Synthesis of the novel benzothiazole compounds from 7-benzylidenebicyclo [3.2.0] hept-2-en-6-ones and 2-aminobenzenethiol

Article abstract of DOI:10.3906/kim-1105-67

Synthesis of the 2-(2-styrylcyclopent-3-enyl)benzo-[d]thiazoles (6a-i) is reported for the first time. Reaction of 7-benzylidenebicyclo [3.2.0] hept-2-en-6-ones (3a-i) and 2-aminobenzenethiol (4) in the presence of p-TsOH as a catalyst, in ethanol under r

Full text of DOI:10.3906/kim-1105-67

Turk J Chem
36 (2012) , 93 – 100.
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˙
c
ꢀ TUBITAK
doi:10.3906/kim-1105-67
Synthesis of the novel benzothiazole compounds from
7-benzylidenebicyclo [3.2.0] hept-2-en-6-ones and
2-aminobenzenethiol
Esra FINDIK
Department of Chemistry, Gaziosmanpa¸sa University, 60250, Tokat-TURKEY
e-mail: esrafndk@gmail.com
Received: 01.06.2011
Synthesis of the 2-(2-styrylcyclopent-3-enyl)benzo-[d]thiazoles (6a-i) is reported for the first time. Re-
action of 7-benzylidenebicyclo [3.2.0] hept-2-en-6-ones (3a-i) and 2-aminobenzenethiol (4) in the presence
of p-TsOH as a catalyst, in ethanol under reflux, resulted in the formation of novel 2-(2-styrylcyclopent-3-
enyl)benzo[d]thiazoles in high yields.
Key Words: Benzothiazole, 2-aminobenzenethiol, synthesis, 2-(2-styrylcyclopent-3-enyl)benzo-[d]thiazole
Introduction
Benzothiazoles are important heterocyclic compounds with multiple applications. They have long been known
to be biologically active,1−3 and their varied biological features are still of great scientific interest nowadays.
Benzothiazoles are widely found in bioorganic and medicinal chemistry with applications in drug discovery and
have a very intensive antitumor,4−11 antiviral,12 anti-HIV,13 and microbiological activity.14,15 Benzothiazoles
are used for treatment of autoimmune and inflammatory diseases,16 in the prevention of solid organ transplant
rejection, epilepsy,17−19 amyotrophic lateral sclerosis,20 and analgesia.21 Further industrial applications as
antioxidants,22,23 vulcanization accelerators,24,25 and a dopant in light emitting organic electroluminescent
devices26 have also been reported.
Numerous methods have been reported in the literature for the synthesis of benzothiazoles. Traditionally
used methods are (i) condensation of 2-aminothiophenols with aldehydes,27−30 carboxylic acids,31−34 acid
chlorides, 35,36 or esters37,38 and (ii) Jacobson’s cyclization of thiobenzanilides.39−42
This paper reports the direct synthesis of the novel benzothiazole compounds, 2-(2-styrylcyclopent-3-
enyl)benzo[d]thiazoles, from the acid-catalyzed reaction of 7-benzylidenebicyclo[3.2.0]hept-2-en-6-ones (2a-i)
with 2-aminobenzenethiol (4).
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Synthesis of the novel benzothiazole compounds from..., E. FINDIK
Experimental
General: Solvents were dried over standard drying agents and freshly distilled prior to use. All commercially
available chemicals were used without further purification. All reactions were performed under nitrogen. 1 H-
NMR (400 MHz) and 13 C-NMR (100 MHz) spectra were measured with a Bruker Avance 400 MHz with
tetramethylsilane as internal standard for solutions in deuteriochloroform. J values are given in Hz. Chemical
shifts were reported in ppm relative to the solvent signal. Multiplicities are indicated as follows: s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublets), and quin (quintet). All column
chromatographic separations were performed using silica gel (Merck 60-230 mesh). Organic solutions were dried
over anhydrous Na2 SO4 and concentrated below 40 ◦ C in vacuo. IR spectra were recorded on a Jasco FTIR-
430 spectrophotometer with NaCl optics. Mass spectra were recorded on a Thermofinnigan Trace GC/Trace
DSQ/A1300 (E.I. Quadrupole, 70 eV) equipped with a SGE-BPX5 MS capillary column (30 m × 0.25 mm i.d.,
0.25 μm). Elemental analyses were obtained from a LECO CHNS 932 Elemental Analyzer.
Synthesis of 7-(2-substitutedbenzylidene)bicyclo[3.2.0]hept-2-en-6-one (3a-i). The starting
compounds (3a-i) were prepared by using the recently reported method.43
(1R,5S,E)-7-(2-Methoxybenzylidene)bicyclo[3.2.0]hept-2-en-6-one (3a) Yellow solid, 97%, mp
180-182 ◦ C. IR (KBr): δ max cm−1 3045, 2936, 1636, 1569; 1 H-NMR (400 MHz, CDCl3): δ = 7.73 (dd, J
= 7.6, 1.2 Hz, 1H, ArH), 7.40-7.36 (m, 2H), 7.02 (t, J = 7.6 Hz, 1H, ArH), 6.93 (t, J = 8.4 Hz, 1H, ArH),
6.04-6.01 (m, 1H), 5.87-5.86 (m, 1H), 4.37-4.36 (m, 1H), 3.91-3.88 (m, 1H), 3.86 (s, 3H, -OCH3), 2.83-2.78 (m,
1H), 2.63-2.55 (m, 1H). 13 C-NMR (100 MHz, CDCl3) δ 204.1, 159.2, 148.8, 133.0, 131.5, 129.5, 128.7, 123.1,
120.6, 119.1, 111.2, 60.3, 55.5, 49.7, 34.6.
(1R,5S,E)-7-(2-Bromobenzylidene)bicyclo[3.2.0]hept-2-en-6-one(3b) Yellow solid, 97%, mp 176-
178 ◦ C. IR (KBr): δ max cm−1 3081, 2834, 1622, 1563; 1 H-NMR (400 MHz, CDCl3): δ 7.80 (d, J = 7.6
Hz, 1H, ArH), 7.64 (d, J = 8.0 Hz, 1H, ArH), 7.38 (t, J = 7.4 Hz, 1H, ArH), 7.28-7.22 (m, 2H), 5.98-5.96 (m,
1H), 5.91-5.89 (m, 1H), 4.37-4.36 (m, 1H), 3.95-3.90 (m, 1H), 2.86-2.81 (m, 1H), 2.65-2.58 (m, 1H). 13 C-NMR
(100 MHz, CDCl3) δ 203.4, 151.2, 133.8, 133.7(2C), 130.9, 128.8(2C), 127.5, 127.1, 122.6, 60.7, 49.5, 34.9.
General procedure for the synthesis of 2-(2-styrylcyclopent-3-enyl)benzo[d]-thiazoles (6a-
i). An equimolar mixture of 7-benzylidenebicyclo [3.2.0] hept-2-en-6-ones (3a-i) and 2-aminobenzenethiol in
ethanol and catalytic amount of p-TsOH was refluxed for 5 h. After the reaction was completed, the mixture
was extracted with 20 mL of ethyl acetate and dried over anhydrous Na2 SO4 , and the solvent was removed
under reduced pressure to give the desired products. Further purification was carried out by short column
chromatography on silica gel (hexane/ethyl acetate (19:1)).
2-((1S,2S)-2-(2-Methoxystyryl)cyclopent-3-enyl)benzo[d]thiazole (6a) Yellow viscous oil, 93%.
IR (KBr): δ max cm−1 3056, 2927, 2852, 1513, 1461, 1290, 1243, 1105, 1027, 971, 754, 728; 1 H-NMR (400
MHz, CDCl3) δ 8.08 (d, J = 8.0 Hz, 1H, ArH), 7.89 (d, J = 8.0 Hz, 1H, ArH), 7.54-7.49 (m, 2H), 7.40 (t,
J = 7.6 Hz, 1H, ArH), 7.26 (t, J = 7.4 Hz, 1H, ArH), 6.98 (t, J = 7.2 Hz, 1H, ArH), 6.94-6.90 (m, 2H),
6.38 (dd,J = 16.8, 8.0 Hz, 1H), 5.99-5.97 (m, 1H), 5.89-5.87 (m, 1H), 4.03-4.01 (m, 1H), 3.88 (s, 3H, -OCH3),
3.86-3.82 (m, 1H), 3.18-3.12 (m, 1H), 3.07-3.00 (m, 1H). 13 C-NMR (100 MHz, CDCl3) δ 174.9, 156.6, 153.2,
135.1, 132.9, 131.9, 129.8, 129.5, 128.3, 127.6, 126.7, 125.8, 125.6, 124.6, 122.7, 121.4, 120.6, 57.2, 55.3, 50.4,
40.1. GC-MS calcd. = 333. Found: M+1 = 333. Anal. Calcd for: C21 H19 NOS: C, 75.64; H, 5.74; N, 4.20; S,
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Synthesis of the novel benzothiazole compounds from..., E. FINDIK
9.62. Found: C, 75.45; H, 5.36; N, 4.42; S, 9.95.
2-((1S,2S)-2-(2-Bromostyryl)cyclopent-3-enyl)benzo[d]thiazole (6b) Yellow viscous oil, 98%. IR
(KBr): δ max cm−1 3056, 2925, 2850, 1643, 1513,1465, 1436, 1311, 1261, 1108, 1022, 964, 755, 727; 1 H-NMR
(400 MHz, CDCl3) δ 8.05 (d, J = 8.0 Hz, 1H, ArH), 7.87 (d, J = 8.0 Hz, 1H, ArH), 7.55 (bd,J = 8.0 Hz, 2H,
ArH), 7.49 (t, J = 7.6 Hz, 1H, ArH), 7.38 (t, J = 7.6 Hz, 1H, ArH), 7.27 (t, J = 7.6 Hz, 1H, ArH), 7.09 (t, J
= 7.6 Hz, 1H, ArH), 6.89 (d, J = 15.6 Hz, 1H), 6.29 (dd, J = 15.6, 8.0 Hz, 1H), 5.98-5.97 (m, 1H), 5.84-5.83
(m, 1H), 4.02-4.01 (m, 1H), 3.81 (dd, J = 8.0 Hz, 1H), 3.15-3.09 (m, 1H), 3.03-2.96 (m, 1H). 13 C-NMR (100
MHz, CDCl3) δ 174.5, 153.1, 137.1, 135.1, 134.5, 132.9, 132.2, 130.3, 129.7, 128.4, 127.4, 127.1, 125.9, 124.8,
123.5, 122.7, 121.5, 56.5, 50.6, 39.9. GC-MS calcd. = 381/383. Found: M+1 = 381/383. Anal. Calcd for:
C20 H16 BrNS: C, 62.83; H, 4.22; N, 3.66; S, 8.39. Found: C, 62.64; H, 4.29; N, 3.87; S, 8.56.
2-((1S,2S)-2-(4-Chlorostyryl)cyclopent-3-enyl)benzo[d]thiazole (6c) Yellow viscous oil, 96%. IR
(KBr): δ max cm−1 3056, 2962, 2850, 1646, 1509, 1436, 1261, 1091, 1012, 802, 727; 1 H-NMR (400 MHz,
CDCl3) δ 8.06 (d, J = 8.0 Hz, 1H, ArH), 7.85 (d, J = 8.0 Hz, 1H, ArH), 7.48 (t, J = 7.4 Hz, 1H, ArH), 7.36
(t, J = 7.4 Hz, 1H, ArH), 7.28 (m, 4H), 6.46 (d, J = 15.6 Hz, 1H), 6.30 (dd, J = 15.6, 8.0 Hz, 1H), 5.96-5.94
(m, 1H), 5.80-5.78 (m, 1H), 3.98-3.94 (m, 1H), 3.79 (dd, J = 16.0, 7.6 Hz, 1H), 3.13-3.09 (m, 1H), 3.01-2.96
(m, 1H). 13 C-NMR (100 MHz, CDCl3) δ 174.6, 153.1, 135.7, 135.0, 132.9, 132.3, 132.2, 130.3, 129.5, 128.6,
127.5, 126.0, 124.8, 122.8, 121.5, 56.5, 50.3, 40.0. GC-MS calcd. = 337/338/339. Found: M+1 = 337/339.
Anal. Calcd for: C20 H16 ClNS: C, 71.10; H, 4.77; N, 4.15; S, 9.49. Found: C, 70.92; H, 4.54; N, 4.36; S, 9.54.
2-((1S,2S)-2-(3-Bromostyryl)cyclopent-3-enyl)benzo[d]thiazole (6d) Yellow viscous oil, 95%. IR
(KBr): δ max cm−1 3056, 2929, 2850, 1590, 1436, 1311, 1241, 1108, 964, 759, 728, 682; 1 H-NMR (400 MHz,
CDCl3) δ 8.05 (d, J = 8.0 Hz, 1H, ArH), 7.85 (d, J = 8.0 Hz, 1H, ArH), 7.55 (bs, 1H), 7.47 (t, J = 7.6 Hz,
1H, ArH), 7.38-7.33 (m, 2H), 7.26 (d, J = 7.6 Hz, 1H), 7.14 (t, J = 8.0 Hz, 1H), 6.42 (d, J = 15.6 Hz, 1H),
6.32 (dd, J = 15.6, 8.0 Hz, 1H), 5.95-5.94 (m, 1H), 5.78-5.76 (m, 1H), 3.97-3.93 (m, 1H), 3.77 (dd, J = 15.6,
8.0 Hz, 1H), 3.14-3.06 (m, 1H), 3.00-2.94 (m, 1H). 13 C-NMR (100 MHz, CDCl3) δ 174.5, 153.1, 139.3, 134.9,
133.5, 133.1, 132.2, 130.4, 130.2, 130.0, 129.4, 129.1, 126.0, 125.0, 124.8, 122.8, 121.6, 56.5, 50.2, 40.0. GC-MS
calcd. = 381/383. Found: M+1 = 381/383. Anal. Calcd for: C20 H16 BrNS: C, 62.83; H, 4.22; N, 3.66; S, 8.39.
Found: C, 62.68; H, 4.08; N, 3.74; S, 8.48.
2-((1S,2S)-2-(3-Methylbromostyryl)cyclopent-3-enyl)benzo[d]-thiazole (6e). Yellow viscous
oil, 97%. IR (KBr): δ max cm−1 3056, 2923, 2854, 1513, 1436, 1311, 1108, 964, 759, 728; 1 H-NMR (400
MHz, CDCl3) δ 8.04 (d, J = 8.0 Hz, 1H, ArH), 7.87 (d, J = 8.0 Hz, 1H, ArH), 7.49 (t,J = 7.2 Hz, 1H), 7.38
(t,J = 7.6 Hz, 1H), 7.23-7.21 (m, 3H), 7.07 (m, 1H), 6.48 (d, J = 16.0 Hz, 1H), 6.31 (dd, J = 16.0, 8.0 Hz,
1H), 5.97-5.94 (m, 1H), 5.81-5.79 (m, 1H), 3.96-3.92 (m, 1H), 3.79 (dd,J = 16.0, 7.2 Hz, 1H), 3.14-3.07 (m,
1H), 3.01-2.96 (m, 1H), 2.37 (s, 3H). 13 C-NMR (100 MHz, CDCl3) δ 174.8, 153.1, 138.0, 137.0, 134.9, 132.6,
131.2, 130.8, 130.1, 128.4, 128.1, 127.0, 125.9, 124.7, 123.4, 122.7, 121.5, 56.7, 50.3, 39.9, 21.4. GC-MS calcd. =
317. Found: M+1 = 317. Anal. Calcd for: C21 H19 NS: C, 79.45; H, 6.03; N, 4.41; S, 10.10. Found: C, 79.23;
H, 5.96; N, 4.21; S, 10.21.
2-((1S,2S)-2-(4-Methylbromostyryl)cyclopent-3-enyl)benzo[d]-thiazole (6f) Yellow viscous oil,
97%. IR (KBr): δ max cm−1 3054, 3021, 2919, 2852, 1513, 1436, 1311, 1241, 1108, 966, 759, 728; 1 H-NMR
(400 MHz, CDCl3) δ 8.05 (d, J = 8.0 Hz, 1H, ArH), 7.87 (d, J = 8.0 Hz, 1H, ArH), 7.50 (t, J = 7.2 Hz, 1H,
ArH), 7.38 (t, J = 7.2 Hz, 1H, ArH), 7.32 (d, J = 8.0 Hz, 2H, ArH), 7.15 (d, J = 8.0 Hz, 2H, ArH), 6.50 (d, J
95

Synthesis of the novel benzothiazole compounds from..., E. FINDIK
= 15.6 Hz, 1H), 6.29 (dd,J = 15.6, 8.0 Hz, 1H), 5.97-5.95 (m, 1H), 5.83-5.81 (m, 1H), 3.97-3.93 (m, 1H), 3.80
(dd, J = 16.0, 7.2 Hz, 1H), 3.15-3.08 (m, 1H), 3.02-2.97 (m, 1H), 2.37 (s, 3H). 13 C-NMR (100 MHz, CDCl3)
δ74.9, 153.1, 137.1, 135.0, 134.3, 132.7, 130.7, 130.4, 130.0, 129.2, 126.2, 125.9, 124.7, 122.7, 121.5, 56.7, 50.4,
39.9, 21.2. GC-MS calcd. = 317/318/319. Found: M+1 = 316/317/318. Anal. Calcd for: C21 H19 NS: C,
79.45; H, 6.03; N, 4.41; S, 10.10. Found: C, 79.54; H, 6.11; N, 4.37; S, 10.25.
2-((1S,2S)-2-((4-Methoxystyryl)cyclopent-3-enyl)benzo[d]thiazole (6g) Yellow viscous oil, 97%.
IR (KBr): δ max cm−1 3056, 2931, 2834, 1606, 1509, 1436, 1249, 1174, 1033, 759, 728; 1 H-NMR (400 MHz,
CDCl3) δ 8.04 (d, J = 8.0 Hz, 1H, ArH), 7.86 (d, J = 8.0 Hz, 1H, ArH), 7.48 (t, J = 7.6 Hz, 1H, ArH),
7.37 (t, J = 7.6 Hz, 1H, ArH), 7.34 (d, J = 8.6 Hz, 2H, ArH), 6.87 (d, J = 8.6 Hz, 2H, ArH), 6.47 (d, J =
15.6 Hz, 1H), 6.19 (dd, J = 15.6, 8.0 Hz, 1H), 5.95-5.93 (m, 1H), 5.81-5.80 (m, 1H), 3.93-3.88 (m, 1H), 3.81 (s,
3H, -OCH3), 3.81-3.75 (m, 1H), 3.13-3.07 (m, 1H), 3.01-2.94 (m, 1H). 13 C-NMR (100 MHz, CDCl3) δ 174.9,
159.1, 153.1, 135.0, 132.8, 130.2, 129.9, 129.3, 127.5, 125.9, 124.8, 122.1, 121.5, 113.9, 56.7, 55.3, 50.5, 39.9.
GC-MS calcd. = 333/334. Found: M+1 = 333/334. Anal. Calcd for: C21 H19 NOS: C, 75.64; H, 5.74; N, 4.20;
S, 9.62. Found: C, 75.72; H, 5.57; N, 4.28; S, 9.77.
2-(1S,2S)-2-((E)-2-(Thiophen-2-yl)vinyl)cyclopent-3-enyl)benzo-[d]thiazole (6h) Yellow vis-
cous oil, 97%. IR (KBr): δ max cm−1 3060, 2927, 2850, 1610, 1513, 1455, 1311, 1241, 1108, 954, 757, 694;
1 H-NMR (400 MHz, CDCl3) δ 8.05 (d, J = 8.0 Hz, 1H, ArH), 7.86 (d, J = 8.0 Hz, 1H, ArH), 7.49 (t, J = 7.2
Hz, 1H, ArH), 7.38 (t, J = 7.2 Hz, 1H, ArH), 7.14 (d, J = 4.8 Hz, 1H, -thienyl), 6.98-6.94 (m, 2H, -thienyl),
6.65 (d, J = 15.6 Hz, 1H), 6.18 (dd, J = 15.6, 8.0 Hz, 1H), 5.96-5.94 (m, 1H), 5.79-5.77 (m, 1H), 3.95-3.91
(m, 1H), 3.78 (dd, J = 16.0, 7.2 Hz, 1H), 3.13-3.06 (m, 1H), 3.00-2.93 (m, 1H). 13 C-NMR (100 MHz, CDCl3)
δ 174.7, 153.1, 142.3, 135.0, 132.3, 131.2, 130.3, 127.3, 125.9, 125.3, 124.8, 124.0, 123.9, 122.7, 121.5, 56.5, 50.2,
40.0. GC-MS calcd. = 309/310. Found: M+1 = 309/310. Anal. Calcd for: C24 H19 NS2: C, 74.77; H, 4.97; N,
3.63; S, 16.63. Found: C, 74.58; H, 4.69; N, 3.74; S, 16.81.
2-((1S,2S)-2-((E)-2-(Furan-2-yl)vinyl)cyclopent-3-enyl)benzo[d]-thiazole (6i) Yellow viscous oil,
93%. IR (KBr): δ max cm−1 3056, 2925, 2852, 1513, 1436, 1311, 1241, 1014, 960, 759, 728; 1 H-NMR (400
MHz, CDCl3) δ 8.02 (d, J = 8.0 Hz, 1H, ArH), 7.86 (d, J = 8.0 Hz, 1H, ArH), 7.49 (t, J = 7.2 Hz, 1H, ArH),
7.39-7.35 (m, 2H, ArH and -furyl), 6.37-6.36 (m, 1H, -furyl), 6.3-6.2 (m, 2H), 6.20 (d, J = 3.2 Hz, 1H, -furyl),
5.94-5.92 (m, 1H), 5.77-5.75 (m, 1H), 3.92-3.88 (m, 1H), 3.76 (dd,J = 16.0, 7.2 Hz, 1H), 3.11-3.0 (m, 1H),
2.97-2.93 (m, 1H); 13 C-NMR (100 MHz, CDCl3) δ 174.8, 153.1, 152.6, 141.7, 134.9, 132.3, 130.2, 130.2, 125.9,
124.7, 122.7, 121.5, 119.2, 111.2, 107.3, 56.4, 50.3, 40.0; GC-MS calcd. = 293/294. Found: M+1 = 293/294;
Anal. Calcd for: C24 H19 NOS: C, 78.02; H, 5.18; N, 3.79; S, 8.68. Found: C, 77.82; H, 5.02; N, 3.91; S, 8.84.
Results and discussion
The starting materials, 7-benzylidenebicyclo-[3.2.0]hept-2-en-6-ones (3a,b and 3c-i43), were firstly synthesized
from the condensation of cis-(1R,5S)-bicyclo[3.2.0]hept-2-en-6-one (1) with substituted benzaldehydes (2a-g),
thiophene-2-carbaldehyde (2g), and furan-2-carbaldehyde (2h) according to our recently published procedure.43
Then the acid-catalyzed reaction of 7-benzylidenebicyclo[3.2.0]hept-2-en-6-ones (3a-i) with 2-aminobenze-
nethiol (4) was examined.44 The reaction of 7-benzylidenebicyclo[3.2.0]hept-2-en-6-ones (1a-i) with 2-aminoben-
96

Synthesis of the novel benzothiazole compounds from..., E. FINDIK
zenethiol (4) in the presence of 10% mol p-TsOH in ethanol under reflux resulted in the formation of the
rearrangement products 6a-i, 2-(2-styrylcyclopent-3-enyl)benzo[d]thiazoles (6a-i), instead of the expected 1,5-
benzothiazepines (5a-i). The products 6a-i were isolated in high yields (in the range of 93%-98%) after the
usual work-up (Scheme 1, Table).
N
S
NH2
Ar
SH
H
H
CHO
O
O
4
5a-i
NaOH
EtOH
H
H
+
p-TsOH
X
EtOH, reflux
N
Ar
3a-i
1
2a-i
5
4
S
1
7
3
2
6
Ar
6a-i
Scheme 1. Synthesis of the 2-(2-styrylcyclopent-3-enyl)benzo[d]thiazoles (6a-i).
Table. Synthesis of the 2-(2-styrylcyclopent-3-enyl)benzo[d]thiazoles (4a-i).
Entry
3
Ar
6
(% yield)
(93)
1
2
3
4
5
6
7
8
9
3a 2-CH3OC6H4 6a
3b
3c
3d
3e
3f
2-BrC6H4
4-ClC6H4
6b
6c
6d
6e
6f
(98)
(96)
3-BrC6H4
3-CH3C6H4
4-CH3C6H4
(95)
(97)
(97)
3g 4-CH3OC6H4 6g
3h 2-thiophenyl 6h
(97)
(97)
3i
2-furyl
6i
(93)
The structures of 6a-i were established by IR, 1 H-NMR, 13 C-NMR, mass, and elemental analyses. The
IR spectra of compounds 6 showed characteristic absorptions in the range of 759-728 cm−1 (C-S bond) and
1612-1590 cm−1 (C=N bond), respectively, which confirm the presence of a thiazole ring. In the 1 H-NMR
spectra of 6a-i, the styryl protons (H6 and H7) are represented as an AB system (A part of AB system, doublet
of doublet, J = 16.8-15.6 and 8.0 Hz and B part of AB system, doublet J = 16.8-15.6 Hz) at the range of
6.89-6.47 and 6.38-6.19 ppm, respectively. The coupling constant J H6, H7 (16.8-15.6 Hz) confirms the trans
configuration in compounds 6. The 13 C-NMR spectra of 6a-i showed the characteristic imine carbon atom
(C=N) with chemical shift at 174.5-174.9 ppm. The mass spectra of compounds (6a-i) gave molecular ions
peaks corresponding to their molecular masses.
97

Synthesis of the novel benzothiazole compounds from..., E. FINDIK
We suggest the following mechanism to explain the rearrangement products (6a-i) (Scheme 2). The
reaction proceeds first by Michael addition via the thiol pair of electrons followed by a reaction of the o-
amino group with the carbonyl of the cyclobutanone moiety to give a hydroxyaminal intermediate 8 and this
hydroxyaminal reverses by cleaving the cyclobutane system followed by elimination of the thiol functionality. At
this stage, the reactants produce an o-mercaptoanilide 9, which undergoes condensation, under acid catalysis,
to give benzothiazole 6 as the final product.
H2N
H
H
H
H
O
H2N
H2N
O
O
O
O
HS
H+
H
4
S
S
Ar
Ar
Ar
Ar
Ar
H
H
3a-i
7
O
H
O
NH
N
N
H+
S
-H2O
S
HS
H
Ar
8
Ar
Ar
6a-i
9
Scheme 2. Proposed formation mechanism of rearrangement products 2-(2-styrylcyclopent-3-enyl) benzo[d]thiazoles
(6a-i).
Conclusion
For the first time, the novel heterocycles 2-(2-styrylcyclopent-3-enyl)benzo[d]-thiazoles (6a-i) were prepared by
the acid-catalyzed rearrangement reaction of 7-benzylidenebicyclo[3.2.0]hept-2-en-6-ones with 2-aminobenzene-
thiol in high yields.
Acknowledgment
We wish to thank the Department of Chemistry, Faculty of Arts and Sciences, Gaziosmanpa¸sa University.
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