Br?nsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water: Access to oxygen-bridged tetrahydro-1,4-oxazepines

Article abstract of DOI:10.1039/c8cc00881g

A Br?nsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides and isoxazoles with water is described. This process provides atom-economical access to oxygen-bridged tetrahydro-1,4-oxazepines, where the bridged oxygen atom originates from water. The unique property of the Br?nsted acid shows distinct chemoselectivity from the corresponding gold-catalyzed cycloadditions.

Full text of DOI:10.1039/c8cc00881g

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Brønsted acid-catalyzed formal [5+2+1] cycloaddition of ynamides
and isoxazoles with water: access to oxygen-bridged tetrahydro-
1,4-oxazepines†
Received 00th January 20xx,
Accepted 00th January 20xx
Yingying Zhao,a,b Chunxiang Wang,a Yancheng Hu,*a and Boshun Wan*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
Brønsted acid-catalyzed formal [5+2+1] cycloaddition of Brønsted acid a highly economical alternative7 for the gold-
ynamides and isoxazoles with water is described. This process catalyzed cycloadditions of ynamides. However, despite these
provides an atom-economical access to oxygen-bridged advances, the development of acid-catalyzed cycloaddition of
tetrahydro-1,4-oxazepines, where the bridged oxygen atom is ynamides that can show distinct reactivity from that of gold
originated from water. The unique property of Brønsted acid catalysis is still desirable and of great importance.
a) Gold and acid catalysis yield the same product:
shows distinct chemoselectivity from the corresponding gold-
R2
EWG
catalyzed cycloadditions.
N
R1
R1
R2
Over the past decade, the cycloaddition of ynamides with
various unsaturated precursors has created remarkable
opportunities for the assembly of complex amino-
functionalized heterocycles. Much progress has been achieved
by transition-metal catalysis, gold catalysis in particular.1‒5 For
example, Liu and coworkers described an elegant gold-
catalyzed [2+2+2] cycloaddition of ynamides with nitriles for
the divergent synthesis of pyrimidines3a and pyridines3b
(Scheme 1a, left). Besides, various unsaturated N–O
heterocycles such as 1,4,2-dioxazoles,4a 1,2,4-oxadiazoles,4b
and 4,5-dihydro-1,2,4-oxadiazoles4c have also served as three-
atom synthons in the gold-catalyzed cycloadditions with
ynamides, thereby yielding amino-substituted oxazoles and
imidazoles (Scheme 1a, right). In comparison with the
significant advances achieved in gold catalysis, it was not until
[Au]
or
Tf2NH; TfOH
N
N
R
R2
N
EWG
EWG
[Au] or Tf2NH
EWG
R1
R1
N
or
N
N
O
R2
RCN
R2
N
R
Y
R1
X
R
X
R
[3+2]
[2+2+2]
EWG
N
N
R
b)
Gold and acid catalysis yield dif f erent products:
O
R1
R3
R4
[Au]
EWG
EWG
[3+2]
N
R2
R1
N
R2
N
R2
N
H
R1
EWG
H O
Tf2NH,
2
+
O
N
ꢀ ꢀ
ꢀ ꢀ
O
formal
[ 5+2+1]
R2 EWG
N
R3
R4
O
N
R
R3
R4
[Pt]
N
O
T his work
R
[5+2]
R4
R1
R3
R
very recently that metal-free catalytic cycloaddtions of Scheme 1. Gold versus Brønsted acid-catalyzed cycloadditions
ynamides were disclosed.5,6 The groups of Sun,5a Chang,5b of ynamides.
Maulide5c,d and Tang5e independently found that nitriles and
Isoxazoles are an important class of heterocycles in organic
ynamides could undergo [2+2+2] cycloadditions in the
synthesis,8 and have recently proved to be versatile precursors
presence of Tf2NH or TfOH to generate pyridines and
for the synthesis of other valuable scaffolds.9-12 In this context,
pyrimidines (Scheme 1a, left). Later, we reported the Tf2NH-
Ye and co-workers discovered that 3,5-dimethylisoxazole could
catalyzed [3+2] cycloadditions of ynamides with dioxazoles6a or
undergo [3+2] cycloaddition with ynamides in the presence of
oxadiazolones,6b enabling an easy access to oxazoles and
a gold catalyst to deliver 2-aminopyrroles (Scheme 1b, upper
imidazoles (Scheme 1a, right). All of these transformations
left).10a,b Very recently, the same group reported a platinum-
yielded the same products as that of gold catalysis, making the
catalyzed formal [5+2] cycloaddition of isoxazoles with
ynamide to afford unexpected 1,3-oxazepines (Scheme 1b,
bottom left).10c Encouraged by these results and in
continuation of our work on the cycloaddition reactions,6,13 we
envisioned that
a Brønsted acid-catalyzed intermolecular
cycloaddition of ynamides with isoxazoles could provide access
to 1,4-oxazepines. Intriguingly, by introducing small amount of
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water, an unexpected formation of oxygen-bridged 1,4- reaction could also be extended to 2-thienyl-derived substrate
oxazepine derivative was observed (Scheme 1b, right), in 1g, which was transformed into the product 3g in 73% yield.15
which isoxazole acts as a five-atom building unit to construct However, unfortunately, this cycloaddition was not suitable for
the seven-membered ring and water acts as the bridged alkyl-substituted ynamides (see the ESI†
).16 The scope of the
oxygen atom source. To the best of knowledge, transition- electron-withdrawing groups on the nitrogen atom was also
metal-free catalytic three-component formal cycloaddition to investigated. The treatment of N-SO2Ph derived ynamide 1h
construct bridged skeletons has not been disclosed yet. Herein, with the standard conditions led to the formation of 3h in 75%
we report our results on the Brønsted acid-catalyzed [5+2+1] yield. Besides, halogen substituents (e.g. F, Cl) on the
cycloaddition of ynamides and isoxazoles with water to afford phenylsulfonyl group were also compatible with the process,
oxygen-bridged
potentially bioactive molecules.14 The unique property of Ynamides
Brønsted acid, as well as introducing a small amount of water, trimethylphenylsulfonyl
shows distinct chemoselectivity from the corresponding gold- groups proved to be suitable substrates as well.17 Notably,
catalyzed cycloadditions.10
when alkyl substituent such as Me was employed in the
tetrahydro-1,4-oxazepines,
which
are providing the desired products in moderate yields (3i
possessing sterically hindered 2,4,6-
and 2-naphthylsulfonyl (1l)
, 3j).
(
1k)
We commenced our investigation with the optimization of sulfonyl group (N-Ms), the cycloaddition proceeded smoothly
the reaction conditions by choosing ynamide 1a and 3,5- to generate the corresponding product 3m in 54% yield. The
dimethylisoxazole 2a as the model substrates (see ESI
details). The optimized reaction conditions were found to be that the electronic and steric factors on the benzyl group of
Tf2NH as catalyst (15 mol%), 1.5 equivalents of 1a, 1.0 ynamides had no significant impact on the reaction (3n 3u),
†
for full substituents on the R2 group were then screened. It was found
–
equivalent of H2O, and -10 °C for 6 h. The structure of 3a was furnishing the corresponding products in 51‒77% yield.
unambiguously confirmed by single-crystal X-ray diffraction Similarly, ynamide 1v bearing a 1-naphthylmethyl group in R2
analysis of its analogue 3g 15
.
It is noteworthy that no 2- underwent the cycloaddition efficiently to deliver 3v in 65%
aminopyrrole was detected during the optimization (including yield. Pleasingly, this transformation was also applicable to N-
the one in the absence of water), revealing the distinct nBu ynamide 1w 17
thus allowing the assembly of oxygen-
catalytic activity of acid catalysis from that of gold catalysis.10
bridged heterocycle 3w in 46% yield. Disappointedly, the more
,
electron-rich oxazolidinone-substituted ynamide failed to
participate in the reaction owing to its relatively lower
reactivity (see the ESI†). Finally, the feasibility of extending the
protocol to other isoxazoles was demonstrated. Replacing the
methyl substituents in R3 and R4 to ethyl groups generated 3x
in a satisfactory yield. Upon exposure of 3-(chloromethyl)-5-
methylisoxazole 2c to the standard conditions, the target
product 3y was isolated in 40% yield. To our surprise, an
unexpected dehydrochlorinated product 3z was obtained
when
4-(chloromethyl)-3,5-dimethylisoxazole
2d
was
employed as the reactant. 3,4,5-Trimethylisoxazole was also a
suitable substrate, generating product 3ge (dr = 5:1) in 48%
yield.
Scheme 2. Substrate scope. The reactions of ynamides 1a‒1f
Scheme 3. Further transformations.
with 3,5-dimethylisoxazole 2a were conducted at -10 °C, while
others were performed at -20 °C. Detailed conditions were
To illustrate the utility of this method, a scale-up experiment
given in the ESI†. Isolated yields are reported.
(1.36 mmol) was carried out for ynamide 1g and isoxazole 2a
(see the ESI†). The reaction proceeded smoothly in the
With the optimized conditions in hand, we then examined
the generality of this process and the results are summarized
in Scheme 2. A variety of aryl-substituted ynamide were well
tolerated regardless of their steric and electronic properties,
presence of 7.5 mol% Tf2NH, providing 3g in 58% yield. The
resulting product can undergo hydrolysis to give highly
functionalized dihydrooxazole 5g in CDCl3 at 50 °C [Scheme 3,
eqn (1)].18 Besides, we found that various dihydrooxazoles
5
leading to the desired products 3a–3f in 40–72% yield. The
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can be directly synthesized from ynamides and isoxazole via a attack the carbonyl group (C7) of Int-3 to facilitate the
one-pot two-step process [Scheme 3, eqn (2)].
intramolecular cyclization, yielding seven-membered ring Int-8,
which then undergoes acid-catalyzed O,O-acetal formation to
deliver
the oxygen atom bridging C3 and C7 in
3
(pathway B). The 18O-labeling experiment implies that
3
originates from water,
thus supporting the possibility of pathways A and B, and ruling
out the pathway C which involves the attack of H2O onto the
carbocation of Int-3 to form the oxygen atom bridging C2 and
C7 in the product. This cationic-type mechanism is also
supported by the failure of alkyl-derived ynamides in the
cycloaddition,16 because alkyl-substituted carbocation (R1
alkyl) in Int-3 is less stable than the aryl analogue (R1 = aryl).
=
Scheme 4. Mechanistic investigations.
To shed light on the reaction mechanism, some control
experiments were conducted. The gold catalyst (IPrAuNTf2)
could not promote this cycloaddition under the optimized
conditions [Scheme 4, eqn (3)]. However, when the reaction
was performed at 25 °C, the formation of 2-aminopyrrole 4g
instead of 3g was observed,10 revealing Brønsted acid gives a
distinct reaction pathway from that of gold catalyst in this
process [Scheme 4, eqn (3)]. When D2O was engaged in the
reaction in the presence of TMSOTf, which can rule out H/D
exchange between Brønsted acid and D2O, the deuterium
atom was partially incorporated into 3g (42% D) at C2 and C6
positions respectively [Scheme 4, eqn (4), right]. Additionally,
when the reaction was carried out with H218O, 18O-labeled
product was obtained in 65% yield with 69% 18O incorporation
[Scheme 4, eqn (4), left]. The splitting of the carbon signal at
104.9 (C3, Δδ = 0.025 ppm) and 103.8 ppm (C7, Δδ = 0.023
ppm) was observed in the 13C NMR spectrum of 3g-[18O], thus
explicitly illustrating that the bridged oxygen atom originates
Scheme 5. Proposed reaction mechanism.
In summary, we have developed a Brønsted acid-catalyzed
formal [5+2+1] cycloaddition of ynamides and isoxazoles with
water, providing an atom-economical access to oxygen-
bridged tetrahydro-1,4-oxazepines with the bridged oxygen
atom originating from water. This protocol will not only enrich
the chemistry of ynamides but also provide important insights
into designing and developing unprecedented acid-catalyzed
cycloadditions with distinct selectivity. Studies along this
direction are currently underway in our laboratory.
from water (see the ESI†). Considering that ynamide can easily
undergo hydrolysis to produce amide in the presence of water
and acid,19 we then speculated that amide might be the
intermediate for this reaction. However, amide 6a, which was
prepared by the Tf2NH-catalyzed hydrolysis of ynamide 1a
,
failed to participate in the cycloaddition under the standard
conditions, thus excluding this hypothesis [Scheme 4, eqn (5)].
On the basis of the above experimental observations and
the precedents on the chemistry of ynamide,5,6 a plausible
Financial support from the National Natural Science
Foundation of China (no. 21572225 and 21772194) is gratefully
acknowledged.
mechanism was proposed in Scheme 5. Ynamide
protonated by Tf2NH to generate keteniminium ion Int-1 20
Then, an N-attack of isoxazole on the α-carbon atom of Int-1
yields the adduct Int-2 which further undergoes ring
1 is first
.
2
,
Notes and references
fragmentation to form carbocation Int-3 through the cleavage
of N‒O bond. A subsequent intramolecular O-attack onto the
carbocation of Int-3 furnishes the seven-membered
heterocycle Int-4. Eventually, the addition of H2O to iminium
ion (C3) and subsequent acid-catalyzed ketalization afford the
1
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the reaction.
3
4
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,
18 The configuration of dihydrooxazoles was confirmed by
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The prices of IPrAuNTf2, Ph3PAuNTf2, Tf2NH and TfOH in
Sigma-Aldrich are $320/mmol, $267/mmol, $7.3/mmol and
$0.23/mmol, respectively.
8
9
For a review see: F. Hu and M. Szostak, Adv. Synth. Catal.,
2015, 357, 2583.
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