Photochromic dihetarylethenes. 12.* Synthesis of 5-alkyl-2- (1,3,4-oxadiazol-2-yl)thiophenes and their photochromic derivatives

Article abstract of DOI:10.1023/A:1015383007839

Photochromic derivatives of 5-alkyl-2-(1,3,4-oxadiazol-2-yl)thiophenes have been synthesized for the first time. Their photochromic and fluorescent properties have been studied.

Full text of DOI:10.1023/A:1015383007839

Chemistry of Heterocyclic Compounds, Vol. 38, No. 2, 2002
PHOTOCHROMIC DIHETARYLETHENES.
12.* SYNTHESIS OF 5-ALKYL-2-
(1,3,4-OXADIAZOL-2-YL)THIOPHENES
AND THEIR PHOTOCHROMIC DERIVATIVES*2
M. M. Krayushkin1, F. M. Stoyanovich1, O. Yu. Zolotarskaya1, E. I. Chernoburova1,
N. N. Makhova1, V. N. Yarovenko1, I. V. Zavarzin1, A. Yu. Martynkin1, and B. M. Uzhinov2
Photochromic derivatives of 5-alkyl-2-(1,3,4-oxadiazol-2-yl)thiophenes have been synthesized for the
first time. Their photochromic and fluorescent properties have been studied.
Keywords:
benzothiazoles,
1,2-dithienylethenes,
1,3,4-oxadiazoles,
perfluorocyclopentene,
photochromes, nondestructive readout of optical information, fluorescence.
One of the conditions determining the practical value of photochromic compounds is the possibility of
reading optical information without destroying it. A promising approach is the method of readout using the
fluorescence inherent in the actual photochrome molecule excited in the region in which a mutual transition of
two forms does not occur. Today there are only a few similar examples [2,3], however no special studies
directed towards obtaining fluorescing photochromic dihetarylethenes have been carried out. The development
of methods for synthesis of thermally irreversible compounds capable of effecting multiple readout of
information is a promising direction in the design of new photochromic structures.
Previously we obtained photochromic dihetarylethenes of general formula 1 [4,5]. Compounds 1a,b are
photochromes, but do not fluoresce. Consequently in the next step we paid attention to structures known to
contain fluorescent fragments.
F
N
X
N
X
S
S
Me Me
1a,b
1 a X = O, b X = S
_______
* For Part 11 see [1].
*2 Dedicated to M. G. Voronkov on the occasion of his Jubilee.
__________________________________________________________________________________________
1 N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moscow 117913, Russia;
2
e-mail: mkray@ioc.ac.ru. Moscow M. V. Lomonosov State University, Moscow 119899, Russia; e-mail:
uzhinov@light.chem.msu.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 185-196,
February, 2002. Original article submitted May 25, 2001.
0009-3122/02/3802-0165$27.00©2002 Plenum Publishing Corporation
165

It is known that 2,5-diaryl-1,3,4-oxadiazoles are efficient organic luminophores [6]. For example, on
investigating the spectral properties of 2-(2-thienyl)-5-aryl-1,3,4-oxadiazoles it was discovered that they possess
a very intense fluorescence [7]. It was expedient therefore to build the photochromic system 2 containing
thiophene, 1,3,4-oxadiazole, and benzene rings. In addition this combination should be capable of increasing the
length of the conjugation chain. It was also interesting to consider the effect of additional functional groups in
this system on the photochromic properties of the compounds.
F
N
N
N
N
R
R
O
O
S
S
Me Me
2a,b
2 a R = H, b R = OMe
The synthesis of the initial bromides 3 and 4 for the corresponding photochromic compounds 2a,b is
shown in Scheme 1. By reacting acid chloride 5 and benzoic acid hydrazide in pyridine at 60°C the
diacylhydrazide 6 was obtained, which after boiling in phosphorus oxychloride for 10 h was converted in
98% yield into bromide 3. 4-Methoxybenzoyl chloride and hydrazide 7, obtained from methyl ester of 4-bromo-
5-methyl-2-thiophenecarboxylic acid (8) by boiling in alcohol with an aqueous solution of hydrazine hydrate,
were used for the synthesis of bromide 4. The resulting hydrazide 9 was converted in 95% yield into compound
4 by boiling in phosphorus oxychloride.
Scheme 1
O
Br
O
C
NHNH2
C
Cl
Me
S
5
Br
Me
O
H
N
C
C
O
R
N
H
S
6 R = H; 9 R = OMe
O
Br
O
C
POCl3
MeO
C
Cl
NHNH2
Me
S
7
Br
N
N
O
R
BuLi
C5F8
Me
S
3, 4
2a,b
3 R = H; 4 R = OMe
The sequential interaction of bromide 3 with BuLi and octafluorocyclopentene in THF at -70°C led to
the photochromic compound 2a in 34% yield. The structure of 2a was established by spectral methods, and the
1
data of elemental analysis were consistent with it. In the H NMR spectrum singlets were observed for the
methyl protons, the thiophene ring proton, and also for the benzene ring protons, which indicate the presence of
one open symmetrical form. The photochromic characteristics are given in Table 1.
166

TABLE 1. Photochromic Characteristics of 1,2-Bis(3-thienyl)-
perfluorocyclopentenes
φ
Quantum yield of the
-1
-1
λmax
ε
Com-
pound
, nm ( , M ·cm )
photoreaction
Cyclicity
→
→
form A
form B
A
B
B
A
311 (48700)
318 (52700)
354 (28300)
351 (38700)
603 (14600)
605 (19300)
633 (10500)
570 (12900)
1.0
0.014
0.01
0.012
0.05
2000
45
600
380
2a
0.85
0.98
0.74
2b
10b
25
The photochromic compound 2b was obtained analogously in 30% yield from bromide 4. The structure
of 2b was established by spectral methods and was consistent with the data of elemental analysis. Singlets were
observed in the 1H NMR spectrum for the methyl protons of the OCH3 group and for the thiophene ring proton.
There were also two signals for the aromatic protons, which were doublets with J ~8 Hz.
It turned out unexpectedly that in spite of the presence of fluorophoric fragments, compounds 2a,b did
not possess fluorescent properties. We have therefore made attempts in the present work to synthesize
derivatives of the photochromic system 10 containing simultaneously thiophene, 1,3,4-oxadiazole, and
benzothiazole rings, which in our opinion may display fluorescent properties.
F
N
N
N
N
N N
MeO
OMe
O
S
S
O
S
S
R
R
10a,b
10 a R = Me, b R = C6H13
A convenient starting material for the synthesis of substances 10a,b is 2-carbamoyl-6-
methoxybenzothiazole (11), a method of obtaining which was recently developed by us [8]. Amide 11 is also
interesting as it serves as a starting material for the synthesis of D(-)-2-(6-hydroxy-2-benzothiazolyl)-3,4-
dihydrothiazole-4-carboxylic acid (luciferin), the substance providing the glow of fireflies [9], and widely used
for some time in biophysical investigations. Starting from benzothiazole 11, the synthesis of 2-(4-bromo-5-
methyl-2-thienyl)-5-(6-methoxy-2-benzothiazolyl)-1,3,4-oxadiazole (12) was effected as in Scheme 2.
6-Methoxybenzothiazole-2-carboxylic acid (13), obtained on saponification of amide 11 in 10%
aqueous NaOH solution, was converted into the corresponding methyl ester 14, which gives hydrazide 15 after
boiling in alcohol with an aqueous solution of hydrazine hydrate. Interaction of compound 15 with 4-bromo-5-
methyl-2-thienoyl chloride (5) leads to the diacylhydrazine 16. Its cyclization to oxadiazole 12 under the action
of phosphorus oxychloride at atmospheric pressure did not proceed to completion. Even on extended boiling the
ratio of starting material to reaction product was 10:3. After heating in an ampul for 20 h at 140°C the yield of
bromide 12 was 40% and at 160-170°C for 16 h the yield was successfully increased to 70%.
Regretably, our numerous attempts to obtain the photochromic compound 10a from bromide 12 were
unsuccessful due to its poor solubility in THF, and in mixtures of THF–toluene, THF–HMPA at -70°C. Starting
material was recovered in 30-50% yield, the remainder (70-50%) were resinous products. It should be
mentioned that an increase in the reaction time of bromide 12 with BuLi led to darkening of the reaction mixture
and to an increase in the formation of resinification products.
167

Scheme 2
O
C
O
C
N
S
N
S
CH2N2
10% NaOH
NH2
OH
MeO
MeO
11
13
O
C
O
C
N
S
N
S
5
N2H4
EtOH
NHNH2
POCl3
OMe
Py
MeO
MeO
15
14
Br
O
N
H
OMe
C
N
H
N
C
O
Me
S
S
16
Br
N
N
N
10a
OMe
Me
S
O
S
12
To increase the solubility of the initial bromide it was decided to replace the methyl residue in
position 2 of the thiophene ring by hexyl (Scheme 3). 4-Bromo-5-hexylthiophene-2-carboxylic acid (18) was
obtained for this purpose by the bromination of 5-hexylthiophene-2-carboxylic acid (17) in acetic acid in the
presence of FeCl3. Interaction of its acid chloride 19 with hydrazide 15 and subsequent cyclization of the diacyl
hydrazine 20 under the same conditions as for compound 16 gave product 21.
Scheme 3
Br
SOCl2
Br2
C6H13
CO2H
COCl
C6H13
CO2H
N
S
17
S
18
AcOH
Br
Br
C6H13
O
H
N
15
OMe
C6H13
C
N
H
C
O
S
S
S
19
20
Br
C6H13
N
N
N
POCl3
OMe
S
O
S
21
On sequential interaction of bromide 21 with BuLi and C5F8 the photochromic compound 10b was
1
synthesized (Scheme 4) in low yield (2%) and was characterized by H NMR and mass spectrometry. In the
1H NMR spectrum a singlet and two doublets (J ~8 Hz) were observed for the protons of the benzothiazole
fragment, a singlet for the thiophene ring proton, and signals for the protons of the hexyl group.
168

Scheme 4
BuLi
C5F8
21
F
N
N
N
N
N N
OMe
MeO
O
S
S
O
S
S
C6H13
C6H13
10b
On photochemical investigation of compound 10b it was discovered that its open form possesses
fluorescence (Table 1).
By the action of an excess of octafluorocyclopentene on the Li derivative 22, obtained from bromide 21,
1
we synthesized (Scheme 5) the mono derivative 23 in 9% yield and characterized it by H NMR and mass
spectrometry.
Scheme 5
Li
N
N
N
C5F8
BuLi
OMe
21
C6H13
S
O
S
22
F
N
N
N
F
OMe
C6H13
S
O
S
23
The unsymmetrical compound 25 was synthesized in 29% yield by the reaction of fluoride 23 with
3-bromo-2-methylbenzothiophene (24) (Scheme 6). The structure of product 25 was established by data of
elemental analysis and by spectral methods. Signals were observed in the 1H NMR spectrum for the protons of
the benzothiazole fragment, viz. a singlet and two doublets (J ~8 Hz), a singlet for the proton of the thiophene
ring, signals for the protons of the benzothiophene fragment, and also signals for the protons of the methyl,
hexyl, and methoxy groups.
Scheme 6
F
Br
BuLi
N
N
N
23
+
OMe
Me
S
S
S
O
S
Me
C6H13
24
25
It was discovered in the photochemical investigation of compound 25 that its open form possesses
fluorescence (Table 1).
169

A photochemical study of compounds 2a,b, 10b, and 25 was carried out in acetonitrile. Photocyclization
of A→ B was effected on irradiating with light of λ 313 and 365 nm, and the reverse reaction B → A on
irradiating with light of λ 578 nm.
F
F
S
S
S
S
Alk
Alk
Alk
Alk
B
A
Isosbestic points were observed in the absorption spectra. The coincidence of their position on carrying
out the forward and reverse reactions indicate the complete reversibility of photocyclization and the absence of
side processes (see Figs. 1, 2).
a
b
Fig.1. Change in absorption spectrum for acetonitrile solutions of a) compound 2a and
b) compound 2b on irradiating with light of λ 313 (forward reaction) and 578 nm (reverse reaction).
170

a
b
Fig. 2. Change in absorption spectrum for acetonitrile solutions of a) compound 10b and
b) compound 25 on irradiating with light of λ 365 (forward reaction) and 578 nm (reverse reaction).
The differences in the positions of the long wave absorption bands of compounds 2a and 2b were
insignificant. The large bathochromic shift of the long wave absorption bands of 10bB relative to 25B is caused
primarily by the significant lengthening of the conjugation chain (the cyclic forms of the photochromes with
benzothienyl substituents on the perfluorocyclopentene ring absorb in a shorter wave region than their analogs
with alkyl- and aryl-substituted thienyl fragments [10]), and secondly by the presence of a second n-hexyl
substituent in place of methyl at position 2 of the thiophene ring. The dark ring fission reaction (B → A) was
absent for 2a, 2b and 25, and the cyclic form 10bB was thermally unstable. After 100 h exposure in the dark the
optical density of this form was reduced by 2% (after 530 h by 15%).
171

Compounds 2a and 2b did not fluoresce. The open forms 25A and 10bA fluoresce, the intensity of the
fluorescence fell sharply after cyclization under the action of UV light with λ 365 nm. The fluorescence
emission maximum of 25A was at λ 428 nm and of 10bA at λ 422 nm. The fluorescence excitation spectrum,
repeating the absorption spectrum, indicates that the observed fluorescence belongs to the fluorescence of 10bA.
The presence of fluorophoric fragments in the structures of photochromic dithienylethenes does not by
any means always lead to the development of fluorescence by the latter, and the presence of fluorescence in 25A
and 10bA is probably explained by the adequate separation of the benzothiazole substituent from the
perfluorocyclopentene fragment directly involved in the photochromic conversion.
EXPERIMENTAL
The 1H, 13C, and 19F NMR spectra were taken on Bruker WM 200 (200 MHz) and AM 300 (300 MHz)
spectrometers for solutions in CDCl3, (CD3)2CO, or (CD3)2SO. Mass spectra were obtained on a Finnigan MAT
INCOS 50 (70 eV) instrument in chromatography mode (capillary column RSL 200 of length 30 m) or by direct
insertion. The composition of reaction mixtures was checked by 1H NMR and thin-layer chromatography (TLC)
on Silufol plates in systems of various polarity. Preparative separation was carried out by column
chromatography on silica gel L 40-100 mesh with eluents of various polarity.
All reactions with organolithium compounds were carried out in a dry argon atmosphere. Reactants and
solvents were dried using standard procedures. Reactants were introduced into previously dried apparatus using
rubber stoppers and disposable syringes. Melting points were determined on a Boetius stage. The elemental
analysis of compounds obtained was carried out in the Microanalysis Laboratory of the Institute of Organic
Chemistry of the Russian Academy of Sciences. The irradiation of samples in the photochemical investigations
was carried out with a DRSh-500 high pressure mercury lamp. The radiation intensity of the mercury lamp was
determined with an F4 photocell calibrated with a ferrioxalate actinometer [11] for λ 313, 365, 405, and 436 nm
and an actinometer based on Reinecke salt [12] for λ 546 and 578 nm. Absorption spectra were recorded on a
Shimadzu UV 2101PC spectrophotometer. Fluorescence was investigated with the aid of a Perkin–Elmer LS 50
spectrofluorimeter. To determine quantum yield a solution of the substance in ethanol was irradiated with light
at λ 365 nm when carrying out photocyclization and at 578 nm for the reverse reaction using light filters to
separate the lines of the mercury spectrum. The duration of irradiation was gradually increased from 5 sec to
1-2 min (7-10 experimental points in all), the absorption spectrum of the irradiated solution was recorded for
each exposure.
To determine cyclicity an undegassified solution of the photochrome was subjected to irradiation over a
wide range, including the lines of the mercury spectrum at 313, 365, 405, 436, 546, and 578 nm. The ratio of the
intensities of the lines in the mercury spectrum in the UV and visible regions was varied using wide-band filters.
The time of irradiation during which one photocycle was effected was determined from the formula:
c0 φB→A
Ja
τ
=
B
where c0 is the initial concentration of substance, φB → the quantum yield for the conversion of B to A, JaB is
A
the intensity of light absorbed by form B in the photostationary state.
4-Bromo-5-methylthiophene-2-carboxylic Acid Chloride (5). Thionyl chloride (6 ml, 0.08 mol) was
added to 4-bromo-5-methylthiophene-2-carboxylic acid [13] (7 g, 0.03 mol) and the mixture was boiled under
reflux for 2 h. The solution was then cooled to room temperature and left overnight. Unreacted SOCl2 was
removed, the residue distilled in vacuum, collecting the fraction with bp 137-138°C, P = 15 torr. Yield of
5 78%; mp 32.5-34.5°C.
172

1-(4-Bromo-5-methyl-2-thienoyl)-2-benzoylhydrazine (6). Acid chloride 5 (2.48 g, 0.01 mol) was
added to a stirred solution of benzoic acid hydrazide (1.36 g, 0.01 mol) in pyridine (10 ml) at 10°C. The mixture
was stirred at room temperature for 20 min, then heated for 2 h at 60°C. The reaction mixture was brought to
room temperature and left overnight. The mixture was poured into water (50 ml), the precipitated solid filtered
off, washed with water, and dried. Ethanol (100 ml) was added to the substance obtained, the mixture heated
1
under reflux for 1 h, the solid was then filtered off, and dried. Yield 61%; mp 286-288°C. H NMR spectrum
(DMSO-d6), , ppm: 2.45 (3H, s, CH3); 7.52 (2H, m, H arom.); 7.60 (1H, m, H arom.); 7.81 (1H, s,
δ
H thiophene); 7.91 (2H, m, H arom.); 10.48 (2H, cis-s, NH). Mass spectrum, m/z (Irel, %): 340 [M]+ (44%).
4-Bromo-5-methyl-2-(5-phenyl-1,3,4-oxadiazol-2-yl)thiophene (3). Phosphorus oxychloride (20 ml)
was added to diacylhydrazine 6 (1 g, 2.9 mmol) and the mixture boiled under reflux for 10 h. The unreacted
phosphorus oxychloride was removed, the residue was washed on the filter with water, and dried. Yield 98%;
mp 168.5-169.5°C (EtOH). 1H NMR spectrum (DMSO-d6), , ppm: 2.46 (3H, s, CH3); 7.60 (1H, br d, p-H); 7.65
δ
(2H, br t, m-H); 7.87 (1H, s, H thiophene); 8.07 (2H, br d, o-H). Mass spectrum, m/z (Irel, %): 320 [M]+ (38%).
Found, %: C 48.36; H 2.84; Br 24.38; S 9.78. C13H9BrN2OS. Calculated, %: C 48.61; H 2.82; Br 24.88; S 9.98.
4-Bromo-5-methylthiophene-2-carboxylic Acid Hydrazide (7). An aqueous solution (3 ml) of
N2H4.H2O (3 ml) was added to a solution of 4-bromo-5-methylthiophene-2-carboxylic acid methyl ester (1.3 g,
5.5 mmol) in methanol (5 ml) and the mixture boiled under reflux for 2.5 h. The precipitated solid was filtered
1
off, washed with water, and with alcohol. Yield 90%; mp 167-168°C (EtOH). H NMR spectrum (DMSO-d6),
, ppm: 2.39 (3H, s, CH3); 4.5 (2H, s, NH2); 7.61 (1H, s, H thiophene); 9.76 (1H, s, NH). Found, %: C 30.93;
δ
H 2.92; Br 34.17; S 13.71. C6H7BrN2OS. Calculated, %: C 30.65; H 3.0; Br 33.99; S 13.64.
1-(4-Bromo-5-methyl-2-thienoyl)-2-(4-methoxybenzoyl)hydrazine (9) was obtained analogously to
diacylhydrazine 6 from hydrazide 7 (1.5 g, 6.4 mmol) and 4-methoxybenzoyl chloride (1.1 g, 6.5 mmol).
Yield 95%; mp 264-265°C. 1H NMR spectrum (DMSO-d6), , ppm: 2.45 (3H, s, CH3); 3.85 (3H, s, OCH3); 7.08
δ
(2H, d, H arom.); 7.81 (1H, s, H thiophene); 7.9 (2H, d, H arom.); 10.95 (2H, s, NH).
2-(4-Bromo-5-methyl-2-thienyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole (4) was obtained
1
analogously to compound 3 from diacylhydrazine 9. Yield 70%; mp 167-168°C (EtOH). H NMR spectrum
(DMSO-d6), , ppm: 2.49 (3H, s, CH3); 3.88 (3H, s, OCH3); 7.16 (2H, d, H arom.); 7.81 (1H, s, H thiophene);
δ
8.01 (2H, d, H arom.). Found, %: C 47.56; H 2.98; Br 22.51; S 8.90. C14H11BrN2O2S. Calculated, %: C 47.88;
H 3.16; Br 22.75; S 9.13.
1,2-Bis[2-methyl-5-(5-phenyl-1,3,4-oxadiazol-2-yl)-3-thienyl]hexafluorocyclopentene (2a). A solution
of BuLi (0.9 ml, 1.78 mmol) in hexane was added to a stirred suspension of compound 3 (0.5 g, 1.56 mmol) in
abs. THF (10 ml) at -70°C in an atmosphere of Ar, and the mixture stirred at this temperature for 15 min.
Octafluorocyclopentene (0.16 g, 0.78 mmol) in abs. THF (1 ml) was then added at -70°C and the mixture stirred
for 2 h. The temperature of the reaction mixture was brought up to room temperature and the mixture left
overnight in an atmosphere of Ar. The mixture was cooled to -5°C, alcohol (5 ml) was added, and the mixture
stirred at 20°C for 40 min. The solvent was removed, the residue dissolved in CHCl3, the solution washed with
5% NaHCO3, with water, and dried over CaCl2. The solvent was removed. The product was separated on a
column of silica gel (eluent CHCl3). Yield 34%; mp 236-238°C. 1H NMR spectrum (CDCl3), , ppm: 2.09 (3H,
δ
s, CH3); 7.50-7.51 (3H, m, H arom.); 7.81 (1H, s, H thiophene); 8.14 (2H, m, H arom.). Mass spectrum, m/z
(Irel, %): 656 [M]+ (10%). Found, %: C 56.45; H 2.58. C31H18F6N4O2S2. Calculated, %: C 56.71; H 2.76.
1,2-Bis{2-methyl-5-[5-(4-methoxyphenyl)-1,3,4-oxadiazol-2-yl]-3-thienyl}hexafluorocyclopentene
(2b) was obtained analogously to 2a from bromide 4. The product was isolated by column chromatography
1
(eluent hexane–ethyl acetate from 10:1 to 3:1). Yield 30%; mp 219-220.5°C. H NMR spectrum (CDCl3),
, ppm: 2.19 (3H, s, CH3); 3.89 (3H, s, OCH3); 7.01 (2H, d, H arom.); 7.75 (1H, s, H thiophene); 8.02 (2H, d,
δ
H arom.). Found, %: C 56.04; H 3.13; S 9.05. C33H22F6N4O4S2. Calculated, %: C 55.31; H 3.09; S 8.95.
6-Methoxybenzothiazole-2-carboxylic Acid (13). A 10% NaOH solution (75 ml) was added to amide
11 (3 g, 14 mmol) and the mixture was boiled under reflux for 45 min. The suspension was cooled, the solid
173

filtered off, washed with benzene, dried, dissolved in water (400 ml), and the solution acidified with conc. HCl.
The precipitated solid was filtered off, washed with water, and dried. Yield 80%; mp 106°C (105-108°C in [9]).
6-Methoxybenzothiazole-2-carboxylic Acid Methyl Ester (14). An ether solution of CH2N2 (1 g,
24 mmol) was added during 15 min to a stirred solution of acid 13 (2 g, 9.56 mmol) in THF (50 ml) at 1°C. The
temperature of the reaction mixture was brought to room temperature, and the mixture stirred for 2 h. The
solvent was removed, and the residue crystallized from methanol. Yield of ester 14 was 66%; mp 142-143°C
(MeOH) (142-142.8°C in [9]).
6-Methoxybenzothiazole-2-carboxylic Acid Hydrazide (15). An aqueous solution (2 ml) of N2H4·H2O
was added to a solution of ester 14 (1.3 g, 5.8 mmol) in methanol (20 ml) and the mixture boiled under reflux
for 2.5 h. The precipitated solid was filtered off, washed with water, and then with alcohol. Yield 87%;
1
mp 223-225°C (EtOH). H NMR spectrum (DMSO-d6), , ppm: 3.88 (3H, s, OCH3); 4.7 (2H, br. s, NH2); 7.21
δ
(1H, d, H arom.); 7.78 (1H, s, H arom.); 8.0 (1H, d, H arom.); 10.3 (1H, br. s, NH). Found, %: C 48.29; H 3.78;
S 13.89. C9H9N3O2S. Calculated, %: C 48.42; H 4.06; S 14.36.
1-(4-Bromo-5-methyl-2-thienoyl)-2-(6-methoxy-2-benzothiazolyl)hydrazine (16) was obtained
1
analogously to diacylhydrazine 6 from hydrazide 15 and acid chloride 5. Yield 78%; mp 259-260°C. H NMR
spectrum (DMSO-d6), , ppm: 2.47 (3H, s, CH3); 3.90 (3H, s, OCH3); 7.19 (1H, d, H arom.); 7.70 (1H, s,
δ
H thiophene); 7.8 (1H, s, H arom.); 8.2 (1H, d, H arom.); 10.61 (1H, s, NH); 10.9 (1H, br. s, NH).
2-(4-Bromo-5-methyl-2-thienyl)-5-(6-methoxy-2-benzothiazolyl)-1,3,4-oxadiazole (12). A solution
of compound 16 (1 g, 2.3 mmol) in POCl3 (17 ml) was heated for 16 h in an ampul at 160-170°C. The
phosphorus oxychloride was removed, the residue washed on the filter with water, and dried. Yield 70%;
1
mp 277-278°C (Py). H NMR spectrum (CDCl3), , ppm: 2.52 (3H, s); 3.96 (3H, s, OCH3); 7.21 (1H, d,
δ
H arom.); 7.41 (1H, s, H arom.); 7.79 (1H, s, H thiophene); 8.11 (1H, d, H arom.). Mass spectrum, m/z (Irel, %):
407 [M]+ (100). Found, %: C 43.85; H 2.35; Br 19.32; S 15.35. C15H10BrN3O2S2. Calculated, %: C 44.13;
H 2.47; Br 19.57; S 15.70.
5-Hexylthiophene-2-carboxylic Acid (17). An ether solution of BuLi (66 ml, 97 mmol) was added to a
stirred solution of 2-hexylthiophene [14] (15 g, 89 mmol) in ether (150 ml) at room temperature in an
atmosphere of Ar. The mixture was stirred for 0.5 h at room temperature, boiled for 1 h under reflux, and stirred
for 1 h at 20°C. The solution was poured into a mixture of dry ice (200 g) and ether (500 ml). After 2 h water
(200 ml) was added, the layers were separated, the organic layer was washed with 5% NaOH solution. The
combined aqueous fraction was acidified with conc. HCl. The precipitated solid was filtered off, and washed
with water. Yield 87%; mp 60-62°C (65°C in [15]).
4-Bromo-5-hexylthiophene-2-carboxylic Acid (18). A solution of bromine (7.5 g, 47 mmol) in acetic
acid (9 ml) was added during 1.5 h to a stirred solution of acid 17 (10 g, 47 mmol) in acetic acid (100 ml) at
room temperature in the presence of FeCl3 (1.3 g, 8 mmol). The mixture was stirred for 2 h, boiled under reflux
for 6 h, then cooled to room temperature. The mixture was poured into water (1 liter), the precipitated solid was
1
filtered off, washed with water, and dried. Yield 65%, mp 86-86.5°C. H NMR spectrum (CDCl3), , ppm:
δ
0.85-0.99 (3H, m); 1.38-1.49 (6H, m); 1.66-1.78 (2H, m); 2.85 (2H, t); 7.71 (1H, s, H thiophene). Found, %:
C 45.19; H 4.95. C11H15BrO2S. Calculated, %: C 45.37; H 5.19.
4-Bromo-5-hexyl-2-thienoyl Chloride (19) was obtained analogously to acid chloride 5 from acid 18.
After removing the SOCl2 in vacuum the acid chloride 19 was used without further purification.
1-(4-Bromo-5-hexyl-2-thienoyl)-2-(6-methoxy-2-benzothiazolyl)hydrazine (20) was obtained
1
analogously to diacylhydrazine 6 from hydrazide 15. Yield 98%; mp 153-156°C. H NMR spectrum (CDCl3),
, ppm: 0.9 (3H, s, CH3); 1.32-1.65 (8H, m, CH2); 2.79 (2H, t, CH2); 3.9 (3H, s, OCH3); 7.17 (1H, d, H arom.);
δ
7.38 (1H, s, H thiophene); 7.56 (1H, s, H arom.); 7.96 (1H, d, H arom.); 9.39 (1H, br. s, NH); 9.84 (1H, br. s,
NH). Mass spectrum, m/z (Irel, %): 495 [M]+ (6).
2-(4-Bromo-5-hexyl-2-thienyl)-5-(6-methoxy-2-benzothiazolyl)-1,3,4-oxadiazole (21) was obtained
1
analogously to compound 12 from diacylhydrazine 20. Yield 60%; mp 157.5-158°C (AcOEt). H NMR
spectrum (CDCl3), , ppm: 0.91 (3H, t); 1.31-1.45 (6H, m); 1.65-1.79 (2H, m); 2.85 (2H, t); 3.92 (3H, s, OCH3);
δ
174

7.19 (1H, d, H arom.); 7.41 (1H, s, H arom.); 7.79 (1H, s, H thiophene); 8.10 (1H, d, H arom.). Mass
spectrum, m/z (Irel, %): 477 [M]+ (55). Found, %: C 49.91; H 4.0; Br 16.38; S 13.21. C20H20BrN3O2S2.
Calculated, %: C 50.20; H 4.21; Br 16.70; S 13.40.
1,2-Bis{2-hexyl-5-[5-(6-methoxy-2-benzothiazolyl)-1,3,4-oxadiazol-2-yl]-3-thienyl}perfluorocyclo-
pentene (10b) was obtained analogously to product 2a from bromide 21 and was isolated (6 mg, 2%) on a
1
chromatographic column (eluent benzene–CHCl3, 1:1). Mp 203-204°C. H NMR spectrum (CDCl3), , ppm:
δ
0.75-0.95 (3H, m); 1.1-1.45 (8H, m); 2.35 (2H, t); 3.95 (3H, s, OCH3); 7.29 (1H, d, H arom.); 7.45 (1H, s,
H arom.); 7.96 (1H, s, H thiophene); 8.14 (1H, d, H arom.). Mass spectrum, m/z (Irel, %): 970 [M]+ (10).
1-{2-Hexyl-5-[5-(6-methoxy-2-benzothiazolyl)-1,3,4-oxadiazol-2-yl]-3-thienyl}heptafluorocyclo-
pentene (23). A solution (2.1 ml, 3.0 mmol) of BuLi in hexane was added to a stirred solution of bromide 21
(1.3 g, 2.7 mmol) in abs. THF (30 ml) at -70°C in an atmosphere of Ar. Stirring was continued at the same
temperature for 15 min, then octafluorocyclopentene (0.7 ml, 5.4 mmol) was added at -70°C and the mixture
stirred for 2 h. The reaction mixture was brought to room temperature and left overnight. The mixture was
cooled to -5°C, alcohol (5 ml) was added, and the mixture stirred for 40 min at 20°C. The solvent was removed,
the residue was washed on the filter with water, and dried. The product was separated on a chromatographic
1
column (eluent ether–hexane, 1:1). Yield 9%; mp 136-138°C. H NMR spectrum (CDCl3), , ppm: 0.86-0.97
δ
(3H, m); 1.3-1.43 (6H, m); 1.7-1.8 (2H, m); 2.85 (2H, t); 3.95 (3H, s, OCH3); 7.21 (1H, d, H arom.); 7.42 (1H, s,
H arom.); 7.88 (1H, s, H thiophene); 8.11 (1H, d, H arom.). Mass spectrum, m/z (Irel, %): 591 [M]+ (100).
1-{2-Hexyl-5-[5-(6-methoxy-2-benzothiazolyl)-1,3,4-oxadiazol-2-yl]-3-thienyl}-2-[2-methyl-3-benzo-
thiophenyl]perfluorocyclopentene (25). A solution (0.35 ml, 0.5 mmol) of BuLi in hexane was added at -68°C
in an atmosphere of Ar to a solution of compound 24 [16] (0.11 g, 0.5 mmol) in abs. THF (20 ml) and the
mixture was stirred at this temperature for 15 min. Compound 23 (0.12 g, 0.2 mmol) in abs. THF (5 ml) was
then added at -70°C, and the mixture stirred for 2 h at the same temperature. The reaction mixture was brought
up to room temperature and left overnight in an atmosphere of Ar. The mixture was cooled to -5°C, alcohol
(5 ml) was added, and the mixture stirred for 0.5 h at 20°C. The solvents were removed, water (5 ml) was added
to the residue, the solid filtered off, and dried. The product was separated on a chromatographic column (eluent
1
ethyl acetate–hexane, 1:4). Yield was 29%; mp 207-208°C. H NMR spectrum (CDCl3), , ppm: 0.85 (3H, t);
δ
0.9-1.36 (8H, m); 2.05-2.21 (2H, m); 2.30 (3H, s, CH3); 3.95 (3H, s, OCH3); 7.20 (1H, d, H arom.); 7.28 (1H, m,
H arom.); 7.36 (1H, m); 7.41 (1H, s, H arom.); 7.61 (1H, m, H arom.); 7.79 (1H, d, H thiophene); 7.99 (1H, s,
H arom.); 8.04 (1H, d). Mass spectrum, m/z (Irel, %): 719 [M]+ (50).
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