Redox Cyclization of Amides and Sulfonamides with Nitrous Oxide for Direct Synthesis of Heterocycles

Article abstract of DOI:10.1021/acs.orglett.0c00397

Herein we report a redox cyclization of amides and sulfonamides with nitrous oxide (N₂O) for the direct synthesis of heterocycles. Various amides and sulfonamides could undergo directed ortho metalation (DoM) by treatment with BuLi, and the lithium intermediate could be trapped by N₂O gas to achieve redox cyclization. N₂O serves as a N-atom donor to mediate the intramolecular coupling of lithium species toward heterocycle formation with free external oxidant. This protocol offers a direct synthesis of heterocycles with features of readily available starting materials, simple operation, and a broad substrate scope.

Full text of DOI:10.1021/acs.orglett.0c00397

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Letter
Redox Cyclization of Amides and Sulfonamides with Nitrous Oxide
for Direct Synthesis of Heterocycles
Zhencheng Lai, Chaorong Wang, Jiaming Li, and Sunliang Cui*
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sı Supporting Information
ABSTRACT: Herein we report a redox cyclization of amides and
sulfonamides with nitrous oxide (N O) for the direct synthesis of
2
heterocycles. Various amides and sulfonamides could undergo directed
ortho metalation (DoM) by treatment with BuLi, and the lithium
intermediate could be trapped by N O gas to achieve redox cyclization.
2
N O serves as a N-atom donor to mediate the intramolecular coupling of
2
lithium species toward heterocycle formation with free external oxidant.
This protocol offers a direct synthesis of heterocycles with features of
readily available starting materials, simple operation, and a broad
substrate scope.
7
enzotriazin-4(3H)-ones and the sulfonyl isosteres are
important heterocyclic scaffolds with a wide variety of
biological activities (Figure 1). In addition, they are versatile
oxidant and a N-atom source in chemistry. For example, N O
2
B
can oxidize various carbenes, N-heterocyclic olefins, and
transition-metal complexes. Meanwhile, N O was reported
to replace N-oxide to promote the Pauson−Khand cyclo-
addition. N O was also found to trap organolithium and
1
,2
8
2
9
2
10
organomagnesium reagents to access hydrazones. Further-
more, Severin reported that N O could serve as a nitrogen
2
donor to mediate the coupling of lithium amides and
11
organomagnesium compounds for the synthesis of triazenes.
This indicated that the investigation of N O as a nitrogen
2
donor building block to achieve more complex molecules
would be promising.
Recently, the redox cyclization of amides has become a
distinct and powerful approach to heterocycles. For example,
Guimond disclosed that the O-pivaloyl hydroxamic acid was a
type of unique benzamide to engage in Rh(III)-catalyzed redox
neutral cyclization with alkynes and alkenes for the synthesis of
Figure 1. Representative examples of biologically active benzotriazin-
4
(3H)-ones and sulfonyl isostere.
12
isoquinolinones (Figure 2a). In this process, the amide group
has a dual role as a directing group in C−H activation and an
internal oxidant to regenerate the Rh(III) catalysis. Meanwhile,
Glorius also reported a similar Rh(III)-catalyzed [4 + 2]
building blocks in organic synthesis. For example, Murakami
and Liu disclosed that benzotriazin-4(3H)-ones and the
sulfonyl isosteres were denitrogenative cyclization partners
3
with allenes and alkynes. Consequently, the synthesis of these
1
3
cyclization of these amides with allenes (Figure 2a).
heterocycles is attractive. In conventional methods, they were
accessed through a multistep route from anthranilates and
anthranilamides using diazotization of the amino functional
Additionally, Rovis developed a Rh(III)-catalyzed [4 + 1]
redox cyclization of amides and diazo compounds to deliver
14
4
isoindolones (Figure 2b). Our group also established a redox
4 + 3] cyclization of amides with vinyl diazo compounds to
group. Recently, the transition-metal catalyzed annulation
[
reaction and C−H amination/diazotization have also been
15a
5
afford azepinones (Figure 2c). We believe that the easily
developed toward these molecules. However, most of these
available amides are versatile cyclization components, and we
methods suffer from preactivation treatment, a multistep
synthetic route, and a narrow substrate scope. Therefore, the
direct and efficient approach to these molecules remains
considerably important and interesting.
Received: January 31, 2020
Nitrous oxide (N O, laughing gas) is a side product in
2
industrial processes and is identified as the current most potent
6
ozone-depleting substance. N O can also be used as an
2
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c00397
A
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a
Table 1. Reaction Optimization
b
entry
RLi
ligand
none
temp. (°C)
N O gas
yield (%)
2
1
2
3
s-BuLi
s-BuLi
s-BuLi
s-BuLi
s-BuLi
s-BuLi
s-BuLi
n-BuLi
t-BuLi
s-BuLi
s-BuLi
s-BuLi
−45
−45
−78
−45
−78
0
balloon
balloon
balloon
52
71
42
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
HMPA
c
4
bubbling
bubbling
bubbling
bubbling
bubbling
bubbling
bubbling
bubbling
bubbling
85 (82)
45
5
6
7
8
9
69
64
54
85
49
83
74
25
−45
−45
−45
−45
−45
10
11
12
DMPU
TMEDA
d
a
Reaction conditions: 1b (0.5 mmol) and ligand (2.5 equiv) were
dissolved in THF (5 mL) and cooled to corresponding temperature
under argon, followed by the addition of RLi (2.5 equiv), and the
solution was kept for 2 h. Then, argon was vacuumed, and N O gas
2
b
was induced (balloon or bubbling) and kept for 1 h. Yield refers to
c
d
isolated products. 10 mmol scale in parentheses. Et O was used as
2
solvent.
Figure 2. Selected redox cyclizations of amides.
obtained, demonstrating the practicability of this reaction.
When the same bubbling was conducted at −78, 0, and 25 °C,
the yield would decrease to 45, 69, and 64%, respectively
(entries 5−7). Replacing s-BuLi with n-BuLi would lead to a
lower yield (entry 8), whereas the utilization of t-BuLi would
give a comparable yield (entry 9). Besides, the ligands were
also screened. For example, HMPA (hexamethylphosphor-
amide) was less effective (entry 10), and DMPU (1,3-
dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone) was as
good as TMEDA (entry 11). On the contrary, the use of
hypothesized that amides might undergo cyclization with N O
2
to deliver interesting heterocyclic molecules. In a continuation
15b−e
of our interests in redox cyclization,
herein we report a
directed ortho metalation (DoM)/redox cyclization of amides
and sulfonamides with nitrous oxide (Figure 2d).
We initially investigated the reaction using N-(pivaloyloxy)-
benzamide 1a and N O in the presence of Rh(III) catalysis.
2
We thought that 1a could form a rhodacycle species to
undergo cyclization with N O. However, many trials failed, and
2
1a was recovered, probably due to the low intrinsic reactivity of
Et O as a solvent would decrease the yield to 74% (entry 12).
2
N O, which was unable to trap the organo-rhodium
With the optimized reaction condition in hand, we next
investigated the scope of this redox cyclization. The
benzamides could be easily prepared from benzoyl chloride
and amines. As shown in Scheme 1, a variety of para-
substituted benzamides with valuable functional groups like
methyl, cyclohexanyl, phenyl, tetrahydropyran (THP)-pro-
2
7
intermediate. In this stage, we envisioned that the DoM of
16
benzamides could deliver a lithium complex, which might be
active enough to be intercepted by N O. Snieckus and
2
coworkers have made a great contribution to DoM and have
17
demonstrated its powerful utility in arene functionalization.
Hence we turned our attention to using N-tert-butyl
benzamide 1b as a model substrate. Upon standard conditions
of DoM, 1b was treated with s-BuLi in THF at −45 °C for 2 h;
tected alcohol, and alkyne could react smoothly with N O to
2
afford corresponding benzotriazin-4(3H)-ones (3b−f) in
moderate to excellent yield (40−89%), thus offering ample
opportunity for further derivatization. Other functional groups
such as fluoro, chloro, methoxy, methoxymethyl (MOM)-
protected phenolic hydroxy, methylthio, dimethylamino, and
Boc-protected amino were all tolerated in this process to
deliver the products smoothly (3g−m). It should be noted that
these functional groups were not tolerated in conventional
synthetic routes. In addition, the ortho-methoxy benzamide
(1o) was applicable in this redox cyclization to give 3n in
moderate yield. When 2-naphthoyl amide was used, the redox
cyclization could also occur, and the product 3o could be
formed in 72% yield. Furthermore, the heterocyclic amides
were also prepared and subjected to this redox cyclization. The
heterocyclic amides with the scaffold of pyridine and thiophene
could engage well in this cyclization to deliver the
corresponding products using DMPU as the ligand, albeit in
slightly low yields (3p and 3q). The structure of 3p was
then, a balloon gas of N O was introduced. To our delight, a
2
new product 3a was observed and isolated in 52% yield (Table
1
, entry 1). 3a was identified as a benzotriazin-4(3H)-one
compound, and the structure was unambiguously confirmed by
X-ray analysis, which was consistent with the standard
analysis of NMR and mass spectroscopy. This surprising result
1
8
indicated that a cyclization indeed occurred, and N O acted as
2
a N-donor source. In addition, the cyclization was a redox
process because there was not any external oxidant used. Next,
TMEDA (N,N,N′,N′-tetramethylethylenediamine) was added
as a ligand to facilitate the reaction, and the yield was
significantly improved to 71% (entry 2). When the temper-
ature was shifted to −78 °C, the yields would decrease to 42%
(
entry 3). When N O was bubbled at −45 °C, to our delight,
2
the yield of 3a could further increase to 85% (entry 4). The
reaction could be scaled up to 10 mmol scale with 82% yield
B
https://dx.doi.org/10.1021/acs.orglett.0c00397
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Scheme 1. Substrate Scope of Amides
molecules from readily available starting material, along with
the achievement of structural diversity and molecular complex-
ity.
Furthermore, we used sulfonamides as a cyclization partner
to investigate this protocol because they are a typical isostere
of amides and a fundamental synthon in organic synthesis.
Upon treatment with a combination of n-BuLi and potassium
t-pentylate (superbase; for the reaction conditions survey, see
21
the Supporting Information), a series of sulfonamides could
cyclize with N O to furnish 2H-benzo[e][1,2,3,4]thiatriazine
2
1,1-dioxides in moderate yield (Scheme 2). For example, N-(t-
a
Scheme 2. Substrate Scope of Sulfonamides
a
Reaction conditions: n-BuLi (2 mmol) and KO(Et)Me (2 mmol)
2
were dissolved in 2-MeTHF (5 mL) at rt under argon for 30 min and
cooled to −78 °C followed by the addition of 4 (0.5 mmol) and kept
for 1 h; then, argon was vacuumed and N O gas was bubbled and kept
2
for another 1 h. Yields refer to isolated products.
butyl)benzenesulfonamide 4a could engage in the redox
cyclization to deliver 5a in 68% yield, which was confirmed
22
a
by X-ray analysis. The scope of the substituents on the
nitrogen atom of 4 was examined, where primary, secondary,
and tertiary alkyl groups were suitable, including methyl,
isopropyl, cyclopropanyl, cumyl, and (S)-1-phenylethan-1-yl
(5b−f). On the contrary, variation of the aryl ring was also
conducted, and various functional groups including methyl,
tert-butyl, phenyl, methoxy, and chloro, could be well tolerated
in this redox cyclization process (5g−k), albeit the chloro
substitution gave a slightly low yield (5k). This method
provides a direct and efficient approach toward these
molecules from easily available sulfonamides.
Reaction conditions: 1 (0.5 mmol) and TMEDA (1.25 mmol) were
dissolved in THF (5 mL) at −45 °C under argon followed by the
addition of s-BuLi (1.25 mmol) and kept for 2 h; then, argon was
vacuumed and N O gas was bubbled and kept for another 1 h. Yields
refer to isolated products. DMPU was used as ligand.
2
b
19
confirmed by X-ray analysis. On the contrary, the N-
substitution scope of the amides was also tested. Apart from
the tert-butyl group, a series of alkyl, aryl, and methoxyamino
functionalities including methyl, isopropyl, cyclopropanyl,
cumyl, (S)-1-phenylethan-1-yl, and para-methoxyphenyl
(
PMP) were found to be compatible with this process to
In this stage, the representative synthetic application was
also conducted via derivatization of the products. As shown in
Scheme 3, compound 3u could undergo a N-deprotection of
the cumyl group upon treatment with TFA to deliver 6 in 82%
yield on a gram scale. Then, 6 could be divergently modified
with various functional groups. For example, 6 could be
functionalized with bromoethanol, allyl bromide, propargyl
bromide, t-butyl bromoacetate, and 2-(2-bromoethyl)-
furnish the structural divergent benzotriazin-4(3H)-ones (3r−
x) in good yield. The structure of 3t was confirmed by X-ray
analysis. It should be noted that benzotriazin-4(3H)-ones
with stereocenters like 3v could be accessed rapidly from
simple benzamide material 1w. Given the importance of
benzotriazin-4(3H)-ones in chemistry and biology, this
method provides a direct and efficient approach toward these
2
0
C
https://dx.doi.org/10.1021/acs.orglett.0c00397
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
a
Scheme 3. Representative Derivatization of the Products
Figure 3. Proposed reaction mechanism.
a
beginning, benzamides 1 would undergo a directed ortho
metalation upon treatment with s-BuLi to deliver lithium
species A, and the addition of TMEDA could improve the
solubility of A. The aryl lithium group of A could undergo an
(
a) K CO , bromoethanol, acetone, reflux; (b) K CO , allyl
2 3 2 3
bromide, acetone, reflux; (c) K CO , proparyl bromide, acetone,
reflux; (d) K CO , tert-butyl bromoacetate, acetone, reflux; and (e)
K CO , 2-(2-bromoethyl)isoindoline-1,3-dione, acetone, reflux.
2
3
2
3
2
3
addition to N O to form intermediate B. Intermediate B would
2
isoindoline-1,3-dione to deliver a series of benzotriazin-4(3H)-
ones with various functionalities, such as hydroxy, alkene,
alkyne, ester, and protected amino group (7a−e). These
functional groups provide ample opportunities for further
modification of these molecules. Therefore, this late-stage
derivatization method provides facile and complementary
access to a range of benzotriazin-4(3H)-ones and would be
synthetically useful.
isomerize to C for the lithium amide group, which would
further undergo an intramolecular addition between the
lithium amide group and diazotate for the generation of
product 3 with the release of Li O. Thus the whole redox
2
cyclization process has been achieved. The mechanism for
sulfonamides is similar, whereas the utilization of a super base
would generate aryl potassium species to improve their
nucleophilic addition activity because the arylsulfonyl group
is more electron-withdrawing than the benzoyl group.
To probe the reaction mechanism, a control reaction was
conducted. As shown in Scheme 4, N-(tert-butyl)-2,6-
In summary, a redox cyclization of amides and sulfonamides
with N O for the direct synthesis of heterocycles has been
2
successfully developed. N O acts as a nitrogen donor and
Scheme 4. Control Reaction
2
mediates the intramolecular coupling of DoM-generated
lithium species. This protocol offers general and direct access
to privileged molecules with features of readily available
starting materials, easy operation, and a broad substrate scope.
ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00397.
Full experimental procedures, characterization data, X-
ray crystallographic data, optimization details, and NMR
spectra data (PDF)
difluorobenzamide 1z was prepared and subjected to the
standard condition. The aromatic ring of 1z was 2,6-difluoro-
substituted and could avoid the formation of aryl lithium, and
thus only lithium amide 1z-Li could be formed. Under the
standard conditions, there was not any new product observed,
and 1z was recovered (Scheme 4a). This indicated that the in
Accession Codes
CCDC 1959698, 1969707, 1972557, and 1972989 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
situ generated lithium amide 1z-Li was unable to attack N O.
2
On the contrary, the para-methoxyphenyl lithium 8 was
prepared and was found to engage in addition to N O at −45
2
°
C to deliver the (E)-1,2-bis(4-methoxyphenyl)diazene 9 in
1
223 336033.
5
5% yield (Scheme 4b), which is consistent with the literature,
23
where the aryl lithium could be intercepted by N O.
2
AUTHOR INFORMATION
Corresponding Author
■
Considering that the DoM of benzamides would simulta-
neously generate aryl lithium and lithium amide species, these
control reactions indicated that aryl lithium would have the
Sunliang Cui − Institute of Drug Discovery and Design, College
of Pharmaceutical Sciences, Zhejiang University, Hangzhou
310058, China; orcid.org/0000-0001-9407-5190;
Email: slcui@zju.edu.cn
priority to attack N O first.
On the basis of these results and the literature,
2
7
,10,11
a
plausible reaction mechanism is depicted in Figure 3. At the
D
https://dx.doi.org/10.1021/acs.orglett.0c00397
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Authors
pubs.acs.org/OrgLett
Letter
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Zhencheng Lai − Institute of Drug Discovery and Design,
College of Pharmaceutical Sciences, Zhejiang University,
Hangzhou 310058, China
Chaorong Wang − Institute of Drug Discovery and Design,
College of Pharmaceutical Sciences, Zhejiang University,
Hangzhou 310058, China
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Complete contact information is available at:
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6
Notes
(
The authors declare no competing financial interest.
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We are grateful to the National Natural Science Foundation of
China (21971222), the Key R&D Plan of Zhejiang Province
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E
https://dx.doi.org/10.1021/acs.orglett.0c00397
Org. Lett. XXXX, XXX, XXX−XXX