10.20 mmol, 5.0 equiv) in
dry THF (20 ml) was stirred at 0°C for 30 min, then a
solution of compound 20 (1.00 g, 2.14 mmol, 1.0 equiv) in
dry THF (10 ml) was added dropwise, and the reaction
mixture was stirred at 0°C for 2 h. Then the reaction
mixture was evaporated, suspended in CH2Cl2 (20 ml), and
centrifuged. Silica gel column chromatography (CH2Cl2)
provided product 21. Yield 676 mg (72%), colorless solid.
1
Rf 0.36 (CH2Cl2). HPLC: tR 8.41 min. H NMR spectrum
(CDCl3), δ, ppm (J, Hz): 1.99 (1H, br. s, OH), 4.95 (2H, s,
3
3
CH2); 7.33 (4H, t, J = 7.5, H Ar); 7.46 (4H, t, J = 7.5,
3
3
3
7.28 (4H, t, J = 7.5, H Ar); 7.37 (4H, t, J = 7.5, H Ar);
H Ar); 7.56 (4H, d, J = 7.5, H Ar); 7.73 (2H, s, H Ar);
3
3
7.45 (4H, d, J = 7.5, H Ar); 7.64 (2H, s, H Ar); 7.79 (1H,
7.76 (1H, s, H Ar); 8.17 (4H, d, J = 7.5, H Ar). 13C NMR
3
s, H Ar); 7.93 (1H, s, H-8 ); 8.10 (4H, d, J = 7.5, H Ar);
spectrum (CDCl3), δ, ppm: 64.6; 109.8; 120.5; 120.6;
123.8; 123.9; 124.3; 126.3; 139.7; 140.7; 144.9. Found, m/z:
439.1835 [M+H]+. C31H23N2O. Calculated, m/z: 439.1805.
9.36 (1H, s, H tetrazole). 13C NMR spectrum (CDCl3, 50°C),
δ, ppm: 24.7; 26.3; 46.8; 47.2; 109.7; 119.8; 120.7; 120.9;
124.0; 124.6; 125.1; 126.4; 138.9; 139.6; 140.5; 140.6;
141.8; 148.1; 151.7; 154.2. Found, m/z: 692.2542 [M+H]+.
C42H34N11. Calculated, m/z: 692.2993.
Supplementary information file containing experimental
procedures for compounds 2, 8, 14–16 and 1H and
13C NMR spectra of compounds 4, 9a–d, 10a, 11a,b, 12,
13, 17, 18a–c, 20, and 21, is available at the journal
website http://hgs.osi.lv.
9-(2-Chloroethyl)-6-(piperidin-1-yl)-2-(1H-tetrazol-
1-yl)-9H-purine (18c) was prepared analogously to com-
pound 11a from compound 17 (400 mg, 1.48 mmol,
1.0 equiv), 2-chloroethanol (0.11 ml, ρ 1.20 g/cm3, 1.63 mmol,
1.1 equiv), Ph3P (466 mg, 1.78 mmol, 1.2 equiv), and
DIAD (0.35 ml, ρ 1.03 g/cm3, 1.78 mmol, 1.2 equiv) in
THF (8 ml) for 3 h at 20°C. The product was purified by
silica gel column chromatography (eluent CH2Cl2 in
MeCN, gradient 0–6%). Yield 312 mg (63%), colorless
solid. Rf 0.15 (CH2Cl2–MeCN, 20:1). HPLC: tR 5.14 min.
1H NMR spectrum (CDCl3, 50°C), δ, ppm (J, Hz): 1.67‒
This work was supported by the ERDF 1.1.1.1 activity
project No. 1.1.1.1/16/A/131 ''Design and Investigation of
Light Emitting and Solution Processable Organic Molecular
Glasses''.
References
1. Rosemeyer, H. Chem. Biodiversity 2004, 1, 361.
2. Legraverend, M. Tetrahedron 2008, 64, 8585.
3. Arsenyan, P.; Vasiljeva, P.; Domracheva, I.; Kanepe-Lapsa, I.
Chem. Heterocycl. Compd. 2020, 56, 776. [Khim. Geterotsikl.
Soedin. 2020, 56, 776.]
4. Musiyak, V. V.; Gruzdev, D. A.; Kravchenko, M. A.;
Vakhrusheva, D. V.; Levit, G. L.; Krasnov, V. P.; Charushin, V. N.
Mendeleev Commun. 2019, 29, 11.
5. Manvar, A.; Shah, A. Tetrahedron 2013, 69, 8105.
6. Novosjolova, I.; Bizdena, E.; Turks, M. Eur. J. Org. Chem.
2015, 3629.
7. Saito, Y.; Hudson, R. H. E. J. Photochem. Photobiol. C 2018,
36, 48.
3
1.88 (6H, s, 3CH2); 3.96 (2H, t, J = 5.7, CH2); 4.10‒4.50
3
(4H, m, 2CH2); 4.57 (2H, t, J = 5.7, CH2); 7.88 (1H, s,
H-8); 9.41 (1H, s, H tetrazole). 13C NMR spectrum (CDCl3,
50°C), δ, ppm: 24.7; 26.3; 42.5; 46.0; 47.0; 119.8; 139.9;
141.9; 147.8; 151.4; 154.1. Found, m/z: 334.1268 [M+H]+.
C13H17ClN9. Calculated, m/z: 334.1290.
Methyl 3,5-di(9H-carbazol-9-yl)benzoate (20).43
(1R,2R)-Cyclohexane-1,2-diamine (0.96 ml, ρ 0.95 g/cm3,
7.96 mmol, 0.6 equiv) was added to a solution of methyl
3,5-dibromobenzoate (19) (3.90 g, 13.27 mmol, 1.0 equiv),
566

Chemistry of Heterocyclic Compounds 2021, 57(5), 560–567
8. Xu, W.; Chan, K. M.; Kool, E. T. Nat. Chem. 2017, 9, 1043. Savy, P. P. A.; Seward, E. M.; Ultsch, M.; Sutherlin, D. P.
9. Matarazzo, A.; Hudson, R. H. E. Tetrahedron 2015, 71, 1627.
10. Dumas, A.; Luedtke, N. W. Chem.–Eur. J. 2012, 18, 245.
11. Šišuļins, A.; Bucevičius, J.; Tseng, Y.; Novosjolova, I.;
Traskovskis, K.; Bizdēna, Ē.; Chang, H.; Tumkevičius, S.;
Turks, M. Beilstein J. Org. Chem. 2019, 15, 474.
J. Med. Chem. 2012, 55, 7686.
26. Carrera, D. E.; Sheng, P.; Safina, B. S.; Li, J.; Angelaud, R.
Org. Proc. Res. Dev. 2013, 17, 138.
27. Kapilinskis, Z.; Novosjolova, I.; Turks, M. Molbank 2018,
M1024.
12. Yang, Y.; Cohn, P.; Dyer, A. L.; Eom, S. H.; Reynolds, J. R.;
Castellano, R. K.; Xue, J. Chem. Mater. 2010, 22, 3580.
13. Yang, Y.; Cohn, P.; Eom, S.-H.; Abboud, K. A.; Castellano, R. K.;
Xue, J. J. Mater. Chem. C 2013, 1, 2867.
14. Wang, Z.; Yao, J.; Zhan, L.; Gong, S.; Ma, D.; Yang, C. Dyes
Pigm. 2020, 180, 108437.
15. Traskovskis, K.; Mihailovs, I.; Tokmakovs, A.; Kokars, V.;
Rutkis, M. Proc. SPIE 2012, 8434, 84341P.
16. Traskovskis, K.; Ruduss, A.; Kokars, V.; Mihailovs, I.; Lesina, N.;
Vembris, A. New J. Chem. 2019, 43, 37.
28. Hellerman, L. J. Am. Chem. Soc. 1927, 49, 1735.
29. Huang, R.; Xie, C.; Huang, L.; Liu, J. Tetrahedron 2013, 69, 577.
30. Bhakuni, D. S.; Gupta, P. K.; Chowdhury, B. L. Indian J.
Chem., Sect. B: Org. Chem. Incl. Med. Chem. 1984, 23B, 1286.
31. Li, J. J.; Gui, X. X. Chin. Chem. Lett. 2014, 25, 1341.
32. Sebris, A.; Turks, M. Chem. Heterocycl. Compd. 2019, 55,
1041. [Khim. Geterotsikl. Soedin. 2019, 55, 1041.]
33. Zaķis, J. M.; Ozols, K.; Novosjolova, I.; Vilšķērsts, R.;
Mishnev, A.; Turks, M. J. Org. Chem. 2020, 85, 4753.
34. Ozols, K.; Cīrule, D.; Novosjolova, I.; Stepanovs, D.;
Liepinsh, E.; Bizdena, E.; Turks, M. Tetrahedron Lett. 2016,
57, 1174.
17. Brændvang, M.; Gundersen, L. L. Bioorg. Med. Chem. 2005,
13, 6360.
18. Lagisetty, P.; Russon, L. M.; Lakshman, M. K. Angew.
Chem., Int. Ed. 2006, 45, 3660.
35. Novosjolova, I.; Bizdēna, Ē.; Turks, M. Tetrahedron Lett.
2013, 54, 6557.
19. Qu, G. R.; Zhang, H. L.; Niu, H. Y.; Xue, Z. K.; Lv, X. X.;
Guo, H. M. Green Chem. 2012, 14, 1877.
20. Wang, S. Ben; Jin, P.; Li, F. N.; Quan, Z. S. Eur. J. Med.
Chem. 2014, 84, 574.
36. Satoh, Y.; Marcopulos, N. Tetrahedron Lett. 1995, 36, 1759.
37. Kovaļovs, A.; Novosjolova, I.; Bizdēna, Ē.; Bižāne, I.;
Skardziute, L.; Kazlauskas, K.; Jursenas, S.; Turks, M.
Tetrahedron Lett. 2013, 54, 850.
21. Zhong, M.; Nowak, I.; Cannon, J. F.; Robins, M. J. J. Org.
Chem. 2006, 71, 4216.
38. Smirnova, N. B.; Postovskii, I. Y. Zh. Vses. Khim. O-va im.
D. I. Mendeleeva 1964, 9(6), 711.
22. Elzein, E.; Palle, V.; Wu, Y.; Maa, T.; Zeng, D.; Zablocki, J.
J. Med. Chem. 2004, 47, 4766.
23. Nilov, D. I.; Komarov, D. Y.; Panov, M. S.; Karabaeva, K. E.;
Mereshchenko, A. S.; Tarnovsky, A. N.; Wilson, R. M. J. Am.
Chem. Soc. 2013, 135, 3423.
24. Safina, B. S.; Sweeney, Z. K.; Li, J.; Chan, B. K.; Bisconte, A.;
Carrera, D.; Castanedo, G.; Flagella, M.; Heald, R.; Lewis, C.;
Murray, J. M.; Nonomiya, J.; Pang, J.; Price, S.; Reif, K.;
Salphati, L.; Seward, E. M.; Wei, B.; Sutherlin, D. P. Bioorg.
Med. Chem. Lett. 2013, 23, 4953.
25. Murray, J. M.; Sweeney, Z. K.; Chan, B. K.; Balazs, M.;
Bradley, E.; Castanedo, G.; Chabot, C.; Chantry, D.; Flagella, M.;
Goldstein, D. M.; Kondru, R.; Lesnick, J.; Li, J.; Lucas, M. C.;
Nonomiya, J.; Pang, J.; Price, S.; Salphati, L.; Safina, B.;
39. Huang, L. K.; Cherng, Y. C.; Cheng, Y. R.; Jang, J. P.;
Chao, Y. L.; Cherng, Y. J. Tetrahedron 2007, 63, 5323.
40. Kwon, T. H.; Kim, M. K.; Kwon, J.; Shin, D. Y.; Park, S. J.;
Lee, C. L.; Kim, J. J.; Hong, J. I. Chem. Mater. 2007, 19, 3673.
41. Vabre, R.; Legraverend, M.; Piguel, S. Dyes Pigm. 2014, 105,
145.
42. Grigalevicius, S.; Tavgeniene, D.; Krucaite, G.; Griniene, R.;
Li, W. C.; Luo, D.; Chang, C. H. Dyes Pigm. 2018, 152, 100.
43. Feng, K.; Zuniga, C.; Zhang, Y.-D.; Kim, D.; Barlow, S.;
Marder, S. R.; Brédas, J. L.; Weck, M. Macromolecules 2009,
42, 6855.
44. Yoon, S. H.; Shin, J.; Um, H. A.; Lee, T. W.; Cho, M. J.;
Kim, Y. J.; Son, Y. H.; Yang, J. H.; Chae, G.; Kwon, J. H.;
Choi, D. H. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 707.
567