Metal-Free [3,3]-Sigmatropic Rearrangement/[3+2] Annulation Cascade of N-Phenoxy Amides with Terminal Alkynes for the Diastereoselective Synthesis of trans-Dihydrobenzofurans

Article abstract of DOI:10.1002/adsc.201900527

An efficient synthetic route for the diastereoselective construction of trans-dihydrobenzofurans via cascade [3,3]-sigmatropic rearrangement/[3+2] annulation sequences has been developed. This protocol represents a metal-free transformation of the promisi

Full text of DOI:10.1002/adsc.201900527

Title: Metal-Free [3,3]-Sigmatropic Rearrangement/[3+2] Annulation
Cascade of N-Phenoxy Amides with Terminal Alkynes for the
Diastereoselective Synthesis of trans-Dihydrobenzofurans
Authors: Fu-Xiaomin Liu, Weijie Chen, Guoxun Zhu, Zhi Zhou, Hui
Gao, and Wei Yi
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201900527
Link to VoR: http://dx.doi.org/10.1002/adsc.201900527

10.1002/adsc.201900527
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Metal-Free [3,3]-Sigmatropic Rearrangement/[3+2] Annulation
Cascade of N-Phenoxy Amides with Terminal Alkynes for the
Diastereoselective Synthesis of trans-Dihydrobenzofurans
Fu-Xiaomin Liu,† Weijie Chen,† Guoxun Zhu, Zhi Zhou,* Hui Gao and Wei Yi*
Key Laboratory of Molecular Target & Clinical Pharmacology and the State Key Laboratory of Respiratory Disease,
School of Pharmaceutical Sciences & the Fifth Affiliated Hospital, Guangzhou Medical University, Guangzhou,
Guangdong 511436, P. R. China
Fax: (+86)-20-37104196; phone: (+86)-20-37104196; e-mail: zhouzhi@gzhmu.edu.cn; yiwei@gzhmu.edu.cn
†These authors contributed equally to this work.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract: An efficient synthetic route for the catalyst.[8] Considering the good reactivities of
diastereoselective
dihydrobenzofurans
rearrangement/[3+2] annulation sequences has been
developed. This protocol represents metal-free
transformation of the promising N-phenoxy amides with
terminal alkynes and features mild conditions, good
functional group compatibility, and practical synthetic
potential.
construction
via cascade
of
trans-
ODGs-containing substrates and in a search for
innovative reaction models, we became interested
in developing the novel metal-free catalytic
system for the efficient conversion of N-phenoxy
amides into meaningful structures,[9] which was
more practical and environmentally friend but
rarely investigated in comparison with the
transition metal-catalyzed transformations.[10]
[3,3]-sigmatropic
a
Keywords:
Diastereoselectivity;
[3,3]-sigmatropic
rearrangement; N-phenoxyacetamides; terminal alkynes;
dihydrobenzofurans.
Scheme 1 C-H functionalization modes of N-phenoxy
amides.
Dihydrobenzofuran represents
a
privileged
structural fragment widely existing in natural
products and biologically active molecules.[1] As
a consequence, continued efforts have been
devoted to develop efficient and practical
protocols including transition metal (TM)-
catalyzed C–H functionalizations and domino
cyclization reactions for the synthesis of this
scaffold.[2] Despite the notable advances, further
development of novel synthetic approaches for
the
construction
of
dihydrobenzofuran
Taking advantage of metal-free annulations
and in combination with our continuing efforts to
construct the highly functionalized heterocyclic
skeletons,[3i,3m,4g,7c,11] we would like to disclose
herein the diastereoselective synthesis of trans-
dihydrobenzofurans via base-mediated cascade
[3,3]-sigmatropic rearrangement/[3+2] annulation
sequences of N-phenoxy amides with terminal
alkynes (Scheme 1b). This transformation
frameworks, in particular, to construct highly
functionalized dihydrobenzofurans with good
stereoselectivity is still in demand.
N-Phenoxy amides have been developed as
versatile substrates containing the oxidizing
directing group (ODG), which participates in
diversified transition metal-catalyzed C–H
functionalization with different coupling partners
including alkynes,[3] alkenes,[4] heteroarenes[5] and
diazo compounds[6] to generate a variety of ortho-
substituted phenols or oxygen-containing
heterocycles (Scheme 1a).[7] Among these
transformations, the O-N bond of N-phenoxy
amides is easily cleaved by low valent transition
metals, leading to the regeneration of the active
featured
high
atom
economy,
good
diastereoselectivity and mild reaction conditions,
representing a new approach for the direct
construction of valuable 2-amino substituted
trans-dihydrobenzofuran framework.
At the outset of our investigation, N-
phenoxyacetamide (1a) and methyl propiolate (2a
)
1
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10.1002/adsc.201900527
Advanced Synthesis & Catalysis
moderate to good yields (3a-h). The structure of
were chosen as the model substrates to react
under the base-mediated conditions. To our
delight, the desired 2-amino substituted
dihydrobenzofuran derivative 3a was furnished in
30% yield under the initial conditions with the
assistance of DMAP (Table 1, entry 1). It was
complex 3d was further confirmed by X-ray
diffraction analysis (CCDC 1410799).[12] Notably,
ethyl propiolate could also be totally converted
into the methyl ester product 3a through
interesterification under the standard conditions.
Moreover,
phenoxyacetamides
when
meta-substituted
were employed,
N-
two
noteworthy that 3a was formed in
diastereoselective manner, thus giving the
specific trans-configuration via [3+2]
a
inseparable regioisomers were obtained and the
ratios was largely dependent on the nature of the
substituent at the meta-position (3e-g). In
a
annulation. Further screening of various bases
and solvents suggested that NH3·MeOH was ideal
for the [3+2] process, since other organic or
inorganic bases and solvents all gave inferior
results (entries 2-13). After a number of trials to
screen the reaction temperature, the amounts of
NH3·MeOH, and the proportion of substrates (see
Table S1 in Supporting Information for details),
we were pleased to identify the optimal
addition, we were pleased to find that the
naphthalene substrate was a viable reactant to
provide the desired trans-dihydrobenzofuran 3i in
88% yield. Other substrates with different
substituents on the nitrogen atom were also tested
under the standard conditions. The results
revealed that N-phenoxypropionamide reacted
smoothly to yield the desired dihydrobenzofuran
conditions as follows: 1a (1 equiv), 2a (3 equiv) 3j, while isopropionyl- and pivaloyl-substituted
N-phenoxy amides were failed to provide the
dihydrobenzofuran products, which was probably
due to the different steric hindrances between
these substituents.
and NH3·MeOH (1 equiv) in MeOH at 50 °C for
48 h under an atmosphere of air, furnishing the
desired trans-dihydrobenzofuran product 3a in 85%
yield.
Table 1 Optimization of reaction conditions.[a]
Scheme 2 Scope of substrates for the synthesis of
trans-dihydrobenzofurans.[a]
Entry
1
Base (x equiv)
DMAP (1.5)
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
Yield[b] (%)
30
0
2
Et3N (1.5)
3
DABCO (1.5)
DIPEA (1.5)
13
45
0
4
5
morpholine (1.5)
NH3·H2O (1.5)
2M-NH3·MeOH (1.5)
K2CO3 (1.5)
6
50
76
0
7
8
9
CsOAc (1.5)
41
67
10
12
15
85
10
11
12
13
14[c]
2M-NH3·MeOH (1.5)
2M-NH3·MeOH (1.5) Toluene
2M-NH3·MeOH (1.5)
2M-NH3·MeOH (1.5)
2M-NH3·MeOH (1)
THF
CH3CN
MeOH
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol) and
o
base (x equiv) in solvent (0.1 M) at 50 C for 24 h under air.
[b] 1H-NMR yield.
[c] 3 equiv of 2a was used and the reaction was conducted for
48 h.
[a]
Reaction conditions:
1 (0.2 mmol), 2 (0.6 mmol) and
o
NH3·MeOH (1 equiv) in MeOH (0.1 M) at 50 C for 48 h
[b]
under air, isolated yields were reported.
The yield with
With the established reaction conditions in
hand, we next examined a variety of N-phenoxy
amides as well as terminal alkynes to demonstrate
the scope and generality of this transformation.
As shown in scheme 2, various N-
phenoxyacetamides bearing different substituents
at either para-, meta- or ortho-position were fully
ethyl propiolate was reported in the parentheses. [c] The ratio
was determined by 1H-NMR.
To better probe the versatility of this protocol
for the diastereoselective synthesis of trans-
dihydrobenzofurans, a diverse array of alkynes
were further investigated. The results showed that
the electron-deficient alkynes were indispensable
tolerated to react with methyl propiolate 2a
delivering the corresponding products in
,
2
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10.1002/adsc.201900527
Advanced Synthesis & Catalysis
for following the [3+2] annulation. Gratifyingly, of complex products rather than the desired
several N,N-disubstituted propiolamides bearing dihydrobenzofuran derivative (Scheme 3b).
morpholine, pyrrolidine and alkyl substituents
readily participated in this transformation, alkyne and the N–H bond were crucial for the
leading to the facile construction of a series of [3+2] annulation. Deuterium-labeling experiment
These results implied that both the terminal
trans-dihydrobenzofuran frameworks that were was next conducted in the presence of deuterated
installed by various commonly encountered methanol. To the end, 3i was generated smoothly
functional groups including alkyl (3l
halogen (3n 3w and 3y), phenyl (3r and 3z), and 3-positions, as well as the methyl moiety
trifluoromethyl (3s), nitro (3t) and ester group (Scheme 3c).
-m, 3u), with obvious deuterium incorporation at both 2-
-q,
(
3x). It was worth noting that all products were
On the basis of our experimental observations
and precedent literatures, a plausible mechanism
obtained in a diastereoselective manner to give
the specific trans-products, demonstrating the involving the sequential [3,3]-sigmatropic
good compatibility and exclusive rearrangement/intramolecular nucleophilic
diastereoselectivity of the developed metal-free addition/cyclization process was proposed for the
[3+2] annulation.
metal-free [3+2] annulation (Scheme 4). As
demonstrated in path a, the deprotonation of 1a is
firstly occurred in the presence of base, followed
by the Michael-type addition to methyl propiolate
to give the intermediate
O–N bond is achieved by the [3,3]-sigmatropic
rearrangement process, furnishing the
intermediate , which can be easily converted
into ortho-substituted phenol via the
aromatization. Finally, the intermediate
undergoes an intramolecular nucleophilic
addition to produce the desired
dihydrobenzofuran framework. Alternatively,
when the electron-withdrawing N-
phenoxyacetamide was employed as the substrate,
a similar rearrangement process involving two
molecules of methyl propiolate was proposed for
the construction of 2,5-dihydrobenzo[b]oxepines
via the selective [4+3] annulation (path b).
A. Then, the cleavage of
B
Very interestingly, when electron-withdrawing
group-substituted N-phenoxyacetamides were
employed as the substrates to react with methyl
C
C
propiolate
2a
,
the
distinctive
2,5-
dihydrobenzo[b]oxepines were obtained as the
major products (eqn 1), revealing that the
electronic property of the substituent at the
phenyl ring had a clear influence on determining
the reaction outcome.
Scheme 3 Mechanistic studies.
Scheme 4 Plausible mechanism.
Finally, gram-scale synthesis and derivation of
the product 3d were conducted to further
demonstrate the synthetic potentiality of the
developed [3+2] annulation. As shown in Scheme
5a, a decent isolated yield of 61% was obtained
under the modified standard conditions to furnish
3d in a gram level. Further derivation of 3d
resulted in the facile formation of several useful
motifs (Scheme 5b). For example, 3d could be
converted to the 2-hydroxy benzofuran derivative
To gain more insights into the reaction
mechanism of the developed [3+2] annulation, a
series of experimental studies were then carried
out. Kinetic isotope effect was measured via the
competitive reaction of 1a and deuterium-labeled
1a-d5 with 2a, giving a low value of KH/KD = 1.13,
which revealed that the C–H bond cleavage
process was not involved in the rate-determining
step (Scheme 3a).[3a,13] Control experiment
disclosed that the internal alkyne was not
compatible for this transformation, and N-methyl
substituted 1a resulted in an inseparable mixture
5
in the presence of aqueous NaOH solution,
while the 3-(aminomethylene)benzofuran-2(3H)-
3
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10.1002/adsc.201900527
Advanced Synthesis & Catalysis
Acknowledgements
one
6 was formed in the presence of TsOH·H2O.
In addition, treatment of 3d with dilute
We thank NSFC (21877020), Guangdong Natural
Science Funds for Distinguished Young Scholar
(2017A030306031) and Natural Science Foundation
of Guangdong Province (2018A030313274) for
financial support on this study.
hydrochloric acid underwent the hydrolysis and
decarboxylation, thereby giving the product
7.
Intriguingly, exposure of 3d to PhI(OAc)2 led to
the ring-opening process furnishing the oxirane
derivative 8. Taken together, the developed [3+2]
annulation was practical for the synthesis of
functionalized trans-dihydrobenzofurans with
profound synthetic utility.
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derivatives with good functional group
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dihydrobenzofurans and the synthetic utility in
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equiv), terminal alkynes 2 (0.6 mmol, 3.0 equiv) and
NH3∙MeOH solution (2 M, 1.0 equiv) in MeOH (2.0 mL)
o
was stirred in a sealed tube at 50 C for 48 h without
exclusion of air or moisture. Afterwards, the mixture was
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concentrated. The crude product was purified by
preparative TLC to afford the corresponding products 3.
4
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10.1002/adsc.201900527
Advanced Synthesis & Catalysis
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via
5
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10.1002/adsc.201900527
Advanced Synthesis & Catalysis
COMMUNICATION
Metal-Free [3,3]-Sigmatropic Rearrangement/[3+2]
Annulation Cascade of N-Phenoxy Amides with
Terminal Alkynes for the Diastereoselective
Synthesis of trans-Dihydrobenzofurans
Adv. Synth. Catal. Year, Volume, Page – Page
Fu-Xiaomin Liu, Weijie Chen, Guoxun Zhu, Zhi
Zhou,* Hui Gao and Wei Yi*
6
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