InCl3-catalyzed 5- exo-dig cyclization/1,6-conjugate addition of N -propargylamides with p -QMs to construct oxazole derivatives

Article abstract of DOI:10.1039/c9ob02651g

An InCl₃-catalyzed atom-economic intramolecular 5-exo-dig cyclization/1,6-conjugate addition/aromatization of N-propargylamides with p-QMs to produce oxazoles tethering diarylmethane has been successfully developed. InCl₃ not only se

Full text of DOI:10.1039/c9ob02651g

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DOI: 10.1039/C9OB02651G
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InCl3-catalyzed 5-exo-dig cyclization/1,6-conjugate addition of N-
propargylamides with p-QMs to construct oxazole derivatives
Guang-Ming Nan,a Xue Li,a Tian-Yu Yao,b Ting-Xun Yan,b Li-Rong Wen,b,* and Ming Lia,b,*
Received 00th January 20xx,
Accepted 00th January 20xx
An InCl3-catalyzed atom-economic intramolecular 5-exo-dig cyclization/1,6-conjugate addition/aromatization of N-
propargylamides with p-QMs to produce oxazoles tethering diarylmethane has been successfully developed. InCl3 not only
served as Lewis acid to catalyze the cyclization of propargylic amides but also activated the carbonyl of p-QMs to achieve
the 1,6-addition process in a one-pot manner. The reaction has attractive features, including mild reaction conditions, broad
scope of substrates, good yields, and scalability.
DOI: 10.1039/x0xx00000x
the above successful examples and our ongoing interest in
heterocycle synthesis,12 we here report an InCl3-catalyzed
Introduction
intramolecular
addition/aromatization of N-propargylamides with p-QMs to
construct oxazole derivatives bearing diarylmethane group
5-exo-dig
cyclization/1,6-conjugate
The exploration of privileged structures in drug discovery is a rapidly
emerging theme in medicinal chemistry.1 Among them, oxazoles and
their derivatives are important nitrogen containing heterocycles
generally endowed with biological properties, making them a
privileged scaffold for applications in medicinal chemistry and
pharmaceuticals.2 Additionally, oxazole derivatives are important
intermediates in organic synthesis and ligands for metal catalysis.3
Therefore, continuous efforts have been devoted to the
development of novel methods for the synthesis of novel oxazole
derivatives.2
a
(Scheme 1c). Herein InCl3 not only served as Lewis acid to catalyze
the cyclization of N-propargylamides but also activated the carbonyl
of p-QMs in one pot. In this transformation, all the atoms
participating in the reaction enter the product and no by-product are
formed. This ene–ene reaction is useful method for carbon–carbon
bond formation with high atom-economy.
A rapidly increasing recognition of the rich and fascinating
chemistry of N-propargylamides as versatile building blocks in
organic synthesis has been brought out in the past decades.4 Various
strategies for the construction of oxazoles, particularly by the
cyclization reactions of N-propargylamides, have been developed by
metal catalysis,5 such as Au, Pd, Cu, Ag, Fe, Zn, Ru, and others.6
Recently, Xu reported a Zn(II)/Sc(III) bimetallic catalytic approach to
construct functionalized oxazole derivatives from easily available N-
propargylamides (Scheme 1a).7 Hashmi reported a convenient
protocol for the synthesis of oxazole α-hydroxy esters from N-
propargylamides and alkylglyoxylates (Scheme 1b).8 On the other
hand, para-quinone methides (p-QMs) are prominent 1,6-acceptors
due to the possibility of aromatization after the 1,6-addition.9 A lot
of nucleophiles10 have been added to p-QMs in a 1,6-fashion under a
variety of conditions. All the elegant achievements mentioned above
in the transformations of p-QMs mainly focused on transition metal
catalysis.11
Results and discussion
Optimization of reaction conditions was carried out using p-quinone
methide 1a and N-(prop-2-yn-1-yl)benzamide (2a) under various
conditions, and the results are summarized in Table 1. An initial
experiment was conducted in DCE at 70 oC in the presence of 10 mol %
Zn(OTf)2, the desired product 3aa was obtained in 55% yield after 24
To the best of our knowledge, no reaction of N-propargylamides
as nucleophiles with p-QMs has been reported so far. On the basis of
a. University and College Key Lab of Natural Product Chemistry and Application in
Xinjiang, Yili Normal University, Yining 835000, China
b. State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and
Molecular Engineering, Qingdao University of Science & Technology, Qingdao
266042, China
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Scheme 1 Strategies to construct oxazole derivatives.
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Journal Name
h (Table 1, entry 1). In contrast, the reaction couldn’t proceed when
no catalyst was employed, indicating a Lewis acid catalyst was crucial
to the transformation (Table 1, entry 2). Then other Lewis acids such
as Cu(OTf)2, Al(OTf)3, In(OTf)3 were also examined (Table 1, entries 3-
5). The results showed In(OTf)3 was the most efficient and gave 85%
yield (Table 1, entry 5). Next, InCl3 was tested. Encouragingly, the
yield of 3aa was increased to 90%, and the reaction time also was
shortened significantly to 3 h from 24 h (Table 1, entry 6). However,
both decreasing and increasing amount of InCl3 decreased the yield
of 3aa (Table 1, entries 7 and 8). No comparative yields were
obtained when other solvents such as MeCN and EtOH were used
(Table 1, entries 9 and 10). Gratifyingly, the yield of 3aa increased to
94% when the concentration of 1a was raised up to 0.2 M (Table 1,
entry 11). However, further increasing in concentration didn’t give a
superior yield (Table 1, entry 12). Screening of the reaction
temperature and the ratio of 2a/1a couldn’t provide satisfactory
results (Table 1, entries 13-16). Taking temperature and reaction
time into consideration, the entry 11 was selected as the best
reaction conditions.
Table 2 Substrate scope of N-propargylamides 2a, b
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DOI: 10.1039/C9OB02651G
Having optimized conditions in hand, substrate scope was
evaluated using a wide range of N-propargylamides 2 with 1a (Table
2). As shown in Table 2, irrespective of the electronic nature, the
aromatic N-propargylamides bearing various substituents on the
phenyl ring all successfully were transformed to the desired products
3aa−3ak in good to excellent yields. N-propargylamides bearing
Table 1 Optimization of the reaction conditionsa
catalyst
(mol %)
solvent
(mL)
entry
t (oC)
time (h)
yield (%)b
aUnless otherwise noted, all reactions were performed with 1a (0.5 mmol), 2
(0.75 mmol) and InCl3 (0.05 mmol) in DCE (2.5 mL) for 3 h. bIsolated yield
based on 1a.
1
2
Zn(OTf)2 (10)
-
DCE (2)
DCE (2)
DCE (2)
DCE (2)
DCE (2)
DCE (2)
DCE (2)
DCE (2)
MeCN (2)
EtOH (2)
DCE (1)
DCE (0.5)
DCE (1)
DCE (1)
DCE (1)
DCE (1)
70
70
70
70
70
70
70
70
70
70
70
70
50
80
70
70
24
24
24
24
24
3
55
N.R.
53
3
Cu(OTf)2 (10)
Al(OTf)3 (10)
In(OTf)3 (10)
InCl3 (10)
InCl3 (5)
4
15
5
85
electron-deficient substituents offered higher yields than that
bearing electron-rich substituents. Gratifyingly, replacing the
aromatic groups R with 2-naphthyl, piperonyl, 2-thienyl and 2-furyl,
afforded products 3al-3ao in 54-70% yields. Unfortunately, when
alkyl N-propargylamides such as Bn, CH3 and t-Bu (2p-2r) were
employed, the reaction system became sluggish and the
corresponding products were hardly observed, which troubled the
1,6-conjugate addition process.
Next, the scope of p-QMs was investigated under the standard
conditions (Table 3). As shown in Table 3, p-QMs bearing both
electron-rich and electron-deficient substituents on the phenyl ring
were smoothly transformed to their corresponding products
(3ba−3ga) in moderate to excellent yields, except for 3ha and 3ia
which had only 13% and 18% yields, respectively due to the 2-bromo
substituted one with steric hindrance and the special property of
6
90
7
3
68
8
InCl3 (20)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
3
82
9
3
63
10
11
12
13
14
15c
16d
3
complex
94
3
3
82
24
3
92
88
24
24
86
80
aUnless otherwise noted, all reactions were performed with 1a (0.2 mmol),
2a (0.3 mmol). bIsolated yield based on 1a. cWith 0.24 mmol 2a. dWith 0.2
mmol 2a. N.R. = no reaction
2 | J. Name., 2012, 00, 1-3
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alkyl N-propargylamide substrate 2r also was investigVaiteewdArbtiycleinOnsliitnue
Table 3 Substrate scope of p-QMs 1a, b
13a
1H NMR (Scheme 1, eq 4), and two cyclizatiDoOnIi:n1t0e.1r0m3e9d/Cia9tOeBs0M2615'1G
and M2'14 also were observed in a ratio of 3:97. It was found that M2
was much more than M1, which resulted in the alkyl N-
propargylamides 2p-2r couldn't provide the corresponding products.
Based on the above experimental results and relevant reports,4 a
plausible mechanism as outlined in Scheme 2 was proposed. Firstly,
InCl3 as Lewis acid could activate the triple bond of 2a to form B
through A losing HCl, followed by regioselective intramolecular 5-
exo-dig cyclization to give the intermediate C5c,5i,7 which, on
hydrolysis with HCl generated in situ, resulted in the oxazoline
intermediate M1. At the same time, InCl3 coordinated with the
carbonyl group of p-QMs to form an electrophilic intermediate D,
then 1,6-conjugate addition process of D with M1 via E gave F with
losing a proton. Then on hydrolysis with HCl generated in situ, F
produced the final product 3aa with releasing InCl3.
To demonstrate the practicality of this transformation, a scaled-up
synthesis was performed under the standard conditions (Scheme 3,
eq 1). Gratifyingly, the reaction of 1.01 g (3.1 mmol) p-QM 1a with N-
propargylamide 2a produced the corresponding product 3aa in 92%
yield (1.35 g). Additionally, the phenol part of 3aa could be easily
oxidized to the formal of quinone in the presence of 1 equiv. of 2,3-
aUnless otherwise noted, all reactions were performed with 1 (0.5 mmol), 2a
(0.75 mmol) and InCl3 (0.05 mmol) in DCE (2.5 mL) for 3 h. bIsolated yields
based on 1.
trifluoromethyl group. When R1 group of p-QMs are 2-naphthyl (3ja),
piperonyl (3ka) or 2-thienyl groups (3la), the reactions proceeded
well, affording products in 83-87% yields. Moreover, the tert-butyl
group of p-QMs were replaced with other alkyl groups such as methyl
and isopropyl or phenyl group, the reactions proceeded smoothly,
affording the desired products 3ma-3oa in 72-80% yields.
The structures of all new compounds 3 were identified by their IR,
1H NMR, 13C NMR and HRMS spectra, and unequivocally confirmed
by X-ray diffraction analysis of single-crystal of 3ad (see the
Supporting Information for details).
Some control experiments were conducted to elucidate the
reaction mechanism. Firstly, to explore the active intermediate, an
in situ 1H NMR experiment with 2a was performed, and two
cyclization intermediates M113 and M214 (1:2.5) were observed
(Scheme 1, eq 1). Then M1 and M2 were prepared using N-
propargylamide 2a in the presence of Zn(OTf)2 or InCl3, respectively
(see SI). Next, the isolated M1 and M2 were employed to react with
1a, respectively. The results revealed that without any catalyst the
reaction of M1 with 1a did not occur, but provided 3aa in 80% yield
by InCl3 catalysis (Scheme 1, eq 2). In contrast, the reaction of M2
with 1a afforded only trace 3aa under the standard conditions
(Scheme 1, eq 3), which suggested that p-QM 1a could capture M1
quickly, preventing the isomeration of M1 to M2. The above
experimental results indicate that the real active intermediate is M1,
and InCl3 not only catalyzes the cyclization of N-propargylamides but
also promotes the 1,6-conjugate addition process. Additionally, an
Scheme 2 Control experiments.
Scheme 3 Plausible mechanism.
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4
5
Scheme 4 Gram-scale and follow-up reactions.
dichloro-5.6-dicyano-1,4-benzoquinone (DDQ) at room temperature
within 10 min, affording compound 4 in 90% yield (Scheme 3, eq 2).
Conclusions
In summary, we have successfully developed a novel, simple and
economical, environmentally friendly one-pot reaction to synthesize
novel functionalized oxazoles tethering diarylmethane in good to
excellent yields. It is the first report of preparation of title
compounds by use of N-propargylamides with p-QMs promoted by
InCl3 catalysis, in which InCl3 plays dual roles: catalyzing the
cyclization of N-propargylamides to give oxazoles, and activating the
p-QMs to achieve the 1,6-addition. The reaction pathway involves
intramolecular
5-exo-dig
cyclization/1,6-conjugate
addition/aromatization. The reaction has attractive features,
including mild reaction conditions, environmentally friendly, high
chemo- and regioselectivities, broad scope of substrates, good yields,
and scalability. Undoubtedly, this domino synthetic strategy provides
a convenient and green way to construct the target molecules in an
atom-economic manner.
6
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Conflicts of interest
The authors declare no competing financial interest.
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Acknowledgements
We thank Natural Science Foundation of China (21572110) and the
Natural Science Foundation of Shandong Province (ZR2019MB010)
for financial support.
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