One-pot cyclization/decarboxylation of α-keto acids and acylhydrazines for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles under transition-metal-free conditions

Article abstract of DOI:10.1016/j.tetlet.2016.09.007

A one-pot KI/TBHP-mediated oxidative cyclization of α-keto acids with acylhydrazines was developed. A series of functional 2,5-disubstituted 1,3,4-oxadiazoles were synthesized through a tandem keto amine condensation followed by oxidative cyclization and decarboxylation reactions. This procedure was achieved under transition-metal-free conditions and showed advantages including readily available materials, mild reaction conditions and good group tolerance.

Full text of DOI:10.1016/j.tetlet.2016.09.007

Tetrahedron Letters xxx (2016) xxx–xxx
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Tetrahedron Letters
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One-pot cyclization/decarboxylation of a-keto acids and
acylhydrazines for the synthesis of 2,5-disubstituted 1,3,4-
oxadiazoles under transition-metal-free conditions
⇑
⇑
Peng Gao , Juan Wang, Zijing Bai, Hualei Cheng, Jian Xiao, Mengnan Lai, Desuo Yang, Mingjin Fan
Shaanxi Key Laboratory of Phytochemistry, College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji, Shaanxi 721013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A one-pot KI/TBHP-mediated oxidative cyclization of
a
-keto acids with acylhydrazines was developed. A
Received 30 August 2016
Accepted 2 September 2016
Available online xxxx
series of functional 2,5-disubstituted 1,3,4-oxadiazoles were synthesized through a tandem keto amine
condensation followed by oxidative cyclization and decarboxylation reactions. This procedure was
achieved under transition-metal-free conditions and showed advantages including readily available
materials, mild reaction conditions and good group tolerance.
Keywords:
Cyclization
Ó 2016 Elsevier Ltd. All rights reserved.
Decarboxylation
Oxidation
Transition-metal-free reactions
Iodide
Introduction
In recent years, the decarboxylative functionalization has
drawn considerable attention and become a powerful strategy for
2,5-Disubstituted 1,3,4-oxadiazoles and their derivatives repre-
sent an important scaffold that has been found in many natural
and synthesized molecules because of their remarkable biological
activities1 and unique physical characteristics2 (Fig. 1). As a conse-
quence, many methods have been developed for the synthesis of
2,5-disubstituted 1,3,4-oxadiazoles, such as dehydration of dia-
cyl-hydrazines,3 oxidative cyclization of N-acylhydrazones,4 and
some transition metal-catalyzed coupling reactions.5 Compared
with these methods, one-pot annulation of acylhydrazines and car-
bonyl compounds is a more efficient approach.6 The Bahramnejad
research group disclosed the synthesis of 2,5-disubstituted 1,3,4-
oxadiazoles via CAN-mediated cyclization of acylhydrazides and
organic molecules design.7 In this impressive field, a variety of
noble transition metals such as Pd, Cu, Ag or Rh, catalyzed decar-
boxylative coupling reactions have well-studied.8 On the other
hand, the metal-free decarboxylative methodologies also have
prosperously developed.9–11 In particular, construction of hetero-
cycles via metal-free decarboxylation has attracted broad inter-
ests.11 For example, Wang and co-workers reported synthesis of
pyridines, imidazo[1,5-a]pyridines and quinazolinones via iodine-
mediated decarboxylation of amino acids.11a–c Nachtsheim and
his group also presented an iodine-catalyzed decarboxylation of
amino acids for the construction of oxazoles.11d Although various
heterocycles were successfully synthesized by these metal-free
decarboxylative procedures, some associated problems still
limited their applications, such as low yields of products, poor
group tolerance, and excess use of amino acids and iodides.
Moreover, only amino acids were reported as activated
carboxylic acids for the synthesis of heterocycles. Thus,
exploration of new activated carboxylic acids and development
of new methods for heterocycle constructions are still desirable.
aromatic aldehydes.6a The Rostamizadeh reported
a one-pot
P2O5-promoted cyclization of acylhydrazide and acyl halides gave
2,5-disubstituted 1,3,4-oxadiazoles.6b Recently, Wu and co-
workers developed an iodine-catalyzed direct annulation of
acylhydrazides with methyl ketones containing an unexpected
C–C cleavage procedure6c and
a copper-catalyzed domino
protocol cyclization of isatins and hydrazides.6d
In consideration of these, we envision that
a a-imine acid
intermediate instead of the previous reported imine, might make
the decarboxylative construction of heterocycles easy to realized
(Scheme 1). Herein, we report a KI-catalyzed cyclization and
decarboxylation strategy to construct 2,5-disubstituted 1,3,4-
⇑
Corresponding authors. Tel.: +86 917 356 6589.
E-mail addresses: gaopeng_hx@bjwlxy.edu.cn (P. Gao), mingjinfan@163.com
oxadiazoles from
a-keto acids and acylhydrazines using tert-
butyl hydroperoxide (TBHP) as a terminal oxidant.
(M. Fan).
http://dx.doi.org/10.1016/j.tetlet.2016.09.007
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
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2
P. Gao et al. / Tetrahedron Letters xxx (2016) xxx–xxx
O
N
or triethylamine instead of Na2CO3 as a base, led to a decrease of
Cl
OH
F
O
HN
the product yields (Table 1, entries 3–5). A screen of iodides includ-
ing KI, NaI, and nBu4NI revealed that KI was the most powerful cat-
alyst to promote the cyclization procedure (Table 1, entries 6–8).
To improve the yield of 3a, various solvents were tested in this
reaction. Among these solvents, including 1,4-dioxane, toluene,
DMSO and dimethyl carbonate (DMC), 1,4-dioxane proved to be
the best choice (Table 1, entries 9–11). When other oxidants, such
as di-tert-butyl peroxide (DTBP), K2S2O8 or mCPBA, replaced TBHP
in this transformation, lower yields of products were isolated
(Table 1, entries 12–14). In addition, decreasing the temperature
to 100 °C or 80 °C apparently reduced the product yield (see Sup-
porting information, Table S1). Eventually, the optimized reaction
conditions was chosen 1a (0.5 mmol), 1 equiv of 2a, 1.2 equiv of
Na2CO3, 4 equiv of TBHP and 10 mol % of KI in 3 mL of 1,4-dioxane
at 120 °C for 5 h in a sealed tube.
Cl
H
N
H
Cl
O
O
N
O
N
N
N
N
O
CPOMDCPO,
insecticidal activitie
Raltegravir,
treatment of HIV infection
O
O
N
N
N
N
HIPAO,
PPTO,
OLED material
fluoroscent material
Figure 1. Representative examples of functional 2,5-disubstitutied 1,3,4-
oxadiazoles.
Previous works:
With the optimized reaction conditions in hand, the scope and
generality of this KI-catalyzed oxidative cyclization of acylhydrazi-
nes with phenylglyoxylic acid was examined (Table 2). Benzoyl
hydrazines bearing electronically neutral (–H, –Me and –tBu) and
electron-donating substituents (–OMe) proceeded smoothly to
give the corresponding 2,5-disubstitutied-1,3,4-oxadiazoles in
moderate to excellent yields (59–90%, 3a–3f). Comparing to the
meta- or para-position alkyl substituent in the phenyl ring of ben-
zoyl hydrazines, highly reduced yield of the product 3c might be
owing to the steric effect of o-methyl group. Benzoyl hydrazines
bearing electron withdrawing groups on the aromatic ring, such
as 4-Cl and 4-CF3, reacted well and afforded the corresponding
products in the yield of 81% and 55% respectively (3g, 3h). Unfor-
tunately, when 4-nitrobenzoyl hydrazines were employed in the
reaction, only a trace amount of product was detected (3i). Gratify-
ingly, a fused ring or heteroaryl acylhydrazine was successfully
converted to corresponding products (3j–3m), which usually have
unique optical properties.7d,e Furthermore, this method was
extended to alkyl acylhydrazines, such as phenylacetic hydrazide
and acetohydrazide. To our satisfaction, both moderate yields of
products were obtained (3n, 3o).
R1
R1
R2
O
NH2
R1
N
Ar
R2
Nu'
N
CO2H
R2 CO2H
Nu
-CO2
-CO2
Nu
This work:
R1
HO2C
R2
O
R1 HN
N
R2
NH
N
N
O
R1
R2
O
O
HO2C
H2N
Scheme 1. Metal-free decarboxylative strategies to synthesize heterocycles.
Results and discussion
Initially, our efforts were focused on the optimization of the
reaction conditions using benzohydrazide 1a and phenylglyoxylic
acid 2a as the model substrates. Delightfully, the desired 2,5-
diphenyl-1,3,4-oxadizole 3a was obtained in 58% yield using
4 equiv of TBHP in the presence of 10 mol % of KI at 120 °C in
1,4-dioxane for 5 h (Table 1, entry 1). Encouraged by this result,
various reaction parameters such as bases, iodides, oxidants, sol-
vents and temperatures were evaluated. When 1.2 equiv of Na2CO3
was employed in the reaction, yield of 3a was greatly improved to
88% (Table 1, entry 2). Use of other bases, such as K2CO3, pyridine
Encouraged by these results with acylhydrazines, we then
investigated the cyclization and decarboxylation reaction with
various
a-keto acids. As shown in Table 3, a series of substituted
a-keto acids bearing diverse functional groups and substitution
Table 1
Optimization of the reaction conditionsa
O
O
NH2
OH
O
conditions
N
H
O
N
N
2a
3a
1a
Entry
Iodide
Base
Oxidant
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
KI
KI
KI
KI
KI
NaI
I2
nBu4NI
KI
KI
—
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DTBP
K2S2O8
mCPBA
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
Toluene
DMSO
DMC
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
58
88
82
60
0
80
61
57
13
0
Na2CO3
K2CO3
Pyridine
Et3N
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
9
10
11
12
13
14
KI
KI
KI
KI
80
0
84
49
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), 10 mol % iodide, 1.2 equiv base, 4 equiv oxidant in solvent (3 mL) at 120 °C for 5 h.
Isolated yield.
b
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P. Gao et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Table 2
20 mol% KI
O
O
4 equiv TBHP
1.2 equiv Na2CO3
Substrate scope of acylhydrazinesa,b
OH
NH2
N
O
H
(a)
O
N N
2 equiv TEMPO
O
O
1, 4-dioxane,120 oC
1a
2a
KI, T BHP, Na2CO3
1,4-dioxane, 120 oC
O
NH2
Ph
R
Ph
3a,
71%
OH
R
N
H
Ph
20 mol% KI
N N
3a-o
O
O
O
4 equiv TBHP
1.2 equiv Na2CO3
1a-o
2a
N N
H H
O
(b)
(c)
1, 4-dioxane,120 o
C
N
N
4a
O
O
O
O
Ph
Ph
Ph
Ph
3a, N. D.
N
N
N N
N
N N
N
O
N
20 mol% KI
3a, 88%
3b, 85%
3c
3d, 90%
4 equiv TBHP
1.2 equiv Na2CO3
, 59%
NH
O
tBu
MeO
Cl
Ph
F3C
Ph
1, 4-dioxane,120 o
C
N N
CO2H
5a
O
O
O
O
Ph
3a, 92%
N N
N N
N N
N N
Scheme 2. Control experiments.
3f, 83%
3g
3h, 55%
3e, 76%
, 81%
O2N
O
O
N N
Ph
N
Ph
N
O
O
Ph
Ph
CO2
O
S
TBHP
I
R2
N N
R
O
3
N N
3k
, 91%
N
N
3l
, 80%
3i, trace
3j, 71%
I
O
Ph
O
O
O
Ph
Ph
O
N N
N
N N
IO
6
N N
3m, 42%
N N
3o, 65%
O
R2
H
N
HO2C
R
O
R
R2
O
8
1
3n
, 55%
N
O
HO2C
R
5
H2N
b
R2
a
N
H
s
H
N
e
e
a
s
I
R2
Reaction conditions: 1 (0.5 mmol), 2a (0.5 mmol), 0.05 mmol KI, 0.6 mmol
Na2CO3, 2 mmol TBHP, in 1,4-dioxane (3 mL) at 120 °C for 5 h.
a
b
2
N
b
O
Isolated yield.
O2C
R
7
Scheme 3. Possible mechanism.
Table 3
Substrate scope of
a
-keto acidsa,b
withdrawing groups, including –Cl, –Br and –CF3, resulted in
51–77% yields (3g0–3t). Moreover,
-keto acids containing a naph-
O
O
KI, TBHP, Na2CO3
1,4-dioxane,120 o
a
R2
OH
O
R1
O
NH2
R2
R1
N
H
thyl or furan moiety furnished the 2,5-disubstituted-1,3,4-oxadia-
C
O
N
N
3
zoles in useful yields. (3j0, 3k0).
1
2
Notably, different cyclization partners might give the same 2,5-
disubstituted 1,3,4-oxadiazoles. For example, 3b was produced in
85% from 4-methyl-benzoyl hydrazine and phenyl glyoxylic acid,
and also could be formed in 77% yield (3b0) via cyclization and
decarboxylation of benzoyl hydrazine with 4-methyl-phenyl gly-
oxylic acid. Mostly, similar yields of products were obtained
through both of the approaches in this reaction system. Only a
small part of products were isolated with big gaps in yields
between the two ways (3j and 3j0, 3k and 3k0). Thus, depending
on different situations, a potentially viable strategy should be
selected in the future synthesis of 2,5-disubstituted 1,3,4-
oxadiazoles.
O
O
Ph
Ph
Ph
Ph
Ph
N
N N
N
N
N
3c', 74%
3p, 76%
3b', 77%
OMe
Cl
O
O
O
Ph
N
OMe
N N
N
N
N
3q
, 75%
3f'
3g'
, 79%
, 77%
Br
Br
CF3
O
O
O
Ph
Ph
S
N N
N
N
N
N
With the scope of the method established, we then focused on
the possible mechanistic pathways of this reaction. When 2 equiv
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), a known radical-
trapping reagent, was employed in the reaction, 2,5-diphenyl-
1,3,4-oxadiazole (3a) was obtained in 71% yield (Scheme 2, a). This
result suggested the mechanism probably did not follow a radical
reaction pathway. When treatment of N0-benzoylbenzohydrazide
(4a) was performed under the standard conditions, no desired
products were detected, thus suggesting that 4a were not formed
in the reaction. In the following experiments, the presynthesized
acylhydrazone 5a was treated under the optimized conditions
and an excellent yield of 3a was obtained. This result showed 5a
may be the key intermediate in this transformation.
3r, 72%
3h', 57%
3s, 51%
O
O
Ph
Ph
O
N
N
N
3k', 32%
N
3j', 54%
a
Reaction conditions: 1 (0.5 mmol), 2a (0.5 mmol), 0.05 mmol KI, 0.6 mmol
Na2CO3, 2 mmol TBHP, in 1,4-dioxane (3 mL) at 120 °C for 5 h.
b
Isolated yield.
On the basis of above observations and previous literature,10,11
a possible mechanism is illustrated in Scheme 3. Initially, keto
patterns produced the desired products in yields arranging from
32% to 88%. Aryl glyoxylic acids with electron-donating groups,
such as methyl and methoxyl, were well tolerated and gave good
yields of products (3b0–3r). Pleasingly, substrates with electron-
amine condensation between
a-keto acid 1 and acylhydrazine 2
would take place formed intermediate 5. Then, hypoiodite, which
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P. Gao et al. / Tetrahedron Letters xxx (2016) xxx–xxx
generated from oxidation of iodide with TBHP,12 iodinated imine
nitrogen atom of 5 to produce the cyclization intermediate 7.
Finally, decarboxylative aromatization of 7 and releasing of iodide
gave the desired 2,5-disubstituted 1,3,4-oxadiazoles.
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Conclusion
In summary, we have established a one-pot cyclization proce-
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oxadiazoles between acylhydrazines and
a-keto acids using
KI/TBHP system, with a strategic transition-metal-free decarboxy-
lation reaction that drives the cyclization step. A proposed reaction
pathway may involve a tandem keto amine condensation, followed
by oxidative cyclization and decarboxylative aromatization.
Further studies on the development of transition-metal-free decar-
boxylation and cyclization reactions for the formation of heterocy-
cles are underway in our laboratory.
Acknowledgments
We thank Shannxi Provincial Key Laboratory Project (No.
15JS004) and Baoji University of Arts And Sciences (No. ZK15046)
for financial support.
Supplementary data
Supplementary data (experimental procedures, characteriza-
tion data, copies of the 1H NMR, 13C NMR spectra for the products
and X-ray data of 3a) associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.09.
007.
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Please cite this article in press as: Gao, P.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.09.007