Bifunctional Furfuryl Cations Strategy: Three-Component Synthesis of Enamidyl Triazoles

Article abstract of DOI:10.1002/chem.201504330

A new multicomponent synthesis of functionalized enamidyl triazoles starting from simple and readily available starting materials is described. A simple treatment of a dichloromethane solution of an azide, amine, and 5-bromo-2-furylcarbinol with a Lewis acid provides the enamidyl triazole in good to high yield. A triple domino sequence, formal [3+2] cycloaddition/ring-opening/amidation, is involved in this new skeleton-generating reaction.

Full text of DOI:10.1002/chem.201504330

DOI: 10.1002/chem.201504330
Communication
&
Multicomponent Reactions
Bifunctional Furfuryl Cations Strategy: Three-Component
Synthesis of Enamidyl Triazoles
[
a]
[b]
[a]
[a]
[a]
[a]
Hengtuo Yang, Jing Gou, Jiawei Guo, Dongyu Duan, Yu-Ming Zhao, Binxun Yu,*
[a]
and Ziwei Gao*
arising from the low resonance energy of the furan ring have
Abstract: A new multicomponent synthesis of functional-
ized enamidyl triazoles starting from simple and readily
available starting materials is described. A simple treat-
ment of a dichloromethane solution of an azide, amine,
and 5-bromo-2-furylcarbinol with a Lewis acid provides
the enamidyl triazole in good to high yield. A triple
domino sequence, formal [3+2] cycloaddition/ring-open-
ing/amidation, is involved in this new skeleton-generating
reaction.
[10]
rarely been used in MCRs. Herein, we describe our develop-
ment of a three-component reaction (3-CR) of a 5-halo-furylcar-
binol, azide, and amine under Lewis acid conditions
(
Scheme 1). To the best of our knowledge, the reaction repre-
sents the first example of MCRs based on the furfuryl carbinol
recyclization, enabling the rapid construction of a diverse set
of highly functional enamidyl triazoles.
The synthesis of small polyfunctionalized heterocyclic com-
pounds remains one of the most interesting subjects in the
[1]
drug discovery process and in isolation and structural identifi-
[2–3]
cation of biological macromolecules.
Within this context,
development of highly efficient construction of heterocyclic
skeletons by combined use of multicomponent reactions
(
MCRs) and domino processes has been a frequent target for
[4]
synthetic chemistry efforts. For designing MCRs, a typical
guiding principle is to devise a novel multifunctional starting
material that is appropriately functionalized, allowing it to be
engaged in the subsequent domino process.
Scheme 1. Three-component synthesis of functionalized enamidyl triazoles.
Furans play an important role in organic chemistry not only
due to their presence as key structural scaffolds in many natu-
The derivatives containing a 1,2,3-triazole conjugated with
a a, b-unsaturated amide moiety are of great importance be-
cause they have been proven to show significant inhibition ac-
[5]
ral products and in important pharmaceuticals, but they have
also been versatile tools in synthetic chemistry and a lot of
[
11]
tivities for matrix metalloproteases (MMPs), histone deacety-
[
6]
[12]
[13]
transformation reactions have been documented for them.
The furan ring allows for its facile recyclization reactions into
lase, class I b-lactamase, and so forth. A recent synthetic
methodology for the construction of the enamidyl triazole
scaffold has been focused primarily on the 1,3-dipolar azide–
[
7]
[8]
different carbo and heterocycles, particularly furfuryl cations
have also served as well-known reactive intermediates. For ex-
ample, the Piancatelli reaction converts furfuryl carbinols into
[
14]
alkyne cycloaddition (AAC), but tedious multi-step synthesis
of precursors limits the further exploitation of its application
potential. We have disclosed that the reaction between the
furfuryl cation and an azide leads to the (E)-stereospecific pro-
[9]
hydroxycyclopentenones under acidic conditions. This rear-
rangement process has seen a significant use in complex mole-
cule synthesis. However, the high reactivity and low stability
[
15]
duction of the enonyl triazole. We next wondered about the
behavior of the furfuryl cation in the presence of another
active functionality on it, to probe if it could perform a bifunc-
tional platform for connecting other two parts in MCRs. We
chose 5-halo-2-furylcarbinol as the furfuryl cation source, envi-
saging that it could achieve a formal [3+2] cycloaddition with
an azide, and the resulting a, b-unsaturated acyl halide from
furan ring-opening could be trapped with an amine for the
construction of the enamidyl triazole.
[
a] H. Yang, J. Guo, D. Duan, Prof. Dr. Y.-M. Zhao, Prof. Dr. B. Yu,
Prof. Dr. Z. Gao
Key Laboratory of Applied Surface and Colloid Chemistry
Ministry of Education, School of Chemistry
Chemical Engineering, Shaanxi Normal University
Xi’an 710062 (P.R. China)
E-mail: yubx@snnu.edu.cn
[
b] Dr. J. Gou
School of Chemistry and Materials Science, Shaanxi Normal University
With a goal of investigating the proposed three-component
reaction, we employed the 5-bromo-furfurylcarbinol (2a), ani-
line (1a), and cyclohexyl azide (3a) as the standard substrates
Xi’an 710062 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504330.
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Communication
that more basic aliphatic amines may be able to interfere in
[
a]
Table 1. Optimization of the reaction conditions.
trapping of the acidic byproducts with Et N. A comparison of
3
the yields of primary with secondary amines indicated that the
basicity is more important than steric factors; for example, al-
though the steric profile is evidently increased from n-butyl, to
isobutyl to tert-butyl amine, the products showed similar yields
[
b]
Entry
Acid [equiv]
Solvent
T [8C]/t [min]
Yield [%]
(
4b, 4e, and 4h). Meanwhile, cyclic and more basic amines,
1
2
3
4
5
6
7
8
9
AlCl
FeCl
ZnCl
InCl
3
(1.1)
(1.1)
(1.1)
(1.1)
CH
CH
2
Cl
Cl
2
2
À20 to rt/20
À20 to rt/20
rt–reflux/120
rt–reflux/120
rt–reflux/120
rt–reflux/120
rt–reflux/120
À20 to rt/20
À20 to rt/20
À20 to rt/20
À20 to rt/20
10
5
trace
such as azetidine, pyrrolidine and morpholine, generated the
corresponding products in relatively low yields (4i, 4k, and 4l).
The yield of compound 4c also supports this trend. The termi-
nal alkyne substrate was well tolerated (4 f). The structure of
3
2
2
DCE
DCE
DCE
DCE
DCE
[
c]
3
NR
[
d]
d]
d]
Sc(OTf)
Ir(OTf)
Dy(OTf)
3
(0.5)
(0.5)
(0.5)
(1.1)/iPr
(1.1)/pyridine(1.5)
ND
ND
ND
[
[
3
4
m was determined by single-crystal X-ray diffraction analy-
3
[
16]
[
[
[
[
e]
e]
e]
e]
sis. The cis-stereospecificity of double bonds for the other
products was assigned by analogy to 4m and also with
TiCl
TiCl
TiCl
TiCl
4
4
4
4
2
EtN(1.5)
CH
2
CH
2
CH
2
CH
2
Cl
Cl
Cl
Cl
2
2
2
2
66
59
75
20
1
1
0
(1.1)/Et
(0.5)/Et
3
3
N(1.5)
N(1.5)
H NMR spectroscopy.
11
We next examined various azides using 2a and phenyl
amine (1a) or n-butyl amine (1b) as the reaction partners
[
(
a] Conditions: 5-bromo-furfurylcarbinol (2a; 1.0 mmol), cyclohexyl azide
3a; 1.1 mmol), aniline (1a; 1.1 mmol), and solvent (5 mL). [b] Isolated
yield. [c] NR = no reaction [d] ND = not detected, 2a was fully consumed
but 4a was not found in the complex mixture. [e] Only the Z isomer was
(
Table 3). Aromatic azides smoothly participated in the three-
component coupling reaction to afford the corresponding
products 4n and 4q in moderate yields. The n-butyl azide un-
derwent [3+2] cycloaddition with 2a, followed by amidation
1
observed (as determined by H NMR spectroscopy). DCE=1,2-dichloro-
ethane.
[a]
Table 2. Reaction scope of amines.
to embark on screening an effective cation-generating
reaction conditions (Table 1). Gratifyingly, the optimized
conditions were simple, in which stoichiometric TiCl in
4
the presence of Et N as base at ambient temperature
3
in dichloromethane as solvent were required for the
formation of the three-component coupling product
4
a (entry 10). Other bases, such as iPr EtN and pyri-
2
dine, also gave the desired products in similar yields
entries 8 and 9). A low product yield was observed
(
when the reaction was performed with 50 mol% of
TiCl (entry 11). Of note, only (Z)-enamide, which result-
4
ed from the furan ring opening, was observed; the (E)-
1
isomer was not detected according to the H NMR of
the crude reaction mixture. We also examined the use
of other strong Lewis acids. AlCl and FeCl (entries 1
3
3
and 2) furnished the desired products in 10 and 5%
yields, respectively. The relatively weak Lewis acid
ZnCl2 gave a trace product in 1,2-dichloroethane at
reflux when the reaction time was expanded to 2 h
(
(
entry 3). No product was observed with lnCl condition
3
entry 4), while the moisture stable rare-earth Lewis
acids, such as Sc(OTf) , Ir(OTf) , and Dy(OTf) , were also
3
3
3
not suitable for this reaction, and led to a complicated
reaction mixture at reflux (entries 5–7).
Using the standard conditions, we explored the
scope of the present three-component reaction. First,
we focused on the variation of the most readily avail-
able component, the amine (Table 2). Various aromatic
and aliphatic amines participated in the reaction with
2
a and 3a to afford the corresponding Z-enamidyl tria-
zoles 4a–4l in moderate to high yields. Electron-poor
aromatic amines, which are more reactive than aliphat-
ic amines, afforded the desired products 4g, 4j, and
[
a] Conditions: 2a (1.0 mmol), 3a (1.1 mmol), amine 1 (1.1 mmol), TiCl
4
(1.1 mmol),
Et N(1.5 mmol), and CH Cl
yields of at least two runs; in all cases only Z isomer was determined by H NMR
spectroscopy. [b] 3b was used instead of 3a. Cy=cyclohexyl.
3
2
2
(5 mL) at À208C to rt; yields given are average isolated
1
4
m in high yields. This could be ascribed to the fact
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Communication
[
a]
[a]
Table 3. Reaction scope of azides.
Table 4. Reaction scope of furfuryl alcohols.
[b]
[c]
[b]
[c]
Substrate
Product :Yield
Substrate
Product :Yield
[a] Conditions: 2 (1.0 mmol), 3a (1.1 mmol), 1 (1.1 mmol), TiCl
4
(1.1 mmol),
Et N(1.5 mmol), and CH Cl
3
2
2
(5 mL) at À208C to rt. [b] In all cases only Z
1
isomer was determined by H NMR spectroscopy. [c] Yields given are aver-
age isolated yields of at least two runs. Cy=cyclohexyl. [d] 1b was used
instead of 1a.
bonyl group and triazole were individually located at the cis
position to the double bond. Secondary alcohols with a side
chain, like methyl, ethyl, and n-butyl, generated the corre-
sponding products in similar yields compared with primary al-
cohols (4ab, 4ac, and 4ad). Homoallylic alcohol 2g was well
tolerated during the reaction, affording 4ae in 51% yield.
To further demonstrate the synthetic utility of this reaction,
the Weinreb amide 4af was synthesized by the optimal reac-
[
(
a] Conditions: 2a (1.0 mmol), 3a (1.1 mmol),
1.1 mmol), Et N(1.5 mmol), and CH Cl
1 (1.1 mmol), TiCl
4
(5 mL) at À208C to rt; yields given
3
2
2
are average isolated yields of at least two runs; in all cases only Z isomer
was determined by H NMR spectroscopy. [b] 1b was used instead of 1a.
1
[
17]
in one pot, leading to 4o in good yield. Cyclic and benzyl sec-
ondary azides also afforded the desired 3-CR products in mod-
erate yields (4r, 4s, and 4w). A tertiary azide was also reacted
with 5-bromo-furfurylcarbinol and the corresponding 4p was
obtained in a moderate yield, although prolonged reaction
time was required, presumably due to the more steric bulki-
ness of the substrate. The aromatic ring of benzyl azides bear-
ing an electron-donating (-OMe) or -withdrawing (-Br) group
were all compatible, furnishing the corresponding enamides
tion conditions (Scheme 2).
It was easily transformed into
alkyl or aryl ketones with organolithium or Grignard reagents
in high yields. Surprisingly, the aldehyde 7, which was obtained
with DIBAL-H reduction of 4af, was not quite stable. It could
easily change into the trans isomer after being passed through
an Et N/SiO column. This could be attributed to a reversible
3
2
Michael reaction, which may favorably react between more re-
active a,b-unsaturated aldehyde and a tertiary amine, affording
a thermodynamic more stable (E)-enealdehyde.
4
x and 4y in 52 and 55% yields, respectively. The double
bonds on allylic or cinnamyl substrates, which are usually con-
sidered as Lewis acid sensitive groups, had little influence on
the reaction outcome (4u, 4v, and 4z).
By varying the 5-halo-furylcarbinol component using the
readily attained starting materials 2b–2g, six further examples
of enamidyl triazole compounds 4c–4ae were synthesized
(
Table 4). The reaction of substrate 2b, which had a chlorine
group instead of bromine on the 5-position of the furan ring,
produced the product 4c in a lower yield (55%), indicating the
more electron-withdrawing Br group, as an amidation evoca-
tor, strongly increases the substrate reactivity in this type of 3-
CR. 3-Methyl-substituted furan 2c was found to be effective to
generate the corresponding trisubstituted olefin 4aa. The car-
Scheme 2. Subsequent transformations of Weinreb amides.
Chem. Eur. J. 2016, 22, 129 – 133
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Communication
The mechanism of this 3-CR transformation is proposed in
Scheme 3. The regioselective [3+2] cycloaddition between ox-
ocarbenium I and the azide could give spiro-dihydrofuran
cation III, which is configured to aromatize to triazole by b-H
elimination in the vicinity of oxygen, followed by ring-opening
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Acknowledgements
We thank NSFC (21302118, 21202074 and 21271124), the 111
Project (B14041), Natural Science Basic Research Plan in Shaan-
xi Province of China (2015JQ5140) and the Fundamental Re-
search Funds for the Central Universities (GK261001296) for fi-
nancial support. We are also grateful to Professors J.-F. Wei and
D. Xue for helpful discussions, Mr. M.-Z. Wang for help with
NMR analysis, Dr. H.-M. Sun for X-ray crystallographic analysis
of compound 4m, and Ms. J. Fan for mass spectrometric analy-
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Keywords: azides · cyclization · cycloaddition · heterocycles ·
ring opening · synthetic methods
[
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Received: October 28, 2015
Published online on November 30, 2015
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim