Trichloroisocyanuric acid-mediated one-pot synthesis of unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles at ambient temperature

Article abstract of DOI:10.1080/00397910802054289

An efficient method for the one-pot synthesis of unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles has been developed using trichloroisocyanuric acid (TCCA) at ambient temperature. A wide variety of aromatic as well as heterocyclic aldehydes exhibit condensation with a variety of acylhydrazines followed by oxidative cyclization to yield corresponding unsymmetrical 2,5-disubstituted 1,3,4-oxadiazoles. The mild nature of the synthesis and short reaction time are notable advantages of the developed protocol. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802054289

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Trichloroisocyanuric
Acid–Mediated One-Pot
Synthesis of Unsymmetrical
2,5-Disubstituted 1,3,4-
Oxadiazoles at Ambient
Temperature
D. M. Pore a , S. M. Mahadik a & U. V. Desai a
a Department of Chemistry, Shivaji University ,
Kolhapur, India
Published online: 09 Sep 2008.
To cite this article: D. M. Pore , S. M. Mahadik & U. V. Desai (2008)
Trichloroisocyanuric Acid–Mediated One-Pot Synthesis of Unsymmetrical 2,5-
Disubstituted 1,3,4-Oxadiazoles at Ambient Temperature, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
38:18, 3121-3128
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Synthetic Communications1, 38: 3121–3128, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802054289
Trichloroisocyanuric Acid–Mediated One-Pot
Synthesis of Unsymmetrical 2,5-Disubstituted
1,3,4-Oxadiazoles at Ambient Temperature
D. M. Pore, S. M. Mahadik, and U. V. Desai
Department of Chemistry, Shivaji University, Kolhapur, India
Abstract: An efficient method for the one-pot synthesis of unsymmetrical 2,5-
disubstituted 1,3,4-oxadiazoles has been developed using trichloroisocyanuric
acid (TCCA) at ambient temperature. A wide variety of aromatic as well as
heterocyclic aldehydes exhibit condensation with a variety of acylhydrazines
followed by oxidative cyclization to yield corresponding unsymmetrical 2,5-
disubstituted 1,3,4-oxadiazoles. The mild nature of the synthesis and short reac-
tion time are notable advantages of the developed protocol.
Keywords: Acylhydrazines, acylhydrazones, oxidative cyclization, 1,3,4-
oxadiazoles, trichloroisocyanuric acid
1,3,4-Oxadiazoles are an important class of heterocyclic compounds that
have received attention in medicinal chemistry[1] as surrogates of car-
boxylic acids, esters, and carboxamides. The interest in their synthesis
stems from their antimicrobial,[2] antifungal,[3] anti-inflamatory,[4] and
antihypertensive[5] actvities. They have proved to be useful in material
science as probes for their fluorescence and scintillation properties.[6] In
addition, oxadiazole derivatives have been widely used as electron-
conducting and hole-blocking materials in molecule-based as well as
polymeric light-emitting devices (LEDs).[7] Among several multistep
approaches reported so far for the synthesis of 1,3,4-oxadiazoles 1, the
cyclodehydration of diacylhydrazines 2 and the oxidative cyclization of
acylhydrazones 3 are the most popular ones (Scheme 1). For cyclodehy-
dration of diacylhydrazines, several dehydrating agents such as SOCl2,[8]
Received February 16, 2008.
Address correspondence to D. M. Pore, Department of Chemistry, Shivaji
University, Kolhapur 416 004, India. E-mail: p_dattaprasad@rediffmail.com
3121

3122
D. M. Pore, S. M. Mahadik, and U. V. Desai
Scheme 1. Pathways for the synthesis of 1,3,4-oxadiazoles.
[13]
BF3-(OEt)2,[9] P2O5,[10] POCl3,[11] H2SO4,[12] and ZrCl4
have been
reported, and oxidative cylization has been achieved by lead tetrace-
tate,[14a] lead(IV) oxide,[14b] KMnO4,[14c] electrochemical methods,[14d]
iodobenzene diacetate (IBD),[14e] and chloramine T.[14f] In addition, few
reliable and operationally facile methods for the one-step synthesis of
1,3,4-oxadiazoles have been reported, especially using carboxylic acids
and acylhydrazines.[9,15] However, to the best of our knowledge, there
are only two reports for the one-pot synthesis of 1,3,4-oxadiazoles start-
ing from aldehydes and acylhydrazines (Route 2, Scheme 1). They are
plagued by disadvantages such as long reflux time (ceric ammonium
nitrate [CAN],[16a] 11 h) and microwave irradiation (KMnO4).[16b] Thus,
the development of an efficient protocol operable at ambient temperature
for the one-pot synthesis of 1,3,4-oxadiazoles is warranted.
Trichloroisocyanuric acid (TCCA) is a versatile reagent and is
becoming increasingly popular because of its commercial availability at
low cost[17a] and lesser corrosiveness.[17b] It is worth mentioning that
although it gives rise to a plethora of reactions,[17c] its usefulness in an
oxidative cyclization reaction has not yet been explored. Our continued
interest in the development of useful synthetic methodologies[18]
prompted us to explore the feasibility of TCCA for the one-pot synthesis
of 2,5-disubstituted-1,3,4-oxadiazoles.
Initially, we decided to check the feasibility of TCCA in oxidative
cyclization of acylhydrazones. Accordingly, TCCA (30 mol%) was added
to an ethanolic solution of 1-(4-methoxybenzoyl)-2-(4-methyl benzylidine
hydrazine (Table 1, entry 8; Scheme 2). The desired 1,3,4-oxadiazole was
obtained (thin-layer chromatography, TLC) in a very short time at

Unsymmetrical 2,5-Disubstituted 1,3,4-Oxadiazoles
3123
Table 1. Oxidation of acylhydrazones to 1,3,4-oxadiazoles
Entry
R
R0
Time (min)
Yield (%)a
1
2
3
4
5
6
7
8
Ph
Ph
4-ClC6H4
Ph
Ph
20
15
20
20
15
15
20
15
80
85
82
80
85
79
75
80
4-OCH3C6H4
4-OCH3C6H4
4-ClC6H4
4-OCH3C6H4
4-CH3C6H4
4-ClC6H4
4-OCH3C6H4
4-CH3C6H4
4-ClC6H4
4-OCH3C6H4
4-CH3C6H4
aYields refer to pure isolated products.
ambient temperature. A series of acylhydrazones derived from conden-
sation between a variety of aldehydes and acylhydrazines were then
demonstrated to undergo smooth cyclization to yield corresponding
1,3,4-oxadiazoles in moderate to excellent yields as shown in Table 1.
Encouraged by these results, our attention was then focused toward
the one-pot synthesis of 1,3,4-oxadiazoles. Accordingly, an equimolar
mixture of p-methylbenzaldehyde and p-methoxybenzhydrazide was stir-
red by employing the conditions outlined previously. The careful workup
of the reaction mixture yielded three different products [viz, desired 1,3,4-
oxadiazoles (20%), diacylhydrazine (25%, comparison with authentic
sample) and unreacted aldehyde (40%)]. The attempts to improve
the yield by increasing the amount of TCCA were useful to a moderate
extent. Precisely, with the use of 1 eq. of TCCA, the corresponding
1,3,4-oxadiazole was obtained in a slightly improved yield (55%) along
with the unreacted aldehyde (20–25%).
In attempts to improve the yields, we surmised that (i) the protonated
form of aldehyde will be more electrophilic than aldehyde and (ii) TCCA,
if added to ethanol released HCl, can be used to protonate aldehyde. So,
we modified the procedure slightly.[19] p-methylbenzaldehyde (2 mmol)
was added to a solution of TCCA (1 mmol) in ethanol. This was followed
by addition of a solution of p-methoxybenzhydrazide (2 mmol) in
Scheme 2. Oxidation of acylhydrazones of 1,3,4-oxadiazoles.

3124
D. M. Pore, S. M. Mahadik, and U. V. Desai
Scheme 3. Synthesis of 1,3,4-oxadiazoles from aldehydes and acylhydrazines.
Table 2. One-pot synthesis of 2,5-disubstituted-1,3,4-oxadiazoles
Entry Product
Time (min) Yield (%)a,b
1
2
3
4
5
6
7
8
9
10
R ¼ R0 ¼ Ph
25
45
30
30
30
30
30
30
35
45
72
81
74
67
85
74
87
84
72
56
R ¼ Ph; R0 ¼ 4-OCH3C6H4
R ¼ 4-ClC6H4; R0 ¼ 4-OCH3C6H4
R ¼ 4-ClC6H4; R0 ¼ Ph
R ¼ R0 ¼ 4-OCH3C6H4
R ¼ Ph; R0 ¼ 4-CH3C6H4
R ¼ 4-OCH3C6H4; R0 ¼ 4-CH3C6H4
R ¼ R0 ¼ 4-CH3C6H4
R ¼ 4-ClC6H4; R0 ¼ 4-CH3C6H4
R ¼ 4-OCH3C6H4; R0 ¼ 3,4-(OCH3)C6H3
11
12
13
14
R0 ¼ 4-ClC6H4; X ¼ N; n ¼ 2
R0 ¼ Ph; X ¼ O; n ¼ 1
55
45
50
60
68
72
75
72
R0 ¼ Ph; X ¼ S; n ¼ 1
R0 ¼ 4-OCH3C6H4; X ¼ S; n ¼ 1
aYields refer to pure isolated products.
bAll products gave satisfactory spectroscopic (IR, 1H NMR and 13C NMR)
analysis.

Unsymmetrical 2,5-Disubstituted 1,3,4-Oxadiazoles
3125
ethanol, which resulted in the formation of the corresponding aroylben-
zylidene hydrazine that subsequently underwent oxidation by the
addition of remaining amount of TCCA (1 mmol) (Scheme 3). The modi-
fication proved to be worthy, and the desired 1,3,4-oxadiazole was
obtained in almost quantitative yield in a very short time.
To establish the scope and generality of this modified procedure, a
wide variety of aldehydes were reacted with a range of acylhydrazines,
and the results on the formation of corresponding 1,3,4-oxadiazoles are
summarized in Table 2.
In all the cases, irrespective of the presence of electron-donating or
electron-withdrawing groups in an aldehyde moiety, the desired 1,3,4-
oxadiazoles were obtained in quantitative yield. These reaction con-
ditions seem to be almost equally efficient for heterocyclic aldehydes
(Table 2, entries 11–14). Moreover, it is worthwhile to note that variation
in the electronic nature of the substituent on the aromatic ring from acyl-
hydrazines has also been successfully used for synthesis of unsymmetrical
1,3,4-oxadiazoles.
CONCLUSION
In summary, the collected data reveal the broad potential for the one-pot
synthesis of unsymmetrical 1,3,4-oxadiazioles using TCCA at ambient
temperature. This one-pot procedure was found to be quite general and
worked well for a variety of aryl and heterocyclic aldehydes as well as
for a variety of acylhydrazines. The simplicity of the experimental
procedure and the ready accessibility of the catalyst render this an
experimentally attractive method for the preparation of unsymmetrical
1,3,4-oxadiazoles.
EXPERIMENTAL
Typical Procedure for the One-Pot Synthesis of 2,5-Disubstituted
1,3,4-Oxadiazoles
To a well-stirred solution of TCCA (1 mmol) in ethanol (5 mL), an alde-
hyde (2 mmol) was added. After stirring the reaction mixture for 10 min,
a solution of acylhydrazide (2 mmol) in ethanol (5 mL) was added to
furnish the corresponding acylhydrazone. To the resultant reaction mix-
ture, the remaining part of TCCA (1 mmol) was added and allowed to stir
untill completion of the reaction (TLC). The reaction mixture was
extracted with ether (30 mL) and dried over anhydrous sodium sulfate;
the ether was evaporated. The residue obtained was chromatographed

3126
D. M. Pore, S. M. Mahadik, and U. V. Desai
over silica gel. Elution with petroleum ether–ethyl acetate (9:1, v=v)
furnished pure 2,5-disubstituted-1,3,4-oxadiazole.
Spectroscopic Data of Unknown Compounds
2-(3,4-Dimethoxyphenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole (entry 10):
mp 140–142ꢀC; IR (KBr): 1614, 1495, 1256, 1022, 832, 739 cmꢁ1;
1H NMR (CDCl3, 200 MHz) d: 3.89 (3H, s), 3.96 (3H, s), 4.0 (3H, s),
6.94 (1H, s), 7.01 (2H, d, J ¼ 8 Hz), 7.63 (2H, t, J ¼ 8 Hz), 8.05 (2H,
d, J ¼ 8 Hz); 13C NMR (CDCl3, 50 MHz) d: 55.35, 56.06, 56.26, 109.42,
110.97, 112.40, 113.55, 114.41, 116.49, 116.64, 120.16, 128.57, 128.72,
149.32, 151.85, 162.18, 164.2; MS(EI): 312 (Mþ ), 165, 135, 119, 107, 92, 77.
ꢀ2-(4-Chlorophenyl)-5-pyridyl-1,3,4-oxadiazole (entry 11): mp 150–
1
152 C; IR (KBr): 1605, 1479, 1090, 835, 732 cmꢁ1; H NMR (CDCl3,
200 MHz) d: 7.45–7.57 (m, 3H), 7.91 (1H, dt, J ¼ 8 Hz, 2 Hz), 8.15–
8.23 (2H, m), 8.33 (1H, dt, J ¼ 8 Hz, 2 Hz), 8.81 (1H, m); 13C NMR
(CDCl3, 50 MHz) d: 122.14, 123.36, 125.87, 126.61, 128.61, 129.50,
130.83, 132.00, 133.63, 137.24, 150.30; MS(EI): 257 (Mþ ), 258, 201,
166, 139, 141, 111, 113, 78, 51.
2-(4-Methoxyphenyl)-5-thiophenyl-1,3,4-oxadiazole (entry 14): mp
ꢀ
1
139–141 C; IR (KBr): 1614, 1498, 1261, 1019, 798, 738 cmꢁ1; H NMR
(CDCl3, 300 MHz) d: 3.80 (s, 3H), 6.92 (2H, d, J ¼ 8 Hz), 7.11 (1H, d,
J ¼ 4 Hz), 7.47 (1H, d, J ¼ 8 Hz, 3 Hz), 7.70 (1H, d, J ¼ 4 Hz), 7.95
(2H, d, J ¼ 8 Hz); 13C NMR (CDCl3, 75 MHz) d: 55.56, 114.44,
116.13, 125.42, 127.26, 128.05, 128.69, 129.37, 129.76, 162.30, 162.41;
MS(EI): 258 (Mþ ), 187, 135, 159, 111, 92, 77.
ACKNOWLEDGMENT
Authors (D.M.P., U.V.D.) thank University Grants Commission, New
Delhi, for financial assistance [F.33–260=2007(SR)].
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