5-Aryl-2-furaldehydes in the synthesis of 2-substituted 1,3-benzazoles

Article abstract of DOI:10.1023/B:RUJO.0000010217.17000.e6

2-[2-(5-Aryl-2-furyl)ethenyl]-1,3-benzazoles were synthesized by reaction of 5-aryl-2-furaldehydes with 2-methylbenzoxazole, 2-methylbenzothiazole, 2-methylbenzimidazole, and 2-cyanomethylbenzimidazole. The corresponding benzazoles were also obtained by reaction of N-(5-arylfurfurylidene)anilines with 2-methylbenzoxazoles and of 3-(5-aryl-2-furyl)-2-cyanopropenoyl chlorides with o-phenylenediamine. The acylation of o-aminophenol with 3-(5-aryl-2-furyl)-2-cyanopropenoyl chlorides occurs at the amino group without subsequent oxazole ring closure.

Full text of DOI:10.1023/B:RUJO.0000010217.17000.e6

Russian Journal of Organic Chemistry, Vol. 39, No. 9, 2003, pp. 1295 1300. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 9, 2003,
pp. 1368 1373.
Original Russian Text Copyright
2003 by Ganushchak, Lesyuk, Fedorovich, Obushak, Andrushko.
5-Aryl-2-furaldehydes in the Synthesis
of 2-Substituted 1,3-Benzazoles
N. I. Ganushchak, A. I. Lesyuk, I. S. Fedorovich,
N. D. Obushak, and V. N. Andrushko
Ivan Franko L’vov National University, ul. Kirilla i Mefodiya 6, L’vov, 79005 Ukraine
e-mail: obushak@in.lviv.ua
Received November 6, 2002
Abstract 2-[2-(5-Aryl-2-furyl)ethenyl]-1,3-benzazoles were synthesized by reaction of 5-aryl-2-furaldehydes
with 2-methylbenzoxazole, 2-methylbenzothiazole, 2-methylbenzimidazole, and 2-cyanomethylbenzimidazole.
The corresponding benzazoles were also obtained by reaction of N-(5-arylfurfurylidene)anilines with
2-methylbenzoxazoles and of 3-(5-aryl-2-furyl)-2-cyanopropenoyl chlorides with o-phenylenediamine. The
acylation of o-aminophenol with 3-(5-aryl-2-furyl)-2-cyanopropenoyl chlorides occurs at the amino group
without subsequent oxazole ring closure.
2-Arylethenyl-substituted benzazoles are used as
organic luminophores and optical bleaching agents
[1 4]. Luminescence properties of these compounds
are determined by the length of the -conjugation
system. The conjugation system can be extended, e.g.,
via introduction of an arylfuryl group into the 2-posi-
tion of benzazole ring. It is known that arylfurans also
exhibit properties intrinsic to luminophores, bleaching
agents, and scintillators [1 3]. Therefore, a combina-
tion of two fluorophoric fragments in a single mole-
cule may be interesting from the practical viewpoint.
and 2-cyanomethylbenzimidazole. Aldehydes Ia Ig
were found to react with 2-methylbenzoxazole (II)
and 2-methylbenzothiazole (III) under conditions
analogous to those described in [5], namely in the
presence of anhydrous zinc(II) chloride on heating to
120 180 C (depending on the substituent R) without
a solvent. As a result, compounds IVa IVg and
Vb Ve were formed (Scheme 1, Table 1).
The reaction is favored by the presence of electron-
acceptor substituents R in the aromatic ring of alde-
hydes Ia Ig. By contrast, in the reaction of 2-methyl-
benzoxazole (II) with 5-(4-methoxyphenyl)-2-furalde-
hyde we failed to isolate the corresponding condensa-
tion product, while the yield of benzoxazole IVa from
In the present work we examined the reaction of
5-aryl-2-furaldehydes Ia Ig with 2-methylbenzoxa-
zole, 2-methylbenzothiazole, 2-methylbenzimidazole,
Scheme 1.
II, IV, X = O; III, V, X = S; I, IV VII, R = H (a), 4-Cl (b), 4-Br (c), 2,5-Cl2 (d), 3-NO2 (e), 4-NO2 (f), 2-Cl (g);
VI, R = H; VII, R = OMe.
1070-4280/03/3909-1295$25.00 2003 MAIK Nauka/Interperiodica

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GANUSHCHAK et al.
Table 1. Yields, melting points, and elemental analyses of 2-substituted benzazoles IVa IVg, Vb Ve, and IXa IXc
Found, %
H
Calculated, %
H
Comp.
no.
Yield,
%
mp,
C
Formula
C
N
C
N
IVa
IVb
IVc
IVd
IVe
IVf
IVg
Vb
Vc
Vd
Ve
IXa
IXb
IXc
36
63
79
64
54
51
75
67
68
71
58
36
44
51
97 98
79.27
70.80
62.18
64.15
68.41
68.57
70.59
67.28
59.82
61.17
65.40
64.05
68.58
68.76
4.43
3.63
3.34
3.04
3.52
3.65
3.73
3.46
3.20
2.88
3.35
3.41
3.87
3.82
4.97
4.32
3.71
3.69
8.50
8.32
4.44
4.13
3.54
3.63
8.11
7.69
12.75
12.56
C19H13NO2
79.42
70.92
62.32
64.07
68.67
68.67
70.92
67.55
59.70
61.30
65.51
64.24
68.88
68.88
4.56
3.76
3.30
3.11
3.64
3.64
3.76
3.58
3.16
2.98
3.47
3.40
3.95
3.95
4.88
4.35
3.82
3.93
8.43
8.43
4.35
4.15
3.66
3.76
8.04
7.89
12.68
12.68
176 177
161 162
165 166
195 196
236 237
130 131
150 151
164 165
171 172
176 177
152 153
235 237
272 273
C19H12ClNO2
C19H12BrNO2
C19H11Cl2NO2
C19H12N2O4
C19H12N2O4
C19H12ClNO2
C19H12ClNOS
C19H12BrNOS
C19H11Cl2NOS
C19H12N2O3S
C19H12Cl2N2O
C19H13N3O3
C19H13N3O3
5-phenyl-2-furaldehyde (Ia) was poor. We also tried
to synthesize 2-substituted benzazoles IV and V from
Schiff bases which were prepared by reaction of alde-
hydes Ia If with aniline and p-methoxyaniline. Com-
pounds VIa VIf and VIIa VIIf thus obtained
(Table 2) were brought into reaction with 2-methyl-
benzoxazole under the conditions reported in [6]
(DMF, KOH). The products were benzoxazole deriva-
tives IVa IVf (Scheme 1). The reaction occurred
under milder conditions (110 130 C) than the con-
densation of aldehydes Ia Ig with 2-methylbenzoxa-
zole, and in both cases (methods a and b) the yields
of IVa IVf were comparable. We also found that,
from the preparative viewpoint, it is more convenient
to use Schiff bases VI which are more reactive than
their p-methoxy-substituted analogs VII. Benzazoles
IV and V are bright yellow or orange substances, and
most of them show fluorescence in benzene solution.
while under more severe conditions only tars were
obtained. Condensation products IXa IXc were
isolated when the the reaction was carried out in
acetic anhydride (method c) or pyridine (method d)
(Scheme 2).
The yields of IXa IXc were moderate (Table 1),
and no corresponding products were isolated in the
reaction with arylfuraldehydes I having electron-donor
substituents in the aromatic ring. On the other hand,
2-cyanomethylbenzimidazole (X) in which the methyl
group is activated by cyano group readily reacted with
aldehydes I at room temperature to afford condensa-
tion products XIa XIf in high yields (method e;
Scheme 3, Table 3). Compounds XIa XIf were also
obtained by independent procedure based on the reac-
tion of o-phenylenediamine with carboxylic acid
chlorides to build up benzimidazole ring system [5 7]
(method f, Scheme 3).
As might be expected [5], 2-methylbenzimidazole
(VIII) turned out to be less reactive toward aldehydes
I than azoles II and III. Compound VIII failed to
react with aldehydes I under the given conditions,
Initial carboxylic acid chlorides XIIa XIIf were
prepared from 3-(5-aryl-2-furyl)-2-cyanopropenoic
acid esters which were in turn synthesized by con-
densation of 5-aryl-2-furaldehydes I with ethyl cyano-
Scheme 2.
R = 2,5-Cl2 (a), 3-NO2 (b), 4-NO2 (c).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003

5-ARYL-2-FURALDEHYDES IN THE SYNTHESIS OF 2-SUBSTITUTED 1,3-BENZAZOLES
1297
Table 2. Yields, melting points, and elemental analyses of Schiff bases VIa VIf and VIIa VIIf
Found, %
H
Calculated, %
H
Comp.
no.
Yield,
%
mp,
C
Formula
C
N
C
N
VIa
VIb
VIc
VId
VIe
VIf
VIIa
VIIb
VIIc
VIId
VIIe
VIIf
60
73
91
86
65
79
96
85
71
83
76
87
71 72
151 152
103 104
93 94
133 134
162 163
86 87
143 144
175 176
89 90
156 157
123 124
82.43
72.19
62.29
64.41
69.58
69.80
77.56
69.08
60.51
62.27
67.17
66.82
5.22
4.18
3.70
3.47
4.09
4.05
5.40
4.43
3.87
3.64
4.31
4.29
5.74
4.93
4.35
4.29
9.37
9.69
4.98
4.36
3.90
4.11
8.52
8.73
C17H13NO
82.57
72.47
62.60
64.58
69.86
69.86
77.96
69.35
60.69
62.45
67.08
67.08
5.30
4.29
3.71
3.51
4.14
4.14
5.45
4.53
3.96
3.78
4.38
4.38
5.66
4.97
4.29
4.43
9.58
9.58
5.05
4.49
3.93
4.05
8.69
8.69
C17H12ClNO
C17H12BrNO
C17H11Cl2NO
C17H12N2O3
C17H12N2O3
C18H15NO2
C18H14ClNO2
C18H14BrNO2
C18H13Cl2NO2
C18H14N2O4
C18H14N2O4
acetate. These reactions were considered in detail
previously [8].
closure occurred, and the products were the corre-
sponding anilides XIIIa and XIIIb (Scheme 4). The
yields, melting points, and analytical data of products
XIIIa and XIIIb are given in Experimental, and their
1H NMR spectra are presented in Table 4.
The electron absorption spectra of the condensation
products (Table 5) are characterized by the presence
of a strong K-band whose position is fairly sensitive
to substituent in the aromatic ring of the arylfuryl
It is known that an analogous scheme is applicable
to the synthesis of benzoxazoles and benzothiazoles
(via reaction of carboxylic acid chlorides with
o-aminophenol or o-aminobenzenethiol, respectively)
[9 11]. We made an attempt to prepare 2-substituted
benzoxazoles by reaction of acyl chlorides XIIa and
XIIc with o-aminophenol; however, no oxazole ring
Scheme 3.
Scheme 4.
XIII, R = H (a), 4-Br (b).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003

1298
GANUSHCHAK et al.
Table 3. Yields, melting points, and elemental analyses of 2-substituted benzimidazoles XIa XIf
Found, %
H
Calculated, %
Comp.
no.
Yield,
%
mp,
C
Formula
C
N
C
H
N
XIa
XIb
XIc
XId
XIe
XIf
60
73
91
86
65
79
250 252
292 294
288 290
178 179
273 274
324 326
76.93
69.17
61.48
63.05
67.36
67.19
4.24
3.42
3.06
2.93
3.27
3.35
13.38
12.20
10.66
10.94
15.59
15.81
C20H13N3O
77.16
69.47
61.56
63.18
67.41
67.41
4.21
3.50
3.10
2.92
3.39
3.39
13.50
12.15
10.77
11.05
15.72
15.72
C20H12ClN3O
C20H12BrN3O
C20H11Cl2N3O
C20H12N4O3
C20H12N4O3
fragment. The absorption band shifts to longer wave-
length in going from benzoxazole to benzimidazole
derivatives [1]. Even stronger red shift is observed on
introduction of a cyano group (compounds XIa XIf).
Thus 5-aryl-2-furaldehydes are convenient reagents
for the synthesis of 2-substituted benzazoles contain-
ing arylfuran fragments.
off, washed with petroleum ether, and recrystallized
from a 1:2 mixture of petroleum ether with toluene
or from petroleum ether.
b. A mixture of 7.5 mmol of Schiff base VIa VIf
or VIIa VIIf, 1 g (7.5 mmol) of 2-methylbenzoxazole
(II), and 0.8 g of KOH in 15 ml of DMF was heated
for 2.5 3 h at 120 140 C. The mixture was then
poured into hot water, and the precipitate was treated
as described above in a.
EXPERIMENTAL
2-[2-(5-Aryl-2-furyl)ethenyl]benzothiazoles
Vb Ve were synthesized by the procedure described
above in a, by heating of the reaction mixture con-
taining the corresponding aldehyde I and 2-methyl-
benzothiazole for 0.5 h. Products Vb Ve were recrys-
tallized from hexane or hexane benzene (1:1).
1
The H NMR spectra were recorded from solutions
in DMSO-d6 on a Bruker AM-300 spectrometer
(300 MHz); the spectra of compounds XIb and XIe
were measured at 400 MHz. The signal from residual
protons in the solvent ( 2.50 ppm) was used as
internal reference. The UV spectra were measured on
a Specord M-40 spectrophotometer in toluene (c = 5
2-[2-(5-Aryl-2-furyl)ethenyl]benzimidazoles
IXa IXc. c. A solution of 10 mmol of 5-aryl-2-fur-
aldehyde Id If in 5 6 ml of acetic anhydride was
mixed with a solution of 2 g (15 mmol) of 2-methyl-
benzimidazole (VIII) in 4 ml of acetic anhydride.
Anhydrous ZnCl2, 0.4 g, was added, and the mixture
was heated for 3 3.5 h under reflux. It was then
cooled to 90 95 C, 8 ml of 20% hydrochloric acid
and 6 ml of water were added, and the mixture was
refluxed for 0.5 h. The precipitate was filtered off,
washed first with hot acetic acid and then with water
(until neutral reaction), and dried. We thus isolated
the corresponding hydrochloride of IXa IXc which
was dispersed in acetone or dioxane, and the suspen-
sion was treated with aqueous ammonia until the
precipitate dissolved. The solution was diluted with
water, and the precipitate was filtered off, washed
with water and alcohol, and recrystallized from DMF
alcohol (1:2).
5
10 M, layer thickness 10.7 mm).
5-Aryl-2-furaldehydes Ia Ig were synthesized by
the procedure reported in [12], and acid chlorides
XIIa XIIf were prepared as described in [8].
N-(5-Arylfurfurylidene)anilines VIa VIf and
VIIa VIIf. A mixture of 20 mmol of 5-aryl-2-furalde-
hyde Ia If, 23 mmol of aniline or p-anisidine, and
3.28 g of sodium acetate in 10 ml of anhydrous al-
cohol was heated for 6 h under reflux. The mixture
was cooled, diluted with 30 ml of water, and extracted
with ether. The extract was dried over MgSO4 and
evaporated, the residue was treated with ether, and
the precipitate was filtered off and recrystallized from
alcohol.
2-[2-(5-Aryl-2-furyl)ethenyl]benzoxazoles IVa
IVg. a. A mixture of 1 g (7.5 mmol) of freshly dis-
tilled 2-methyl-1,3-benzoxazole (II), 7.5 mmol of
5-aryl-2-furaldehyde Ia Ig, and 0.5 g of ZnCl2
(preliminarily dehydrated by melting) was stirred for
4 h at 150 160 C with protection from atmospheric
moisture. The product was dissolved in DMF and
precipitated with water. The precipitate was filtered
d. A solution of 5 mmol of 5-aryl-2-furaldehyde
Id If in 3 4 ml of pyridine was mixed with a solution
of 1 g (7.5 mmol) of 2-methylbenzimidazole (VIII),
0.2 g of anhydrous ZnCl2 was added, and the mixture
was heated for 2.5 3 h under reflux. It was then
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003

5-ARYL-2-FURALDEHYDES IN THE SYNTHESIS OF 2-SUBSTITUTED 1,3-BENZAZOLES
1299
Table 4. 1H NMR spectra ( , ppm; J, Hz) of condensation products IVb IVg, Vb Ve, XIb, XIe, XIIIa, and XIIIb
Comp.
no.
3-H, d 4-H, d
HA (1H)
HB, X
Harom
(1H)
(1H)
IVb
IVc
IVd
IVe
6.96
(3.3)
7.07
7.04 d 7.32 7.39 m (2H, 5-H, 6-H), 7.57 7.70 m 7.46 d (2H), 7.84 d (2H),
(16.2) (3H, HB, 4-H, 7-H) J = 8.4
6.95
(3.0)
7.07
7.03 d 7.32 7.39 m (2H, 5-H, 6-H), 7.60 7.70 m 7.59 d (2H), 7.78 d (2H),
(16.4) (3H, HB, 4-H, 7-H) J = 7.5
7.02
(3.0)
7.34a
7.33a
7.15 d 7.63 d (1H, HB), 7.59 d (1H) and 7.68 d (1H) (4-H, 7-H); 7.33 7.40 m (4H,
(16.2) 4-H, 5-H, 6-H, 3-Harom), 7.53 m (1H, 4-Harom), 8.03 m (1H, 6-Harom
)
7.00
7.10 d 7.27 7.42 m (3H, 4-H, 5-H, 6-H), 7.55 7.79 m (4H, HB, 4-H, 7-H, 5-Harom
(16.5) 8.13 d (1H, 6-Harom), J = 7.5; 8.24 d (1H, 4-Harom), J = 6.9; 8.58 s (1H,
1-Harom
)
)
IVf
7.04
(2.4)
7.36a
7.26
7.16 d 7.32 7.42 m (3H, 4-H, 5-H, 6-H), 7.58 8.10 d (2H), 8.28 d (2H),
(16.2) 7.72 m (3H, HB, 4-H, 7-H) J = 9.0
IVg
7.00
(3.0)
7.06 d 7.32 7.41 m (3H, 5-H, 6-H, 5-Harom), 7.58 7.71 m (3H, HB, 4-H, 7-H)
(16.2)
7.45 t (1H, 4-Harom), 7.52 d (1H, 3-Harom), J = 8.4; 8.04 d (1H, 6-Harom),
J = 7.2
Vb
Vc
Vd
Ve
6.90
(4.2)
7.04
7.06
7.33
7.31
7.32 7.50 m (6H, HA, HB, 5-H, 6-H, 3-Harom, 5-Harom), 7.91 d (1H, 4-H), J = 7.2;
7.97 d (1H, 7-H), J = 8.4, 7.83 d (2H, 2-Harom, 6-Harom), J = 7.8
6.90
(3.0)
7.33 7.50 m (4H, HA, HB, 5-H, 6-H), 7.92 d (1H, 4-H), 7.59 d (2H), 7.77 d (2H), J =
J = 8.1; 7.97 d (1H, 7-H), J = 7.8
7.35 7.57 m (6H, HA, HB, 5-H, 6-H, 3-Harom, 4-Harom), 7.93 d (1H, 4-H), J = 7.8;
7.98 d (1H, 7-H), J = 7.8; 8.03 m (1H, 6-Harom
7.36 7.55 m (4H, HA, HB, 5-H, 7-H), 7.93 d (1H, 4-H), 7.74 t (1H, 5-H), J = 7.8;
8.7
6.97
(3.3)
)
6.97
(3.9)
J = 7.8; 7.99 d (1H, 7-H), J = 7.2
8.15 d (1H, 6-H), 8.26 d
(1H, 4-H), 8.60 s (1H,
2-H)
XIb
XIe
7.15
(3.0)
7.24
7.39
8.13 s 7.11 m (2H, 5-H, 6-H), 7.44 m (3H, 4-H, 3-Harom, 5-Harom), 7.56 d (1H,
7-H), 7.85 d (2H, 1-Harom, 6-Harom), J = 8.4; 12.80 br.s (1H, NH)
7.31
(3.6)
8.18 s 7.10 7.17 m (2H, 5-H, 6-H), 7.45 m (1H, 7.72 t (1H, 5-H), 8.15 m
4-H), 7.56 m (1H, 7-H), 12.85 br.s (1H,
NH)
(1H, 6-H), 8.26 d (1H,
4-H), J = 7.2; 8.68 s (1H,
2-H)
XIIIa
XIIIb
7.23
(2.7)
7.45a
7.47
8.13 s 6.81 t (1H, 5-H), 6.93 m (2H, 3-H, 4-H), 7.37 7.55 m (4H, 4-H, 3-H,
8.09 d (1H, 6-H), J = 6.9; 8.97 br.s (1H,
4-H, 5-H), 7.91 d (2H,
OH), 10.06 br.s (1H, NH)
2-H, 6-H), J = 8.1
7.31
(2.7)
8.14 s 6.80 t (1H, 5-H), 6.88 7.01 m (2H, 3-H, 7.66 d (2H), 7.85 d (2H), J =
4-H), 8.07 d (1H, 6-H), J = 8.4; 8.96 br.s
8.1
(1H, OH), 10.06 br.s (1H, NH)
a
Overlapped by other proton signals.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003

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GANUSHCHAK et al.
Table 5. Electron absorption spectra of compounds IVa IVf, IXa IXc, and XIa XIf
Compound no.
max, nm
log
Compound no.
max, nm
log
IVa
IVb
IVc
IVd
IVe
IVf
367
398
405
389
377
414
392
386
4.80
4.87
4.76
4.83
4.73
4.69
4.84
4.79
IXc
XIa
XIb
XIc
XId
XIe
XIf
413
410
446
450
433
440
431
4.77
4.88
4.93
4.81
4.88
4.79
4.77
IXa
IXb
cooled to 80 85 C, 8 ml of hydrochloric acid and
6 ml of water were added, and the mixture was heated
for 0.5 1 h at the boiling point and was left to stand
until a solid precipitated. The precipitate was treated
as described above in c.
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the mixture was heated for 0.5 1 h under reflux. It
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recrystallized from DMF alcohol.
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in 5 ml of DMF was heated for 1 h under reflux. The
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N-(2-Hydroxyphenyl)-3-(5-aryl-2-furyl)-2-cyano-
propenamides XIIIa and XIIIb. A mixture of 0.8 g
(3.1 mmol) of compound XIIa, 0.34 g (3.1 mmol)
of o-aminophenol, and 45 ml of triethylamine was
stirred for 1 h on heating to the boiling point. The
mixture was then cooled and acidified with dilute
hydrochloric acid to pH 1. The precipitate was filtered
off, washed with water and alcohol and recrystallized
from alcohol DMF (4:1). Yield of XIIIa 0.65 g
(65%), mp 264 265 C. Found, %: C 72.45; H 4.16;
N 8.40. C20H14N2O3. Calculated, %: C 72.72; H 4.27;
N 8.48. Compound XIIIb was synthesized in a similar
way. Yield 61%, mp 315 316 C. Found, %: C 58.52;
H 3.13; N 6.91. C20H13BrN2O3. Calculated, %:
C 58.70; H 3.20; N 6.85.
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 9 2003