Studies with 2-(acetonylthio)benzothiazole. New routes to isoxazoles, isoxazolo[3,4-b]pyridines, pyrazolo[1,5-a]pyrimidines, pyridines and quinolizines

Article abstract of DOI:10.3184/030823409X464412

Derivatives of isoxazole, isoxazolo[3,4-blpyridine, pyrazolo-[1,5- alpyrimidine and pyridin-2(1H)-one incorporating a benzothiazole-2-thio residue have been synthesised. The structures of these newly synthesised compounds are characterised by elemental an

Full text of DOI:10.3184/030823409X464412

JOURNAL OF CHEMICAL RESEARCH 2009
JULY, 433-436
RESEARCH PAPER 433
Studies with 2-(acetonylthio)benzothiazole. New routes to isoxazoles,
isoxazolo[3,4-b]pyridines, pyrazolo[1,5-a]pyrimidines, pyridines and
quinolizines
Fatima AI-Omran* and Adal A. EI-Khair
Chemistry Department, Kuwait University, Faculty of Science, PO Box 5969, Safat 13060, Kuwait
Derivatives of isoxazole, isoxazolo[3,4-blpyridine,
pyrazolo-[1,5-alpyrimidine
and pyridin-2(1H)-one
incorporating
a benzothiazole-2-thio residue have been synthesised. The structures of these newly synthesised compounds are
characterised by elemental analysis, IR, lH and 13C NMR, NOE and MS.
Keywords: enaminones, benzothiazoles, quinolizines, monocyclic and fused pyridines, isoxazoles, pyrazoles, fused pyrimidines
The interesting biological activIties of 2-mercaptobenzo-
thiazole, pyridine, isoxazole, pyrazolo[I,5-a]pyrimidine,
quinolizine and isoxazolo[3,4-b ]pyridine derivatives have
stimulated considerable research work in recent years.I-7
In connection with our previous interest in the synthesis
with the carbonyl group, all indicated the formation of the
enaminone 3 and ruled out structure 4 (Scheme I). The
structure of enaminone 3 was assigned as the Z-configuration
on the following evidence. The nonequivalence of the N-
methyl groups is due to resonance of the nitrogen lone pair
of electrons with the a,13-unsaturated carbonyl group, giving
rise to partial double bond character of the C-N bond and
thus restricted rotation (resonance form 3a in Scheme 1).The
electron-withdrawing C=O group shifts the 13-alkeneproton
to the deshielded position (iSH 8.19), which suggests that the
alkene has the dimethylamino group trans to the carbonyl
group. Firm evidence for this conclusion was obtained from
nuclear Overhauser effect (NOE) experiments. Thus, on
irradiating the alkene proton at iSH 8.19 ppm both the methyl
and N,N-dimethylamino signals were enhanced, while
irradiating the methyl or the dimethylamino protons enhanced
only the alkene proton (cf Scheme I)
of polyfunctionally
substituted heterocycles utilising
enaminones,6-9 it was of interest to study the behaviour of
the hitherto unreported (Z)-3-(benzothiazol-2'-ylthio)-4-
(N,N-dimethylamino)but-3-en-2-one (3) towards a variety of
reagents in order to obtain a series of compounds in which a
sulfur atom links the 2-benzothiazole unit with heteroaromatic
rings such as pyridine, isoxazole, pyrazolo[I,5-a]pyrimidine,
quinolizine and isoxazolo[3,4-b ]pyridine derivatives, in
the hope of obtaining compounds that have biological and
pharmaceutical applications.
Results and discussion
The reactivity of 3 towards nitriles with active methylene
groups was investigated. Treatment of compound 3 with
malononitrile in refluxing ethanol containing a catalytic
amount of piperidine afforded a single product which may be
either 5 or 6. The spectral data did not clearly discriminate
between the 2,3,5,6-tetrasubstitution in pyridine 5 and the
2,3,4,5-arrangement in 6.7,11-13However, since the initial
The key intermediate I-(benzothiazol- 2'-yIthio)propan- 2-one
(2) used in our experiments was readily prepared in good
yield by the treatment of 2-mercaptobenzothiazole (1) with
a-chloroacetone in refluxing acetone containing potassium
carbonate.10 Treatment of 2 with DMFDMA afforded
a
product which could have been either the enaminone 3 or its
isomer 4 (cf Scheme I). The structure was established to be
3 on the basis of its spectral data. The IR spectrum of product
showed an absorption band at 1646 cm-1corresponding to the
carbonyl group. The absence of a methylene singlet at iSH 4.27
ppm in the IH NMR spectrum indicates that the methylene
group in 2 was involved in the reaction. Moreover, the IH
NMR spectrum revealed three singlet signals at iSH 2.37, 3.24
and 3.35 ppm corresponding to a C-methyl and two N-methyl
groups, while the higher frequency singlet signal at iSH 8.19
ppm corresponding to a 13-protonon a double bond conjugated
condensation product from the reaction of enaminone
3
with malononitrile failed to react with elemental sulfur in
DMF/EtOH containing catalytic piperidine to provide the
thienopyridine derivative 7 in a reaction characteristic of
azines with vicinal methyl and carbonitrile substituents,11,14
the structure 6 was ruled out and the product assigned to be 5.
The formation of5-(benzothiazol-2'-ylthio )-6-methyl-2-oxo-l,
2-dihydropyridine-3-carbonitrile 5 is considered most likely
based on its similarity to the well-established behaviour
(X
N
b
a
I }-SH
--
-
~
S
4
Scheme
1
Reagents and conditions: a: CH3COCH2CI/K2C03,
Me2CO; b: DMFDMA, heat.
* Correspondent. E-mail: fatima.alomran@ku.edu.kw

434 JOURNAL OF CHEMICAL RESEARCH 2009
r
NMe2
1
Bt S
-
Bt-S)~ClN_~
HC
3
N
0
~
_
XXCN
3
H3C
0
NH
H
5
Bt (XI N)-
~
S
CH3
Bt-S, X'>'--NH
"-Sl~(N
l"l~
2
N
H
0
N
H
0
7
6
Scheme
2
Reagents and conditions: a: H2C(CNh,EtOH/pip,reflux,H20/HCI;b: PhCH=C(CNh,pyridine/reflux; H20/HCI.
of enaminones towards methylene nitriles.7,s This was
assumed to proceed via initial Michael addition of the active
methylene reagent to the double bond in enaminone 3 with
subsequent cyclisation, then Dimroth-type rearrangement and
aromatisation via loss of dimethylamine to yield the 2,3,5,6-
tetrasubstituted pyridine 5. (Scheme 2).
The 13CNMR spectrum showed two downfield signals at Be
168.6 and 154.0 ppm. The signal at Be 168.6 corresponded
to a tertiary carbon, while that at Be 154.0 was coupled to a
proton. As generally found in the isoxazole system, the carbon
resonating at lowest field corresponds to C_S.15,16The data
are therefore consistent with structure 10. The formation of
10 is assumed to proceed via addition of the hydroxylamine
NH2 group to the active double bond in compound 3 to form
the non-isolable intermediate 9 which readily undergoes
intramolecular cyclisation to the isoxazole derivative 10 by
loss of dimethylamine and water molecules (Scheme 3).
These results prompted us to investigate the behaviour of
compound 3 towards heterocyclic amines such as 5-amino-
3-methyl-IH-pyrazole 12 and 3-amino-5-methylisoxazole 16
as potential approaches to fused heterocyclic systems. Thus,
treatment of enaminone 3 with 12 in refluxing ethanol afforded
the condensation product with elimination of dimethylamine
and water molecules (13, Scheme 4). This is consistent with
our recent report9 in which structure 13 rather than structure 14
was assigned. The IH NMR spectrum reveals a singlet signal
at BH 8.59 which integrates for one proton and is attributed to
the pyrimidine CH-2 in structure 13 and not CH-4 in structure
14. The mass spectrum of the reaction product revealed a
molecular ion peak at m/z 312. The absence of carbonyl group
Treatment of 5 with benzylidenemalononitrile in refluxing
pyridine afforded the aminoquinolizinone
8 in excellent
yield (Scheme 2). The mass spectrum of the reaction product
showed a molecular ion peak at m/z 451. The IH NMR
spectrum displayed a broad signal integrating for two protons
which readily underwent HID exchange upon addition of
D20. Also, it revealed, in addition to aromatic signals, two
downfield singlets at BH 8.54 and 8.56 ppm assignable to
the H-2 and H-9 quinolizine ring protons, respectively.
The formation of product 8 can be explained from the
formation of the Michael adduct intermediate followed by
cyclisation and aromatisation (Scheme 2).
Treatment of compound 3 with hydroxylamine hydrochloride
in refluxing ethanol afforded an isoxazole derivative in
good yield that was assigned the structure 10 rather than
11, based on the IH NMR and 13CNMR spectra. The mass
spectrum of the product revealed a molecular ion peak at
m/z 248. The IR spectrum was free of carbonyl absorption.
3
9
11
10
Scheme
3
Reagents and conditions: a: NH20H.HCI,EtOH,reflux.

JOURNAL OF CHEMICAL RESEARCH 2009
435
a
•
12
~
N=CH?NMe2
H3C
8H
-j~ v "~
15
Scheme
4
Reagents and conditions: a: 3, EtOH, reflux; b: DMFDMA, reflux; c: 2, pyr, reflux; H20/HCI.
Scheme
5
Reagents and conditions: a: 3, EtOH/pip, reflux 3 h; H20/HCI; b: 3, EtOH, reflux 0.5 h; c: EtOH/pip, reflux 2 h; H20/HCI.
absorption in the IR spectrum indicates that the carbonyl
group was involved in the reaction. (Scheme 4)
Experimental
The IR spectra (KEr) were recorded on a Perkin Elmer FTIR System
2000 instrument. IH, l3C NMR and NOE spectra were recorded
on a Broker 400 MHz spectrometer, and Broker AVANCE II-600
with DMSO-d6 or CDCI3 as solvent and tetramethylsilane (TMS)
as internal standard; chemical shifts are reported as i5 (ppm from
TMS). Mass spectra were measured on a GC/MS DFS, THERMO
instrument. Microanalyses are obtained using a LECO 932 CHNS
analyser. Compound 15 was prepared according to our recent
report.?
1-(Benzothiazol-2'-ylthio) propan-2-one (2): The 2-mercapto-
benzothiazo1e 1 (1.67 g, 10 mmol), chloroacetone (0.79 g, 10 mmol)
and potassium carbonate (1.38 g, 10 mmol) in acetone (100 ml) were
heated to reflux on a water bath for 2 h. The solvent was then removed
under reduced pressure. The residual solid was collected by filtration
and recrystallised from ethanol as pale yellow crystals (1.47 g, 66%),
m.p. 67-68°C. (Lit.1Om.p. 6rC). IR: Ymox 1721 cm-I (C=O). NMR:
Further
obtained by independent
via condensation 12 with DMF DMA and subsequent
evidence
for the proposed
structure
13 was
synthesis of the same product
condensation of the so-formed amidine 15 with the S-acetonyl
compound 2 in pyridine, to afford a product identical (m.p.
and mass spectra) with that obtained from the reaction of
3
with 12. The formation of pyrimidine
assumed to proceed via the addition of the exocyclic amino
group in 12 to the a,13-unsaturated moiety in compound
followed by cyclisation and aromatisation. It is significant to
note that although the ring nitrogen in compound 12 is the
most nucleophilic centre,17,18 it is also the most sterically
hindered site. Therefore, addition takes place at the exocyclic
NH2 to afford the pyrazolopyrimidine derivative 13 as shown
derivative 13 is
3
i5H (CDCI3) 2.43 (s, 3H, CH3), 4.27 (s, 2H, CH2), 7.28-7.48 (m, 2H,
benzothiazole-H), 7.76 (d, IH, J= 7 Hz, benzothiazole-H), 7.84 ppm
(d, 1H, J = 7 Hz, benzothiazole-H); i5c (DMSO-d6) 199.0 (keto),
164.4 (C-2'), 143.9, 134.2, 129.4, 125.4, 122.9, 122.7 (benzothiazole-
C), 44.2 (CH2) and 29.7 ppm (CH3). Anal. Found C, 53.97; H, 3.74;
N, 6.38. ClOH9NOS2 (223.31) requires: C, 53.87; H, 4.06; N, 6.27%.
in Scheme 4. This is consistent with our recent report.9
In the same manner, enaminone 3 reacted with 3-amino-5-
methylisoxazole
(16) in refluxing ethanol afforded
a single
product that could be formulated as 17 or its isomer 18. The
mass spectrum of the reaction product revealed a molecular
ion peak at m/z 313. The structure 18 was readily ruled out
on the basis of the isolated intermediate 19 that was cyclised
to 17 in refluxing ethanol in the presence of piperidine
(ef Scheme 5).
(2)-3 -(Benzothiazol-2 '-ylthio)-4-(N,N-dimethylamino)
but-3-en-
2-one (3): Compound 2 (2.23 g, 10 mmol) and dimethylformamide
dimethylacetal (DMFDMA) (1.33 g, 10 mmol) were heated for
5 minutes in an oil bath. The reaction mixture was allowed to cool
to room temperature and then triturated with ethanol (20 mL).
The solid product so formed was collected by filtration and
recrystallised from ethanol as off-white crystals (2.43 g, 82%), m.p.
Conclusion
103-105°C C. IR: Ymox 1646 cm-I (keto CO). NMR: i5 (CDCI3) 2.37
H
(s, 3H, CH3); 3.24 (s, 3H, NCH3), 3.35 (s, 3H, NCH3), 7.24-7.42
(m, 2H, benzothiazole-H), 7.72 (d, IH, J = 7 Hz, benzothiazo1e-
H), 7.83 (d, 1H, J= 7 Hz, benzothiazole-H), 8.19 ppm (s, 1H, H-4);
i5c(DMSO-d6) 194.4 (C=O), 175.1 (C-2'), 156.8, 155.2, 135.4, 126.6,
124.3, 122.1 and 121.5 (benzothiazole-C and C-4), 91.9 (C-3), 48.5
(N,N-Me2) and 26.6 ppm (CH3). Anal. Found C, 56.23; H, 4.92; N,
10.22. Cl3HI4NpS2 (278.39) requires C, 56.08; H, 5.07; N, 10.07%.
The synthesis of novel pyridine, isoxazole, pyrazolo[I,5-a]
pyrimidine,quinolizine
incorporating benzothiazole-2-thio
(Z)-3-(benzothiazol- 2'- y lthio )-4-(N,N-dimethy
and isoxazolo[3,4-b ]pyridine derivatives
moiety, utilising novel
lamino )but-3-
a
en-2-one, is reported, with a view to the possible biological and
pharmaceutical applications of these compounds.

436
JOURNAL OF CHEMICAL RESEARCH 2009
5-(Benzothiazol-2 '-ylthio)-6-methyl-2-oxo-l ,2-dihydropyridine-3-
carbonitrile (5): A mixture of3 (2.78 g, 10 mmo1) and ma10nonitri1e
(0.66 g, 10 mmo1) in ethanol (20 mL) containing a few drops of
piperidine was refluxed for 2 h. The reaction mixture was allowed to
cool to room temperature and neutralised with aqueous hydrochloric
acid (10%). The solid product so formed was collected by filtration
and recrystallised from ethanol as off-white crystals (2.42 g, 81%)
m.p. 180-182°C. IR: Ymox 3432 (NH), 2226 (CN), 1656 cm-' (amide
CO). NMR (DMSO-d6): OH 2.43 (s, 3H, CH3), 7.34 (t, lH, J= 7 Hz,
5-(Benzothiazol-2' -ylthio)-3, 4-dimethylisoxazolo[3, 4-bjpyridine
hydrochloride (17): A mixture of the enaminone 3 (2.78 g, 10 mmo1)
and the isoxazo1amine 16 (0.97 g, 10 mmo1) in etbano1 (20 mL)
containing a few drops of piperidine was refluxed for 3 h. The
reaction mixture was allowed to cool, then poured into ice-cold
water and neutralised with 10% HCI. The solid product so formed
was collected by filtration and recrystallised from ethanol as pale
brown crystals (73%), m.p. 120-122°C. NMR (CDC13): OH 2.38
(3H, s, CH3), 2.48 (3H, s, CH3), 7.27-7.99 (m, 4H, benzotbiazo1e-H),
8.22 (s, lH, H-6); oe 168.4 (C-3), 160.1 (C-2'), 154.7, 153.4, 152.9
(C-7a, C-3a' and C-6), 149.4 (C-5), 135.1, 135.0 (C-4 and C-7a'),
121.3, 122.2, 124.6, 125.4 (benzotbiazo1e-C), 103.3 (C-3a), 15.0 and
12.7 ppm (2CH3). MS (EI): m/z 313 [M+-HC1].Anal. FoundC, 51.31;
H, 3.62; N, 12.29. C'sH12C1N30S2 (349.86) requires: C, 51.49, H,
3.45; N, 12.01%.
3-(Benzothiazol-2 '-ylthio)-4-(5" -methylisoxazol-3 "-yl-amino )but-
3-en-2-one (19): A mixture of tbe enaminone 3 (2.78 g, 10 mmo1)
and the isoxazo1amine 16 (0.97 g, 10 mmo1) in ethanol (20 mL) was
refluxed for 0.5 h. The reaction mixture was allowed to cool, then
poured into ice-cold water and neutralised with 10% HCI. The solid
product so formed was collected by filtration and recrystallised from
ethanol as off-white crystals (71%), m.p. 101-102°C. NMR (CDC13):
OH 2.38 (3H, s, CH3), 2.42 (3H, s, CH3), 4.25 (bs,lH, NH), 5.98 (s,
lH, H-4"), 7.23-7.89 (m, 5H, benzothiazo1e-H, H-4); oe 201.8 (C-2),
168.2 (C-3"), 156.1 (C-2'), 155.1 (C-5"), 153.3 (C-3'a), 152.6 (C-4),
135.0 (C-7'a), 126.1 (C-3), 124.9, 124.0, 121.8, 121.5 (benzothiazo1e
-C), and 99.7 (C-4"), 15.9, 12.7 ppm (2CH3). MS (EI): m/z 331 [M+].
Anal. Found C, 54.21; H, 3.82; N, 12.73. C'sH13N302S2 (331.41)
requires: C, 54.36, H, 3.95; N, 12.76%.
benzotbiazo1e-H), 7.46 (t, 1H, J
= 7 Hz, benzothiazo1e-H), 7.84 (d,
lH, J= 7 Hz, benzotbiazo1e-H), 7.97 (d, 1H, J = 7 Hz, benzothiazo1e-
H), 8.53 (s, lH, H-4), 13.35 (br.s, lH, NH, Dp exchangeable); oe
168.8 (CO), 161.7, 161.0 (C-2', C-6), 156.2 (C-4), 154.7, 135.6,
127.4, 125.4, 122.6, 121.8 (benzothiazo1e-C), 116.3 (CN), 104.5
(C-5), 102.7 (C-3) and 19.6 ppm (CH3). Anal. Found C, 56.33;
H, 3.18; N, 14.08. C'4H9N30S2 (299.37) requires: C, 56.16; H, 3.03;
N,14.04%.
6-Am ino-l- (benzothiazol-
2 '-yl thi 0)-4- oxo-8-phenyl-4H-
quinolizine-3,7-dicarbonitrile
(8): mixture of the pyridone
A
5
(2.78 g, 10 mmo1) and benzylidenema10nonitri1e (1.54 g, 10 mmo1) in
pyridine (20 mL) was refluxed for 8 h. The reaction was poured into
ice-cold water and neutralised with 10% HCI. The solid product so
formed was collected by filtration and recrystallised from a mixture
of EtOH: DMF (2: 1) as brown crystals (3.65 g, 81%), m.p. 158-
160°C. IR: Ymox 3441-3425 (NH2), 2226 (2CN), 1652 cm-' (amide
CO). NMR (DMSO-d6): OH 7.34-7.99 (m, 9H, benzothiazo1e and
phenyl-H),
NH2, Dp-exchangeab1e);
8.54 (s, lH, H-2o)e, 81.6596.7(s,(ClH-6,),H1-96)2,.313(C.3-72)p,pm161(b.0r.s(,C2OH),,
156.2 (benzothiazo1e C-2), 154.5 (benzothiazo1e C-3a), 135.8 (C-8),
135.2 (benzothiazo1e C-7a), 132.1, 130.9, 128.9, 127.4 (phenyl C),
125.3, 125.0, 122.2, 121.8 (benzotbiazo1e C), 122.6 (C-9a), 117.2
(C-1), 116.3 (C-9), 115.9, 115.0 (2CN), 104.4, 102.3 ppm (C-3, C-7).
MS (EI): m/z 451 [M+]. Anal. Found: C, 63.92; H, 2.66; N, 15.33.
C24H13NsOS2(451.52) requires: C, 63.84; H, 2.90; N, 15.51%.
This work was financed by the University of Kuwait research
grant SC07/04. We are grateful to the Faculty of Science,
Chemistry
Department,
SAF facility for the spectral and
analytical data (Project GS 01101, GS 03/01 and GS 01103).
2-(5-Methylisoxazol-4-ylthio)benzothiazole
(10): A mixture of
3
(2.78 g, 10 mmo1) and hydroxylamine hydrochloride (0.69 g,
Received 7 October 2009; accepted 6 May 2009
Paper 09/0394 doi: 10.3184/030823409X464412
Published online: 15 July 2009
10 mmo1) in ethanol (20 mL) was refluxed for 2 h. The reaction
mixture was allowed to cool to room temperature. The solid product
which separated was collected by filtration and recrystallised from
ethanol as pale brown crystals (1.61 g, 65%) m.p. 144-146°C. NMR
(CDC13): OH 2.39 (s, 3H, CH3), 7.29-7.93 (m, 4H, benzothiazo1e-H),
8.43 ppm (s, lH, H-3'); oe 168.6 (C-5'), 163.7 (C-2), 154.0 (C-3'),
152.7, 135.2, 126.5, 124.5, 122.4, 121.7 (benzotbiazo1e-C), 102.0
(C-4'), 11.2 ppm (CH3). MS (EI): m/z 248 [M+]. Anal. Found C,
53.04; H, 3.43; N, 11.43. CllH8N20S2 (248.32) requires: C, 53.20;
H, 3.24; N, 11.28%.
6-(Benzothiazol-2 '-ylthio)-2, 7-dimethylpyrazolo[1, 5-ajpyrimidine
(13): Method A: A solution of the enaminone 3 (2.78 g, 10 mmo1)
and the pyrazo1amine 12 (0.97 g, 10 mmo1) in ethanol (20 mL)
was refluxed for 3 h. The reaction mixture was allowed to cool to
room temperature. The solid product so formed was collected by
filtration and recrystallised from etbano1 as beige crystals (81%),
m.p.112-114°C.
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10mmo1) in pyridine (30mL) was treated with tbe amidine 157 (1.52 g,
10 mmo1). The mixture was refluxed for 3 h. tban was allowed to
cool to room temperature and neutralised with hydrochloric acid
(10%). The solid product so formed was collected by filtration and
recrystallised from ethanol as brown crystals (74%). NMR (CDC13):
OH 2.60 (s, 3H, CH3), 3.06 (s, 3H, CH3), 6.62 (s, lH, H-3), 7.31
(t, 1H,J= 8Hzbenzotbiazo1e-H), 7.42 (t, 1H,J= 8 Hz, benzothiazo1e-
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17 K.M. Dawood, A.M. Farag and Z.E. Kandee1, J. Chern. Res. (S), 1999,88;
(M),537.
H), 7.69 (d, lH, J
= 8 Hz, benzothiazo1e-H), 7.87 (d, lH, ethanol
8 Hz, benzothiazo1e-H), 8.59 ppm (s, lH, H-5); oe 167.7 (C-7),
157.0, 154.0, 153.5, 152.0 (C-2', C-2, C-5 and C-3a), 149.2, 135.3,
126.5, 124.7, 122.1, 121.0 (benzothiazo1e-C), 108.4 (C-6), 98.0
(C-3), 15.77 and 14.89 ppm (2CH3). MS (EI): m/z 312 [M+]. Anal.
Found C, 57.56; H, 3.90; N, 17.76; S, 20.43. C'sH12N4S2 (312.41)
requires: C, 57.66; H, 3.87; N, 17.93; S, 20.52%.
18 M.H. E1nagdi, N.H. Taha, F.A. Abde1-EL All, R.M. Abde1 MOla1eb and
F.F. Mohmoud, Coli. Czech. Chern. Cornrnun., 1989,54, 1082.