Convenient Vilsmeier Reagent Mediated One-Pot Synthesis of Symmetrical and Asymmetrical 1,3,4-Oxadiazoles

Full text of DOI:10.1080/00304948.2017.1342511

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
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Convenient Vilsmeier Reagent Mediated One-Pot
Synthesis of Symmetrical and Asymmetrical 1,3,4-
Oxadiazoles
Maaroof Zarei & Fatemeh Rasooli
To cite this article: Maaroof Zarei & Fatemeh Rasooli (2017) Convenient Vilsmeier Reagent
Mediated One-Pot Synthesis of Symmetrical and Asymmetrical 1,3,4-Oxadiazoles, Organic
Preparations and Procedures International, 49:4, 355-362, DOI: 10.1080/00304948.2017.1342511
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Date: 23 August 2017, At: 04:09

Organic Preparations and Procedures International, 49:355–362, 2017
Copyright Ó Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2017.1342511
Convenient Vilsmeier Reagent Mediated One-Pot
Synthesis of Symmetrical and Asymmetrical 1,3,4-
Oxadiazoles
Maaroof Zarei and Fatemeh Rasooli
Department of Chemistry, Faculty of Sciences, University of Hormozgan,
Bandar Abbas 71961, Iran
The 1,3,4-oxadiazole structure is considered to be a privileged scaffold in medicinal and
pesticide chemistry.1 These important five-membered heterocycles have diverse biologi-
cal properties such as antibacterial, antifungal, antitubercular, insecticidal, herbicidal,
analgesic, anti-inflammatory, anticonvulsant and anticancer activities.2 Drugs displaying
a 1,3,4-oxadiazole ring include an antiretroviral drug for the treatment of HIV infection
(raltegravir), an anticancer agent (zibotentan) and an antihypertensive (nesapidil)
(Figure 1).
1,3,4-Oxadiazoles have also shown wide applications in material science.3 In line
with their importance, there are numerous methods available for the construction of the
1,3,4-oxadiazole ring.4 A widely used method is dehydrative cyclization of diacylhydra-
zines. Because of harsh dehydration reagents, such as H2SO4,5 POCl3 and polyphos-
6–7
phoric acid (PPA)8, some effort has been made to develop modified and milder
procedures.
1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide,9 Ph3PO and Tf2O,10 propylphos-
phonic anhydride (T3P),11 XtalFluor-E,12 EDCI13 and Burgess reagent14 were reported in
the literature for cyclodehydration of diacylhydrazines to 1,3,4-oxadiazoles. The use of
hazardous materials, high cost, low yield of products, tedious separation of products and
high temperature are the major limitations associated with these procedures. Thus the
development of new and efficient methodologies for synthesis of 1,3,4-oxadiazoles is
important.
Chloromethylenedimethylammonium chloride (Vilsmeier reagent) 1 has mostly
been used as a formylating agent.15 It was also applied as a powerful water scaven-
ger and coupling reagent for the synthesis of esters,16 amides,17 acid chlorides,18
b-sultams19 and b-lactams.20–22 Preparation of this reagent is easy from DMF and
oxalyl chloride or thionyl chloride in dry dichloromethane.23 High safety and ease
of handling of this reagent are its other advantages. Herein, we report a convenient
one-pot synthesis of 1,3,4-oxadiazoles by employing Vilsmeier reagent as acid acti-
vator and coupling reagent.
Received November 28, 2016; in final form March 24, 2017.
Address correspondence to Maaroof Zarei, Department of Chemistry, Faculty of Sciences,
University of Hormozgan, Bandar Abbas 71961, Iran. E-mail: maaroof1357@yahoo.com;
mzarei@hormozgan.ac.ir
355

356
Zarei and Rasooli
Figure 1 1,3,4-Oxadiazole ring and commercially 1,3,4-oxadiazole drugs.
Recently, we reported simple protocols for the one-pot synthesis of acylhydrazines
and symmetrical and asymmetrical diacylhydrazines from carboxylic acids using Vilsme-
ier reagent.24 Reaction of carboxylic acids with excess hydrazine monohydrate (100%) in
the presence of Vilsmeier reagent and triethylamine in dry CH2Cl2 at room temperature
gave acylhydrazines 2a-d. Symmetrical diacylhydrazines 3a-k were directly prepared
from the carboxylic acids via Vilsmeier reagent in dry acetonitrile. Asymmetrical diacyl-
hydrazines 4a-j were obtained by treatment of acylhydrazines 2a-d with a solution of the
corresponding carboxylic acids, triethylamine and reagent 1 in dry acetonitrile
(Scheme 1).24
Scheme 1
The above successful results encouraged us to try the synthesis of 1,3,4-oxadiazoles
from diacylhydrazines using Vilsmeier reagent as a cyclodehydration reagent. Our initial
studies focused on the model reaction of diacylhydrazine 3a (R1 D C6H5OCH2). Treat-
ment of diacylhydrazine 3a with trimethylamine (3.0 equiv) and Vilsmeier reagent (1.0
equiv) in dry acetonitrile at room temperature gave symmetrical 1,3,4-oxadiazole 5a with
91% yield after crystallization from ethanol. The progress of reaction was checked by
TLC monitoring. After completion of the reaction, water was added and extraction by
ethyl acetate was performed.
With these results in hand, the scope of the reaction was extended to a diverse range
of 1,3,4-oxadiazoles. As can be seen from Scheme 2 and Table 1 (Method A), a wide
range of symmetrical and asymmetrical diacylhydrazines containing aromatic and ali-
phatic substituents can all be converted into the corresponding 1,3,4-oxadiazoles in good
to excellent yields and with high purity.

Convenient Vilsmeier Reagent Mediated One-Pot Synthesis
357
Scheme 2
Preparation of 1,3,4-oxadiazoles 5a-k and 6a-j in two and three stages from carbox-
ylic acids using Vilsmeier reagent (Method A), respectively, persuaded us to try a direct
one-pot synthesis of symmetrical 1,3,4-oxadiazoles 5a-k from the carboxylic acids and
asymmetrical 1,3,4-oxadiazoles 6a-j from acylhydrazines 2a-d (Method B). In this way,
the reaction of phenoxyacetic acid (1.0 mmol) with hydrazine hydrate (0.3 mmol) in the
presence of Vilsmeier reagent 1 (1.0 mmol) and triethylamine (1.0 mmol) in dry acetoni-
trile at room temperature overnight gave crude 1,3,4-oxadiazole 5a (Method B). Saturated
NaHCO3 was added to the mixture and extraction with ethyl acetate was performed. Pure
1,3,4-oxadiazole 5a was obtained after crystallization from ethanol with a nearly identical
Table 1
Synthesis of 1,3,4-oxadiazoles 5a-k and 6a-j Using Vilsmeier Reagent
Isolated yield (%)
Entry
R1
R2
Product Method A Method B
1
2
3
4
5
6
C6H5OCH2
–
–
–
–
–
–
5a
5b
5c
5d
5e
5f
91
92
88
85
90
93
89
88
89
90
92
90
2,4-Cl2C6H3OCH2
4-ClC6H4OCH2
PhthN-CH2
2-NaphthOCH2
7
3-OHC6H4
C6H5
–
5g
5h
5i
83
89
93
84
86
86
81
80
82
81
84
87
95
88
84
85
84
88
89
87
85
84
82
86
89
80
85
93
85
87
8
–
9
2,6-(MeO)2C6H3
4-OHC6H4
MeOCH2
–
10
11
12
13
14
15
16
17
18
19
20
21
–
–
5j
5k
6a
6b
6c
6d
6e
6f
6g
6h
6i
C6H5OCH2
C6H5OCH2
4-ClC6H4OCH2
MeOCH2
2,4-Cl2C6H3OCH2 C6H5OCH2
2,4-Cl2C6H3OCH2 4-ClC6H4OCH2
2,4-Cl2C6H3OCH2 MeOCH2
2-NaphthOCH2
2-NaphthOCH2
2-NaphthOCH2
2-NaphthOCH2
4-ClC6H4OCH2
C6H5OCH2
4-ClC6H4OCH2
2,4-Cl2C6H3OCH2
MeOCH2
MeOCH2
6j

358
Zarei and Rasooli
yield with Method A. Pure asymmetrical 1,3,4-oxadiazoles 6a-j were also prepared
directly from acylhydrazines 2a-d (Scheme 2, Method B, Table 1). The synthesized com-
1
13
pounds were characterized by IR, H NMR, C NMR and elemental analysis. The IR
spectra showed the C D N peak at 1604–1627 cm¡1 and the disappearance of the NH and
CO peaks of diacylhydrazines. Furthermore, the 13C NMR spectra showed the C D N
peaks at 154.8–165.3 ppm.
In conclusion, efficient methods for a convenient preparation of symmetrical and
asymmetrical 1,3,4-oxadiazoles have been developed through Vilsmeier reagent-medi-
ated reactions of hydrazine hydrate with diverse carboxylic acids. The method is simple
and the easy workup removes by-products. Pure products are obtained by crystallization
from ethanol without the need for column chromatography. In addition, this reagent offers
advantages over the existing ones by creating milder reaction conditions.
Experimental Section
All required chemicals were purchased from Merck, Aldrich and Acros chemical compa-
nies. The melting points were determined on a silicone oil bath and are uncorrected. IR
spectra were measured on a Galaxy series FT-IR 5000 spectrometer. NMR spectra were
recorded in DMSO-d6 using a Bruker spectrophotometer (1H NMR 300 MHz, 13C NMR
75 MHz) with tetramethylsilane as an internal standard and coupling constants were given
in cycles per second (Hz). Elemental analyses were run on a Vario EL III elemental ana-
lyzer. Thin-layer chromatography was carried out on silica gel 254 analytical sheets
obtained from Fluka. (Chloromethylene)dimethylammonium chloride (Vilsmeier
reagent) 1 was obtained as a white solid by a reported procedure.23 Procedures for the
synthesis of acylhydrazines 2a-d, diacylhydrazines 3a-k, 4a-j and their spectral data have
been previously reported.24 All solvents and triethylamine were drieꢀd prior to use accord-
ing to standard methods 25 and then stored over molecular sieves (4A). All glassware was
dried by hot air, and reactions were carried out under a CaCl2-tube to prevent the entry of
moisture into the reaction vessel.
General Procedure for the Synthesis of Symmetrical and Asymmetrical 1,3,4-
oxadiazoles 5a-k and 6a-j (Method A)
Symmetrical diacylhydrazines 3a-k and asymmetrical diacylhydrazines 4a-j were con-
verted to symmetrical and asymmetrical 1,3,4-oxadiazoles 5a-k and 6a-j, respectively,
by this method. A solution of diacylhydrazine (1.0 mmol), Vilsmeier reagent (1.0 mmol)
and Et3N (3.0 mmol) in dry CH3CN (20 mL) was stirred at room temperature for 8 hours.
Water (20 mL) was added and the mixture was extracted with EtOAc (3 £ 20 mL). The
organic layer was washed with brine (20 mL), dried (Na2SO4), filtered and the solvent
was removed under reduced pressure to give the crude products. The crude residues were
purified by crystallization from 95% ethanol.
General Procedure for One-pot Synthesis of Symmetrical 1,3,4-oxadiazoles 5a-k
(Method B)
Hydrazine hydrate (0.3 mmol) was added to a solution of carboxylic acid (1.0 mmol),
Vilsmeier reagent (1.0 mmol) and Et3N (2.0 mmol) in dry CH3CN (20 mL) at room tem-
perature. After 2 hours Vilsmeier reagent (0.4 mmol) and Et3N (1.0 mmol) were added
and the mixture was stirred at room temperature for 8 hours. Saturated NaHCO3 (20 mL)

Convenient Vilsmeier Reagent Mediated One-Pot Synthesis
359
was added and the mixture was extracted with EtOAc (3 £ 20 mL). The organic layer
was washed with brine (20 mL), dried (Na2SO4), filtered and the solvent was removed
under reduced pressure to give the crude products. The crude residues were purified by
crystallization from 95% ethanol. Spectral data for 5a-b, 5d, 5f-h and 5j have been previ-
ously reported,26–32 and were matched by the products of this work.
2,5-bis((4-Chlorophenoxy)methyl)-1,3,4-oxadiazole (5c)
White solid. mp: 108–110ꢀC. IR (KBr) cm¡1: 1620 (C D N). 1H NMR d 5.20 (2 CH2, s,
4H), 6.81–7.30 (ArH, m, 8H); 13C NMR d 57.1 (CH2), 116.0, 127.1, 129.3, 157.1 (aro-
matic carbons), 157.2 (C D N).
Anal. Calcd for C16H12Cl2N2O3: C, 54.72; H, 3.44; N, 7.98. Found: C, 54.79; H,
3.53; N, 8.02.
2,5-bis((Naphthalen-2-yloxy)methyl)-1,3,4-oxadiazole (5e)
White solid. Mp: 249–251ꢀC. IR (KBr) cm¡1: 1619 (C D N). 1H NMR d 5.19 (2 CH2, s,
4H), 7.04–7.76 (ArH, m, 14H); 13C NMR d 57.2 (CH2), 109.1, 117.6, 124.0, 127.2, 127.3,
128.7, 129.4, 131.1, 135.1, 156.1 (aromatic carbons), 157.1 (C D N).
Anal. Calcd for C24H18N2O3: C, 75.38; H, 4.74; N, 7.33. Found: C, 75.31; H, 4.68; N,
7.30.
2,5-bis(2,6-Dimethoxyphenyl)-1,3,4-oxadiazole (5i)
1
Pale-yellow solid. mp: 152–154ꢀC. IR (KBr) cm¡1: 1627 (C D N). H NMR d 3.78 (4
OMe, s, 12H), 6.70–7.34 (ArH, m, 6H); 13C NMR d 56.4 (OMe), 108.8, 111.0, 131.4,
155.7 (aromatic carbons), 161.4 (C D N).
Anal. Calcd for C18H18N2O5: C, 63.15; H, 5.30; N, 8.18. Found: C, 63.29; H, 5.47; N,
8.27.
2,5-bis(methoxymethyl)-1,3,4-oxadiazole (5k)
White solid. mp 144–146ꢀC. IR (KBr) cm¡1: 1609 (C D N). 1H NMR d 3.37 (2 OMe, s,
6H), 4.42 (2 CH2, s, 4H); 13C NMR d 58.8 (OMe), 60.3 (CH2), 165.3 (C D N).
Anal. Calcd for C6H10N2O3: C, 45.57; H, 6.37; N, 17.71. Found: C, 45.69; H, 6.51;
N, 17.77.
General Procedure for One-pot Synthesis of Asymmetrical 1,3,4-oxadiazoles 6a-j
(Method B)
Acylhydrazines 2a-d (1.0 mmol) were added to a solution of the appropriate carboxylic
acid (1.2 mmol), Vilsmeier reagent (1.2 mmol) and Et3N (2.5 mmol) in dry CH3CN
(20 mL) at room temperature. After 2 hours Vilsmeier reagent (1.0 mmol) and Et3N
(2.5 mmol) were added and the mixture was stirred at room temperature 8 hours. Satu-
rated NaHCO3 (20 mL) was added and the mixture was extracted with EtOAc (3 £
20 mL). The organic layer was washed with brine (20 mL), dried (Na2SO4), filtered and
the solvent was removed under reduced pressure to give the crude products. The crude
residues were purified by crystallization from 95% ethanol. The data for 6f have been pre-
viously reported,33 and matched those from this work.

360
Zarei and Rasooli
2-((4-Chlorophenoxy)methyl)-5-(phenoxymethyl)-1,3,4-oxadiazole (6a)
1
White solid. mp: 224–226ꢀC. IR (KBr) cm¡1: 1622 (C D N). H NMR d 5.18, 5.19 (2
CH2, 2s, 4H), 6.82–7.28 (ArH, m, 9H); 13C NMR d 58.5, 61.7 (CH2), 115.5, 115.9, 122.2,
126.9, 129.1, 129.8, 157.2, 157.3 (aromatic carbons), 158.8, 163.9 (C D N).
Anal. Calcd for C16H13ClN2O3: C, 60.67; H, 4.14; N, 8.84. Found: C, 60.75; H, 4.28;
N, 8.90.
2-(Methoxymethyl)-5-(phenoxymethyl)-1,3,4-oxadiazole (6b)
1
Silver solid. mp: 208–210ꢀC. IR (KBr) cm¡1: 1610 (C D N). H NMR d 3.30 (OMe, s,
3H), 4.47, 5.18 (2 CH2, 2s, 4H), 6.89–7.27 (ArH, m, 5H); 13C NMR d 56.6 (OMe), 58.9,
60.7 (CH2), 116.2, 120.7, 129.7, 157.4 (aromatic carbons), 159.6, 165.0 (C D N).
Anal. Calcd for C11H12N2O3: C, 59.99; H, 5.49; N, 12.72. Found: C, 60.12; H, 5.64;
N, 12.66.
2-((2,4-Dichlorophenoxy)methyl)-5-(phenoxymethyl)-1,3,4-oxadiazole (6c)
1
White solid. mp: 148–150ꢀC. IR (KBr) cm¡1: 1621 (C D N). H NMR d 5.09, 5.19 (2
CH2, 2s, 4H), 6.87–7.39 (ArH, m, 8H); 13C NMR d 57.4, 58.2 (CH2), 115.6, 118.6, 121.7,
125.5, 128.6, 128.7, 129.7, 130.5, 138.1, 153.7 (aromatic carbons), 157.3, 159.3 (C D N).
Anal. Calcd for C16H12Cl2N2O3: C, 54.72; H, 3.44; N, 7.98. Found: C, 54.66; H,
3.53; N, 7.94.
2-((4-Chlorophenoxy)methyl)-5-((2,4-dichlorophenoxy) methyl)-1,3,4-oxadiazole (6d)
1
White solid. mp: 257–259ꢀC. IR (KBr) cm¡1: 1613 (C D N). H NMR d 5.08, 5.19 (2
CH2, 2s, 4H), 6.82–7.39 (ArH, m, 7H); 13C NMR d 56.6, 57.3 (CH2), 115.4, 117.4, 125.8,
127.1, 128.4, 128.7, 129.3, 131.1, 148.4, 153.6 (aromatic carbons), 154.8, 157.3 (C D N).
Anal. Calcd for C16H11Cl3N2O3: C, 49.83; H, 2.88; N, 7.26. Found: C, 49.88; H,
2.97; N, 7.31.
2-((2,4-Dichlorophenoxy)methyl)-5-(methoxymethyl)-1,3,4-oxadiazole (6e)
1
White solid. mp: 160–162ꢀC. IR (KBr) cm¡1: 1612 (C D N). H NMR d 3.43 (OMe, s,
3H), 4.46, 5.13 (2 CH2, 2s, 4H), 6.91–7.41 (ArH, m, 3H); 13C NMR d 57.1 (OMe), 58.0,
60.6 (CH2), 117.6, 125.9, 128.2, 128.8, 131.7, 152.6 (aromatic carbons), 158.2, 165.2
(C D N).
Anal. Calcd for C11H10Cl2N2O3: C, 45.70; H, 3.49; N, 9.69. Found: C, 45.82; H,
3.65; N, 9.61.
2-((4-Chlorophenoxy)methyl)-5-((naphthalen-2-yloxy)methyl)-1,3,4-oxadiazole (6g)
1
White solid. mp: 170–172ꢀC. IR (KBr) cm¡1: 1623 (C D N). H NMR d 5.04, 5.15 (2
CH2, 2s, 4H), 6.82–7.78 (ArH, m, 11H); 13C NMR d 53.3, 57.1 (CH2), 109.7, 116.0,
117.7, 123.4, 127.1, 127.2, 127.6, 128.4, 129.5, 129.6, 131.1, 135.5, 150.3, 156.4 (aro-
matic carbons), 157.5, 157.6 (C D N).
Anal. Calcd for C20H15ClN2O3: C, 65.49; H, 4.12; N, 7.64. Found: C, 65.41; H, 4.06;
N, 7.59.

Convenient Vilsmeier Reagent Mediated One-Pot Synthesis
361
2-((2,4-Dichlorophenoxy)methyl)-5-((naphthalen-2-yloxy)methyl)-1,3,4-oxadiazole
(6h)
Off-white solid. mp: 187–189ꢀC. IR (KBr) cm¡1: 1611 (C D N). 1H NMR d 4.93, 5.01 (2
CH2, 2s, 4H), 6.80–7.72 (ArH, m, 10H); 13C NMR d 58.0, 58.2 (CH2), 108.0, 117.5,
117.9, 124.5, 125.8, 127.0, 127.4, 128.0, 128.5, 128.8, 129.7, 130.1, 132.5, 135.5, 147.5,
153.7 (aromatic carbons), 157.0, 157.4 (C D N).
Anal. Calcd for C20H14Cl2N2O3: C, 59.87; H, 3.52; N, 6.98. Found: C, 59.99; H,
3.66; N, 7.06.
2-(Methoxymethyl)-5-((naphthalen-2-yloxy)methyl)-1,3,4-oxadiazole (6i)
1
White solid. mp: 176–178ꢀC. IR (KBr) cm¡1: 1604 (C D N). H NMR d 3.32 (OMe, s,
3H), 4.45, 5.13 (2 CH2, 2s, 4H), 7.09–7.78 (ArH, m, 7H); 13C NMR d 56.5 (OMe), 57.7,
60.6 (CH2), 109.1, 117.3, 124.2, 127.1, 127.5, 128.5, 129.4, 132.2, 136.0, 156.2 (aromatic
carbons), 157.3, 165.0 (C D N).
Anal. Calcd for C15H14N2O3: C, 66.66; H, 5.22; N, 10.36. Found: C, 66.58; H, 5.31;
N, 10.33.
2-((4-Chlorophenoxy)methyl)-5-(methoxymethyl)-1,3,4-oxadiazole (6j)
1
Cream-colour solid. mp: 213–215ꢀC. IR (KBr) cm¡1: 1616 (C D N). H NMR d 3.39
(OMe, s, 3H), 4.41, 5.01 (2 CH2, 2s, 4H), 6.84 (ArH, d, 2H, J D 7.4), 7.28 (ArH, d, 2H,
J D 7.4); 13C NMR d 57.6 (OMe), 58.9, 60.9 (CH2), 116.1, 127.5, 129.5, 157.4 (aromatic
carbons), 157.9, 164.7 (C D N).
Anal. Calcd for C11H11ClN2O3: C, 51.88; H, 4.35; N, 11.00. Found: C, 51.97; H,
4.48; N, 11.07.
Acknowledgments
Financial support from the Research Council of the University of Hormozgan is gratefully
acknowledged.
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