Synthesis of thiacyanine dyes containing coumarin moieties at benzothiazole rings

Article abstract of DOI:10.1016/j.mencom.2013.07.011

New bischromophoric dyes (thiacarbocyanine and thiadicarbocyanine) containing coumarin moieties at each of the two benzothiazole systems, as well as monomethinecyanine containing coumarin moiety at only one heterocyclic system, manifest significant batho-chromic shift compared to relative coumarin-free dyes.

Full text of DOI:10.1016/j.mencom.2013.07.011

Mendeleev
Communications
Mendeleev Commun., 2013, 23, 212–214
Synthesis of thiacyanine dyes containing
coumarin moieties at benzothiazole rings
Zhanna A. Krasnaya,*a Vladimir A. Kuz’min,b
Lyubov V. Litvinkova,b Evgeniya O. Dorofeevaa and Sergei G. Zlotina
a N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian
Federation. Fax: +7 499 135 5328; e-mail: kra@ioc.ac.ru
b N. M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, 119334 Moscow, Russian
Federation
DOI: 10.1016/j.mencom.2013.007.011
New bischromophoric dyes (thiacarbocyanine and thiadicarbocyanine) containing coumarin moieties at each of the two benzothiazole
systems, as well as monomethinecyanine containing coumarin moiety at only one heterocyclic system, manifest significant batho-
chromic shift compared to relative coumarin-free dyes.
O2N
Coumarin derivatives constitute an important group of organic
luminophores and laser dyes that efficiently generate radiation
in the region of 400–560 nm.1–3 Coumarin polymethines have
efficient fluorescence that makes them promising probes in bio-
chemistry and medicine.4–7 Psoralens (furocoumarins) became
important for photobiology and photochemotherapy.8–10
Currently, cyanine dyes that form strongly fluorescenting com-
plexes with nucleic acids and proteins have found broad applica-
tion inchemistry and biochemistry for identification of biomacro-
molecules.11,12 It can be expected that cyanine dyes containing
coumarin moieties would possess valuable properties.
The purpose of this study was to synthesize cyanine dyes con-
taining coumarin moieties at heterocyclic systems. We intended
to obtain symmetric bischromophoric dyes (thiacarbocyanine and
thiadicarbocyanine) containing coumarin fragments at each of
the two benzothiazole systems, as well as monomethinecyanine
containing coumarin fragment at only one heterocyclic system.
Both symmetric and unsymmetric thiacarbocyanine dyes became
popular for identification of DNA molecules.
S
S
HNO3
H2SO4
NH2NH2·H2O
Me
Me
N
N
2
3
H2N
N2
Cl–
S
S
NaNO2
HCl
Me
Me
N
N
4
5
N
S
Me
O
O
6
O
O
1
Et
N
Me
S
TsOEt
The key compounds were quaternary salts obtained from
2-methylbenzothiazole bearing one coumarin moiety. It is expe-
dient to obtain such salts from the corresponding bases. Syn-
thesis of the latter was an independent and rather complicated task.
The key stage in their preparation supposed to be the Meerwein
reaction between aryldiazonium halides of benzothiazole type with
a,b-unsaturated carbonyl compounds (in our case coumarin). The
Meerwein reaction is a complicated process and usually accom-
panied by strong resinification, hence purification and identifica-
tion of the target product are often laborous. Despite these com-
plications, the Meerwein reaction seems nearly the only method
to incorporate a coumarin moiety into a heterocyclic system.
6-(Coumarin-2-yl)-3-ethyl-2-methylbenzothiazolium tosylate
1 was synthesized according to Scheme 1.† 2-Methylbenzo-
TsO
O
O
7
Scheme 1
hydrazine hydrate.17 Reduction of 2-methyl-6-nitrobenzothiazole 3 with
hydrazine hydrate in PriOH in the presence of ‘Norit’ activated carbon and
iron trichloride gave amine 4 in 80% yield, mp 123–124°C. UV (EtOH,
max/nm): 223, 290. H NMR (CDCl3) d: 2.75 (s, 3H, Me), 3.78 (br.s,
1
l
2H, NH2), 6.78 (dd, 1H, H-5), 7.05 (br.s, 1H, H-7), 7.7 (d, 1H, H-4).
3-(2-Methylbenzo[d]thiazol-6-yl)-2H-chromen-2-one 1. A solution of
sodium nitrite (7.6 g) in water (12 ml) was added dropwise at –5°C to a
stirred mixture of hydrochloric acid (25 ml), water (20 ml) and 6-amino-
2-methylbenzothiazole 4 (16.4 g) to give diazonium salt 5. Then the solu-
tion of 5 was filtered and added dropwise to a vigorously stirred mixture of
coumarin 6 (14.6 g), acetone (120 ml), AcONa·3H2O (27.5 g), CuCl2·2H2O
(5 g) and H2O (10 ml) cooled to 0°C. Nitrogen evolution continued for
1.5 h. Meanwhile, the reaction mixture reached room temperature. The
reaction mixture was steam distilled to collect ~1 dm3 of the distillate. After
cooling, CHCl3 (500 ml) was added to the residue in the distillation flask.
The flask contents were stirred with a mechanical stirrer and a dark resinous
precipitate was filtered off on a Büchner funnel. The precipitate was washed
with CHCl3. The filtrate was transferred into a separating funnel to separate
the lower chloroform layer, which was concentrated to 1/3 in vacuo. The
residue was subjected to column chromatography on Al2O3 using CHCl3
as the eluent. According to UV spectroscopic data, the first portions of the
†
1H NMR spectra were recorded using a Bruker WM-250 instrument
(250 MHz). The absorption spectra of the dyes were measured on a
Specord UV-VIS spectrophotometer. High resolution mass spectra were
obtained on a Bruker microTOF instrument by means of electrospray
ionization (ESI). The reactions were monitored using UV spectroscopy.
2-Methyl-6-nitrobenzothiazole 3 was obtained in 75% yield by nitration
of 2-methylbenzothiazole 2 with nitrating mixture in conc. H2SO4.13 A few
methods were tried for the reduction of 2-methyl-6-nitrobenzothiazole 3
into amine 4.14–16 It was found to be most convenient to use the method
suggested for the catalytic reduction of aromatic nitro compounds with
© 2013 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2013, 23, 212–214
S
S
Me2SO4
NaOH
SH
SMe
N
N
5
13
12
6
O
4
S
7
O
3
N
Et2SO4
7
Et
SMe
CH(OEt)3
1S
KI, Et3N
7
Et
N
EtSO4
N
Et
11
S
Et
TsO
I
N
O
O
8
O
S
S
O
Et
N
(EtO)2CHCH2CH(OEt)2
Et
N
O
O
7
Et
S
N
10
Scheme 3
2
S
TsO
formate in dry pyridine under reflux gave thiacarbocyanine
8 in 80% yield, whereas condensation of compound 7 with
1,1,3,3-tetraethoxypropane in dry pyridine under reflux led to
thiadicarbocyanine 9 in 52% yield.
O
O
9
Scheme 2
thiazole 2 was nitrated with a HNO3 + H2SO4 mixture to afford
nitro derivative 3 which was reduced to 6-amino-2-methylbenzo-
thiazole 4. Diazotization of amine 4 gave benzothiazolyldiazonium
chloride 5. The Meerwein reaction of the latter with coumarin 6
provided a small yield of 6-(coumarin-2-yl)-2-methylbenzo-
thiazole 1 which was isolated in pure form by steam removal of
other components, chromatography on Al2O3 and recrystallization
from EtOH. The structure and purity of base 1 were confirmed
Reaction of compound 7 with salt 11 in anhydrous EtOH in the
presence of dry pyridine and triethylamine followed by addition
(d, 2H, a-H, g-H, J 12.5 Hz), 7.4 (t, 1H, b-H, J 12.5 Hz), 7.15 (d, 2H,
H-5 and H-5'), 7.2 (d, 2H, H-4'', TsO–, J 7.5 Hz), 7.55 (d, 2H, H-3'', TsO–,
J 7.5 Hz), 8.02 (br.s, 2H, H-7 and H-7'), 8.18 (s, 2H, H-8 and H-8'),
7.6–7.8 (m, 10H, H-4,4',10,10',11,11',12,12',13,13'). Found (%): C, 66.75;
H, 4.21; N, 3.18. Calc. for C46H36N2O7S3 (%): C, 66.97; H, 4.40; N, 3.40.
3-Ethyl-2-{(1E,3E,5E)-5-[3-ethyl-6-(2-oxo-2H-chromen-3-yl)benzo-
[d]thiazol-2-(3H)-ylidene]penta-1,3-dienyl}-6-(2-oxo-2H-chromen-3-yl)-
benzo[d]thiazolium tosylate 9. A mixture of tosylate 7 (0.06 g, 0.12 mmol),
1,1,3,3-tetraethoxypropane (0.03 g, 0.13 mmol) and dry pyridine (0.5 ml)
was heated for 30 min to give a precipitate. Diethyl ether (10 ml) was
added to the cooled reaction mixture, the precipitate was separated and
washed with water, EtOH and Et2O. Recrystallization from EtOH gave
0.026 g (52%) of dye 9 as black crystals, mp 240–243°C (decomp.).
1
by its H NMR spectrum and high resolution mass spectrum
(micrOTOF). The formation of base 1 during the synthesis and
its purity were monitored by UV spectroscopy. Tosylate 7 was
obtained in 53% yield from base 1 by treatment with ethyl
p-toluenesulfonate.
Dyes 8, 9 (Scheme 2) and 10 (Scheme 3) were synthesized
from tosylate 7.‡ Condensation of compound 7 with ethyl ortho-
1
UV [lmax/nm (e)]: 700 (97396, EtOH), 720 (90312, CHCl3). H NMR
eluate contained coumarin (lmax = 215, 280 and 312 nm), while the next
portions were yellow and contained compound 1. These portions of the
eluate were concentrated in vacuo and the residue was recrystallized from
EtOH to give 2.1 g (7.2%) of 1 as cream-coloured crystals with mp 193–
195°C. UV spectrum [EtOH, lmax/nm (e)]: 215 (41320), 235 (sh, 19533),
(DMSO-d6) d: 1.3 (t, 6H, NCH2Me), 2.28 (s, 3H, MeC6H4SO3–), 4.35 (q,
4H, NCH2Me), 6.30–6.62 (m, 3H, a,g,e-H), 7.12 (d, 2H, H-4'', TsO–,
J 7.5 Hz), 7.35 (d, 2H, H-3'', TsO–, J 7.5 Hz), 7.41–7.80 (m, 16H, b,d-H,
H-5,5',7,7',4,4',10,10',11,11',12,12',13,13'). Found (%): C, 67.44; H, 4.62;
N, 3.15. Calc. for C48H38N2O7S3 (%): C, 67.74; H, 4.50; N, 3.29.
3-Ethyl-2-{[3-ethylbenzo[d]thiazol-2(3H)-ylidene]methyl}-6-(2-oxo-
2H-chromen-3-yl)benzo[d]thiazolium iodide 10. Dry pyridine (0.8 ml)
and NEt3 (0.04 ml) were added with stirring to a mixture of salts 7 (0.14 g,
0.28 mmol) and 11 (0.17 g, 0.51 mmol) in anhydrous EtOH (8.2 ml). The
mixture was stirred for 1 h at 40°C, then a 25% aqueous solution of KI
(0.5 ml) was added to give a fine precipitate. The mixture was cooled in
a refrigerator and the precipitate was separated, washed with Et2O and
recrystallized from MeOH to give 0.06 g (35%) of dye 10 as greenish-
grey crystals, mp > 265°C. UV [lmax/nm (e)]: 440 (50155, EtOH), 443
(56256, CHCl3). 1H NMR (DMSO-d6) d: 1.4 (t, 6H, NCH2Me), 4.75 (q,
4H, NCH2Me), 6.80 (s, 1H, a-H), 8.35 (s, 1H, H-7), 8.55 (s, 1H, H-8),
7.3–8.2 (m, 10H, Ar). Found (%): C, 55.32; H, 3.61; N, 4.25. Calc. for
C28H23IN2O2S2 (%): C, 55.08; H, 3.80; N, 4.59.
1
270 (13147), 310 (sh, 21411), 335 (25167). H NMR (CDCl3) d: 2.85
(s, 3H, Me), 7.7 (d, 1H, H-5, J 8 Hz), 7.9 (s, 1H, H-7), 8.0 (d, 1H, H-4,
J 8 Hz), 8.25 (s, 1H, H-8), 7.2–7.6 (m, 4H, Ar). MS (ESI), m/z: 294.0610
[M+H]+ (calc. for C17H11NO2S: 294.059). Base 1 is insoluble in diethyl
ether but soluble in ethanol on heating.
3-Ethyl-2-methyl-6-(2-oxo-2H-chromen-3-yl)benzo[d]thiazolium tosylate 7.
A mixture of 1 (0.18 g, 0.6 mmol) and ethyl p-toluenesulfonate (0.14 g,
0.7 mmol) was refluxed for 4 h at 140°C. After cooling, the reaction mixture
was ground into powder and dry acetone (1.5 ml) was added. The mix-
ture was kept for 1 h at –5°C and a grey precipitate was separated, which
was repeatedly washed with acetone to give 0.16 g (53%) of tosylate 7,
mp 96–97°C. UV [EtOH, lmax/nm (e)]: 315 (sh), 335. 1H NMR (CDCl3)
d: 2.2 (s, 3H, MeC6H4SO3–), 1.5 (t, 3H, NCH2Me), 3.2 (s, 3H, Me), 4.8
(q, 2H, NCH2Me), 7.0 (d, 2H, MeC6H4SO3–), 7.6 (d, 2H, MeC6H4SO3–),
7.3–8.3 (m, 6H, Ar), 7.9 (s, 1H, H-7), 8.65 (s, 1H, H-8).
Ethyl 3-ethyl-2-methylthiobenzo[d]thiazolium sulfate 11 was obtained
in 90% yield by heating of equimolar mixture of compound 12 and freshly
distilled diethyl sulfate for 2 h in a bath (100°C); mp 47–50°C. 1H NMR
(CDCl3) d: 1.25 (t, 3H, OCH2Me), 1.55 (t, 3H, NCH2Me), 3.10 (s, 3H,
SMe), 4.10 (q, 2H, OCH2Me), 4.70 (q, 2H, NCH2Me), 7.60 (t, 1H, H-5),
7.75 (t, 1H, H-6), 8.00 (d, 1H, H-7), 8.23 (d, 1H, H-4).
2-Methylmercaptobenzothiazole 12 was obtained from 2-mercapto-
benzothiazole 13 using a reported procedure;18 yield 60%, mp 43–45°C
(EtOH). 1H NMR (CDCl3) d: 2.80 (s, 3H, Me), 7.28 (t, 1H, H-5, J 10 Hz),
7.42 (t, 1H, H-6, J 10 Hz), 7.75 (d, 1H, H-7, J 10 Hz), 7.88 (d, 1H, H-4,
J 10 Hz), assignment to H-5/H-6 and to H-4/H-7 was made arbitrarily.
‡
3-Ethyl-2-{(1E,3E)-3-[3-ethyl-6-(2-oxo-2H-chromen-3-yl)benzo[d]-
thiazol-2(3H)-ylidene]prop-1-enyl}-6-(2-oxo-2H-chromen-3-yl)benzo-
[d]thiazolium tosylate 8. A mixture of tosylate 7 (0.45 g, 0.9 mmol), dry
triethyl orthoformate (0.5 g, 3.4 mmol) and dry pyridine (3 ml) was heated
for 15 min. After cooling, Et2O (15 ml) was added to the reaction mixture
containing a dark precipitate. The precipitate was separated and recrys-
tallized from EtOH. Drying resulted in 0.3 g (81%) of dye 8 as dark grey
crystals with mp 245–247°C (lit.,20 mp 230–232°C). UV [lmax/nm (e)]:
585 (145412, EtOH), 600 (143304, CHCl3). 1H NMR (DMSO-d6) d: 1.3
(t, 6H, NCH2Me), 2.28 (s, 3H, MeC6H4SO3–), 4.3 (q, 4H, NCH2Me), 6.45
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Mendeleev Commun., 2013, 23, 212–214
of 25% aqueous KI solution gave thiamethinecyanine 10 in 35%
5 V. B. Kovalska, K. D. Volkova, A. V. Manaev, M. Yu. Losytskyy, J. N.
Okhrimenko, V. F. Traven and S. M. Yarmoluk, Dyes Pigm., 2010, 84,
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L. Gillespie, D. Brown and F. P. Gasparro, J. Dermatol. Sci., 1999,
19, 78.
9 A. E. da Hora Machado, J. A. de Miranda, A. M. de Olivenia Campus,
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13 Y. Mizuno and K. Adachi, J. Pharm. Soc. Jpn., 1952, 72, 745.
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yield.
1
The structures of dyes 8–10 were confirmed by H NMR
and UV spectra and by elemental analyses. The maxima in the
absorption spectra of dyes 8–10 have a long-wave shift in com-
parison with the non-substituted dyes, which indicates that con-
jugation exists between the coumarin moiety and the polymethine
chain of the dyes. The absorption bands of the coumarin fragment
of the dyes synthesized lie in the 280–300 nm region, and their
intensity is much lower than that of the polymethine chain band
for these dyes.
The formation of noncovalent complexes of the new bis-
chromophoric cyanine dyes with nucleic acids can be used in
nucleic acid identification by the fluorescent method. On the
other hand, photochemical activation of the coumarin moiety in
these dyes provides a unique opportunity for creating fluorescent
covalent labels in nucleic acid molecules by a photochemical
reaction with one of the bases in the structure of nucleic acids.19
The results on photochemical and photophysical studies of the
dyes 8–10 for fluorescent detection of biomacromolecules will
be published elsewhere.
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Received: 26th March 2013; Com. 13/4087
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