Sonocatalyzed facile synthesis of 2-aryl benzoxazoles using MnO2 nanoparticles as oxidant agent under mild conditions

Article abstract of DOI:10.1080/00397911.2017.1365371

Nano MnO₂ was found to be an efficient oxidant agent for the synthesis of 2-substituted benzoxazoles through one-pot reaction of o-aminophenol and different aromatic aldehydes in acetonitrile under ultrasonic irradiation. This method was performed under mild conditions with high yields, inexpensive and readily available oxidant agent, facile and easy experimental procedure, simple purification of final products, and short reaction times. The prepared nano MnO₂ has been characterized by FTIR, XRD, and SEM techniques. The pure products were identified and characterized by physical and spectroscopic data such as; melting point, IR, ¹H NMR, and ¹³C NMR.

Full text of DOI:10.1080/00397911.2017.1365371

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Sonocatalyzed facile synthesis of 2-aryl
benzoxazoles using MnO2 nanoparticles as oxidant
agent under mild conditions
Hossein Naeimi, Zahra Rouzegar & Soraya Rahmatinejad
To cite this article: Hossein Naeimi, Zahra Rouzegar & Soraya Rahmatinejad (2017):
Sonocatalyzed facile synthesis of 2-aryl benzoxazoles using MnO2 nanoparticles as oxidant agent
under mild conditions, Synthetic Communications, DOI: 10.1080/00397911.2017.1365371
To link to this article: http://dx.doi.org/10.1080/00397911.2017.1365371
View supplementary material
Accepted author version posted online: 21
Aug 2017.
Submit your article to this journal
Article views: 1
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=lsyc20
Download by: [Australian Catholic University]
Date: 27 August 2017, At: 12:58

Sonocatalyzed facile synthesis of 2-aryl benzoxazoles using
MnO2 nanoparticles as oxidant agent under mild conditions
Hossein Naeimi*
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan
Zahra Rouzegar
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan
Soraya Rahmatinejad
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan
Address correspondence to Hossein Naeimi, 87317-51167, I.R. Iran; Tel: 983155912388; Fax:
983155912397. E-mail: naeimi@kashanu.ac.ir
ABSTRACT
Nano MnO2 was found to be an efficient oxidant agent for the synthesis of 2-substituted
benzoxazoles through one-pot reaction of o-aminophenol and different aromatic aldehydes in
acetonitrile under ultrasonic irradiation. This method was carried out under mild conditions with
high yields, inexpensive and readily available oxidant agent, facile and easy experimental
procedure, simple purification of final products and short reaction times. The prepared nano MnO2
has been characterized by FT-IR, XRD, and SEM techniques. The pure products were identified
1

and characterized by physical and spectroscopic data such as; melting point, IR, 1H NMR and 13C
NMR.
GRAPHICAL ABSTRACT
Ultrasound irradiation was implemented for the convenient synthesis of 2-substitued
benzoxazoles through the reaction of 2-aminophenol with aromatic aldehydes in the presence of
nano-MnO2 as catalyst in acetonitrile solvent.
KEYWORDS: 2-aryl benzoxazole, nano mno2, o-aminophenol, oxidant, ultrasound
Introduction
Benzoxazole and its derivatives have been receiving much attention in the recent years
owing to their pharmaceutical and biological applications.[1] These heterocyclic systems have been
2

reported to show antifungal,[2] antitumor,[3] antibacterial,[4] anti-inflammatory,[5] anticonvulsant,
antitubercular,[6] DNA topoisomerase inhibitor,[7] anti-microbial, anti-HIV-1[8] and antiulcer[9]
activities. Furthermore, some of them have been used as dye releaser in instant color
chromatography[10] herbicides, fluorescent whitening agents,[8] dye lasers,[11] photoluminescent
devices[12] and as ligands in asymmetric synthesis[13] (Figure 1).
A number of methods have been reported for the preparation of benzoxazoles including
cyclization of 2-halo-N-acylanilines,[14] ring closure of o-aryl oximes,[15] annulation of 1,2-
dihalobenzenes with primary amides,[8] ortho-C-H functionalization of phenols with
nitroalkanes,[16] reaction of 2-aminophenol with carbon–carbon triple bonds,[17] 2-aminophenols
and β-diketones,[18] coupling of 2-aminophenols with different carbonyl derivatives such as
aldehydes,[19] carboxylic acids,[20] acyl halides,[21] nitriles,[22] isocyanides,[23] and carbon
monoxide.[24]
Ultrasound is an efficient, green, simple, virtually innocuous, inexpensive and valuable
tool in synthetic chemistry.[25] The advantages of this powerful technique in driving chemical
reactions are (a) low rate of energy consumption, (b) minimization of wastes, (c) utilization of less
hazardous reactants and solvents, (d) generation of product with increased selectivity and
improved yield, (e) the use of low temperatures and pressures, (f) high reaction rate.[26–28] Hence,
ultrasonic irradiation, a means of activation, has been widely used in organic synthesis.[29–31]
In continuation of our previous works on the synthesis of heterocyclic compounds by
[32–34]
ultrasonic irradiation
and considering importance of benzoxazole derivatives, herein, we
decided to explore the ability of nano MnO2 as a mild oxidant for the sonochemical preparation of
2-substituted benzoxazoles. Therefore, nano MnO2 was used as an oxidant agent for the synthesis
3

of these compounds by the condensation reaction of o-aminophenol with aromatic aldehydes under
ultrasound irradiation.
Results and Discussion
Preparation of nano MnO2 started by grinding reaction between (NH4)2C2O4.H2O and
MnSO4.H2O with a mole ratio of 1:1.2, followed by calcination of manganese oxalate (MnC2O4)
o
precursors at 400 C and acid-treatment process using the previous reported method.[35] After
preparation of MnO2 NPs, the oxidant was characterized by FT-IR, SEM and XRD techniques. In
the FT-IR spectra of the MnO2 NPs, the absorption bands in the region of 527 cm-1 is attributed to
the stretching vibrations of the metal–oxygen bond and confirms the formation of the MnO2 (Fig.
1S, see Supporting Information (SI)). The position and relative intensities of all peaks confirm
well with standard XRD pattern of MnO2 (JCPDSNo. 14-0644) indicative of the nanocrystalline
structure of the prepared MnO2 (Fig. 2S, see SI). The average crystalline size of MnO2 particles
was 17 nm (determined from the Scherer equation). The scanning electron microscope (SEM) of
MnO2 particles show nanoparticles with dimensions of about 35.5-46.3 nm (Fig. 3S, see SI).
In this research, we have developed a synthetic method in which nano MnO2 efficiently
catalyzes preparing of 2-substituted benzoxazoles through treatment of o-aminophenol and
aromatic aldehydes under ultrasound conditions (Scheme 1).
In order to optimize the reaction conditions, the reaction of o-aminophenol (1.1 mmol, 0.12
g) with 2-chlorobenzaldehyde (1 mmol) was selected as a simple model and the efficiency of
oxidant amounts, different solvents and the power of applied ultrasound have been studied.
Initially, the model reaction was carried out without nano MnO2 but no product was
obtained (Table 1, entry 1). Hence, nano MnO2 as a reactive and appropriate oxidant agent was
4

used in this reaction. In order to optimize the amount of nano MnO2, the model reaction was carried
out in the presence of different quantities of MnO2 nanoparticles in CH3CN solution. The results
are summarized in Table 1. It was shown that 1 mmol of MnO2 nanoparticles lead to the best
results compared to other runs (Table 1, entry 4). Therefore, 1 mmol of nano MnO2 was
appropriate and sufficient for this reaction.
In continuation of this research, to investigate the effect of various powers of ultrasonic
irradiation the model reaction was performed under silent condition and different powers of
ultrasound irradiation (Table 2). The results confirmed positive effect of sonication on our reaction
system. It was decreased the time of reaction and enhanced the yield of the product (Table 2, entry
3), whereas, the reactions without ultrasonic irradiation was carried out in low yields and long
reaction times (Table 3, entry 1 and 2). The reaction in the presence of nano MnO2 and ultrasonic
irradiation with power of 80 W afforded the best yield of product, with 86% isolated yield after
15 min (Table 2, entry 5).
As it is described in “hot spot” theory, collapse of cavitating bubbles leads to production
of intensive local temperatures and pressures. This process provides the kinetic energy that drives
the chemical reaction. High temperature, pressure and reactant contact surface area related to
cavitation phenomenon and formation of hot spots are driving force for the fast and high yield
formation of product under ultrasound irradiation. High temperature and pressure, formation of
free radicals, high reactant contact surface area, rapid mixing and renewing of the liquid at the
solid-liquid interface are responsible for increasing the yields of products and enhancing the rate
of sonocatalyzed chemical reactions.[27], [28]
Solvents have important effect on the organic reactions. Hence, the model reaction, with 1
mmol of nano MnO2, was investigated in different solvent systems under reflux and sonic
5

conditions. The results are summarized in Table 3. It can be seen from the Table 3 that the best
solvent for this reaction is acetonitrile. Furthermore, ultrasound mediated reaction in acetonitrile
solution is faster and more efficient than the reaction performed under reflux condition.
Using the optimized conditions, we extended the study with o-aminophenol and several
aldehydes for the synthesis of 2-substituted benzoxazoles in the presence of nano MnO2 NPs. The
pertinent results are presented in Table 4. The cyclocondensation-oxidation reaction between o-
aminophenol and aromatic aldehydes proceeded well and afforded the desired products. The yields
of the products vary in the range of 67–90% yields. The reaction of o-aminophenol with propanal
as an aliphatic aldehyde was any product yield that can be due to less stability of 2-alkyl
benzoxazole than the 2-aryl benzoxazole (Table 4, entry 14). This method offered various
advantages including good yields, mild reaction conditions, high purity, high reaction rates and
easy work up procedures. The structures of products were completely characterized by
1
13
spectroscopic data such as; IR, H NMR, C NMR and melting point and they were consistent
with those of authentic samples.[36–43]
The suggested mechanism for synthesis of 2-substituted benzoxazoles using aromatic
aldehydes and o-aminophenol by nano MnO2 under ultrasonic condition is presented in Scheme 2.
The reaction proceeds via a cyclocondensation-oxidation reaction. First, nucleophilic attack of the
nitrogen of o-aminophenol on electrophilic carbon atom of aldehyde and elimination of H2O afford
species 1. Then, nucleophilic attack of ortho-hydroxyl group on the carbon atom of imine 1 gives
benzoxazoline 2. Radical oxidation of benzoxazoline by nano MnO2 and elimination of Mn(OH)2
afford the corresponding product 3. Surely, all steps are promoted by sonication. The physical
effects of ultrasound (mixing of the reagents, dispersion of solids and emulsification of liquids) in
6

combination with the chemical effects (cavitation) can increase the reactivity of chemical species
and accelerate the rate of reaction.[27]
In conclusions, in this study, nano MnO2 were prepared and were investigated in the
synthesis of 2-substitued benzoxazoles via reaction of a variety of aryl aldehydes with o-
aminophenol in acetonitrile solution under ultrasound irradiation at room temperature. The
reaction was efficiently preceded under ambient conditions, giving the corresponding products in
good to high isolated yields. This method has several advantages including available and
inexpensive oxidant, short reaction time, and simple workup procedure. The products were
1
13
confirmed by spectroscopic and physical data such as; IR, H NMR, C NMR and the melting
point.
Experimental
All commercially available reagents were used without further purification and purchased
from Merck and Aldrich Company in high purity. The used solvents were purified by standard
procedure. FT-IR spectra were obtained with KBr pellets in the range 400–4000 cm−1 with a
Perkin Elmer 550 spectrometer. 1H NMR and 13C NMR were recorded in CDCl3 solvent on a
Bruker DRX-400 spectrometer with tetramethylsilane as the internal reference. A BANDELIN
ultrasonic HD 3200 with probe model KE 76, with the diameter of 6 mm, was used to produce
ultrasonic irradiation and homogenize the reaction mixture. Piezoelectric crystal of this kind of
probe normally worked in the range of 700 kHz, but by use of some proper clamps, the output
frequency of piezoelectric crystal was controlled and reduced to 20 kHz in the reaction mixture.
X-ray diffraction patterns of samples were taken by using CuKα radiation (λ =1.5406A) on a
Rigaku D−Max CIII diffractometer. Scanning electron microscope (SEM) of nano MnO2 was
7

performed on a XL30 Philips SEM instrument. Melting points obtained with a Yanagimoto micro
melting point apparatus and are uncorrected. The purity determination of the substrates and
reaction monitoring were accomplished by TLC on silica-gel polygram SILG/UV 254 plates (from
Merck Company).
Shortly, MnSO4.H2O (1 mmol, 0.169 g) and (NH4)2C2O4.H2O (1.2 mmol, 0.17 g) were
taken in a mortar. After grinding at room temperature for 40 min, the mixture was placed in a water
o
bath of 80 C. A dry mixture was obtained after several hours. The mixture was washed with
distilled water, dried at 110 oC for 10 h and calcined at 400 oC for 10 h. Then, the calcined product
o
in 2 M H2SO4 solution was stirred magnetically at 80 C for 2 h. The final product was washed
with distilled water, filtered, and dried at 105 oC.
A mixture of 2-aminophenol (1.1 mmol, 0.12 g), aromatic aldehyde (1 mmol), and MnO2
nanoparticles (1 mmol, 0.086 g) in acetonitrile (4 mL) was sonicated in ultrasonic apparatus with
the power 80 Watt for the time indicated in Table 4. Progress of the reaction was monitored by
thin-layer chromatography (petroleum ether/ethyl acetate, 6/2). After completion, the reaction
mixture was cooled to room temperature. The minerals were separated by simple filtration. Then
filtrate solution was poured onto ice. The solid crude product was filtered, washed with ice-water
and recrystallized from ethanol/H2O (1:1) to give the pure product. All of the 2-substituted
benzoxazole products were identified by physical and spectroscopic data.[36–43]
Acknowledgments
The authors are grateful to the University of Kashan for supporting this work (grant no.
159148/69).
Supporting Information
8

Supporting Information: Full experimental detail, the IR, 1H and 13C NMR spectra, XRD,
FT-IR and SEM. This material can be found via the “Supplementary Content” section of this
article’s webpage.
References
<Number>[1]</Number> Razavi, H.; Palaninathan, S. K.; Powers, E. T.; Wiseman, R. L.; Purkey,
H. E.; Mohamedmohaideen, N. N.; Deechongkit, S.; Chiang, K. P.; Dendle, M. T.;
Sacchettini, J. C. Angew. Chem. Int. Ed. 2003, 42, 2758–2761.
<Number>[2]</Number> Kaplancikli, Z. A.; Turan-Zitouni, G.; Revial, G.; Guven, K. Arch.
Pharm. Res. 2004, 27, 1081–1085.
<Number>[3]</Number> Aiello, S.; Wells, G.; Stone, E. L.; Kadri, H.; Bazzi, R.; Bell, D. R.;
Stevens, M. F. G.; Matthews, C. S.; Bradshaw, T. D.; Westwell, A. D. J. Med. Chem. 2008,
51, 5135–5139.
<Number>[4]</Number> Singh, G.; Kaur, M.; Mohan, C.; Prashar, S. Int. J. Psychol. Relig. 2013,
3, 6113–6118.
<Number>[5]</Number> Seth, K.; Garg, S. K.; Kumar, R.; Purohit, P.; Meena, V. S.; Goyal, R.;
Banerjee, U. C.; Chakraborti, A. K. ACS Med. Chem. Lett. 2014, 5, 512–516.
<Number>[6]</Number> Klimešová, V.; Kočí, J.; Waisser, K.; Kaustová, J.; Möllmann, U. Eur.
J. Med. Chem. 2009, 44, 2286–2293.
<Number>[7]</Number> Kaur, A.; Wakode, S.; Pathak, D. P. Int. J. Pharm. Pharm. Sci. 2014, 7,
16.
<Number>[8]</Number> Viirre, R. D.; Evindar, G.; Batey, R. A. J. Org. Chem. 2008, 73, 3452–
3459.
<Number>[9]</Number> Meghumi, Y. Y. S.; Kazako, K.; Fukio, K. Chem. Pharm. Bull. 1998,
46, 445–451.
<Number>[10]</Number> Mohammadpoor-Baltork, I.; Moghadam, M.; Tangestaninejad, S.;
Mirkhani, V.; Zolfigol, M.; Hojati, S. J. Iran Chem. Soc. 2008, 5, S65–S70.
<Number>[11]</Number> Ma, X.; Qiu, X.; Jin, R.; Kang, C., Gao, L. High Perform. Polym. 2014,
1–8.
<Number>[12]</Number> Ko, C. W.; Tao, Y. T.; Danel, A.; Krzeminska, L.; Tomasik, P. Chem.
Mater. 2001, 13, 2441–2446.
<Number>[13]</Number> Kianfar, A. H.; Mahmood, W. A. K.; Dinari, M.; Azarian, M. H.;
Khafri, F. Z. Spectrochim. Acta Mol. Biomol. Spectrosc. 2014, 127, 422–428.
<Number>[14] </Number>Evindar, G.; Batey, R. A. J. Org. Chem. 2006, 71, 1802–1808.
<Number>[15]</Number> Guru, M. M.; Ali, M. A.; Punniyamurthy, T. Org. Lett. 2011, 13, 1194–
1197.
<Number>[16]</Number> Aksenov, N. A.; Aksenov, A. V.; Nadein, O. N.; Aksenov, D. A.;
Smirnov, A. N.; Rubin, M. RSC Adv. 2015, 5, 71620–71626.
<Number>[17]</Number> Xie, H.-Z.; Gao, Q.; Liang, Y.; Wang, H.-S.; Pan, Y.-M. Green Chem.
2014, 16, 2132–2135.
<Number>[18]</Number> Mayo, M. S.; Yu, X.; Zhou, X.; Feng, X.; Yamamoto, Y.; Bao, M. J.
Org. Chem. 2014, 79, 6310–6314.
9

<Number>[19]</Number> Matloubi Moghaddam, F.; Rezanejade Bardajee, G.; Ismaili, H.;
Maryam Dokht Taimoory, S. Synth. Commun. 2006, 36, 2543–2548.
<Number>[20]</Number> Yildiz‐Oren, I.; Tekiner‐Gulbas, B.; Yalcin, I.; Temiz‐Arpaci, O.; Akı‐
Sener, E.; Altanlar, N. Arch. Pharm. 2004, 337, 402–410.
<Number>[21]</Number> Pottorf, R. S.; Chadha, N. K.; Katkevics, M.; Ozola, V.; Suna, E.;
Ghane, H.; Regberg, T.; Player, M. R. Tetrahedron Lett. 2003, 44, 175–178.
<Number>[22]</Number> Moskvichev, Y. A.; Gerasimova, N.; Pashinin, A.; Korikov, P.;
Nozhnin, N.; Alov, E.; Kozlova, O. Heterocycl. Compd. 2001, 37, 1162–1167.
<Number>[23]</Number> Boissarie, P. J.; Hamilton, Z. E.; Lang, S.; Murphy, J. A.; Suckling, C.
J. Org. Lett. 2011, 13, 6256–6259.
<Number>[24]</Number> Neumann, K. T.; Lindhardt, A. T.; Bang-Andersen, B.; Skrydstrup, T.
Org. Lett. 2015, 17, 2094–2097.
<Number>[25]</Number> Cravotto, G.; Cintas, P., Chem. Eur. J. 2007, 13, 1902–1909.
<Number>[26]</Number> Mason, T. J. Ultrason. Sonochem. 2003, 10, 175–179.
<Number>[27]</Number> Mason, T. J. Phil. Trans. R. Soc. A 1999, 357, 355–369.
<Number>[28]</Number> Cravotto, G.; Cintas, P. Chem. Soc. Rev. 2006, 35, 180–196.
<Number>[29]</Number> Naeimi, H.; Rabiei, K. Ultrason. Sonochem. 2012, 19, 130–135.
<Number>[30]</Number> Khaligh, N. G. Ultrason. Sonochem. 2012, 19, 736–739.
<Number>[31]</Number> D’Anna, F.; Marullo, S.; Vitale, P.; Noto, R. Ultrason. Sonochem.
2012, 19, 136–142.
<Number>[32]</Number> Naeimi, H.; Didar, A. Ultrason. Sonochem. 2017, 34, 889–895.
<Number>[33]</Number> Naeimi, H.; Shaabani, R. Ultrason. Sonochem. 2017, 34, 246–254.
<Number>[34]</Number> Naeimi, H.; Rahmatinejad, S.; Nazifi, Z. S. J. Taiwan Inst. Chem. Eng.
2016, 58, 1–7.
<Number>[35]</Number> Yuan, A.; Wang, X.; Wang Y.; Hu, Y. Energy Convers. Manage. 2010,
51, 2588–2594.
<Number>[36]</Number> Chen, Y. X.; Qian, L.-F.; Zhang, W.; Han, B. Angew. Chem. Int. Ed.
2008, 47, 9330–9333.
<Number>[37]</Number> Praveen, C.; Kumar, K. H.; Muralidharan, D.; Perumal, P.,
Tetrahedron 2008, 64, 2369–2374.
<Number>[38]</Number> Sharghi, H.; Aberi, M.; Doroodmand, M. M. J. Iran Chem. Soc. 2012,
9, 189–204.
<Number>[39]</Number> Leng, Y.; Yang, F.; Zhu, W.; Wu, Y.; Li, X. Org. Biomol. Chem. 2011,
9, 5288–5296.
<Number>[40]</Number> El-Sheikh, M. I.; Marks, A.; Biehl, E. R. J. Org. Chem. 1981, 46,
3256–3259.
<Number>[41]</Number> Guru, M. M.; Ali, M. A.; Punniyamurthy, T. J. Org. Chem. 2011, 76,
5295–5308.
<Number>[42]</Number> Más-Montoya, M.; Usea, L.; Espinosa Ferao, A.; Montenegro, M. F.;
Ramírez de Arellano, C.; Tárraga, A.; Rodríguez-López, J. N.; Curiel, D. J. Org. Chem.
2016, 81, 3296–3302.
<Number>[43]</Number> López-Ruiz, H.; Briseño-Ortega, H.; Rojas-Lima, S.; Santillán, R.;
Farfán, N. Tetrahedron Lett. 2010, 51, 2633–2635.
10

Table 1. Optimization of nano MnO2 amounts for synthesis of 2-(2-chlorophenyl)-1,3-
benzoxazolea.
Entry
Oxidant amount (mmol)
Time (min)
Yield (%)b
1
2
3
4
5
6
–
0.5
0.8
1
30
15
15
15
20
15
–
30
65
86
86
86
1
1.2
aReaction conditions: o-aminophenol (1.1 mmol), 2-chloro benzaldehyde (1 mmol), and MnO2 nanoparticles in acetonitrile
solution (4 mL) under ultrasonic irradiation (80 W) at room temperature.
bIsolated yields.
11

Table 2. Survey the effect of ultrasonic irradiation on the formation of 2-(2-chlorophenyl)-1,3-
benzoxazolea
Entry
Power (W)
Time (min)
Yield (%)b
trace
1
2
Silent (r.t.)
90
80
10
Silent (80C)
3
4
5
6
7
70
75
80
85
80
15
15
15
15
20
49
68
86
86
86
aReaction conditions: o-aminophenol (1.1 mmol), 2-chlorobenzaldehyde (1 mmol), and MnO2 nanoparticles (1 mmol) in
acetonitrile as solvent (4 mL) at room temperature.
bIsolated yields.
12

a
Table 3. Effect of solvent on the rate of sonocatalyzed reaction using nano MnO2
Entry
Solvent
Yield (%)b
USc (Silentd)
0 (0)
1
2
3
4
5
H2O
H2O/CH3CN (1:1)
CH3CN
0 (0)
86 (17)
EtOH
67 (10)
MeOH
75 (15)
aReaction conditions: o-aminophenol (1.1 mmol), 2-chlorobenzaldehyde (1 mmol), and MnO2 nanoparticles (1 mmol) in selected
solvent (4 ml).
bIsolated yields of products.
cRoom temperature, 15 min.
dReflux, 2 h.
13

Table 4. Preparation of 2-substituted benzoxazoles under optimized conditionsa
Entry
Substrate
Product
Time
(min)
16
Yield
(%)b
87
1
2
15
86
3
4
14
13
88
89
5
6
14
16
88
87
7
8
17
17
87
85
14

9
16
44
85
86
10
11
13
67
12
13
12
10
89
90
14
CH3-CH2-CHO
-
80
-
15

Figure 1. Representative compounds containing benzoxazole.
16

Scheme 1. Synthesis of 2-substituted benzoxazoles.
17

Scheme 2. Proposed reaction mechanism for the sonocatalyzed formation of 2-substitued
benzoxazoles by nano MnO2 as an oxidant agent.
18