Synthesis of Furoxans and Isoxazoles via Divergent [2 + 1 + 1 + 1] Annulations of Sulfoxonium Ylides and tBuONO

Article abstract of DOI:10.1021/acs.orglett.9b01876

We have presented a simple and new method for the divergent assembly of furoxans and isoxazoles in which the [2 + 1 + 1 + 1] annulation reaction of sulfoxonium ylides is reported for the first time. When the reaction was performed using ^tBuONO

Full text of DOI:10.1021/acs.orglett.9b01876

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Furoxans and Isoxazoles via Divergent [2 + 1 + 1 + 1]
t
Annulations of Sulfoxonium Ylides and BuONO
Zhonghe Tang,† Yao Zhou,† and Qiuling Song*,†,‡,§
†Institute of Next Generation Matter Transformation, College of Material Sciences Engineering at Huaqiao University, 668 Jimei
Boulevard, Xiamen, Fujian 361021, China
‡Key Laboratory of Molecule Synthesis and Function Discovery, Fujian Province University, College of Chemistry at Fuzhou
University, Fuzhou, Fujian 350108, China
§State Key Laboratroy of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
S
* Supporting Information
ABSTRACT: We have presented a simple and new method
for the divergent assembly of furoxans and isoxazoles in which
the [2 + 1 + 1 + 1] annulation reaction of sulfoxonium ylides
is reported for the first time. When the reaction was performed
t
using BuONO as the nitrogen source without metal catalyst,
the desired furoxans were obtained in decent yields with wide
substrate scope. Isoxazoles bearing three carbonyl groups were
achieved when the reaction was conducted using Cu(TFA)2 as
catalyst.
eterocyclic compounds are ubiquitously embedded in
many pharmaceuticals, materials, and bioactive mole-
cules. Among myriad heterocycles, furoxans and isoxazoles are
two subclasses of privileged pharmacophores (Figure 1).
valuable furoxans and isoxazoles is of great interest in organic
chemistry and pharmaceutical chemistry.
H
Since sulfoxonium ylides could serve as carbene surrogates
of diazo compounds, considerable growth in the field of
sulfoxonium ylides has been witnessed. In the past years,
sulfoxonium ylides have been proven to be versatile building
blocks in organic synthesis to forge intricate molecules
(Scheme 1a).9 In particular, sulfoxonium ylides were
prominent coupling partners in transition-metal-catalyzed C−
H activation in which diverse ketone-functionalized (hetero)-
arenes and heterocycles were achieved varied from different
directing groups.10 Given the bipolarity of sulfoxonium ylides,
they could also be utilized as ketone precursors to afford
various ketones.11 Aside from these transformations, the C−H
cross-coupling of sulfoxonium ylides and the application of
sulfoxonium ylides as weak directing group for the synthesis of
substituted sulfoxonium ylides were also sporadically re-
ported.12 Despite this undisputed advance of sulfoxonium
ylides, only one molecule of sulfoxonium ylide was involved in
most of these excellent transformations. To our knowledge, the
transformation involving multiple molecules of sulfoxonium
ylides has yet to be explored, and the [2 + 1 + 1 + 1]
annulation of sulfoxonium ylides has never been documented
so far.
Figure 1. Examples of biologically active molecules containing
furoxan and isoxazole rings.
Furoxan derivatives have loomed as important NO donors1
and feature bioactivities such as anticancer,2 antibacterial,3
anti-inflammatory,4 and so on.5 In addition, a round of
promising furoxan-based derivatives has been investigated as
the core framework of melt-cast eutectic materials and blasting
materials.6,7 As for isoxazoles, the isoxazole ring ranks 33rd
among the 351 ring systems in commercially available drugs,8
which unarguably represents one of the most important
skeletons in pharmaceuticals and agrochemicals. Given the
particular physiological activity of furoxans and isoxazoles, the
development of simple and mild routes to assemble the
tBuONO has emerged as a brilliant nitrogen source to
streamline synthesis of N-heterocycles in synthetic chemistry.13
14
t
As part of our ongoing interest in BuONO and carbene
chemistry,15 herein we report a novel [2 + 1 + 1 + 1]
Received: May 31, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01876
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Reactions of sulfoxonium ylides
Table 1. Optimization of the Reaction Conditions
entry
catalyst
CuI
Cu(OTf)2
Pd(OAc)2
FeCl3
base
solvent
T (°C)
yield (%)
1
2
3
4
5
6
7
8
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
K2CO3
DABCO
DBU
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
Toluene
CH3CN
DCE
80
80
80
80
80
80
80
80
80
80
80
80
100
60
40
rt
84
83
85
82
86
NR
NR
NR
59
39
42
NR
84
82
90
73
86
88
90
69
9
10
11
12
13
14
15
16
DMF
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
b
17
40
40
40
40
c
18
19
annulation reaction involved of multiple molecules of
sulfoxonium ylides for the divergent synthesis of furoxans
and isoxazoles using sulfoxonium ylides as carbene precursors
d
e
20
a
t
Reaction conditions: 1 (0.3 mmol), 2 (1.5 mmol), and NaOAc (0.45
and BuONO as the nitrogen source (Scheme 1b).
To commence our study, sulfoxonium ylide 1a and tBuONO
(2) were selected as benchmark substrates to test our
assumption. When the reaction was performed in the presence
of CuI and NaOAc in dioxane under nitrogen atmosphere at
80 °C for 12 h, the isolated yield of the furoxan 3a was found
to be 84% (Table 1, entry 1). We also screened some other
metals. The yields of furoxan 3a using Cu(OTf)2, Pd(OAc)2,
and FeCl3 as catalysts were 83%, 85%, and 82%, respectively
(Table 1, entries 2−4). Since the yields of 3a when different
metals were used were similar, we considered whether the
metal had any effect on this transformation. To our surprise
and delight, when the reaction was conducted without the
metal, 3a could be achieved with 86% isolated yield as well
(Table 1, entry 5), which indicated that metal was unnecessary
for the annulation reaction to provide furoxan. Subsequently, a
number of bases were examined. However, the tested bases,
such as K2CO3, DABCO, and DBU all failed to promote this
transformation (Table 1, entries 5−8). We also inspected a
range of solvents (Table 1, entries 9−12); only marginal
improvements were acquired, and it was proven that dioxane
was the best reaction medium to deliver the furoxan 3a. As a
further investigation, we screened the effect of reaction
temperature for this annulation reaction (Table 1, entries
13−16). It was found that 40 °C was the best reaction
temperature, affording the targeted furoxan 3a in 90% yield
(Table 1, entry 15). The attempt to shorten the reaction time
was triumphant (Table 1 entries 17−20), and the desired
furoxan 3a could be readily obtained in high yield when we
shortened the reaction time to 2 h.
mmol) were stirred in dioxane (2 mL) at 40 °C for 12 h under N2.
Isolated yields are given. 6 h. 4 h. 2 h. 1 h.
b
c
d
e
Scheme 2. Scope for the Synthesis of Furoxans
After the optimal reaction conditions were determined, the
substrate scope for the assembly of furoxans 3 was then
explored. As showcased in Scheme 2, a sequence of
sulfoxonium ylides having different electronic properties were
proven to be good carbene precursors, enabling the production
of furoxans 3a−3f in 54−92% yields. The structure of 3c was
assuredly confirmed by X-ray single-crystal diffraction. In
addition to para-substituted aryl sulfoxonium ylides, ortho- and
meta-substituted aryl sulfoxonium ylides were well compatible
in this transformation as well, delivering the targeted furoxans
B
DOI: 10.1021/acs.orglett.9b01876
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
3g−3k in decent yields. The sterically demanding naphthalenyl
sulfoxonium ylides 1l and 1m were also good candidates in this
reaction, and the corresponding furoxans (3l and 3m) could be
isolated in 90% and 89% yield, respectively. To our delight,
alkenyl-derived sulfoxonium ylide 1n was amenable to this [2 +
1 + 1 + 1] annulation as well, leading to the formation of 3n in
48% yield. The treatment of heterocyclic sulfoxonium ylides
(1o and 1p) with TBN was also successful, in which the
desired furoxans 3o and 3p were achieved in high yield of 98%.
Aside from the aromatic sulfoxonium ylides, aliphatic
sulfoxonium ylides were also found to be suitable carbene
donors in this reaction, furnishing the expected furoxans 3q
and 3r in 96% and 60% yields, respectively.
When we screened the metal catalysts for the construction of
furoxans (Table 1, entries 1−4), a low yield of isoxazole 4a was
observed. Subsequently, we turned our attention to optimize
the reaction conditions for the synthesis of isoxazoles, which
are summarized in Table S6−S10. When 0.6 mmol of
sulfoxonium ylide 1a was added, the isoxazole 4a could be
isolated in 34% yield using Cu(TFA)2·H2O as catalyst and
NaOAc as base. To enhance the yield of isoxazole 4a, we also
investigated the effect of other reaction parameters on this
transformation (see the SI for details). Disappointingly, only
marginal improvements were obtained (Tables S6−S10).
Given the multimolecular competitive reactions, it might be
challenging to obtain high yield of isoxazole 4a via this novel
[2 + 1+1 + 1] annulation. Therefore, we determined that 34%
yield was the optimal reaction yield.
Scheme 4. Synthetic Applications and Transformations
the (4-amino-1,2,5-oxadiazol-3-yl)(4-methoxyphenyl)-
methanone 7 could be achieved in 38% yield (Scheme 4b).17
In summary, we have developed the first [2 + 1 + 1 + 1]
annulation reaction of sulfoxonium ylides for the divergent
synthesis of furoxans and isoxazoles. When the reaction was
carried out using NaOAc as base without metal catalyst, two
molecules of sulfoxonium ylides and two molecules of
tBuONO were incorporated into the product, in which a
range of furoxans were achieved in decent yields with wide
substrate scope. When the reaction was conducted using
Cu(TFA)2 as catalyst, trimolecular sulfoxonium ylides and one
molecule of tBuONO were incorporated into the product, and
isoxazoles bearing three carbonyl groups were constructed in
one pot. The current protocol features simple operation, easily
available starting materials, and good yields. Further
application of sulfoxonium ylides as the carbene precursors is
underway in our laboratory.
Under the optimal reaction conditions for the assembly of
isoxazoles, we also inspected the substrate range of isoxazoles.
As demonstrated in Scheme 3, a number of sulfoxonium ylides
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 3. Scope for the Synthesis of Isoxazoles
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01876.
Experimental procedures, characterization data, copies of
NMR spectra (PDF)
Accession Codes
CCDC 1904440 and 1904505 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: qsong@hqu.edu.cn. Fax: 86-592-6162990.
ORCID
could afford the desired isoxazoles 4a−4d in 28%−38% yields.
The structure of product 4a was unambiguously identified by
X-ray single-crystal diffraction.
Yao Zhou: 0000-0002-3500-7355
Qiuling Song: 0000-0002-9836-8860
Notes
The utility of this protocol is illustrated by its application to
transform the furoxans 3 in situ. When the reaction of
sulfoxonium ylide 1a, tert-butyl nitrite 2, and norbornene 5 was
conducted in one pot using chloroform as the solvent, the
product ((3aS,4S,7R,7aS)-3a,4,5,6,7,7a-hexahydro-4,7-
methanobenzo[d]isoxazol-3-yl)(phenyl)methanone 6 was ob-
tained in moderate yield (Scheme 4a).16 For further trans-
formation of the furoxans, the treatment of furoxan 3c with
ammonium hydroxide in MeOH was also executed, in which
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation (21772046) and the Natural Science Foundation
of Fujian Province (2016J01064) is gratefully acknowledged.
We also thank the Instrumental Analysis Center of Huaqiao
University for analysis support. Z.T. acknowledges the
C
DOI: 10.1021/acs.orglett.9b01876
Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b01876
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