Efficient one-pot synthesis of disubstituted 1,3,4-oxadiazole derivatives from the reaction of (N-isocyanimino)triphenylphosphorane, acetaldehyde, a secondary amine, and an electron-poor (E)-cinnamic acid

Article abstract of DOI:10.1002/hc.20660

The iminium intermediate formed by the reaction of a secondary amine with acetaldehyde was reacted by (N-isocyanimino) triphenylphosphorane in the presence of an electron-poor (E)-cinnamic acid derivative to give the corresponding iminophosphorane intermediate, whose intramolecular the aza-Wittig reaction led to disubstituted 1,3,4-oxadiazole derivatives. The reactions were completed under neutral conditions at room temperature, and the corresponding disubstituted 1,3,4-oxadiazole derivatives were produced in excellent yields.

Full text of DOI:10.1002/hc.20660

Heteroatom Chemistry
Volume 22, Number 1, 2011
Efficient One-Pot Synthesis of Disubstituted
1,3,4-Oxadiazole Derivatives from the Reaction
of (N-Isocyanimino)triphenylphosphorane,
Acetaldehyde, a Secondary Amine, and an
Electron-Poor (E)-Cinnamic Acid
Ali Ramazani, Yavar Ahmadi, and Roghayeh Tarasi
Chemistry Department, Zanjan University, 45195-313, Zanjan, Iran
Received 11 September 2010; revised 27 October 2010
[1]. This principle, therefore, is highly efficient in
ABSTRACT: The iminium intermediate formed by
terms of time as well as resources [2]. Among the
the reaction of a secondary amine with acetalde-
known multicomponent reactions to date, the most
hyde was reacted by (N-isocyanimino) triphenylphos-
valuable reactions are those based on isocyanides.
phorane in the presence of an electron-poor (E)-
Isocyanide-based multicomponent reactions (abbre-
cinnamic acid derivative to give the corresponding
viated as IMCRs by Ugi and Do¨mling) by virtue
iminophosphorane intermediate, whose intramolecu-
of their synthetic potential, their inherent atom
lar the aza-Wittig reaction led to disubstituted 1,3,4-
efficiency, convergent nature, ease of implemen-
oxadiazole derivatives. The reactions were completed
tation, and the generation of molecular diversity,
under neutral conditions at room temperature, and the
have attracted much attention because of the ad-
corresponding disubstituted 1,3,4-oxadiazole deriva-
vantages that they offer to the field of combinatorial
ꢀ
C
tives were produced in excellent yields.
2010 Wi-
chemistry [3].
Iminophosphoranes are a class of special type
ley Periodicals, Inc. Heteroatom Chem 22:79–84, 2011;
View this article online at wileyonlinelibrary.com. DOI
10.1002/hc.20660
of zwitterions, which bear a strong nucleophilic
electron-rich nitrogen. The electron distribution
around the P+ N− bond and its consequent chem-
ical implications have been probed and assessed
through theoretical, spectroscopic and crystallo-
graphic investigations [4]. The proton affinity of
these iminophosphoranes can be used as a molecu-
lar guide to assess their utility as synthetic reagents
and their function as ligands in coordination and
organometallic chemistry [4–6].
The intramolecular version of the aza-Wittig-
type reaction has attracted considerable attention
recently because of its high potential for the syn-
thesis of a wide variety of nitrogen heterocycles,
which can be attributed, in good measure, to the
rapid progress in the preparation of functional-
ized iminophosphoranes. The nucleophilicity at the
INTRODUCTION
Recently, multicomponent condensation reactions
have become one of the most powerful methods for
the synthesis of small molecule libraries, because
products are formed in a single step by simultane-
ous reactions of several reagents and the molecular
diversity required for such combinatorial libraries
can be achieved by simply varying each component
Correspondence to: Ali Ramazani; e-mail: aliramazani@
gmail.com.
Contract grant sponsor: Zanjan University.
2010 Wiley Periodicals, Inc.
c
ꢀ
79

80 Ramazani, Ahmadi, and Tarasi
nitrogen is a factor of essential mechanistic im-
portance in the use of these iminophosphoranes as
aza-Wittig reagents. Iminophosphoranes are impor-
tant reagents in synthetic organic chemistry, espe-
cially in the synthesis of naturally occurring prod-
ucts, compounds with biological and pharmaco-
logical activity [4–6]. However, the organic chem-
istry of (N-isocyanimino)triphenylphosphorane 4
remains almost unexplored. (N-Isocyanimino)
triphenylphosphorane 4 is expected to have syn-
thetic potential because it provides a reaction sys-
tem in which the iminophosphorane group can re-
act with a reagent having a carbonyl functionality
[7,8]. In recent years, we have established a one-
pot method for the synthesis of organophospho-
rus compounds [9–17]. As part of our ongoing pro-
gram to develop efficient and robust methods for
the preparation of heterocyclic compounds [18–26],
we wish to report the synthesis of a disubstituted
1,3,4-oxadiazole derivatives 5 by a four-component
condensation of secondary amine 2, acetalde-
hyde 1, (N-isocyanimino)triphenylphosphorane 4,
and an electron-poor (E)-cinnamic acid deriva-
tive 3 in excellent yields under neutral conditions
(Scheme 1).
acids, esters, and carboxamides. They are an im-
portant class of heterocyclic compounds that have
a wide range of pharmaceutical and biological ac-
tivities including antimicrobial, antifungal, anti-
inflammatory, antihypertensive, analgesic, antibac-
terial, hypoglycemic, antimalarial, antitubercular,
and antidepressant [27]. Several methods have been
reported in the literature for the synthesis of 1,3,4-
oxadiazoles. These protocols are multistep in nature
[28]. The most general method involves the cycliza-
tion of diacylhydrazides with a variety of reagents,
such as thionyl chloride, phosphorous oxychloride,
or sulfuric acid, usually under harsh reaction condi-
tions. Few reliable and operationally simple exam-
ples have been reported for the one-step synthesis of
1,3,4-oxadiazoles, especially from readily available
carboxylic acids and acid hydrazides [29].
RESULTS AND DISCUSSION
The imine intermediate generated by the reaction of
secondary amine 2 with acetaldehyde 2 is trapped
by the (N-isocyanimino)triphenylphosphorane in
the presence of an electron-poor (E)-cinnamic acid
derivative 3 to lead to the formation of disubstituted
1,3,4-oxadiazole derivatives 5 and triphenylphos-
phine oxide (6, Scheme 1 and Table 1). The reaction
1,3,4-Oxadiazoles have attracted interest in
medicinal chemistry as surrogates of carboxylic
SCHEME 1 Four-component synthesis of disubstituted 1,3,4-oxadiazole derivatives 5a–g (see Table 1 and the text).
Heteroatom Chemistry DOI 10.1002/hc

One-Pot Synthesis of Disubstituted 1,3,4-Oxadiazole Derivatives 81
TABLE
1 Synthesis of Disubstituted 1,3,4-Oxadiazole
closure. Because of the easy availability of the syn-
thetic approach and the neutral ring closure condi-
tions, this new synthetic approach discussed here
has the potential in synthesis of various disubsti-
tuted 1,3,4-oxadiazoles, which are of considerable
interest as potential biologically active compounds
or pharmaceuticals.
Derivatives 5 (see Scheme 1)
Compounds
R
X
Yielda (%)
3a
3b
3c
3d
3e
3f
Tert-butyl
Isopropyl
Ethyl
Tert-butyl
Ethyl
Isopropyl
Tert-butyl
Cl (meta)
Cl (para)
Cl (para)
Cl (para)
Cl (meta)
Cl (meta)
F (para)
82
85
80
83
85
83
82
EXPERIMENTAL
3g
Starting materials and solvents were obtained from
Merck (Darmstadt, Bundesland, Germany) and
Fluka (Buchs, Switzerland) and were used without
further purification. The methods used to follow the
reactions are thin layer chromatography (TLC) and
NMR, which indicated that there is no side prod-
uct. Melting points (mp) were measured on an Elec-
trothermal 9100 apparatus and are uncorrected. IR
spectra were measured on a Jasco 6300 FTIR spec-
aYield of isolated products.
proceeds smoothly and cleanly under mild condi-
tions, and no side reactions were observed.
The structures of the products were deduced
from their 1H NMR, 13C NMR, mass, and IR spectra.
For example, the 1H NMR spectrum of 5a consisted
of a singlet for the 3CH3 of amin (δ = 1.17 ppm), a
3
doublet for CH3 (δ = 1.53 ppm, JHH = 7.0 Hz), and
1
trometer. H- and 13C-NMR spectra were measured
a singlet for the CH3 (δ = 2.37 ppm), a AB-quartet
for CH2 of benzyl group at δ = 3.98 and 4.13 ppm
(2 JHH = 17.3 Hz), a quartet for the CH (δ = 4.76 ppm,
3 JHH = 7.0 Hz), and a multiplet at δ = 6.94–7.52 ppm
for H-aromatic and H-vinylic. The aryl groups ex-
hibited characteristic signals in the aromatic region
(CDCl3) with a Bruker DRX-250 Avance spectrom-
eter at 250.0 and 62.5 MHz, respectively. Elemen-
tal analyses were performed using a Heraeus CHN-
O-rapid analyzer. Mass spectra were recorded on a
Finnigan-MAT 8430 mass spectrometer operating at
an ionization potential of 20 eV. Flash chromatog-
raphy columns were prepared with the Merck silica
gel powder.
of the spectrum. The H decoupled 13C NMR spec-
trum of 5a showed 19 distinct resonances; partial
1
assignment of these resonances is given in the Ex-
perimental section. The H and 13C NMR spectra of
1
compounds 5b–g were similar to those of 5a, except
for the aromatic and aliphatic moieties, which exhib-
ited characteristic signals with appropriate chemical
shifts.
General Procedure
To a magnetically stirred solution of secondary
amine 2 (1 mmol), acetaldehyde 1 (1 mmol), and
(N-isocyanimino)triphenylphosphorane 4 (1 mmol)
in CH2Cl2 (5 mL), a solution of an electron-poor
(E)-cinnamic acid derivative 3 (1 mmol) in CH2Cl2
(5 mL) at room temperature was added dropwise
over 15 min. The mixture was stirred for 5 h. The sol-
vent was removed under reduced pressure, and the
viscous residue was purified by flash column chro-
matography (silica gel powder; petroleum ether–
ethyl acetate (10:2)). The solvent was removed under
reduced pressure, and the products (5a–g) were ob-
tained. The characterization data of the compounds
are given below.
A mechanistic rationalization for this reaction
is provided in Scheme 2. It is conceivable that the
initial event is the condensation reaction of the ac-
etaldehyde 1, secondary amine 2, and electron-poor
(E)-cinnamic acid derivative 3 that leads to an in-
termediate iminium ion 7. Nucleophilic addition of
the (N-isocyanimino)triphenylphosphorane 4 to the
intermediate iminium ion 7 leads to a nitrilium in-
termediate 8. This intermediate may be attacked by
the conjugate base of the acid 3 to form 1:1:1 adduct
9. This adduct may undergo an intramolecular aza-
Wittig reaction of an iminophosphorane moiety with
the ester carbonyl group to afford the isolated disub-
stituted 1,3,4-oxadiazole 5 by the removal of triph-
enylphosphine oxide 6 from intermediate 10.
N-Benzyl-N-(tert-butyl)-N-(1-[5-[(E(-2-(3-chloro-
phenyl)-1-ethenyl]-1,3,4-oxadiazol-2-yl]ethyl)amine
(5a). Yellow oil, yield: 324 mg (82%). IR (neat):
3472, 2977, 1649, 1567, 1475, 1205, 969, 784 cm−1.
1H NMR: 1.17 (s, 9H, CH3 amin); 1.53 (d, 3H,3 JHH
= 7 Hz, CH3 aliphatic); 3.98, 4.13 (AB quartet, 2H,
2 JHH = 17.25 Hz, CH2 of the benzyl group); 4.76
CONCLUSIONS
The reported method offers a mild, simple, and effi-
cient route for the preparation of disubstituted 1,3,4-
oxadiazole derivatives, by a sequence of multicom-
ponent reactions and an intramolecular aza-Wittig
3
(q, 1H, JHH = 7 Hz, CH); 6.94–7.52 (m, 11H, arom
and vinylic).13C NMR: 18.53, 28.64 (4CH3); 56.26 (C
Heteroatom Chemistry DOI 10.1002/hc

82 Ramazani, Ahmadi, and Tarasi
SCHEME 2 Proposed mechanism for the formation of disubstituted 1,3,4-oxadiazole derivatives 5a–g.
(5b). Yellow crystals, MP: 104.3–105.6◦ . Yield:
aliphatic); 47.85 (CH2 of the benzyl group); 48.38
(CH); 111.61, 136.83 (2CH vinylic); 135.03, 136.63,
143.61 (3C arom); 125.43, 126.09, 126.82, 127.39,
127.94, 129.70, 130.21 (9CH arom); 163.48, 168.92
(2C of oxadiazole). Anal. Calcd for C23H26ClN3O
(395.93): C 69.77, H 6.62, N 10.61; Found: C 69.76,
H 6.62, N 10.62. MS (EI): 395 (M+, 2), 380 (8.48),
234 (38.92), 199 (11.78), 174 (12.15), 162 (72.11),
105 (32.49), 91 (100), 84 (39.08), 56 (16.32), 41
(17.52).
324 mg (85%). IR (KBr): 3450, 2965, 2929, 1639,
1
1536, 1492, 1170, 1087, 817 cm−1. H NMR: 0.90
3
(d, 3H, JHH = 6.25 Hz, CH3 amin); 1.14 (d, 3H,
3 JHH = 6.25 Hz, CH3 amin); 1.55 (d, 3H,3 JHH
=
6.75 Hz, CH3); 3.14–3.25 (m, 1H, CH amin); 3.80,
3.85 (AB quartet, 2H, 2 JHH = 15 Hz, CH2 of the ben-
3
zyl group); 4.26 (q, 1H, JHH = 6.75 Hz, CH); 6.99–
7.51 (m, 11H, arom and vinylic).13C NMR: 16.89,
19.46, 21.19 (3CH3); 48.32 (CH2 of the benzyl group);
48.85, 49.61 (2CH); 110.76, 140.75 (2CH vinylic);
133.28, 135.76, 137.14 (3C arom); 126.80, 128.19,
128.22, 128.61, 129.27 (9CH arom); 163.98, 167.88
N-Benzyl-N-(1-[5-[(E(-2-(4-chlorophenyl)-1-ethe-
nyl]-1,3,4-oxadiazol-2-yl]ethyl)-N-isopropylamine
Heteroatom Chemistry DOI 10.1002/hc

One-Pot Synthesis of Disubstituted 1,3,4-Oxadiazole Derivatives 83
and vinylic).13C NMR: 13.80, 18.12 (2CH3); 44.43
(CH2 of ethyl); 52.23 (CH2 of the benzyl group);
54.48 (CH aliphatic); 111.45, 136.35 (2CH vinylic);
135.04, 136.62, 143.60 (3C arom); 125.49, 127.01,
127.40, 128.28, 128.54, 129.79,130.23 (9CH arom);
164.54, 166.65 (2C of oxadiazole). Anal. Calcd for
C21H22ClN3O (367.87): C 68.56, H 6.03, N 11.42,
Found: C 68.59, H 6.05, N 11.43.
(2C of oxadiazole). Anal. Calcd for C22H24ClN3O
(381.90): C 69.19, H 6.33, N 11.0; Found: C 69.15,
H 6.35, N 11.02. MS (EI): 381 (M+, 1.93), 249
(5.66), 233 (16.58), 213 (26.53), 148 (69.35), 105
(21.69), 91 (100), 83 (36.98), 69 (33.01), 56 (61.04), 41
(65.77).
N-Benzyl-N-(1-[5-[(E(-2-(4-chlorophenyl)-1-ethe-
nyl]-1,3,4-oxadiazol-2-yl]ethyl)-N-ethylamine
(5c).
N-Benzyl-N-(1-[5-[(E(-2-(3-chlorophenyl)-1-ethe-
nyl]-1,3,4-oxadiazol-2-yl]ethyl)-N-isopropylamine (5f).
Yellow oil, yield: 317 mg (83%). IR (neat): 3458,
2971, 2935, 1648, 1566, 1455, 1173, 968, 783 cm−1.
Yellow oil, yield: 294 mg (80%). IR (neat): 3459,
2979, 2937, 1645, 1530, 1491, 1089, 970, 815 cm−1.
3
1H NMR: 1.09 (t, 3H, JHH = 7 Hz, CH3 of Et); 1.57
3
(d, 3H, JHH = 7 Hz, CH3); 2.42–2.52 and 2.67–2.78
3
1H NMR: 0.90 (d, 3H, JHH = 6.25 Hz, CH3 amin);
2
(2m, CH2 of Et); 3.54, 3.87 (AB quartet, 2H, JHH
3
1.14 (d, 3H, JHH = 6.25 Hz, CH3 amin); 1.55 (d,
= 14.25 Hz, CH2 of the benzyl group); 4.26 (q, 1H,
3 JHH = 7 Hz, CH); 6.97–7.51 (m, 11H, arom and
vinylic).13C NMR: 13.75, 15.42 (2CH3); 44.46 (CH2
of ethyl); 50.77 (CH2 of the benzyl group); 54.42
(CH aliphatic); 109.14, 137.35 (2CH vinylic); 133.26,
139.78 and 140.38 (3C arom); 126.97, 128.27, 128.49,
128.61, 129.28 (9CH arom); 164.54, 166.65 (2C of
oxadiazole). Anal. Calcd for C21H22ClN3O: C 68.56;
H 6.03; N 11.42; Found: C 68.58; H 6.01; N 11.44.
MS (EI): 367 (M+, 2.1), 277 (5.23), 247 (20.72), 233
(44.99), 205 (16.25), 162 (24.36), 149 (41.56), 134
(100), 105 (22.24), 91 (65.12), 69 (26.47), 56 (42.43),
41 (16.98).
3H,3 JHH = 6.5 Hz, CH3); 3.17–3.22 (m, 1H, CH
amin); 3.77, 3.88 (AB quartet, 2H, 2 JHH = 15 Hz, CH2
of the benzyl group); 4.26 (q, 1H, 3 JHH = 6.5 Hz, CH);
6.98–7.54 (m, 11H, arom and vinylic). 13C NMR:
16.87, 19.47, 21.18 (3CH3), 48.34 (CH2 of the benzyl
group); 48.87 and 49.61 (2CH); 111.62, 136.97 (2CH
vinylic); 135.05, 136.60, 143.65 (3C arom); 125.47,
126.80, 127.40, 128.11, 128.23, 129.75, 130.22 (9CH
arom); 163.81, 168.75 (2C of oxadiazole). Anal.
Calcd for C22H24ClN3O (381.90): C 69.19, H 6.33,
N 11; Found: C 69.21, H 6.30, N 11.02. MS (EI):
381 (M+, 1.6), 247 (10.52), 233 (33.61), 213 (18.96),
205 (28.15), 163 (28.37), 148 (100), 105 (78.74), 91
(75.68), 63 (42.29), 56 (99.02), 41 (41.37).
N-Benzyl-N-(tert-butyl)-N-(1-[5-[(E(-2-(4-chloro-
phenyl)-1-ethenyl]-1,3,4-oxadiazol-2-yl]ethyl)amine
(5d). Yellow crystals, mp: 90.2–91.8◦ . Yield: 328 mg
(83%). IR (KBr): 3472, 2972, 2929, 1637, 1536, 1491,
1085, 997, 817 cm−1. 1H NMR: 1.17 (s, 9H, CH3
N-Benzyl-N-(tert-butyl)-N-(1-[5-[(E(-2-(4-fluoro-
phenyl)-1-ethenyl]-1,3,4-oxadiazol-2-yl]ethyl)amine
(5g). Yellow oil, yield: 311 mg (82%). IR (neat):
3456, 2970, 2934, 1646, 1532, 1236, 979, 835 cm−1.
1H NMR: 1.17 (s, 9H, CH3 amin); 1.53 (d, 3H, 3 JHH
=
3
amin); 1.53 (d, 3H, JHH = 7 Hz, CH3 aliphatic);
2
6.75 Hz, CH3 aliphatic); 3.99, 4.13 (AB quartet, 2H,
3.98, 4.13 (AB quartet, 2H, JHH = 17.25 Hz, CH2 of
2 JHH = 17.25 Hz, CH2 of the benzyl group); 4.76 (q,
3
the benzyl group); 4.76 (q, 1H, JHH = 7 Hz, CH);
3
6.95–7.50 (m, 11H, arom and vinylic).13C NMR:
18.60, 28.63 (4CH3); 56.25 (C aliphatic); 47.86 (CH2
of the benzyl group); 48.38 (CH); 110.76, 137.01
(2CH vinylic); 133.31, 135.70, 143.67 (3C arom);
126.07, 126.81, 127.94, 128.59, 129.25 (9CH arom);
163.64, 168.84 (2C of oxadiazole). Anal. calcd for
C23H26ClN3O (395.93): C 69.77, H 6.62, N 10.61;
Found: C 69.78, H 6.60, N 10.63.
1H, JHH = 6.75 Hz, CH); 6.87–7.56 (m, 11H, arom
and vinylic).13C NMR: 18.65, 28.61 (4CH3); 56.24 (C
aliphatic), 47.88 (CH2 of the benzyl group); 48.39
(CH); 109.96, 137.16 (2CH vinylic); 133.31, 143.70,
163.76 (3C arom); 115.96, 116.31, 126.08, 126.81,
127.95, 129.71, 129.30 (9CH arom); 161.60, 165.76
(2C of oxadiazole). Anal. Calcd for C23H26FN3O
(379.47): C 72.80, H 6.91, N 11.07; Found: C 72.78,
H 6.89, N 11.10.
N-Benzyl -N-(1-[5-[(E(-2-(3-chlorophenyl)-1-ethe-
nyl]-1,3,4-oxadiazol-2-yl]ethyl)-N-ethylamine
(5e).
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