Functionalization of benzo[c,d]indole system for the synthesis of visible and near-infrared dyes

Full text of DOI:10.1002/jhet.39

8
4
Functionalization of Benzo[c,d]indole System for the Synthesis of
Visible and Near-Infrared Dyes
Vol 46
a
Maged Henary, Mariusz Mojzych, Martial Say, and Lucjan Strekowski*
Department of Chemistry, Georgia State University, Atlanta, Georgia 30302-4098
On leave of absence from Chemistry Department, University of Podlasie, ul. 3 Maja 54,
a
08-110 Siedlce, Poland
*E-mail: lucjan@gsu.edu
Received August 25, 2008
DOI 10.1002/jhet.39
Published online 11 February 2009 in Wiley InterScience (www.interscience.wiley.com).
A number of benzo[c,d]indolium derivatives have been synthesized.
J. Heterocyclic Chem., 46, 84 (2009).
INTRODUCTION
Vilsmeier reagent derived from N,N-dimethylformamide
and phosphorus oxychloride (Scheme 1).
Cationic indolium and benzo-fused indolium moieties
are common heterocyclic end units in cyanine and other
polymethine dyes. As a general rule, the dyes containing
benzo[c,d]indolium systems are more stable and show a
batochromic shift in their electronic spectra in compari-
son with indolium, benzo[e]indolium, and benzo[g]indo-
lium counterparts [1,2]. Synthesis of several 1-substi-
tuted 2-methylbenzo[c,d]indolium salts and examples of
their use in the preparation of visible and near-infrared
dyes are described in this report. We have concentrated
on the chemistry of a hydrophobic N-butyl derivative 4
Interestingly, depending on conditions, two different
products are formed, namely an enamine 5 or an unsatu-
rated aldehyde 7. The enamine 5 was independently
transformed to the aldehyde 7 by treatment with potas-
sium hydroxide in methanol under reflux conditions. Ei-
ther compound 5 or 7, can be efficiently condensed with
malononitrile to give a visible dye 6.
On the other hand, condensation of aldehyde 7 with
cyclohexanone under basic conditions furnished a bis(a-
minodien)one 8 ¼¼ O in low yield. This ketone is a visi-
ble dye (k
¼ 645 nm) in methanol under neutral or
max
(
(
Schemes 1 and 2) and its water-soluble analog 13
Scheme 3). The common precursor to these compounds
basic conditions. Under acidic conditions (pH < 2) dye
8
¼¼ O undergoes protonation at the carbonyl group with
is readily available 1,8-naphtholactam (1) [3,4].
the formation of a hydroxy-substituted cyanine 8AOH
that absorbs in the near-infrared region (kmax ¼ 915
nm). The ketone/cyanine conversions are fully reversi-
ble, depending solely on pH conditions. Although the
cyanine 8AOH absorbs mainly in the invisible near-
infrared region, it shows a residual absorption of low in-
tensity in the visible region and is pink to the naked
eye. The color change from deep dirty-green at basic or
neutral pH to beautiful light-pink at low pH is quite dra-
matic. It should be noted that an attempted condensation
of enamine 5 with cyclohexanone under conditions simi-
lar to those indicated above failed to produce dye 8 ¼¼ O.
A different synthetic route to the dye system 8 ¼¼ O/8AOH
is presented in Scheme 2. We have previously used this
approach for the synthesis of other pH-sensitive dyes [7].
In this two-step preparation the substrate 4 was
treated with the Vilsmeier-Haack reagent 9 [8] in acetic
anhydride in the presence of sodium acetate as a base
RESULTS AND DISCUSSION
One of the synthetic routes to N-alkylbenzo[c,d]indo-
lium salts starting with 1 involves the intermediary of a
derivative 2 [5,6]. Following the well-established chemis-
try, compound 2 was alkylated with n-butyl bromide and
the resultant N-butyl derivative 3 was transformed to the
desired iodide salt 4 by treatment with a mixture of acetic
acid, hydrochloric acid, and potassium iodide. We found
that it is not necessary to purify the intermediate product
3
as the synthesis of 4 proceeds in high yield with crude
substrate 3. The 2-methyl group in 4 is activated toward a
reaction with a base such as pyridine to generate an inter-
mediate anhydrobase or an enamine by proton abstraction
(not shown). This intermediate product is nucleophilic
and, as such, can undergo a reaction with the electrophilic
V
C
2009 HeteroCorporation

January 2009
Functionalization of Benzo[c,d]indole System for the Synthesis of Visible
and Near-Infrared Dyes
85
Scheme 1
Scheme 3
catalyst to furnish a chloro-substituted cyanine 10,
which then was transformed to 8 ¼¼ O by the reaction
with N-hydroxysuccinimide. The mechanism of this
highly efficient transformation has been discussed previ-
ously [7]. This synthesis of 8 ¼¼ O (Scheme 2) is more
efficient and experimentally simpler than that shown in
Scheme 1. Because of the high efficiency it was not
necessary to purify the intermediate dye 10. The final
product 8 ¼¼ O was obtained in an analytically pure form
by using simple crystallization.
ubility is provided by the presence of sulfonate groups.
As with other preparations discussed above, the starting
material is 1,8-naphtholactam (1). Compound 1 was
alkylated by the reaction with 1,4-butanesultone to give
a sulfonatobutyl derivative 11, the subsequent treatment
of which with methylmagnesium chloride furnished the
indolium product 13 [9]. The latter Grignard addition
reaction gave compound 13 in low yield, which is due
to low solubility of the potassium salt 11 in solvents
suitable for organometallic reactions. This problem was
overcome by transformation of the potassium salt 11 to
a highly soluble tetrabutylammonium salt 12. The reac-
tion of 12 with methylmagnesium chloride in tetrahydro-
furan furnished the desired product 13 in 85% yield.
The subsequent construction of the dye system 15 ¼¼ O/
The synthesis of a water soluble pH-sensitive system
15 ¼¼ O/15AOH is given in Scheme 3. The aqueous sol-
Scheme 2
1
5AOH is similar to that of 8 ¼¼ O/8AOH. The
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

8
6
M. Henary, M. Mojzych, M. Say, and L. Strekowski
Vol 46
difference is in the use of a sulfonatobutyl derivative 13
Scheme 3) rather than a butyl derivative 4 (Scheme 2)
for the condensation reaction with the Vilsmeier-Haack
reagent 9.
Since compounds 8 and 15 share the same chromo-
phore, they show virtually identical spectral properties
that depend on pH conditions. The transition ketone/cya-
Calcd. For C20
8
17 3
H N : C, 80.24; H, 5.72; N, 14.04. Found: C,
0.22; H, 5.75; N, 13.91.
(
(
1-Butyl-1,2-dihydrobenzo[c,d]indol-2-ylidene)acetalde-
ꢀ
hyde, 7. N,N-Dimethylformamide (1 mL) was stirred at 10 C
and treated dropwise with phosphorus oxychloride (0.21 mL,
2.3 mmol) and then with a solution of 4 (1.9 g, 0.54 mmol)
and pyridine (0.6 mL) in N,N-dimethylformamide (2 mL). The
ꢀ
mixture was heated to 80 C for 40 min, then treated with an
aqueous solution of potassium hydroxide (60%, 2.5 mL) and
nine for both 8 and 15 is characterized by pK ¼ 2, as
a
ꢀ
heated to 80 C for an additional 90 min. After cooling the
mixture was extracted with dichloromethane (4 ꢁ 10 mL), the
obtained by spectrophotometric titrations.
extracts were concentrated, and the residue was purified by
silica gel chromatography eluting with dichloromethane/metha-
ꢀ
1
EXPERIMENTAL
nol (19:1) to furnish 1.6 g (65%) of 7; mp 104–105 C;
H
NMR (deuteriochloroform): d 0.93 (t, J ¼ 7 Hz, 3H), 1.39 (m,
1
-Butyl-2-methylbenzo[c,d]indolium iodide, 4. Conversion
of the commercial substrate 1 to 2-(2,2-dimethyl-4,6-dioxo-
,3-dioxan-5-ylidene)1H-benzo[c,d]indole (2), alkylation of 2
2
1
1
H), 1.66 (m, 2H), 4.00 (t, J ¼ 7 Hz, 2H), 5.85 (d, J ¼ 8 Hz,
H), 7.12 (t, J ¼ 8 Hz, 1H), 7.52 (m, 2H), 7.74 (t, J ¼ 8 Hz,
1
H), 8.07 (d, J ¼ 8 Hz, 1H), 8.60 (d, J ¼ 8 Hz, 1H), 10.33 (d,
with n-butyl bromide to give 3, and then the treatment of
crude product 3 with a mixture of acetic acid, hydrochloric
acid and potassium iodide to give the desired salt 4 were con-
ducted by using general procedures for the preparation of simi-
lar derivatives [5,6]. After crystallization from ethanol/water
þ
J ¼ 8 Hz, 1H); ms (esi): m/z 252 (M ). Anal. Calcd. for
C
6
17
H
18NO: C, 80.92; H, 7.19; N, 5.55. Found: C, 81.05; H,
.88; N, 5.56.
,6-Bis[(1-ethyl-1,2-dihydrobenzo[c,d]indol-2-ylidene)e-
2
thylidene]cyclohexanone, 8 ¼¼ O. A solution of potassium tert-
butoxide (116 mg, 1 mmol) in tert-butanol (15 mL) was stirred
under a nitrogen atmosphere and treated with the aldehyde 7
(
1:5), compound 4 was obtained in a total yield of 40% start-
ꢀ
1
ing from 1, mp > 300 C; H NMR (deuteriochloroform): d
0
3
.95 (t, J ¼ 7 Hz, 3H), 1.45 (m, 2H), 1.78 (m, 2H), 3.20 (s,
(
150 mL, 0.67 mmol) and cyclohexanone (0.34 lL, 0.33
H), 4.17 (t, J ¼ 7 Hz, 2H), 7.07 (d, J ¼ 8 Hz, 1H), 7.47 (t, J
mmol). The mixture was heated under reflux for 12 h, then
cooled and quenched with water (1.0 mL). The resultant pre-
cipitate was collected by filtration, washed with water and
¼
8 Hz, 1H), 7.59 (d, J ¼ 8 Hz, 1H), 7.87 (t, J ¼ 8 Hz, 1H),
8
.02 (d, J ¼ 8 Hz, 1H), 9.10 (d, J ¼ 8 Hz, 1H). Anal. Calcd.
for C16 18IN: C, 54.71; H, 5.17; N, 3.99. Found: C, 54.80; H,
.13; N, 4.06.
H
ꢀ
cold methanol, and dried under reduced pressure at 23 C; yield
5
ꢀ
1
5
4 mg (29%); mp 188–190 C; H NMR (deuteriochloroform):
d 1.00 (m, 6H), 1.48 (m, 4H), 1.80 (m, 4H), 1.99 (m, 2H),
.79 (m, 4H), 3.88 (m, 4H), 6.06 (d, J ¼ 13 Hz, 2H), 6.68 (d,
J ¼ 8 Hz, 2H), 7.29 (d, J ¼ 8 Hz, 2H), 7.40 (t, J ¼ 8 Hz,
H), 7.65 (t, J ¼ 8 Hz, 2H), 7.77 (d, J ¼ 8 Hz, 2H), 8.42 (d, J
in methanol,
1
-Butyl-2-(2-dimethylaminovinyl)benzo[c,d]indolium iodide,
ꢀ
5
. N,N-Dimethylformamide (1.0 mL) was cooled to 0 C and
2
treated dropwise with phosphorus oxychloride (0.21 mL, 2.3
mmol) and then with a solution of salt 4 (0.35 g, 1.0 mmol)
and pyridine (0.2 mL) in N,N-dimethylformamide (2.0 mL).
The mixture was heated to 65 C for 40 min, then treated with
an aqueous solution of potassium hydroxide (60%, 1 mL) and
2
ꢀ
¼ 8 Hz, 2H), 8.77 (d, J ¼ 8 Hz, 2H); vis: k
max
ꢂ1 ꢂ1
645 nm (e 50800 M cm ); nir: kmax in methanol with one
drop of concentrated hydrochloric acid (pH < 2, 8AOH), 915
nm (e 142000 M cm ). Anal. Calcd. for C40H N O: C,
40 2
ꢀ
heated to 45 C for an additional 30 min. Extraction with
ꢂ1
ꢂ1
dichloromethane (4 ꢁ 10 mL) followed by concentration of
the extract and silica gel chromatography of the residue eluting
with dichloromethane/methanol (19:1) gave 0.28 g (70%) of 5;
85.06; H, 7.13; N, 4.96. Found: C, 85.33; H, 7.12; N, 4.95.
Alternative synthesis of 8 ¼¼ O. The Vilsmeier-Haack rea-
gent 9 was obtained as reported previously [8]. Condensation
of 9 with 4 was conducted in ethanol in the presence of so-
dium acetate by using a general procedure [10]. Crude product
10 was collected as a precipitate after treatment of the mixture
with ether (nir: kmax in methanol, 1025 nm) and used for the
subsequent transformation to 8 ¼¼ O without purification. Thus,
crude dye 10, obtained from 351 mg (1.0 mmol) of 4, was dis-
solved in anhydrous N,N-dimethylformamide (15 mL), and this
solution was treated with N-hydroxysuccinimide (0.33 g, 2.9
mmol) and triethylamine (0.5 mL) under a nitrogen atmos-
ꢀ
1
mp 182–183 C; H NMR (deuteriochloroform): d 0.95 (t, J ¼
8
3
Hz, 3H), 1.45 (m, 2H), 1.78 (m, 2H), 3.47 (s, 3H), 3.86 (s,
H), 4.17 (t, J ¼ 8 Hz, 2H), 5.87 (d, J ¼ 12 Hz, 1H), 7.07 (d,
J ¼ 8 Hz, 1H), 7.47 (t, J ¼ 8 Hz, 1H), 7.59 (d, J ¼ 8 Hz,
1
H), 7.87 (t, J ¼ 8 Hz, 1H), 8.02 (d, J ¼ 8 Hz, 1H), 8.96 (d, J
¼
12 Hz, 1H), 9.10 (d, J ¼ 8 Hz, 1H); ms (esi): m/z 279
þ
(
M ). Anal. Calcd. for C19
H
23IN
2
: C, 56.17; H, 5.71; N, 6.89.
Found: C, 56,05; H, 5.72; N, 6.83.
2-(1-Butyl-1,2-dihydrobenzo[c,d]indol-2-ylidene)ethyli-
dene]malononitrile, 6. A solution of salt 5 (63 mg, 0.15
[
ꢀ
mmol), malononitrile (15 mg, 0.22 mmol) and sodium acetate
(16 mg, 0.20 mmol) in anhydrous ethanol (15 mL) was heated
under reflux for 1 h. Silica gel chromatography eluting with
phere. The mixture was stirred at 23 C under nitrogen for 10 h
and then diluted with ether (35 mL), which caused precipita-
tion of 8 ¼¼ O. The product was crystallized by dropwise dilu-
tion of a solution in methanol with tert-butyl methyl ether;
yield 338 mg (60%) from 4. The spectral characteristics of
a8 b ¼¼ o Ov e .t hus obtained were virtually identical with those reported
1-(4-Sulfonatobutyl)-1,2-dihydrobenzo[c,d]indol-2-one,
potassium salt, 11. This compound was obtained by the fol-
lowing modification of the published procedure [9] which did
hexanes/dichloromethane (5:1) gave 27 mg (60%) of 6; mp
ꢀ
1
1
80–181 C; H NMR (deuteriochloroform): d 1.00 (t, J ¼ 7
Hz, 3H), 1.46 (m, 2H), 1.80 (m, 2H), 4.01 (t, J ¼ 7 Hz, 2H),
6
.25 (d, J ¼ 9 Hz, 1H), 6.99 (m, 1H), 7.52 (m, 2H), 7.74 (t, J
¼
8 Hz, 1H), 8.00 (d, J ¼ 8 Hz, 1H), 8.10 (d, J ¼ 8 Hz, 1H),
þ
8
.32 (d, J ¼ 9 Hz, 1H); ms (maldi) m/z 299 (M ) and 300
þ
(
M
þ 1); vis: kmax in methanol, 519 nm and 556 nm. Anal.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

January 2009
Functionalization of Benzo[c,d]indole System for the Synthesis of Visible
and Near-Infrared Dyes
87
not include any characterization of the product. A mixture of 1
1.69 g, 10 mmol), powdered potassium hydroxide (1.12 g, 20
mmol), and N-methyl-2-pyrrolidone (5 mL) was stirred at
and sodium acetate (82 mg, 1 mmol) in acetic anhydride (20
mL) was heated under reflux for 8 h, then cooled and treated
with ether (30 mL). The resultant precipitate of cyanine 14
was crystallized from methanol/ether; yield 115 mg (30%); mp
(
ꢀ
23 C for 15 min and then treated with 1,4-butanesultone (1.49
g, 11 mmol). The mixture was stirred at 80 C for an additional
ꢀ
ꢀ
1
> 300 C; H NMR (deuterated dimethyl sulfoxide): d 1.82 (m,
14H), 2.83 (t, J ¼ 7 Hz, 4H), 4.01 (t, J ¼ 7 Hz, 4H), 6.20 (d,
J ¼ 13 Hz, 2H), 6.88 (d, J ¼ 7 Hz, 2H), 7.29 (d, J ¼ 7 Hz,
2H), 7.42 (t, J ¼ 7 Hz, 2H), 7.73 (t, J ¼ 7 Hz, 2H), 7.84 (d, J
¼ 7 Hz, 2H), 8.04 (d, J ¼ 7 Hz, 2H), 8.37 (d, J ¼ 13 Hz,
10 h, then cooled and treated dropwise with acetone (25 mL),
which caused crystallization of product 11; yield 3.26 g
ꢀ
1
(
95%); mp 235–240 C; H NMR (deuterated dimethyl sulfox-
ide): d 1.63 (t, J ¼ 7 Hz, 2H), 1.78 (m, 2H), 2.45 (m, 2H),
.90 (t, J ¼ 7 Hz, 2H), 7.24 (d, J ¼ 7 Hz, 1H), 7.56 (t, J ¼ 8
3
2H); nir: k
in methanol, 1013 nm. High-resolution ms (esi,
max
35
ꢂ
Hz, 1H), 7.65 (d, J ¼ 8 Hz, 1H), 7.82 (t, J ¼ 8 Hz, 1H), 8.06
negative ion mode): calcd. for C40H38 ClN O S (M ), m/z
2 6 2
(
(
3
d, J ¼ 8Hz, 1H), 8.20 (d, J ¼ 8 Hz, 1H). High-resolution ms
741.1860; found m/z 741.1871.
esi, negative ion mode): calcd. for C H NO S, m/z
1
2,6-Bis[2-[1-(sodium4-sulfonatobutyl)-1,2-dihydrobenzo-
[c,d]indol-2-ylidene)]ethylidene]cyclohexanone, 15 ¼¼ O. Cyanine
14 was allowed to react with N-hydroxy-succinimide in the
presence of triethylamine in N,N-dimethylformamide as
described above for a similar transformation of 10 to 8 ¼¼ O.
Product 15 ¼¼ O was precipitated by addition of ether and crys-
tallized from methanol/ether; yield 115 mg starting with 150
5
14
4
ꢂ
04.0643 (M ); found m/z 304.0655.
-(4-Sulfonatobutyl)-1,2-dihydrobenzo[c,d]indol-2-one,
1
tetrabutylammonium salt, 12. This compound was obtained
by the following modification of the published procedure [9]
which did not include any characterization of the product. A
mixture of salt 11 (3.0 g, 10 mmol) and tetrabutylammonium
ꢀ
1
chloride (3.1 g, 11 mmol) in acetic acid (10 mL) was stirred at
ꢀ
mg of 14 (77%); mp > 300 C; H NMR (deuterated dimethyl
sulfoxide): d 1.86 (m, 14H), 2.85 (t, J ¼ 7 Hz, 4H), 4.02 (t, J
¼ 7 Hz, 4H), 6.20 (d, J ¼ 13 Hz, 2H), 6.90 (d, J ¼ 7 Hz,
2H), 7.32 (d, J ¼ 7 Hz, 2H), 7.45 (t, J ¼ 7 Hz, 2H), 7.75 (t, J
¼ 7 Hz, 2H), 7.87 (d, J ¼ 7 Hz, 2H), 8.06 (d, J ¼ 7 Hz, 2H),
8.42 (d, J ¼ 7 Hz, 2H); vis: kmax in methanol, 645 nm (e
9
0 C, treated dropwise with ethyl acetate, cooled, and filtered
from the precipitate of potassium chloride. The filtrate was
concentrated on a rotary evaporator and the oily residue was
dissolved in dry toluene (50 mL). The azeotropic removal of
traces of water by concentration on a rotary evaporator gave
1
ꢂ1
ꢂ1
salt 12 (5.0 g, 93%) as a viscous oil; H NMR (deuterated di-
50800 M cm ); nir: kmax in methanol with one drop of
concentrated hydrochloric acid (pH < 2, 15AOH), 914 nm (e
142000 M cm ). High-resolution ms (maldi, negative ion
methyl sulfoxide): d 0.99 (t, J ¼ 8 Hz, 12H), 1.43 (m, 8H),
ꢂ1
ꢂ1
1
8
7
7
.65 (m, 8H), 1.95 (m, 4H), 2.90 (m, 2H), 3.27 (t, J ¼ 8 Hz,
H), 3.94 (t, J ¼ 7 Hz), 6.98 (d, J ¼ 7 Hz, 1H), 7.18 (d, J ¼
Hz, 1H), 7.25 (d, J ¼ 7 Hz, 1H), 7.50 (m, 2H), 7.69 (t, J ¼
ꢂ
mode): calcd. for C H N O S (M þ 1), m/z 723.2199;
4
0 39 2 7 2
found m/z 723.2234.
Hz, 1H), 8.02 (t, J ¼ 7 Hz, 1H). High-resolution ms (esi,
ꢂ
negative ion mode): calcd. for C15
H
14NO
4
S (M ) , m/z
3
04.0643 ; found 304.0640.
-(4-Sulfonatobutyl)-2-methylbenzo[c,d]indolium inner salt,
3. This compound was obtained by the following modifica-
1
REFERENCES AND NOTES
1
[
1] Strekowski, L.; Lee, H.; Mason, J. C.; Say, M.; Patonay, G.
tion of the published procedure [9] which did not include any
characterization of the product. Methylmagnesium chloride
(3.0M solution in tetrahydrofuran, 15.0 mL, 45 mmol) was
added dropwise to a stirred solution of salt 12 (5.46 g, 10
J Heterocyclic Chem 2007, 475, 475.
2] Strekowski, L., Ed. Heterocyclic Polymethine Dyes: Synthe-
sis: Properties and Applications; Springer-Verlag: Berlin, 2008.
3] Parker, R. J.; Smith, D. C. C. J Chem Soc (C) 1967, 2194.
[4] Elliger, C. A. Org Prep Proced Int 1985, 17, 419.
5] Deligeorgiev, T. G.; Gadjev, N. I.; Drexhage, K. H. Dyes
Pigments 1991, 15, 215.
[
[
mmol) in anhydrous tetrahydrofuran under a nitrogen atmos-
ꢀ
phere. The mixture was stirred at 60 C for 1 h, then cooled,
neutralized by dropwise addition of 3M hydrochloric acid, and
[
consecutively diluted with ethanol (30 mL) and ether (30 mL).
ꢀ
[
6] Lee, H.; Berezin, M. Y.; Henary, M.; Strekowski, L.; Achi-
Cooling to 0 C for several hours resulted in crystallization of
1
lefu, S. J Photochem Photobiol (A) 2008, 200, 438.
ꢀ
product 13; yield 2.58 g (85%); mp 143–145 ; H NMR (deu-
terium oxide): d 1.77 (t, J ¼ 8 Hz, 2H), 2.03 (m, 2H), 2.84
[
7] Strekowski, L.; Mason, J. C.; Say, M.; Lee, H.; Gupta, R.;
Hojjat, M. Heterocycl Commun 2005, 11, 129.
8] Makin, S. M.; Boiko, L. I.; Shavrygina, O. A. Zh Org
Khim 1977, 13, 1189.
(
m, 2H), 3.01 (s, 3H), 4.50, (t, J ¼ 8 Hz, 2H), 7.72 (t, J ¼ 8
[
Hz, 1H), 7.84 (t, J ¼ 8 Hz, 1H), 8.08 (m, 2H), 8.42 (d, J ¼ 8
Hz, 1H), 8.50 (d, J ¼ 8 Hz, 1H). High-resolution ms (maldi):
[
9] (a) Deroover, G. (Agfa-Gevaert N. V., Belgium), Eur. Pat.
þ
calcd. for C16
H
18NO
3
S (M þ 1), m/z 304.1007; found m/z
Appl. EP 1093 015 A1, 2001; (b) Deroover, G. Chem Abstr 2001,
134, 302961.
3
04.1015.
Cyanine dye 14. A solution of inner salt 13 (303 mg, 1
mmol) Vilsmeier-Haack reagent 9 [8] (180 mg, 0.5 mmol),
[10] Strekowski, L.; Gorecki, T.; Mason, J. C.; Lee, H.; Patonay,
G. Heterocycl Commun 2001, 7, 117.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet