Cyclization of substituted (2-pyridylthio)phenylacetic acids and chromaticity of mesoionic thiazolo[3,2-a]pyridinium-3-olates

Article abstract of DOI:10.1007/s10593-011-0807-z

The cyclization of substituted (2-pyridylthio)phenylacetic acids has been studied. It was established that reaction occurs at two reaction centers with the formation of substituted 3-imino-2-phenyl-2,3-dihydrothieno[2,3-b]pyridine- 2-carboxylic acids and

Full text of DOI:10.1007/s10593-011-0807-z

Chemistry of Heterocyclic Compounds, Vol. 47, No. 5, August 2011 (Russian original Vol. 47, No. 5, May, 2011)
CYCLIZATION OF SUBSTITUTED
(2-PYRIDYLTHIO)PHENYLACETIC ACIDS
AND CHROMATICITY OF MESOIONIC
THIAZOLO[3,2-a]PYRIDINIUM-3-OLATES
K. V. Fedotov1*, T. P. Shumeiko1, N. N. Romanov1,
A. I. Tolmachev1, and A. Ya. Ilchenko1
The cyclization of substituted (2-pyridylthio)phenylacetic acids has been studied. It was established that
reaction occurs at two reaction centers with the formation of substituted 3-imino-2-phenyl-2,3-dihydro-
thieno[2,3-b]pyridine-2-carboxylic acids and substituted mesoionic thiazolo[3,2-a]pyridinium-3-olates.
The direction of cyclization is influenced by the acidity of the medium and the character of the substitu-
ents in the pyridine nucleus. The spectral properties of the synthesized mesoionic compounds were in-
vestigated experimentally and theoretically (by the Pariser–Parr–Pople method).
Keywords: 3-imino-2-phenyl-2,3-dihydrothieno[2,3-b]pyridine-2-carboxylic acid, mesoionic compo-
unds, (2-pyridylthio)phenylacetic acids, thiazolo[3,2-a]pyridinium-3-olates, absorption spectra.
The cyclization of substituted (2-pyridylthio)phenylacetic acids 1a–c into mesoionic 2-phenylthiazolo-
[3,2-a]pyridinium-3-olates 2a–c [1] and their special spectral features [2] were studied previously. Interest in
compounds of this type is caused not only by the fact that physiologically active substances were detected in a
series of condensed thiazolopyridines [3], but also by the fact that mesoionic compounds have extremely intense
absorption in the visible region of the spectrum [2].
R1
R1
S
S
Ph
Ac2O
Ph
N+
N
COOH
O
R
R
1a–c
2a–c
a R = R1 = H, b R = H, R1 = Me, c R = Me, R1 = H
_______________
* To whom correspondence should be addressed, e-mail: fedotovl@km.ru.
1 Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanskaya str., Kyiv 02094,
Ukraine.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 750–759, May, 2011. Original
article submitted April 5, 2010.
0009-3122/11/4705-0622©2011 Springer Science+Business Media, Inc.
622

We have continued to study the rules of forming mesoionic thiazolo[3,2-a]pyridinium-3-olates and their
properties using the example of 3-cyano- and 3-carboxy-substituted (2-pyridylthio)phenylacetic acids 3 and 4.
CN
COOH
R1
S
COOH
S
COOH
Ph
N
N
Ph
R
3
4
It is known that 2-alkylthio-3-cyanopyridines 5 cyclize readily with the formation of derivatives of
3-aminothieno[2,3-b]pyridines 6 [3], but the ethyl ester of 2-(3-cyano-2-pyridylthio)-3-oxobutanoic acid 7
cyclizes with the formation of imino-2,3-dihydrothieno[2,3-b]pyridines 8 [4].
Alk
Alk
NH2
N
Ar
S
N
S
Ar
Alk
N
Alk
5
6
Alk
Alk
NH
N
COOEt
Me
COOEt
Me
S
Alk
N
S
Alk
N
O
O
7
8
Consequently in our case the formation of mesoionic thiazolo[3,2-a]pyridinium-3-olates and also derivati-
ves of thieno[2,3-b]pyridine may be expected. In reality it turned out that on cyclization of substituted (3-cyano-
2-pyridylthio)phenylacetic acids 3 in acetic anhydride the formation of mesoionic compounds 9 and 3-imino-
2-phenyl-2,3-dihydrothieno[2,3-b]pyridine-2-carboxylic acids 10 was observed. It was shown that the direction of
the reaction depends both on the acidity of the medium, and on the nature of the substituents in the heterocycle.
12
N
R1
CN
NH
11
R 1
S
COOH
1
S
R1
8
COOH
Ph
+
Ac2O + Py
Ac O
H
+
7
6
9
2
Ph
+
4
N
Ph
2
N
S
R
N
3
5
O
R
R
10
9a–g
3a–g
10b–d,f
3, 9, 10 a R = R1 = H, b R = Me, R1 = H, c R = R1 = Me, d R = Ph, R1 = H, f R = R1 = Ph;
3, 9 e R = 4-MeOC6H4, R1 = H, g R = 2-furyl, R1 = CF3
In the presence of pyridine the reaction leads to the formation of colored mesoionic compounds 9, and in
acidic medium colorless substituted 3-imino-2-phenyl-2,3-dihydrothieno[2,3-b]pyridine-2-carboxylic acids 10
are formed. Imines 10 are readily converted into substituted 3-aminothienopyridines 6.
623

We note that our attempts to obtain 3-imino-2-phenyl-2,3-dihydrothieno[2,3-b]pyridine-2-carboxylic
acid 10e led only to the mesoionic compound 9e. In addition attempts to isolate compounds 10g and 9d,f,g in
the pure state were not crowned with success.
Derivatives 10b,c could not be isolated in the pure state, since they undergo decarboxylation under the
reaction conditions. This was demonstrated by a counter synthesis of compound 6c from 3-cyano-2-mercapto-
4,6-dimethylpyridine (11) and benzyl bromide, which was obtained during the isolation of 10c.
Me
Me
NH2
N
Ph
10c
Ph
Br
+
S
Me
N
Me
N
SH
11
6c
Like (3-cyano-2-pyridylthio)phenylacetic acid 3a the 3-carboxy derivative 4 is readily converted into
colored compound 12.
COOH
S
Ac2O
Ph
N+
4
O
12
1
In the H NMR spectra of mesoionic compounds 9a and 12 the characteristic low field signal of atom
H-5 was observed being under the deshielding influence of the closely neighboring quaternary nitrogen atom
and the magnetically anisotropic C–O bond, while for 3-imino-2-phenyl-2,3-dihydrothieno[2,3-b]pyridine-
2-carboxylic acid 10f the signal of the given atom is found at significantly higher field.
1
The direction of the cyclization reaction of acid 4 was also confirmed by data of H NMR spectra. The
aliphatic methyl group signal (5.48 ppm) of the initial acid 4 was absent from the reaction product, and a
characteristic low field doublet (9.13 ppm, H-5) was displayed (compound 12).
Mesoionic compounds 9a–g and 12 absorbed light in the visible region of the spectrum and their elec-
tronic spectra in acetonitrile showed broad bands with absorption maxima close to 657 nm (see Table 1).
TABLE 1. Characteristics of the Electronic Spectra of Compounds 2a,
9a–g, 12, 13, 16 in MeCN
Compound
λmax, nm (ε 10–3, l/mol·cm)*
·
2a
9a
9b
9c
9d
9e
9f
461 (15.7); 288 (10.7); 235 (15.8)
507 (9.25); 446 (8.10); 294 (11.9)
521 (8.65); 288 (16.3); 261 (18.9)
496 (10.1); 300 (14.4); 254 (12.2)
550 (–); 462 (–); 331 (–); 270 (–)
532 (5.30); 451 (3.40); 349 (7.70); 304 (12.8)
576 (–); 465 (–); 272 (–)
9g
12
13
16
657 (–); 480 (–); 373 (–); 306 (–); 254 (–)
467.5 (11.2); 296 (14.5); 241.5 (15.7)
603 (27.9); 487 (8.3); 416 (38.4); 302 (15.2)
608 (66.1); 565 (44.5); 404 (29.6); 345 (13.9)
_______
*For compounds 9d,f,g qualitative data are given since quantitative
absorption spectra were not successfully obtained.
624

TABLE 2. Parameters of the Electronic Spectra of Compounds 2a, 9a,
12, 13, 16, Calculated by the Method of Pariser–Parr–Pople
Compound
λtheor, nm (f)
2a
9a
12
13
16
440.5 (0.104)
508.1 (0.070), 368.4 (0.267)
471.9 (0.069), 372.6 (0.386), 308.6 (0.668)
606.1 (0.373), 472.0 (0.203), 439.7 (0.597)
573.2 (0.526), 470.5 (0.503), 424.0 (0.241)
Introduction into position 8 of the initial thiazolo[3,2-a]pyridinium-3-olate 2 of an electron-withdrawing nitrile
(compound 9a) or carboxyl (compound 12) group led to a bathochromic shift of the long-wave absorption bands
(Table 1). For compound 9a this shift was 46 nm, while for its analog 12, containing a carboxyl group in place of
nitrile, only 7 nm. The weaker effect of introducing a carboxyl group into position 8 may be linked with its lower
electron-withdrawing ability (cf. Hammett constant σm: 0.56 for the CN group and 0.37 for the COOH group [5]).
Substituents in positions 5 and 7 also significantly affect the position of the absorption bands of meso-
ionic compounds 9. The presence of a methyl group in position 5 (compound 9b) leads to a bathochromic shift
of 14 nm in the long wave absorption band. At the same time the introduction of a second methyl group into
position 7 (compound 9c) leads to a hypsochromic shift of 11 nm in comparison with the reference compound
9a. Phenyl substituents in positions 5 and 7 of compound 9f exert more influence on its spectral properties than
alkyl substituents (compound 9d). The long-wave absorption band of compound 9d, containing a phenyl
substituent in position 5, is displaced relative to the corresponding band of 9a by 43 nm towards the red region
of the spectrum, and the introduction of one further phenyl group in position 7 (compound 9f) increases this shift
to 69 nm. It is evident that in this case the significant effect of the phenyl substituents is linked with their
positive mesomeric effect, which is strengthened in the activated state due to a reduction of the torsion angle
between the main mesoionic chromophore and the π-systems of the phenyl groups.
Among the investigated compounds maximal intensifying of color in comparison with 2-phenylthiazo-
lo[3,2-a]pyridinium-3-olate (by 150 nm) was observed on introducing 2-furyl and trifluoromethyl substituents
TABLE 3. Distribution of π-Charges on the Atoms of Compounds 9a and
13 Molecules in the Ground (S0), First (S1), and Second (S2) Singlet
Excited States
S0
S1
S2
S0
S1
13
S2
Atom No.
9a
1
2
+0.422
–0.120
+0.011
+0.866
–0.075
0.000
+0.606
–0.031
+0.147
+0.869
–0.245
–0.013
–0.091
–0.151
–0.191
–0.643
–0.046
–0.259
–
+0.356
–0.062
+0.154
+0.796
–0.044
–0.100
–0.055
–0.007
–0.362
–0.641
+0.028
–0.079
–
+0.287
–0.160
+0.068
+0.676
+0.022
–0.119
+0.031
–0.099
–0.044
–0.809
+0.087
–0.158
–0.246
+0.330
+0.005
+0.151
+0.624
–0.130
–0.071
–0.092
–0.129
–0.016
–0.689
+0.035
–0.236
–0.172
+0.235
–0.019
+0.157
+0.618
+0.006
–0.191
–0.016
–0.084
–0.271
–0.660
+0.043
–0.198
–0.182
3
4
5
6
7
–0.007
–0.019
–0.187
–0.851
+0.047
–0.064
–
8
9
10
11
12
13
625

into positions 5 and 7 respectively (compound 9g). In position 5 of mesoionic compounds 9a–g the steric factor
influences the color preferentially, which in the case of a 2-furyl substituent, like methyl [2], must cause a batho-
chromic effect. But for position 7 it is more important that on transforming the molecule to the excited state S1
the density of electrons on the C(7) atom is increased (see Table 3). Electron-withdrawing substituents, such as
the CF3 group, must in this case also show a bathochromic effect. In accordance with this is the strong color
intensifying of compound 9g, although it was not isolated in an analytically pure state. Consequently we propose
that in the present case such ''qualitative'' comparison of the data of electronic spectra in the visible region is
TABLE 4. 1H NMR Spectra of Synthesized Compounds
Compound
Chemical shifts, δ, ppm (J, Hz)*
2a
7.75 (3H, m, p,m-H Ph); 7.86 (2H, m, o-H Ph); 8.11 (1H, t, J = 6.9, H-6); 8.39 (1H,
t, J = 7.5, H-7); 8.57 (1H, d, J = 8.7, H-8); 9.29 (1H, d, J = 6.6, H-5)
3a
3b
3c
3d
3e
5.66 (1H, s, CH); 7.38 (4H, m, m,p-H Ph, 5-H); 7.50 (2H, d, J = 7.5, o-H Ph); 8.25
(1H, d, J = 7.8, 4-H); 8.70 (1H, m, 6-H)
2.53 (3H, s, 6-CH3); 5.63 (1H, s, CH); 7.21 (1H, d, J = 8.0, 5-H); 7.40 (3H, m,
m,p-H Ph); 7.32 (2H, dd, J1 = 8.4, J2 = 1.5, o-H Ph); 8.10 (1H, d, J = 8.4, 4-H)
2.40 (3H, s, 4-CH3); 2.49 (3H, s, 6-CH3); 5.62 (1H, s, CH); 7.14 (1H, s, 5-H); 7.39
(3H, m, m,p-H Ph); 7.52 (2H, d, J = 7.0, o-H Ph)
6.31 (1H, s, CH); 7.90 (3H, m, H Ph, 5-H); 8.14 (5H, m, H Ph); 8.44 (1H, d, J = 7.8,
4-H); 8.80 (3H, m, H Ph)
3.85 (3H, s, OCH3); 5.74 (1H, s, CH); 7.07 (2H, d, J = 9.0, 5-H); 7.42 (3H, m,
H Ph); 7.56 (2H, dd, J = 6.3, o-H Ph); 7.85 (1H, d, J = 8.1, 4-H); 8.22 (3H, m,
H Ph)
3f
3g
4
5.77 (1H, s, CH); 7.40 (3H, m, H Ph); 7.54 (3H, m, H Ph); 7.58 (5H, m, H Ph); 7.75
(2H, m, H Ph); 7.94 (1H, s, 5-H); 8.31 (2H, m, H Ph)
5.55 (1H, s, CH); 6.81 (1H, dd, H Fur); 7.23 (3H, m, H Ph); 7.36 (1H, s, 5-H); 7.61
(2H, m, H Ph); 7.70 (1H, s, H Fur); 8.07 (1H s, H Fur)
5.47 (1H, s, CH); 7.26 (1H, m, 5-H); 7.36 (3H, m, m,p-H Ph); 7.47 (2H, d, J = 7.0,
o-H Ph); 8.23 (1H, d, J = 7.5, 4-H); 8.59 (1H, m, 6-H)
6с
2.51 (3H, s, 4-CH3); 2.78 (3H, s, 6-CH3); 4.87 (2H, s, p-H Ph, NH2); 7.01 (1H, s,
H-5); 7.33 (1H, t, J = 7.5, p-H Ph); 7.50 (2H, t, J = 7.5, m-H Ph); 7.56 (2H, d,
J = 7.5, o-H Ph)
9a
7.24 (1H, t, J = 7.0, p-H Ph); 7.43 (2H, t, J = 7.5, m-H Ph); 7.50 (1H, t, J = 7.0,
H-6); 7.91 (1H, d, J = 7.5, H-7); 8.05 (2H, d, J = 8.5, o-H Ph); 9.11 (1H, d, J = 7.5,
H-5)
9b
9c
9e
3.28 (3H, s, 5-CH3); 7.10 (1H, t, J = 7.5, p-H Ph); 7.34 (3H, m, H-6, m-H Ph); 7.92
(2H, d, J = 7.8, o-H Ph); 8.29 (1H, d, J = 7.5, H-7)
3.47 (3H, s, 7-CH3); 4.28 (3H, s, 5-CH3); 7.68 (1H, s, H-6); 8.05 (1H, t, J = 7.8,
p-H Ph); 8.26 (2H, t, J =7.8, m-H Ph); 8.80 (2H, d, J = 7.5, o-H Ph)
4.23 (3H, s, OСH3); 7.40 (2H d, J = 9.0, o-H 5-Ph); 7.72 (3H, m, p,m-H 2-Ph); 7.77
(2H, d, J = 8.5, m-H 5-Ph); 7.90 (2H, m, o-H 2-Ph); 8.00 (1H, d, J = 8.0, H-6); 8.88
(1H, d, J = 8.0, H-7)
10f
12
7.38 (3H, t, J = 7.5, H Ph); 7.46 (2H, t, J = 7.2, H Ph); 7.64 (5H, m, H Ph, NH);
7.81 (3H, m, H Ph); 8.09 (3H, d, J = 7.2, H Ph); 8.10 (1H, s, H-5)
7.11 (1H, t, J = 7.2, p-H Ph); 7.36 (2H, t, J = 7.8, m-H Ph); 7.75 (1H, t, J = 6.9,
H-6); 8.02 (2H, d, J = 8.1, o-H Ph); 8.47 (1H, d, J = 7.2, H-7); 9.13 (1H, d, J = 6.6,
H-5); 14.56 (1H, br. s, 8-CO2H)
13
3.77 (3H, s, NCH3); 7.01 (1H, d, J = 8.4, H-7); 7.12 (1H, t, J = 7.8, H Ph); 7.18 (1H,
d, J = 8.4, H Ph); 7.21 (1H, d, J = 8.4, H Ph); 7.36 (2H, t, J = 7.8, H Ph); 7.43 (1H,
d, J = 8.1, H Ph); 7.49 (1H, d, J = 8.4, H-6); 7.59 (1H, d, J = 7.5, H Ph); 7.91 (2H,
d, J = 7.2, H Ph); 9.46 (1H, s, H-13)
16
3.31 (3H, s, CH3OSO3); 3.97 (3H, s, NCH3); 4.24 (3H, s, NCH3); 7.27 (1H, d,
J = 9.0, H-6); 7.47 (2H, m, H Het); 7.62 (2H, m, H Het); 7.88 (2H, m, H Het); 8.00
(1H, d, J = 9.5, H-7); 8.10 (2H, m, H Het); 8.70 (1H, s, H-13)
_________________
*Solvents: CF3CO2D (compounds 2a, 9e), DMSO-d6 (compounds 3a–g,
4, 6c, 9b,c, 10f, 12, 16) and CDCl3 (9a, 13).
626

completely correct. Hence in the case of mesoionic compounds 9 the bathochromic shift of their long-wave
absorption bands may be achieved by introducing electron-donating substituents into position 5 and electron-
withdrawing into position 7.
It was shown previously [6] that mesoionic thiazolo[3,2-a]pyridinium-3-olates are capable of reacting
with intermediates used for the synthesis of cyanine dyes at the methyl group in positions 5, 7 and the methine
group in position 2. We found that 8-cyano-5-methyl-2-phenylthiazolo[3,2-a]pyridinium-3-olate (9b) in a
mixture of acetic anhydride and pyridine also reacts with 3-methyl-2-(methylthio)benzothiazolium methyl
sulfate with the formation of monomethinecyanine 13.
N
12
N
8
11
7
1
6
S
4
N+ 9
7'
S
S
5
2''
5''
2
Ac2O + Py
S
6'
5'
SMe
MeSO4
+
Ph
3''
4''
N+
3
13
N+
10
N
–
O
6''
4'
O
Me
Me
Me
13
9b
If (3-cyano-6-methyl-2-pyridylthio)acetic acid (14) is reacted under the same conditions with 3-methyl-
2-(methylthio)benzothiazolium methyl sulfate, the 8-cyano-5-methylthiazolo[3,2-a]pyridinium-3-olate (15)
formed in situ [6] reacts with 2 equivalents salt giving the bis-substituted dyestuff 16 with monomethinecyanine
and nulmethinemerocyanine chromophores.
12
N
S
–
N
11
N
MeSO4
S
7
SMe
8
N+
Me
1
S
6
–
9
4
N+
S
MeSO4
Ac2O
S
S
5
2
N+
13
3
N
COOH
Ac2O + Py
N
O
N
–
10
O
Me
Me
Me
Me
14
15
16
As it can be seen from the absorption spectra of dyestuffs 13 and 16 solutions in acetonitrile given in
Fig. 1, their characteristic special feature is the presence of several absorption bands in the visible region of the
spectrum.
, nm
Fig. 1. Absorption spectra of dyestuffs 13 (―) and 16 (- - -) solutions in acetonitrile.
627

TABLE 5. Characteristics of Synthesized Compounds
Found, %
Calculated, %
Compound
Empirical formula
mp, ºC
Yield, %
N
S
2a
3a
3b
3c
3d
3e
3f
C13H9NOS
C14H10N2O2S
C15H12N2O2S
C16H14N2O2S
C20H14N2O2S
C21H16N2O3S
C26H18N2O2S
C19H11F3N2O3S
C14H11NO4S
C15H14N2S
6.09
6.16
14.06
14.11
170–171
130–135
142–144
170–174
246–247
198–200
214–215
123–125
264–266
52–54
27
26
88
100
66
38
14
27
36
59
75
76
85
15
78
83
62
21
10.33
10.36
11.89
11.86
9.83
9.85
11.32
11.28
9.37
9.39
10.78
10.75
8.12
8.09
9.21
9.26
7.39
7.44
8.55
8.52
6.58
6.63
7.63
7.59
3g
4
6.86
6.93
7.99
7.93
4.80
4.84
11.01
11.08
6c
9a
9b
9c
9e
10f
12
13
16
10.98
11.01
12.73
12.61
C14H8N2OS
10.97
11.10
12.76
12.71
255–256
211–212
198–200
216–218
202–206
287–289
254–255
>300
C15H10N2OS
C16H12N2OS
C21H14N2O2S
C26H18N2O2S
C14H9NO3S
10.49
10.52
11.96
12.04
10.03
9.99
11.42
11.44
7.77
7.82
9.01
8.95
6.69
6.63
7.44
7.59
5.22
5.16
11.76
11.82
C23H15N3OS2
C26H20N4O5S4
10.22
10.16
25.45
25.51
9.33
9.39
21.50
21.49
π-Electron structures were studied and theoretical electronic absorption spectra were calculated for
compounds 2a, 9a, 12 and dyestuffs 13, 16, containing in their structure a thiazolo[3,2-a]pyridinium-3-olate,
using the semiempirical quantum method of Pariser–Parr–Pople [2, 7]. The calculations were carried out with
the initial parameters used previously for dyestuffs, pyridine, thiazole, and benzothiazole derivatives [8],
allowing for the interaction of four once excited configurations (KB 2×2). Models of the molecules have
standard bond lengths (1.40 Å for C–C, C–N, C–O and 1.80 Å for C–S bonds) and valence angles in rings and
chains [8]. Absorption maxima λteor and oscillator strengths f were determined (Table 2) as a result of the cal-
culations.
As it is seen by comparing the data of Tables 2 and 1 the calculated values agreed fairly well with the
experimental λmax. The significant difference of the calculated values of λteor of dyestuff 16 from the expe-
rimental λmax, is evidently caused by the additional bathochromic shift of its long-wave absorption band as a re-
sult of steric hindrance, which causes turning of the benzothiazole residue around the high order bond [8]. Intro-
duction of nitrile (compound 9a) or carboxyl (compound 12) groups into the pyridine nucleus of compound 2a,
as a result of the emergence of electronic interaction of these groups with the π-system of the mesoionic frag-
ment, causes a new absorption band to be displayed and a bathochromic displacement of the long-wave bands.
The same effect is also observed on substituting a carbon atom by a nitrogen atom in the same position 8 [9].
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From the changes of π-charges on excitation it may be said that in compound 9a the first excited state S1
is linked with transfer of electron density from the thiazolate nucleus to the pyridine nucleus (preferentially to
atoms 2, 4, 5) involving the nitrile group (Table 3). In dyestuff 13 the first excited state S1 is linked with transfer
of electron density to the pyridine nucleus and to the nitrile group, not only from the thiazolate nucleus but also
from a carbon atom of the methine chain, i.e. the significant intensifying of color is the result of the strong
interaction of the two chromophores, one of which is localized on the polymethine chain, and the second on the
mesoionic heterocyclic residue.
EXPERIMENTAL
The 1H NMR spectra were obtained on a Bruker Avance DRX 500 spectrometer (at 500 MHz), internal
standard was TMS, and the electronic spectra on a Shimadzu UV-3100. The starting acids 1, 3, 4, and 14 were
synthesized by the procedure given in [10].
4,6-Dimethyl-2-phenylthieno[2,3-b]pyridyl-3-amine (6c). 2-Mercapto-4,5-dimethylpyridine (11)
(0.32 g, 0.2 mmol) was dissolved in DMF (10 ml) and 10% aqueous NaOH solution (4 ml, 1 mmol) was added.
The mixture was heated to 70oC, benzyl bromide (0.34 g, 0.2 mmol) was added dropwise, and the mixture was
maintained at the same temperature for 3 h. The reaction mixture was cooled, the precipitated product filtered
off, washed twice with methanol (10 ml), and dried. The product was crystallized from methanol; yield 0.15 g
(59%).
Substituted Mesoionic Thiazolo[3,2-a]pyridinium-3-olates 9a–g, 12. Acetic anhydride (4 ml) was
added to a solution of the corresponding acid 3a–g, 4 (0,1 mmol) in pyridine (2 ml), and the mixture heated for
15 min at 140oC. The solid was filtered off, washed with methanol, and with ether. The product was
recrystallized from acetic anhydride.
Substituted 3-Imino-2-phenyl-2,3-dihydrothieno[2,3-b]pyridine-2-carboxylic Acids 10b–d,f. Acetic
anhydride (4 ml) was poured into a solution of the corresponding acid 3b–d,f (0.1 mmol) in acetic acid (2 ml).
The mixture was heated at 140oC for 15 min and then left for 6 h in the refrigerator. The solid was filtered off,
washed with methanol, and with ether. The product was recrystallized from ethanol.
8-Cyano-5-{[3-methylbenzothiazol-2(H)-ylidene]methyl}-2-phenylthiazolo[3,2-a]pyridinium-3-ola-
te (13). A solution of 3-methyl-2-methylthiobenzothiazolium methyl sulfate (0.30 g, 0.1 mmol) in acetic
anhydride (5 ml) was added to a solution of compound 9b (0.25 g, 0.1 mmol) in acetic anhydride (5 ml) and the
mixture was heated to 140oC. Then triethylamine (0.10 ml, 0.1 mmol) was added, the mixture heated once again
at 140oC for 20 min, and placed in the refrigerator for 6 h. The precipitated dyestuff was filtered off, washed
with ethanol, and with ether. The product was recrystallized from acetic anhydride.
8-Cyano-2-{3-methylbenzothiazol-2(3H)-ylidene}-5-{[3-methylbenzothiazol-2(3H)-ylidene]methyl}-
3-oxo-2,3-dihydro[1,3]thiazolo[3,2-a]pyridin-4-ium Methyl Sulfate (16). A solution of 3-methyl-2-methyl-
thiobenzothiazolium methyl sulfate (0.60 g, 0.2 mmol) in acetic anhydride (5 ml) was added to a solution of acid
14 (0.25 g, 0.1 mmol) in pyridine (5 ml) and the mixture heated at 140oC for 20 min, then placed in the
refrigerator for 6 h. The precipitated dyestuff was filtered off, washed with ethanol, and with ether. The product
was recrystallized from acetic anhydride.
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