Ultrasound-promoted synthesis of novel 2-imino-3-aryl-2,3-dihydrobenzo[d] oxazol-5-ol 2-iminooxazolidines derivatives

Article abstract of DOI:10.1016/j.tet.2013.01.069

A synthesis of novel 2-iminooxazolidineswas developed by the reaction between arylcyanamides with p-benzoquinone at room temperature under ultrasound irradiation in excellent yields. Also, the chemoselective tosylation of the products were performed at room temperature under ultrasound irradiation in good yields.

Full text of DOI:10.1016/j.tet.2013.01.069

Tetrahedron 69 (2013) 3082e3087
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Ultrasound-promoted synthesis of novel 2-imino-3-aryl-2,3-
dihydrobenzo[d]oxazol-5-ol 2-iminooxazolidines derivatives
,
a
a
Davood Habibi a , Mahmoud Nasrollahzadeh , Hesam Sahebekhtiari ,
*
Richard Vernon Parish b
a Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran
b School of Chemistry, The University of Manchester, Oxford Road, Manchester M13, 9PL, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A synthesis of novel 2-iminooxazolidineswas developed by the reaction between arylcyanamides
with p-benzoquinone at room temperature under ultrasound irradiation in excellent yields. Also, the
chemoselective tosylation of the products were performed at room temperature under ultrasound
irradiation in good yields.
Received 22 November 2012
Received in revised form 6 January 2013
Accepted 22 January 2013
Available online 31 January 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
p-Benzoquinone
Arylcyanamide
2-Iminooxazolidines
Chemoselective tosylation
Ultrasound irradiation
Room temperature
1. Introduction
well as the nucleophile groups.7,8 In this respect cyanamides are
useful, since they have both nucleophilic (amino) and electrophilic
In recent years, cycloaddition reactions have emerged as
a valuable tool for the construction of heterocyclic compounds.1a
Amongst these, 1,4-cycloaddition provides a unique and efficient
route for the synthesis of various five-membered heterocycles. A
diverse array of heterocyclic compounds occurs naturally and their
functions are often of fundamental importance to living systems.
Amongst them, nitrogen-containing heterocycles are important
compounds from a biological and pharmacological point of view.1
It is known that p-benzoquinone can act as a precursor to a va-
riety of organic compounds via conjugate addition of nitrogen-,
sulfur-, oxygen- and carbon-nucleophiles and also of electron-
accepting components for the synthesis of charge-transfer com-
plexes and radical-ion salts.2,3
(cyano) canters. In some cases the cyano group participates in
a specific fashion to give addition, cycloaddition, cyclotrimerization
reactions and complex formation.7,8 Many of these reactions re-
quire relatively forcing conditions, which might be overcome by
using ultrasound (US) energy, which is being increasingly applied
in synthetic organic chemistry.9 Ultrasound irradiation involves
high energies and pressures on an extremely short time scale. In
liquid irradiated with high-intensity ultrasound, the collapse of
bubbles caused by cavitation produces intense local heating and
high pressures, with very short lifetimes; these transient, localized
hot spots drive high energy chemical reactions. The hot spot has an
equivalent temperature of roughly 5000 ꢀC (9000 ꢀF), a pressure of
about 2000 atm, a lifetime considerably less than a microsecond
and heating and cooling rates above 10 billion ꢀC per second.9
To the best of our knowledge, ultrasound irradiation has not so
far been used in the reactions of arylcyanamides, and as part of our
ongoing interest in heterocyclic chemistry,10 we now wish to report
the synthesis of novel 2-imino-3-aryl-2,3-dihydrobenzo[d]oxazol-
5-ol derivatives by the reaction of arylcyanamides with p-benzo-
quinone at room temperature under ultrasound irradiation
(Scheme 1).
Reactivities of p-benzoquinones4e6 in cyc1oaddition reactions
depend on the electronic and steric character of the substituents.
Nucleophilic addition may proceed beyond the initial 2-substituted
mono-adducts up to tri- or even tetra-substituted products. New
heterocycles can be prepared if the adduct contains electrophilic as
* Corresponding author. Tel.: þ98 811 8257407; fax: þ98 8118282807; e-mail
address: davood.habibi@gmail.com (D. Habibi).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.01.069

D. Habibi et al. / Tetrahedron 69 (2013) 3082e3087
3083
Table 2
Formation of 2-imino-3-aryl-2,3-dihydrobenzo[d]oxazol-5-ol derivatives via sec-
ondary arylcyanamides
Entry
Cyanamide
Product
Time
[min]
Yielda
[%]
Scheme 1. Synthesisofnovel2-imino-3-aryl-2,3-dihydrobenzo[d]oxazol-5-olderivatives.
1
150
600
145
300
88
81
82
92
2. Result and discussion
Initially, the reaction of 2,5-dichlorophenylcyanamide and
p-benzoquinone was studied as a model. Conventional conditions
were used with different solvents under reflux reaction conditions
with 50% excess of p-benzoquinone (Table 1, entries 1e3). Polar
protic solvents such as ethanol, methanol, and water attacked the p-
benzoquinone and the desired products were obtained in low yield
due to formation of side products.11 When acetonitrile was used, the
reaction yield was about 25% of the product after 48 h under reflux
condition (Table 1, entry 3). Much better results were achievable
under ultrasound irradiation (Table 1, entry 4). The higher yield and
lower reaction time during the ultrasonic irradiation can be at-
tributed to the implosive collapse of the cavitation period of the
sound waves. The optimum conditions were found to be 1.0 mmol
of p-benzoquinone and 1.0 mmol of cyanamide in acetonitrile
(5.0 mL). Increasing the amount of p-benzoquinone to more than
1.0 mmol showed no substantial improvement in the yield.
2
3
4
Table 1
The model reaction in the synthesis of 3-(2,5-dichlorophenyl)-2-imino-2,3-
dihydrobenzo[d]oxazol-5-ol under thermal and ultrasound irradiation conditionsa
5
6
7
8
180
300
90
80
92
83
85
Entry
Method
p-BQd
[mmol]
Solvent
[mL]
Time
[h]
Yieldg
[%]
1
2
3
4
5
Without USb
Without USb
Without USb
With USc
1.5
1.5
1.5
1.0
1.0
MeOH (12)
EtOH (12)
MeCN (12)
MeCN (5)
MeCN (5)
48
48
10
11
25
81
35
48
10e
10f
With USc
a
2,5-Dichlorophenylcyanamide (1 mmol).
Reaction under reflux conditions.
The ultrasonic power 50 W, irradiation frequency 28 kHz.
p-Benzoquinone.
Room temperature.
At 50 ꢀC.
b
c
d
e
f
g
Isolated yield.
Using equimolar amounts of the reactants and getting high
yields, means no starting material will remain as residue in the
reaction mixture, minimizing waste streams, which is in accor-
dance with the first principle of green chemistry.12
30
In order to further improve the yield of reaction, a higher tem-
perature we used under ultrasound irradiation (Table 1, entry 5),
but the yields decreased. Increasing temperature usually increases
the formation of side products and cyanamide residue in the re-
action mixture.
The reactions of several aromatic cyanamides were tested in the
synthesis of novel 2-iminooxazolidines under ultrasound irradia-
tion at room temperature and the results are indicated in Table 2. In
all cases, reaction times are reduced and the yields increased.
To study the effects of the nature of the substituent groups on
the benzene ring of cyanamide, various 2-imino-3-aryl-2,3-
dihydrobenzo[d]oxazol-5-ol derivatives were obtained in high
yields under ultrasound irradiation from different aromatic cyan-
amides containing both electron-donating and electron-
withdrawing groups (Table 2). Cyanamides having methyl or
methoxy as an electron-donating group (entries 7e9) reacted
completely at room temperature after 30e90 min, while the
9
120
84
80
10
45
a
Isolated yields.

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D. Habibi et al. / Tetrahedron 69 (2013) 3082e3087
species bearing the electron-withdrawing groups such as Cl and Br
(entries 1e5) required higher reaction times, which indicates that
the nature of the substituent has an effect on the reaction time.
The presence of the two NH and CN groups in 1,4-
phenylenecyanamide (Table 2, entry 10) interestingly afforded the
double-addition product.
spectra were recorded at 100and 75 MHz. FT-IR (KBr) spectra were
recorded on a PerkineElmer 781 spectrophotometer. Melting points
were taken in open capillary tubes with a BUCHI 510 melting point
apparatus and are uncorrected. Elemental analysis was performed
using Heraeus CHNeO-Rapid analyzer. TLC was performed on silica
gel polygram SIL G/UV 254 plates.
The possible mechanism for the synthesis of 2-imino-3-aryl-
2,3-dihydrobenzo[d]oxazol-5-ol derivatives under ultrasound ir-
radiation is shown in Scheme 2. The reaction of p-benzoquinone
with cyanamides occurs through the conjugate addition mecha-
nism followed by the intramolecular cycloaddition reaction of
phenolic OH. In the other words, the process does not stop at the
stage of (2,5-dihydroxyphenyl)arylcyanamide (3) formation.
Intramolecular addition of the hydroxyl group to the cyano group
leads to ring closure, yielding 2-imino-3-aryl-2,3-dihydrobenzo[d]
oxazol-5-ol derivatives. The driving force for these reactions is
presumed to be the thermodynamically favored formation of novel
aromatic compounds and a very reactive electrophile species (3).
4.2. Preparation of the arylcyanamides
Cyanamides were prepared according to the lit.10a
4.3. General experimental procedure for the synthesis of 2-
imino-3-aryl-2,3-dihydrobenzo[d]oxazol-5-ol derivatives
A mixture of p-benzoquinone (1.0 mmol) and cyanamide
(1 mmol) in MeCN (5 mL) was irradiated with ultrasound for the
appropriate time at room temperature. After completion (as mon-
itored by TLC), the solid residue was filtered from the reaction
mixture and then was washed with acetone to afford the pure
product. The desired pure products were characterized by 1H NMR,
13C NMR, FT-IR, elemental analysis (CHN), and melting points.
4.3.1. 3-(2-Chlorophenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-ol
(Table 2, entry 1). Yield 88%; mp 199e202 ꢀC; FT-IR (KBr, cmꢁ1):
3329, 3068, 1687, 1620, 1589, 1497, 1477, 1439, 1406, 1306, 1189,
1108, 1066, 1010, 838, 818, 768, 721, 703, 658, 620; 1H NMR
(300 MHz, DMSO-d6): dH 9.26 (br s, 1H), 7.92e7.54 (m, 4H), 7.03 (d,
J¼8.4 Hz, 1H), 6.40 (d, J¼8.4 Hz, 1H), 5.95 (s, 1H); 13C NMR (75 MHz,
DMSO-d6): dC 155.2, 154.5, 137.5, 134.1, 132.7, 132.2, 131.6, 131.4,
131.2, 129.3, 109.6, 107.6, 96.4; CHN: Anal. Calcd for C13H9ClN2O2: C,
59.90; H, 3.48; N, 10.75. Found: C, 59.86; H, 3.08; N, 10.75.
Scheme 2. The possible mechanism for the synthesis of 2-imino-3-aryl-2,3-dihy-
drobenzo-[d]oxazol-5-ol derivatives.
The products were characterized by IR, 1H NMR and 13C NMR
spectroscopy, melting points, and elemental analysis. Formation of
the products was confirmed by IR spectra, which showed two
characteristic NH and OH peaks.
The reactivity of several 2-imino-3-aryl-2,3-dihydrobenzo-[d]
oxazol-5-ols was also tested in the reaction with tosyl chloride and
K2CO3under ultrasound irradiation at room temperature in ethanol
and the results are indicated in Table 3. With those molecules
containing both the hydroxyl and imino groups, only the O-tosy-
lated products were formed, which showed a good chemo-
selectivity. Apparently, the steric hindrance of the aryl ring on the
nitrogen prohibits the N-tosylation (Scheme 3 and Table 3).
The structures of the O-tosylated products were in agreement
with their IR and NMR spectra. In the IR spectra of the products, the
OH peak had disappeared and two strong absorptions bands were
detected. New absorption bands at w1130e1135 cmꢁ1 and
1355e1372 cmꢁ1 in the FT-IR spectra of the O-tosylated products
were attributed to theSO2 of the tosyl group.
4.3.2. 3-(2,5-Dichlorophenyl)-2-imino-2,3-dihydrobenzo[d]ox-azol-
5-ol (Table 2, entry 2). Yield 81%; mp 231e234 ꢀC; FT-IR (KBr,
cmꢁ1): 3332, 3096, 2683, 1681, 1621, 1601, 1560, 1490, 1476, 1426,
1401, 1306, 1194, 1099, 1014, 848, 826, 784, 712, 661, 613, 582, 438;
1H NMR (300 MHz, DMSO-d6): dH 8.00e7.40 (m, 3H), 7.25 (br s, 1H),
6.97 (d, J¼8.1 Hz, 1H), 6.72 (br s, 1H), 6.35 (d, J¼8.1 Hz, 1H), 5.96 (s,
1H); 13C NMR (75 MHz, DMSO-d6): dC 154.6, 154.4, 137.4, 133.6,
133.0, 132.4, 131.5, 131.3, 129.4, 127.6, 109.6, 107.7, 96.5; CHN: Anal.
Calcd for C13H8Cl2N2O2: C, 52.91; H, 2.73; N, 9.49. Found: C, 52.80;
H, 2.65; N, 9.62.
4.3.3. 3-(3-Bromophenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-ol
(Table 2, entry 3). Yield 82%; mp 164e167 ꢀC; FT-IR (KBr, cmꢁ1):
3330, 3065,1674,1618,1590,1488,1478,1438,1394,1307,1199,1168,
1116,1073,1014, 780, 744, 724, 713, 699, 653, 624; 1H NMR (300 MHz,
DMSO-d6): dH 9.40 (br s, 1H), 7.85 (s, 1H), 7.48e7.64 (m, 3H), 7.00 (d,
J¼8.5 Hz,1H), 6.80 (br s,1H), 6.41 (d, J¼8.2 Hz, 2H); 13C NMR (75 MHz,
DMSO-d6): dC 154.7,154.4,137.3,136.9,133.2,131.7,130.5,128.9,124.9,
122.2, 109.6, 107.9, 96.6; CHN: Anal. Calcd for C13H9BrN2O2: C, 51.17;
H, 2.97; N, 10.49. Found: C, 51.29; H, 2.85; N, 10.31.
3. Conclusions
In conclusion, we have developed an efficient and simple pro-
cedure for the synthesis of2-imino-3-aryl-2,3-dihydrobenzo[d]
oxazol-5-ol derivatives under ultrasound irradiation. This method
has the advantages of excellent yields and reaction rates, simple
methodology, mild reaction conditions, excellent chemoselectivity
and easy work-up.
4.3.4. 3-(4-Chlorophenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-
ol(Table 2, entry 4). Yield 92%; mp 210e212 ꢀC; FT-IR (KBr, cmꢁ1):
3361, 3088, 1674, 1620, 1499, 1466, 1382, 1275, 1188, 1159, 1087,
1003, 970, 811, 713, 628; 1H NMR (300 MHz, DMSO-d6): dH 9.35 (br
s,1H), 7.63 (d, J¼8.6 Hz, 2H), 7.58 (d, J¼8.6 Hz, 2H), 7.00 (d, J¼8.4 Hz,
1H), 6.60 (br s, 1H), 6.45 (d, J¼8.4 Hz, 1H), 6.37 (s, 1H); 13C NMR
(75 MHz, DMSO-d6): dC 154.4, 155.2, 137.3, 134.2, 133.4, 132.0, 129.9,
127.9,109.5,107.8, 96.6; CHN: Anal. Calcd for C13H9ClN2O2: C, 59.90;
H, 3.48; N, 10.75. Found: C, 60.03; H, 3.32; N, 10.89.
4. Experimental
4.1. General
All reagents were purchased from the Merck and Aldrich
chemical companies and used without further purification. The
NMR spectra were recorded in DMSO or CDCl3.1H NMR spectra were
recorded on a Bruker Avance DRX 400 and 300 MHz instruments.
4.3.5. 3-(2,4-Dichlorophenyl)-2-imino-2,3-dihydrobenzo[d]ox-azol-
5-ol(Table 2, entry 5). Yield 80%; mp 216e218 ꢀC; FT-IR (KBr, cmꢁ1):
3333, 3100, 2875, 2683, 1678, 1618, 1587, 1561, 1487, 1473, 1306,
The chemical shifts (
d) are reported in parts per million relative to
TMS as an internal standard and J values are given in hertz. 13C NMR

D. Habibi et al. / Tetrahedron 69 (2013) 3082e3087
3085
Table 3
Formation of O-tosylated products
Entry
2-Iminooxazolidines
Product
Time [h]
1.5
Yielda [%]
1
87
2
3
85
3
2.5
84
4
1
86
5
4
85
a
Yields are after work-up.
OH
OTs
3358, 3059, 1666, 1627, 1617, 1583, 1566, 1499, 1404, 1381, 1273,
1190, 1158, 1058, 1001, 969, 820, 711, 684, 634, 622, 501; 1H NMR
(300 MHz, DMSO-d6): dH 9.33 (br s, 1H), 7.88 (d, J¼8.1 Hz, 2H), 7.41
(d, J¼8.1 Hz, 2H), 6.99 (d, J¼8.4 Hz, 1H), 6.70 (br s, 1H), 6.41 (d,
J¼8.4 Hz, 1H), 6.36 (s, 1H); 13C NMR (75 MHz, DMSO-d6): dC 155.1,
154.3, 138.8, 137.3, 135.2, 133.3, 128.2, 109.5, 107.9, 96.63, 93.09;
CHN: Anal. Calcd for C13H9IN2O2: C, 44.34; H, 2.58; N, 7.96. Found:
C, 44.64; H, 2.40; N, 8.22.
TsCl, K2CO3
EtOH, r.t., )))))
N
O
N
O
Ar
Ar
NH
NH
Scheme 3. The N-tosylation of several 2-imino-3-aryl-2,3-dihydrobenzo[d]oxazol-5-ols.
1251, 1216, 1192, 1163, 1029, 1017, 898, 818, 778, 711, 677, 616, 563,
541, 441, 408; 1H NMR (300 MHz, DMSO-d6): dH 7.93 (s, 1H),
7.80e7.76 (m, 1H), 7.62 (s, 1H), 7.30 (s, 1H), 7.00 (d, J¼8.4 Hz, 1H),
6.90 (br s, 1H), 6.40 (d, J¼8.4 Hz, 2H); 13C NMR (75 MHz, DMSO-d6):
dC 154.6, 154.3, 137.1, 135.5, 132.8, 131.6, 129.7, 129.4, 127.9, 125.9,
109.6, 108.1, 96.8; CHN: Anal. Calcd for C13H8Cl2N2O2: C, 52.91; H,
2.73; N, 9.49. Found: C, 52.74; H, 2.58; N, 8.37.
4.3.7. 3-(4-Methylphenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-ol
(Table 2, entry 7). Yield 83%; mp 207e209 ꢀC; FT-IR (KBr, cmꢁ1):
3360, 3064, 1675, 1618, 1580, 1516, 1494, 1475, 1442, 1408, 1382,
1272, 1225, 100, 998, 970, 821, 811, 793, 713, 689, 646, 619, 608; 1H
NMR (300 MHz, DMSO-d6): dH 9.35 (br s, 1H), 7.37 (d, J¼7.6 Hz, 2H),
7.34 (d, J¼7.6 Hz, 2H), 6.99 (d, J¼8.4 Hz, 1H), 6.40 (d, J¼8.4 Hz, 1H),
6.30 (s, 1H), 2.35 (s, 3H); 13C NMR (75 MHz, DMSO-d6): dC 155.9,
154.5, 137.7, 137.3, 134.1, 132.6, 130.5, 126.1, 109.4, 107.6, 96.4, 21.1;
4.3.6. 3-(4-Iodophenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-ol
(Table 2, entry 6). Yield 92%; mp 248e251 ꢀC; FT-IR (KBr, cmꢁ1):

3086
D. Habibi et al. / Tetrahedron 69 (2013) 3082e3087
CHN: Anal. Calcd for C14H12N2O2: C, 69.99; H, 5.03; N, 11.66. Found:
C, 69.82; H, 4.84; N, 11.49.
812, 797, 748, 723, 666, 649, 632, 598, 553; 1H NMR (400 MHz,
DMSO-d6): dH 7.72 (d, J¼8.0 Hz, 2H), 7.67 (d, J¼8.4 Hz, 2H),
7.44e7.41 (dd, J¼2.4, 2.0 Hz, 1H), 7.37 (d, J¼8.0 Hz, 3H), 7.19 (d,
J¼8.8 Hz, 1H), 6.77e6.75 (d, J¼8.8 Hz, 1H), 6.61 (d, J¼2.4 Hz, 1H),
2.49 (s, 3H); 13C NMR (100 MHz, DMSO-d6): dC 156.4, 145.9, 145.8,
142.8,134.1,133.2,132.6,132.4,131.9,131.8,129.9,128.6,127.7,125.2,
117.0, 110.1, 104.0, 21.8; CHN: Anal. Calcd for C20H14Cl2N2O4S: C,
53.46; H, 3.14; N, 6.23. Found: C, 53.81; H, 3.21; N, 6.32.
4.3.8. 3-(2,6-Dimethylphenyl)-2-imino-2,3-dihydrobenzo[d]ox-azol-
5-ol (Table 2, entry 8). Yield 85%; mp 204e207 ꢀC; FT-IR (KBr,
cmꢁ1): 3347, 3315, 2954, 1862, 2688, 1679, 1616, 1475, 1379, 1296,
1178, 1101, 1003, 837, 798, 778, 728, 709, 634, 446; 1H NMR
(300 MHz, DMSO-d6): dH 9.30 (br s, 1H), 7.30e7.27 (m, 3H), 7.02 (d,
J¼8.2 Hz, 1H), 6.43 (br s, 1H), 6.36 (d, J¼7.6 Hz, 1H), 5.79 (d,
J¼2.3 Hz, 1H), 2.07 (s, 6H); 13C NMR (75 MHz, DMSO-d6): dC 154.7,
154.2, 137.7137.3, 133.9, 132.2, 129.5, 129.1, 109.5, 107.1, 95.6, 17.7;
CHN: Anal. Calcd for C15H14N2O2: C, 70.85; H, 5.55; N, 11.02. Found:
C, 70.34; H, 5.46; N, 10.87.
4.4.3. 3-(4-Iodophenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-yl 4-
methylbenzenesulfonate (Table 3, entry 3). Yield 84%; mp
199e201 ꢀC; FT-IR (KBr, cmꢁ1): 3344, 3043, 1704, 1496, 1480, 1355,
1170, 1132, 1092, 893, 857, 819, 753, 592, 553; 1H NMR (400 MHz,
DMSO-d6): dH 7.89 (d, J¼8.4 Hz, 2H), 7.72 (d, J¼8.0 Hz, 2H), 7.36 (d,
J¼8.0 Hz, 3H), 7.24 (d, J¼8.4 Hz, 2H), 7.09 (d, J¼8.8 Hz, 1H), 6.66 (d,
J¼8.8 Hz, 1H), 6.58 (d, J¼2.4 Hz, 1H), 2.50 (s, 3H); 13C NMR
(100 MHz, DMSO-d6): dC 156.2, 145.7, 145.6, 142.9, 139.2, 133.7,
133.2, 131.9, 129.9, 128.7, 127.2, 116.0, 109.4, 103.6, 93.4, 21.8; CHN:
Anal. Calcd for C20H15IN2O4S: C, 47.44; H, 2.99; N, 5.53. Found: C,
47.52; H, 3.10; N, 5.64.
4.3.9. 3-(4-Methoxyphenyl)-2-imino-2,3-dihydrobenzo[d]oxaz-ol-5-
ol (Table 2, entry 9). Yield 84%; mp 177e180 ꢀC; FT-IR (KBr, cmꢁ1):
3336, 3089, 1673, 1617, 1579, 1499, 1479, 1414, 1294, 1196, 1161,
1090, 1011, 969, 833, 811, 744, 706, 625, 501; 1H NMR (300 MHz,
DMSO-d6): dH 9.30 (br s,1H), 7.45 (d, J¼8.1 Hz, 2H), 7.90 (d, J¼8.1 Hz,
2H), 6.98 (d, J¼8.3 Hz,1H), 6.55 (br s,1H), 6.34 (d, J¼8.3 Hz,1H), 6.18
(s, 1H), 3.80 (s, 3H); 13C NMR (75 MHz, DMSO-d6): dC 158.9, 155.5,
154.3, 137.2, 134.4, 127.9, 115.2, 109.3, 107.2, 96.1, 55.8; CHN: Anal.
Calcd for C14H12N2O3: C, 65.62; H, 4.72; N, 10.93. Found: C, 65.36; H,
4.61; N, 11.11.
4.4.4. 3-(2,6-Dimethylphenyl)-2-imino-2,3-dihydrobenzo[d]ox-azol-
5-yl 4-methylbenzenesulfonate (Table 3, entry 4). Yield 86%; mp
121e123 ꢀC; FT-IR (KBr, cmꢁ1): 3329, 3052, 1699, 1609, 1490, 1369,
1212, 1191, 1172, 1091, 988, 956, 864, 840, 822, 722, 745, 665, 652,
630, 599, 556, 525; 1H NMR (400 MHz, DMSO-d6): dH 7.68 (d,
J¼8.4 Hz, 2H), 7.36 (t, J¼7.6 Hz, 1H), 7.31 (d, J¼8.4 Hz, 2H), 7.25 (d,
J¼7.6 Hz, 2H), 7.16 (d, J¼8.8 Hz, 3H), 6.73 (d, J¼8.8 Hz, 1H), 6.17 (d,
J¼2.4 Hz, 1H), 2.46 (s, 3H), 2.09 (s, 6H); 13C NMR (100 MHz, DMSO-
d6): dC 156.8, 145.9, 145.6, 143.0, 137.2, 133.2, 131.8, 130.3, 129.8,
129.6, 129.4, 128.6, 115.9, 109.8, 103.2, 21.7, 17.7; CHN: Anal. Calcd
for C22H20N2O4S: C, 64.69; H, 4.94; N, 6.86. Found: C, 64.78; H, 4.86;
N, 6.94.
4.3.10. 3,30-(1,4-Phenylene)bis-(2-imino-2,3-dihydrobenzo[d]-ox-
azol-5-ol) (Table 2, entry 10). Yield 80%; mp 231e234 ꢀC; FT-IR (KBr,
cmꢁ1): 3329, 3069, 1674, 1618, 1521, 1480, 1396, 1292, 1190, 1161,
1110, 1007, 971, 810, 715, 640, 521; 1H NMR (300 MHz, DMSO-d6):
dH 9.35 (br s, 2H), 7.77 (s, 4H), 6.50e7.15 (m, 4H), 6.40 (d, J¼8.1 Hz,
4H); 13C NMR (75 MHz, DMSO-d6): dC 154.9, 154.4, 137.3, 134.0,
133.5, 127.1, 109.5, 107.7, 96.5; CHN: Anal. Calcd for C20H14N4O4: C,
64.17; H, 3.77; N, 14.97. Found: C, 63.94; H, 3.59; N, 14.78.
4.4. General experimental procedure for the synthesis of 2-
imino-3-aryl-2,3-dihydrobenzo[d]oxazol-5-yl 4-
methylbenzenesulfonate derivatives
4.4.5. 3,30-(1,4-Phenylene)bis(2-imino-2,3-dihydrobenzo[d]ox-azole-
5,3-diyl)bis(4-methylbenzenesulfonate) (Table 3, entry 5). Yield 85%;
mp 115 ꢀC dec; FT-IR (KBr, cmꢁ1): 3338, 3067, 1705,1617, 1598, 1519,
1485, 1454, 1381, 1292, 1257, 1219, 1193, 1180, 1132, 1091, 991, 956,
895, 862, 835, 814, 742, 664, 594, 552, 529; 1H NMR (400 MHz,
DMSO-d6): dH 7.76 (d, J¼8.4 Hz, 4H), 7.66 (s, 4H), 7.39 (d, J¼8.4 Hz,
6H), 7.04 (d, J¼8.8 Hz, 2H), 6.72 (s, 2H), 6.67 (d, J¼8.8 Hz, 2H), 2.50
(s, 6H); 13C NMR (100 MHz, DMSO-d6): dC 156.2, 145.7, 145.5, 142.9,
133.7, 133.2, 132.0, 129.9, 128.7, 126.7, 116.1, 109.4, 103.7, 21.8; CHN:
Anal. Calcd for C34H26N4O8S2: C, 59.81; H, 3.84; N, 8.21. Found: C,
59.90; H, 3.87; N, 8.27.
A mixture of 2-imino-3-aryl-2,3-dihydrobenzo[d]oxazol-5-ol
(1 mmol), p-toluenesulfonyl chloride (1 mmol) and K2CO3
(1 mmol) in EtOH (5 mL) was irradiated with ultrasound for the
appropriate time at room temperature. After completion (as mon-
itored by TLC), the solvent was concentrated and the solid residue
was washed with water to give the crude solid product. Then,
a crystallization step was performed using aqueous ethanol to af-
ford the pure product. The desired pure products were character-
ized by 1H NMR, 13C NMR, FT-IR, elemental analysis (CHN), and
melting points.
Acknowledgements
The authors are thankful to the Bu-Ali Sina University, Hamedan
Iran for the support of this work.
4.4.1. 3-(3-Bromophenyl)-2-imino-2,3-dihydrobenzo[d]oxazol-5-yl
4-methylbenzenesulfonate (Table 3, entry 1). Yield 87%; mp
189e190 ꢀC; FT-IR (KBr, cmꢁ1): 3334, 3086, 1702, 1590, 1495, 1478,
1430, 1388, 1363, 1221, 1192, 1171, 1135, 1092, 1071, 996, 888, 868,
850, 818, 784, 742, 722, 700, 684, 665, 653, 617, 594, 554, 520; 1H
NMR (400 MHz, DMSO-d6): dH 7.72 (d, J¼8.4 Hz, 2H), 7.64e7.61 (m,
2H), 7.45e7.44 (m, 2H), 7.37 (d, J¼8.4 Hz, 3H), 7.09 (d, J¼8.8 Hz, 1H),
6.69 (d, J¼8.8 Hz, 1H), 6.59 (d, J¼2.4 Hz, 1H), 2.49 (s, 3H); 13C NMR
(100 MHz, DMSO-d6): dC 156.2, 145.7, 145.6, 142.9, 135.3, 133.2,
131.9, 131.6, 131.3, 129.9, 128.7, 128.6, 124.2, 123.3, 116.1, 109.4, 103.5,
21.8; CHN: Anal. Calcd for C20H15BrN2O4S: C, 52.30; H, 3.29; N, 6.10.
Found: C, 52.42; H, 3.37; N, 6.21.
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