Novel sulfur-to-nitrogen migration of ethenylmethyl moiety in benz[d]oxazole system via internal radical capture

Article abstract of DOI:10.1080/00397910500522181

2-Ethenylmethylthiobenz[d]oxazoles 9-11, on heating at high temperature in bromobenzene, underwent migration of ethenylmethyl moieties without rearrangement from sulfur to nitrogen in an internal radical capture pathway, instead of [3,3]-sigmatropic change, to furnish 3-ethenylmethylbenz[d]oxazole- 2(3H)-thiones 13-15. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910500522181

Synthetic Communicationsw, 36: 1447–1457, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500522181
Novel Sulfur-to-Nitrogen Migration of
Ethenylmethyl Moiety in Benz[d]oxazole
System via Internal Radical Capture
Sibdas Ray and Sukla Ghosh
Department of Chemistry, University College of Science, University
of Calcutta, Kolkata, India
Nemai C. Ganguly
Department of Chemistry, Kalyani University, Kalyani, India
Abstract: 2-Ethenylmethylthiobenz[d]oxazoles 9–11, on heating at high tempera-
ture in bromobenzene, underwent migration of ethenylmethyl moieties without
rearrangement from sulfur to nitrogen in an internal radical capture pathway,
instead of [3,3]-sigmatropic change, to furnish 3-ethenylmethylbenz[d]oxazole-
2(3H)-thiones 13–15.
Keywords: Sulfur to nitrogen migration, ethenylmethyl, internal radical capture,
benz[d]oazole, AIBN
INTRODUCTION
The structural analogy of benz[d]oxazole with indole, a heterocyclic system
with diverse biological activity, has evoked recent interest in suitably
constructed benz[d]oxazole compounds.[1] In connection with ongoing
studies on Claisen-type sulfur-to-nitrogen rearrangement in a heterocyclic
matrix,[2] we report here the results of studies on the thermal reaction of
2-ethenylmethylthiobenz[d]oxazoles.
Received in India October 11, 2005
Address correspondence to Sibdas Ray, Department of Chemistry, University
College of Science, University of Calcutta, 92, Acharya Prafulla Chandra Road,
Kolkata 700 009, India. E-mail: sibdas2002@yahoo.co.in
1447

1448
S. Ray, S. Ghosh, and N. C. Ganguly
RESULTS AND DISCUSSION
Benz[d]oxazol-2(3H)-one (1) was heated under reflux in dry toluene with
Lawesson’s reagent[3] for 5 h to furnish benz[d]oxazole-2(3H)-thione (2) and
benz[d]oxazol-2-yl trithio-4-methoxyphenylphosphonate (3) (Scheme 1).
However, exclusive formation[4] of 2 by reaction of 1 with Lawesson’s
reagent has been reported. Compound 3 might have resulted from the
reaction of 2 with Lawesson’s reagent (Scheme 1). Alkylation of 2 with
(ethenyl/ethynyl)methyl bromides 4–8 in the presence of benzyltriethylam-
monium chloride (BTEAC) and aqueous sodium bicarbonate solution at 08C
(Scheme 2) led to, as expected, S-alkylated products 2-(ethenyl/ethynyl)-
methylthiobenz[d]oxazoles 9–12. However, N-alkylated product 13 was con-
comitantly formed as a minor product (16%) in the case of alkylation of 2 with
allyl bromide.
Upon heating compounds 9–11 in dry bromobenzene at a sufficiently
high temperature in an autoclave (Table 1), products 13–15 were obtained
in low yields accompanied by some charring. The presence of unrearranged
ethenylmethyl moieties in the products 13–15 was revealed from N-CH2
proton signals in the region of d 4.8–5.0 and the corresponding carbon
signals at d 44.3–48.1 in 13C NMR spectra. This migration might occur
via an ionic or radical route. To ascertain the feature, reaction of 2-(3-
methylbut-2-enyl)thiobenz[d]oxazole (11) was conducted in refluxing
bromobenzene containing a catalytic amount (20 mol%) of azobisisobutyro-
nitrile (AIBN). This led to completion of the rearrangement in 25 h at
1508C, producing 15 in 95% yield, whereas the same substrate was comple-
tely resistant to rearrangement at 1508C for 30 h in the absence of AIBN
(Table 1). Although AIBN usually promotes radical reactions by abstracting
hydrogen by its decomposition product, it is also known to promote
homolysis of a carbon-to-heteroatom bond.[5] In our case, the promotional
role of AIBN in the cleavage of C-S bond is explained in Scheme 3. This
mechanism depicts detachment of the allyl radical from the substrate. This
feature of the rearrangement viz. intra- or intermolecular nature has not been
ascertained by crossover experiment, and, therefore, the suggested mechanism
Scheme 1.

Sulfur-to-Nitrogen Migration of Ethenylmethyl Moiety
1449
Scheme 2.
is only tentative. Formation of 13–15 via a tight radical pair cannot be ruled out.
To the best of our knowledge, this constitutes the first report of migration of a
skeletally unrearranged ethenylmethyl group from sulfur to nitrogen in
preference to thio-Claisen rearrangement. However, there is an early report of
a similar example of sulfur-to-carbon migration[6] without rearrangement of
the ethenylmethyl skeleton accompanied by subsequent ring closure.
Reaction of 2-propargylthiobenz[d]oxazole (12) in bromobenzene under
reflux for 5 h furnished two products, 16 and 17. The UV (ethanol) spectrum
of 16 [lmax (log1) at 286.5 (4.0), 246.5 (3.9) nm and lmin (log1) at 290.5
(4.0), 266.0 (3.8), 237.0 (3.9) nm] compares well with that of 9 with N55C-
S-CH2 moiety [lmax (log1, ethanol) 285 (4.02), 248 (4.0); lmin (log1,
ethanol) at 282 (3.96), 264.6 (3.0), 226 (3.75) nm]. Significantly, the UV
spectral features of 13 with S55C-N-CH2 moiety [lmax (log1, ethanol) at 300
(4.37) and 258.0 (3.90) nm and lmin (log1, ethanol) at 270.5 (3.40), 238.5
(3.38) nm] is quite different. Compound 16 displayed a diagnostic methylene
carbon signal at d 29.1 in addition to eight aromatic methine and two olefinic
methane carbon signals at d 128.8, 125.1, 124.7, 124.3, 124.1, 122.4, 118.6,
110.6, 110.2, and 109.9 in its 13C NMR spectrum. The appearance of a
Table 1. Thermal reaction of 3-ethenylmethylthiobenz[d]oxazoles 9–11 (Scheme 3)
Reaction conditions
Time (h)
Cat.
Yield
(%)
Substrate
R1
R2
Temp (8C)
Product
Mp (8C)
9
H
H
Ph
170
210–220a
6 nil
22 nil
9 nil
13
14
15
80–82 27
128–130 15
10
11
11
H
CH3 CH3 210–220a
CH3 CH3 150
86–88
86–88
26.6
95
25 AIBNb 15
aReaction was carried out in sealed tube.
bAIBN was used in 20 mol%.

1450
S. Ray, S. Ghosh, and N. C. Ganguly
Scheme 3.
methylene signal at d 29.1 is consistent with its attachment to sulfur rather than
nitrogen because it is expected to appear in the vicinity of d 44 in that case. The
product was assigned the structure 16 on the basis of these spectral features and
other possible structures 18, 19, and 20 (Scheme 4) were ruled out.
The other product was assigned the structure 3-(1-propynyl)benz[d]oxa-
1
zole-2(3H)-thione (17) on the basis of its H NMR and 13C NMR character-
istics. It displayed a stretching absorption at 2210 cm21 (C C) in its IR
spectrum. The UV spectral features of 17 showing lmax (log1) 323.5 (3.98),
;;
284.5 (3.8), 222.0 (4.0) nm and lmin (log1) at 294.0 (3.7), 262.0 (3.6) nm
;;
are in agreement with the attachment of -C C- with the N-3 of the benzox-
azole ring. A plausible mechanism of formation of 16 and 17 by thermal
change of 2-propargylthiobenz[d]oxazole (12) is shown in Scheme 4. The
2-thiobenz[d]oxazolyl radical 21 undergoes addition along its S-center to
the CH-terminal of the acetylenic moiety of a second molecule of 12,
resulting in 16. On the other hand, formation of 17 can be rationalized by
internal combination of the allenyl radical formed by isomerization of the

Sulfur-to-Nitrogen Migration of Ethenylmethyl Moiety
1451
Scheme 4.
preformed propargyl radical 22, with the N-3 center of 21 producing 3-allenyl-
benz[d]oxazole-2(3H)-thione (23). This is followed by prototropic change in
the allenyl unit.
EXPERIMENTAL
The melting points are uncorrected. IR spectra were recorded as KBr pellets
on a Perkin-Elmer-782 spectrophotometer, and UV spectra of ethanolic
solution of the compounds were recorded on a Hitachi U-2000 spectropho-
1
tometer. The H NMR spectra in CDCl3 were run on a Bruker AM-300L
instrument operating at 300 MHz with TMS as internal standard, and the
13C NMR spectra were run in CDCl3 on the same instrument operating at
75 MHz. The degree of proton attachment of the carbons was verified using
the Distortionless Enhancement by Polarization Transfer (DEPT) sequence.
Silica gel (60–120 mesh) was used for chromatographic purification. The
solvents used were dried and distilled before use. Light petrol refers to one
with a boiling range 60–808C.

1452
S. Ray, S. Ghosh, and N. C. Ganguly
Synthesis of Benz[d]oxazole-2(3H)-thione (2) and Benz[d]oxazol-2-yl
Trithio-4-methoxyphenylphosphonate (3)
A mixture of benz[d]oxazol-2(3H)-one (1) (3.0 g, 0.022 mol) and Lawesson’s
reagent (6.0 g, 0.0148 mol) in dry toluene (80 mL) was heated under reflux for
10 h until all starting material was consumed (TLC monitoring). The solvent
was removed from the reaction mixture under reduced pressure, and the
residue was subjected to separation by column chromatography on silica gel
using a petrol–chloroform mixture as eluent. Benz[d]oxazol-2-yl trithio-4-
methoxyphenylphosphonate (3) was isolated using a petrol–chloroform
(1 : 1) mixture as eluent. This was followed by isolation of benz[d]oxazole-
2(3H)-thione (2) on elution with a petrol–chloroform (1 : 3) mixture.
Data
Benz[d]oxazole-2(3H)-thione (2): White fluffy crystalline solid (yield 2.77g,
68%), mp 188–1908C (chloroform–petrol); IR: n (cm21) 3600–3400, 1650,
1532, 1500, 1475, 1440; UV: lmax (log1) 299.5 (4.30), 263.0 (3.99), 216.0
1
(4.08)nm; H NMR: d 7.13–7.20 (m, 3H, H-5, H-6 and H-7), 7.41 (dd, J
7.8 Hz and 1.5 Hz, 1H, H-4), 13.82 (s, 1H, N-H);13C NMR: dC 180.3 (s, C-2),
148.3 (s, C-7a), 131.4 (s, C-3a), 125.3 (d, C-5), 123.8 (d, C-6), 110.6 and
110.1 (each d, C-4 and C-7). Found: C, 55.41; H, 3.25; N, 9.06. C7H5NOS
requires C, 55.63; H, 3.31; N, 9.27%.
Benz[d]oxazol-2-yl trithio-4-methoxyphenylphosphonate (3): White crys-
talline solid (yield 1.27 g, 18%), mp 158–1608C (chloroform–petrol); IR: n
(cm21) 2560, 1600, 1570, 1509, 1465, 1440; UV: lmax (log1) 242.5 (4.20)
1
nm; 31P NMR (200.47 MHz, CDCl3): d 94.6 (m); H NMR d 3.86 (s, 3H, -
OCH3), 7.88 (m, 4H, H-4, H-5, H-6, and H-7 of benzoxazole), 8.09 (dd,
3
3
4JP,H 16.0 Hz and JH,H 8.8 Hz, 2H, meta Hs to phosphorus), 8.22 (dd, JP,H
3
22.5 Hz, JH,H 8.8 Hz, 2H, ortho Hs to phosphorus); 13C NMR: dC 134.0,
134.4, and 134.8 (each d, C-4, C-5, and C-6 of benzoxazole), 134.0
2
3
(d, JP,C 23.9 Hz, ortho C to P), 114.3 (d, C-7), 114.1 (d, JP,C 7.7 Hz, meta
C to P), 56.5 (q, CH3); MS: m/z 353(Mþ.), 255, 145. Found: C, 47.75;
H, 3.21; N, 3.75. C14H12NPO2S3 requires C, 47.60; H, 3.40; N, 3.97%.
General Procedure for the Synthesis of 2-(Ethenyl/ethynyl)
methylthiobenz[d]oxazoles 9–12
A solution of sodium bicarbonate (2.58 g, 0.03 mol) in water (40 mL) was
added to a stirred solution of benz[d]oxazole-2(3H)-thione (2) (1.5 g,
0.009 mol) in dichloromethane (100 mL), cooled in an ice bath, at 08C, and
then benzyltriethylammonium chloride (2.27 g, 0.01 mol) in water (20 mL)

Sulfur-to-Nitrogen Migration of Ethenylmethyl Moiety
1453
was also added. A solution of the (ethenyl/ethynyl)methyl bromide 4–8
(0.015 mol) in dichloromethane (10 mL) was added dropwise to the reaction
mixture during 40 min, and stirring continued for 5 h for completion of the
reaction. The organic layer was then removed, and the aqueous layer was
extracted with dichloromethane (3 ꢀ 40 mL). The combined dichloromethane
extract was washed with cold water (2 ꢀ 5 mL) and dried over anhydrous
sodium sulfate. The solvent was removed from the dried extract, and the
residue was subjected to chromatographic separation on silica gel using a
petrol–chloroform mixture as eluent. In case of allyl bromide as the alkylating
agent, a second product, 3-allylbenz[d]oxazole-2(3H)-thione (13), was also
isolated (yield: 16%).
Data
2-Allylthiobenz[d]oxazole (9): Colorless liquid (yield 50%); IR: n (cm21
3100, 1650, 1610, 1510, 1480 1465; UV: lmax (log1) 285.0 (4.02), 278.0
)
1
(4.03), 248 (4.00), 212.0 (3.94) nm; H NMR: d 3.93 (d, J 7.5 Hz, 2H,
S-CH2), 5.37 and 5.19 (m, J 18 Hz and 9 Hz, each 1H, 55CH2), 5.96–6.10
(m, 1H, CH255CH), 7.22 and 7.23 (each t, J 7.5 Hz, each 1H, H-5 and H-
6), 7.41 (d, J 7.5 Hz, 1H, H-7), 7.59 (d, J 7.5 Hz, 1H, H-4); 13C NMR:
dC 164.1 (s, C-2), 151.9 (s, C-7a), 142.0 (s, C-3a), 132.2 (d, CH55CH2),
124.1 and 123.9 (each d, C-5 and C-6), 118.9 (t, CH55CH2), 118.4 (d, C-
4), 109.7 (d, C-7), 34.8 (t, S-CH2). Found: C, 62.82; H, 4.50; N, 7.45.
C10H9NOS requires C, 62.83; H, 4.71; N, 7.33%.
3-Allylbenz[d]oxazole-2(3H)-thione (13): This was isolated as the second
product during the reaction of 2 with allyl bromide under the phase-
transfer-catalyzed condition. Colorless crystalline solid (yield 16%); mp
80–828C (chloroform–petrol); IR: n (cm21) 1650, 1620, 1490, 1460; UV:
1
lmax (log1) 300.0 (4.37), 258.0 (3.90), 213.5 (4.04)nm; H NMR: d 4.85
(distorted d, J 5.7 Hz, 2H, N-CH2), 5.28–5.35 (m, 2H, 55CH2), 5.87–5.99
(m, 1H, CH255CH-), 7.09–7.12 (m, 1H, H-7), 7.22–7.28 (m, 2H, H-4 and
H-5), 7.33–7.36 (m, 1H, H-6); 13C NMR: dC 180.6 (s, C-2), 147.3 (s, C-
7a), 131.8 (s, C-3a), 129.4 (d, CH55CH2), 124.8 and 124.2 (each d, C-5
and C-6), 119.4 (t, CH55CH2), 110.3 (d, C-4), 109.7 (d, C-7), 48.2 (t, N-
CH2). Found: C, 62.68; H, 4.60; N, 7.12. C10H9NOS requires C, 62.83;
H, 4.71; N, 7.33%.
2-Cinnamylthiobenz[d]oxazole (10): Colorless solid (yield 2.22g, 83.7%); mp
53–558C (petrol–chloroform at 2108C); IR: n (cm21) 3030, 1785, 1610, 1590;
UV, lmax (log1) 286.0 (3.99), 278.5 (4.02), 251.5 (4.10)nm;1H NMR: d 4.15
(d, J 8.1 Hz, 2H, S-CH2), 6.42 (m, 1H, 55CH-CH2), 6.73 (d, J 15.6Hz, 1H,
55CHPh), 7.34 (m, 8H, H-4, H-5 and H-7 of benzoxazole ring and C6H5,
7.66 (m, 1H, H-6); 13C NMR: dC 164.3, (s, C-2), 151.9 (s,
C-7a), 142.0 (s, C-3a), 136.3 (s, q-C of C6H5), 134.4 (d, CH55CHPh), 128.5

1454
S. Ray, S. Ghosh, and N. C. Ganguly
(d, two meta Cs of C6H5), 126.9 (d, para C of C6H5), 126.5 (d, two ortho Cs of
C6H5), 124.8, 124.2 and 123.3 (each d, C-5, C-6 and CH55CH2), 118.5 (d, C-4),
109.8 (d, C-7), 34.8 (t, S-CH2); MS: m/z 268 (MHþ., 100%), 246 (14.5%), 238
(19%), 204 (45%), 169 (85%), 167 (67%), 126 (73%), 121 (98%). Found: C,
71.65; H, 4.81; N, 5.45. C16H13NOS requires C, 71.91; H, 4.87; N, 5.24%.
2-(3-Methylbut-2-enylthio)benz[d]oxazole (11): Colorless liquid (yield
0.87 g, 89%); IR: n (cm21) 2980, 1670, 1600, 1505; UV: lmax (log1) 286.0
1
(3.99), 278.5 (4.02), 251.5 (4.10)nm; H NMR: d 1.75 (d, J 3.5 Hz, 6H, two
-CH3), 3.98 (d, J 7.8 Hz, 2H, S-CH2), 5.42 (t, 1H, CH55CMe2), 7.27
(m, 2H, H-5 and H-6), 7.41 (m, 1H, H-7), 7.6 (dd, 1H, H-4); 13C NMR: dC
165.0 (s, C-2), 151.7 (s, C-7a), 142.0 (s, C-3a), 138.7 (s, CH55CMe2),
124.5 (d, C-5), 123.7 (d, C-6), 118.3 (d, C-4), 117.4 (d, 55CHCH2), 109.7
(d, C-7), 30.3 (t, CH2), 25.6 and 17.8 (two q, two CH3). Found: C, 65.59;
H, 5.87; N, 6.11. C12H13NOS requires C, 65.75; H, 5.94; N, 6.39%.
2-Propargylthiobenz[d]oxazole (12): Colorless flakes (yield 1.39 g, 74.5%);
mp 42–438C (petrol–chloroform at 2108C); IR: n (cm21) 3260, 1510, 1460;
UV: lmax (log1) 284.5 (4.05), 277.5 (4.05), 242.5 (4.04) nm;1H NMR: d 2.26
;;
(t, J 2.6 Hz, 1H, C CH), 4.02 (d, J 2.6 Hz, 2H, CH2), 7.24 and 7.22 (each t, J
7.5 Hz, 2H, H-5 and H-6), 7.40 (d, J 7.5 Hz, 1H, H-7), 7.54 (d, J 7.5 Hz, 1H,
H-4);13C NMR: dC 163.0, (s, C-2), 152.2 (s, C-7a), 142.0 (s, C-3a), 124.4
;;
(d, C-5), 124.1 (d, C-6), 118.8 (d, C-4), 109.9 (d, C-7), 78.0 (s, C C-H),
;;
C10H7NOS requires C, 63.49; H, 3.70; N, 7.41%.
72.3 (d,
CH), 20.8 (t, S-CH2). Found: C, 63.45; H, 3.50; N, 7.20.
General Procedure for the Thermal Rearrangement of
2-Ethenylmethylthiobenz[d]oxazoles in the Absence of AIBN
2-Ethenyl-methylthiobenz[d]oxazole 9–11 was heated in dry bromobenzene
(10 mL/1 mmol of the substrate) in an oil bath; bath temperatures are
indicated in Table 1. The progress of the reaction of 9 was monitored intermit-
tently by TLC. After heating for the requisite period, the solvent was evapor-
ated under reduced pressure from the reaction mixture and the residue was
subjected to column chromatography on silica gel with the chloroform–
petrol mixture as the eluent to get the title compound 13–15.
Data
Compound 13: The compound, 3-allylbenz[d]oxazole-2(3H)-thione (yield
27%), was found to be identical with the product obtained during synthesis of 9.
3-Cinnamylbenz[d]oxazole-2(3H)-thione (14): Colorless solid purified
by preparative TLC using petrol containing 30% chloroform solvent as

Sulfur-to-Nitrogen Migration of Ethenylmethyl Moiety
1455
eluent (yield 15%); mp 128–1308C; IR: n (cm21) 1770, 1490, 1450; UV: lmax
(log1) 301.0 (3.60), 253.0 (3.57) nm; H NMR: d 5.01 (dd, J 6.3 Hz and
1
1.5 Hz, 2H, N-CH2), 6.27 (td, J 15.8 Hz and 6.3 Hz, 1H, CH55CHPh), 6.70
(d, J 15.8 Hz, 1H, 55CHPh), 7.16 (m, 1H, 7-H), 7.30 (m, 8H, five aromatic
Hs of cinnamyl group and H-4, H-5 and H-6 of benzoxazole); 13C NMR:
dC 180.3 (s, C-2), 147.2 (s, C-7a), 135.6 (s, q-C of Ph), 134.9 (d, PhCH55),
131.7 (s, C-3a), 128.7 (d, two meta C of Ph), 128.4 (d, para C of Ph),
126.6 (d, two ortho Cs of Ph), 125.0 and 124.3 (each d, C-5 and C-6),
120.5 (d, 55CHCH2), 110.4 (d, C-4), 109.8 (d, C-7), 20.8 (t, NCH2).
Found: C, 71.56; H, 4.62; N, 5.30. C10H9NOS requires C, 71.91; H, 4.87;
N 5.24%.
3-(3-Methylbut-2-enyl)benz[d]oxazole-2(3H)-thione (15): Colorless scale-
like crystalline solid (yield 26.6%); mp 86–888C (chloroform–petrol); IR:
n (cm21) 2970, 1470, 1430, 1400; UV: lmax (log1) 304.5 (4.29), 258.0
(3.77) nm; 1H NMR: d 1.65 and 1.80 [pair of s, each 3H, 55C(CH3)2],
4.83 (d, J 7.0 Hz, 2H, NCH2), 5.28 (t, J ꢀ7.0 Hz, 1H, CH55CMe2),
7.06 (distorted d, 1H, H-7), 7.23–7.34 (m, 3H, H-4, H-5, and H-6); 13C
NMR: dC 180.3 (s, C-2), 147.3 (s, C-7a), 142.0 (s, C-3a), 132.0
(s, 55CMe2), 124.7 and 124.0 (each d, C-5 and C-6), 116.4 (d, 55CHCH2),
110.2 (d, C-4), 109.6 (d, C-7), 44.3 (t, NCH2), 25.5 and 18.4 (each q, two
CH3). Found: C, 65.51; H, 5.84; N, 6.51. C12H13NOSrequires C, 65.75; H,
5.94; N, 6.39%.
Rearrangement of 2-(3-methylbut-2-enyl)thiobenz[d]oxazole (11) in the
Presence of AIBN to 3-(3-methylbut-2-enyl)benz[d]oxazole-2(3H)-thione
(15): 2-(3-Methylbut-2-enyl)thiobenz[d]oxazole (11) (100 mg, 0.46 mmol)
was taken in dry bromobenzene (4 mL) and warmed. At the incipient
boiling condition, AIBN (10 mg, 0.07 mmol) was added. The mixture was
heated under reflux for 25 h (TLC monitoring). The solvent was removed
from the reaction mixture, and the residue on chromatography over silica
gel with a chloroform–petrol mixture as eluent afforded 3-(3-methylbut-2-
enyl)benz[d]oxazole-2(3H)-thione (15) as a colorless flake-like crystalline
solid (95 mg, 95%). It was recrystallized from a chloroform–petrol mixture.
1
The product was identical with 15 in all respects (IR, UV, H NMR, 13C
NMR, co-TLC, and mixed mp).
Rearrangement of 2-propargylthiobenz[d]oxazole (12) to 1,3-bis(benz
[d]oxazol-2-ylthio)-1-propene (16) and 3-(1-propynyl)benz[d]oxazole-
2(3H)-thione (17): 2-Propargylthiobenz[d]oxazole (12) (250mg, 1.32mmol)
in dry bromobenzene (10 mL) was heated under reflux for 12h. The solvent
was removed from the reaction mixture under reduced pressure, and the
residue was chromatographed on silica gel to isolate 1,3-bis(benz[d]oxazol-2-
ylthio)-1-propene (16) upon elution with chloroform–petrol (1: 1) followed
by 3-(1-propynyl)benz[d]oxazole-2(3H)-thione (17) with chloroform contain-
ing 2% methanol as the eluent.

1456
S. Ray, S. Ghosh, and N. C. Ganguly
1,3-Bis(benz[d]oxazol-2-ylthio)-1-propene (16): Light yellow crystalline
solid (yield 20 mg, 9%); mp 99–1008C (chloroform–petrol); IR: n (cm21
2980, 1520, 1500, 1470; UV: lmax (log1) 300.0 (4.10), 286.5 (4.00), 246.5
)
1
(3.91) nm; H NMR: d 3.87 (d, J 8.0 Hz, 2H, -SCH2), 6.21 (q, J 8.0 Hz, 1H,
55CHCH2), 6.46 (d, J 8.0 Hz, 1H, SCH55), 6.93–6.98 (m, 2H, H-7 of both
rings), 7.12–7.14 (m, 6H, H-4, H-5, and H-6 of both rings); 13C NMR: dC
163.2 (s, C-2 of both rings), 152.1 (s, C-7a of both rings), 141.8 (s, C-3a of
both rings), 128.8 (d, CH2-CH55), 124.1, 124.3, 124.7, and 125.1 (each d,
C-5 and C-6 of both rings), 122.5 (d, 55CHS), 110.6 and 118.6 (each d, C-
4 of both rings), 109.9 and 110.2 (each d, C-7 of both rings), 29.1
(t, SCH2). Found: C, 59.65; H, 3.41; N, 8.09. C17H12N2O2S2 requires
C, 60.00; H, 3.53; N, 8.24%.
3-(1-Propynyl)benz[d]oxazole-2(3H)-thione (17): Light yellow crystalline
)
solid (yield 25 mg, 10%); mp 81–828C (chloroform–petrol); IR: n (cm21
2210, 1620, 1520, 1470; UV: lmax (log1) 323.5 (3.98), 284.50 (3.80), 222.0
1
(4.00)nm; H NMR: d 2.20 (s, 3H, CH3), 6.80–7.34 (m, 4H, H-4, H-5, H-
6, and H-7);13C NMR: dC 185.7 (s, C-2), 150.8 (s, C-7a), 130.9 (d, C-5),
128.6 (s, C-3a), 126.0 (d, C-6), 121.9 and 121.5 (both d, C-7 and C-4),
;;
77.8 (s, N-C ), 12.6 (q, CH3). Found: C, 63.65; H, 3.57; N, 7.26.
C10H7NOS requires C, 63.49; H, 3.70; N, 7.41%.
ACKNOWLEDGMENT
We thank A. Acharya for 1H NMR and 13C NMR spectral measurements. One
of the authors (S. G.) is thankful to UGC, New Delhi, and Council for
Scientific and Industrial Research (CSIR), New Delhi, for awarding a
research fellowship. The authors are also thankful to DST, India, for
providing support to the Departments of Chemistry, University of Calcutta
and University of Kalyani, by way of the Department of Science and
Technology (DST)–Fund for Improvement of Science and Technology
(FIST) program.
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