Copper-Catalyzed Annulation of Oxime Acetates with α-Amino Acid Ester Derivatives: Synthesis of 3-Sulfonamido/Imino 4-Pyrrolin-2-ones

Article abstract of DOI:10.1021/acs.orglett.0c00870

A copper-catalyzed annulation of oxime acetates and α-amino acid ester derivatives for the easy preparation of 4-pyrrolin-2-ones bearing a 3-amino group has been developed. This process features the oxidation of amines with oxime esters as the internal oxidant to produce the active 1,3-dinucleophilic and 1,2-dielectrophilic species concurrently. The subsequent nucleophilic cyclization realizes the efficient construction of 4-pyrrolin-2-one derivatives.

Full text of DOI:10.1021/acs.orglett.0c00870

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Letter
Copper-Catalyzed Annulation of Oxime Acetates with α‑Amino Acid
Ester Derivatives: Synthesis of 3‑Sulfonamido/Imino 4‑Pyrrolin-2-
ones
Chun-Bao Miao,* An-Qi Zheng,‡ Li-Jin Zhou,‡ Xinyu Lyu, and Hai-Tao Yang*
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ABSTRACT: A copper-catalyzed annulation of oxime acetates and α-amino
acid ester derivatives for the easy preparation of 4-pyrrolin-2-ones bearing a 3-
amino group has been developed. This process features the oxidation of
amines with oxime esters as the internal oxidant to produce the active 1,3-
dinucleophilic and 1,2-dielectrophilic species concurrently. The subsequent
nucleophilic cyclization realizes the efficient construction of 4-pyrrolin-2-one
derivatives.
he efficient construction of functional heterocycles in a
simple way is an important task in synthetic chemistry.
Among the widely existing five-membered nitrogen-containing
heterocycles, the 4-pyrrolin-2-one core represents an important
motif, which has been found in many natural products such as
cyanogramide1 and violacein2 (Figure 1) and reported to
unsaturated acid chlorides,15 and photooxidation of 2-
substituted furans.16 However, some drawbacks still existed,
such as not easily available raw materials, substrate limit, and
the use of expensive catalyst.
T
Oxime esters, which can be easily prepared from carbonyl
compounds, have a very high reactivity due to the existence of
an easily broken N−O bond. In the past several years, oxime
esters have emerged as a versatile platform for the synthesis of
diverse nitrogen-containing heterocycles via [3+n]-type
reaction.17 Therein, various five-membered heterocycles
including pyrroles,18 pyrrolines,19 oxazoles,20 thiazoles,21
imidazoles,22 pyrazoles,23 triazoles24 (Scheme 1a), and six-
membered heterocycles such as pyridines25 were constructed
efficiently. Exploring their application in the synthesis of new
heterocyclic scaffolds is challenging and interesting. Most
recently, we developed a Cu(OAc)2-promoted reaction of
malonate-tethered oxime ester with indole derivatives for the
synthesis of polysubstituted 3-pyrrolin-2-ones.26 In continuing
our efforts aimed at extending the application of oxime esters
for constructing new nitrogen-containing frameworks, herein,
we reported a redox strategy to synthesize 4-pyrrolin-2-ones
from oxime esters and α-amino acid ester derivatives.
Figure 1. Biologically active and functional molecules containing a 4-
pyrrolin-2-one moiety.
exhibit various biological activities including reducing choles-
terol uptake,3 antitumoral,4 antibiotic,5 and analgesic proper-
ties,6 etc. Diketopyrrole, which has two fused 4-pyrrolin-2-one
skeletons, has been used as a versatile building block in organic
solar cells as an electron acceptor7 as well as a fluorescent
probe for the detection of biologically important species.8
Therefore, the exploitation of new methods for construction of
such scaffolds with structural diversity is of high interest, which
can be seen from the recent numerous works, such as copper-
mediated reaction of enamines with α-bromocarbonyls or N-
(2-pyridylcarbonyl) amino acid ester,9 K2S2O8/TEMPO-
induced oxidative cyclization of N-unprotected enaminocar-
bonyls,10 Eosin Y/Cu(OAc)2 or Mn(OAc)2-catalyzed annula-
tion of vinyl azides with ketene silyl acetals or 4-
hydroxycoumarins,11 Cu-catalyzed aminodifluoroalkylation of
alkynes and bromodifluoroacetamides,12 Rh-catalyzed diazo-
oxindoles with vinyl isocyanates or N-pivaloyloxy acryla-
mides,13 oxidative dearomatization of pyrroles,14 copper-
catalyzed tandem reaction of vinylalkynes, imines, and α,β-
Amines were very easily oxidized to form imines or imine
cations in situ, which reacted with nucleophiles to afford a
series of α-functionalized amines.27 It has been reported that
amines acted as electrophilic carbon synthons in their reactions
with oxime esters catalyzed by [Ru], [Fe], or I2 to form
Received: March 8, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00870
© XXXX American Chemical Society
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A

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a
Scheme 1. Construction of Five-Membered Nitrogen-
Containing Heterocycles Based on the Oxime Esters
Table 1. Screening of the Reaction Conditions
b
b
entry
catalyst
CuCl
CuCl
CuCl
CuCl
solvent
additive
yield (%)
1
2
3
4
CH3CN
-
-
-
-
trace
60
69
1,4-dioxane
CH3CO2Bun
DCE
73
5
6
7
8
CuCl
CuI
CuBr
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
-
-
-
-
-
-
-
-
80
67
16
12
Cu(OAc)2
CuCl2
CuCN
CuSCN
Cu(OTf)2
Pd(PPh3)4
FeCl2
CuCl
9
48
10
11
12
13
14
15
16
17
18
trace
trace
trace
trace
trace
9
77
78
0
69
-
-
K2CO3
DMAP
DABCO
-
-
CuCl
CuCl
-
c
pyridines accompanied by the C−N bond cleavage.28 Herein,
we introduced an electrophilic center on the α-carbon of
secondary amines and envisaged using oxime esters as an
internal oxidant to oxidize the amines to imine intermediates I
having two vicinal electrophilic centers. Meanwhile, an active
1,3-dinucleophilic enamino-CuII species II was generated
simultaneously. The further interaction would lead to
structurally diverse five-membered 3-amino nitrogen-contaning
heterocycles (Scheme 1b).
To validate this idea, our initial studies were carried out
using oxime acetate 1a and diethyl 2-((4-methylphenyl)
sulfonamido) malonate 2a as the substrates (Table 1). When
CH3CN was used as the solvent, only a trace of the desired
product 3aa was formed in the presence of CuCl (0.1 equiv)
under a N2 atmosphere at 80 °C (entry 1). A solvent survey
showed that toluene was the best solvent to realize the
cyclization to give 3aa in 80% yield within 4 h (entries 2−5).
Other copper salts including CuI, CuBr, Cu(OAc)2, CuCl2,
CuCN, CuSCN, and Cu(OTf)2 were also screened, and CuCl
was still the most effective (entries 5−12). Pd(PPh3)4 and
FeCl2, which were commonly used in the transformation of
oxime esters, did not exhibit catalytic activity (entries 13 and
14). The addition of K2CO3 blocked the reaction severely
(entry 15). An organic base such as DMAP and DABCO as
additives has no significant influence on the yield (entries 16
and 17). In the absence of CuCl, no 3aa was generated (entry
18). Further decreasing the amount of CuCl to 5 mol % led to
a lower yield with a longer reaction time (entry 19).
19
CuCl
a
Reaction conditions: 1a (0.48 mmol), 2a (0.40 mmol), catalyst (10
b
mol %), solvent (4 mL), N2 atmosphere, 80 °C. Isolated yield based
on substrate 2a. 5 mol % of catalyst loading.
c
from hydrogen to a methyl or phenyl group, the reaction also
worked to give the final products 3pa and 3qa, albeit in lower
yield. It was interesting that a bromo group on the α-carbon of
oxime acetate was also tolerated in the reaction, giving 55%
yield of 3ra. This was very useful for the further 4-
functionalization of the generated product through various
coupling reactions. When R1 was an alkenyl or ester group, the
desired product 3oa or 3xa was also obtained in 72% and 48%
yield, respectively. Furthermore, cyclic ketoxime acetates led to
the generation of polycyclic products 3sa and 3ta in moderate
yield. However, when linear aliphatic ketoxime acetates were
introduced to the reaction, the reaction was much more
complicated. The oxime acetate derived from pinacolone
yielded the desired product 3ua in 58% yield, while pentan-3-
one oxime acetate provided the isomerized products 3va′ and
3va″ having an exocyclic double bond in 28% and 17% yield,
respectively. Pentan-2-one oxime acetate afforded the normal
product 3wa in 24% yield accompanied by the isomerized
product 3wa′ in 20% yield. Next, the scope of α-sulfonamido
malonates was examined. Regardless of either electron-rich or
electron-deficient aryl groups of R3, the reaction was
performed well to give 3ab−3ad in acceptable yield.
Furthermore, when R3 was an alkyl or N,N-dimethylamino
group, the desired products 3ae and 3af also could be prepared
in 76% and 61% yield, respectively.
With the optimized cyclization procedure established, the
generality of the CuCl-catalyzed annulation was examined
(Scheme 2). The scope of the oxime esters was first explored
with 2a as the coupling partner. Various aryl ethyl ketoxime
acetates bearing either an electron-donating or electron-
withdrawing group on the phenyl ring were broadly tolerated,
giving the annulation products 3aa−3ja in moderate to good
yield (51−80%). Heterocyclic oxime acetates linking a pyridyl,
pyrazinyl, furyl, and thienyl group on the oxime acetates were
also compatible with this process, providing the corresponding
products 3ka−3na in 54−70% yields. Changing the R2 group
Direct introduction of an amino group on the 3-carbon of 2-
pyrrolones always needed several steps of transformations.29
Using our developed method, a sulfonamido could be easily
installed in the 3-position of 2-pyrrolone scaffolds. Encouraged
by the good results obtained as discussed above, we turned our
attention to N-aryl glycine esters. Ethyl p-tolylglycinate 4a was
selected as a model substrate to react with 1a (Scheme 3, more
conditions can be seen in Table S1 in the Supporting
Information) under the standard conditions, and a very
B
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of P to 5aa. Then, we increased the amount of CuCl to 2
equiv. Although the yield was improved to 23%, it was still
unsatisfactory (entry 3). Further screening of copper salts
suggested that CuI was the most effective oxidant, giving 5aa in
62% yield (entries 4−7). An extensive survey of solvents
revealed that CH3CN was the best solvent for the reaction,
affording 5aa in 70% yield (entries 8−11). To run the reaction
with a catalytic amount of CuI, an external oxidant was added.
When di-tert-butyl peroxide (DTBP) was added in combina-
tion with 0.2 equiv of CuI and 1.2 equiv of K2CO3, 5aa was
obtained in 32% yield after stirring at 80 °C for 12 h (entry
12). Increasing the temperature to 100 °C significantly
improved the yield to 68% after 6 h (entry 13), indicating
that a higher temperature was beneficial to the oxidation of low
valent copper to high valent copper. The effect of K2S2O8 was
inferior to DTBP. Using tert-butyl peroxybenzoate as the
oxidant resulted in the failure of reaction, and tert-butyl
hydroperoxide only gave 9% of the product (entries 14−16).
Using DMAP instead of K2CO3 led to a lower yield, while
DABCO resulted in the failure of reaction (entries 17−18).
When the amount of CuI was reduced to 0.1 equiv, the yield
decreased to 57% (entry 19).
Scheme 2. Substrate Scope of Oxime Acetates and α-
Sulfonamido Malonates
To evaluate the generality of this process, various oxime
acetates were subjected to the reaction with 4a (Scheme 4).
Regardless of the functional groups at the para-positions, a
large range of functional groups on the benzene ring, such as
methoxyl, methyl, chlorine, fluoro, and nitro, were all tolerated,
affording the desired 3-imine-4-pyrrolin-2-ones 5aa−5ha and
5ka. When polysubstituted oxime acetate including a cyclic
keleton was used, this reaction occurred normally to furnish
5ia or 5ja. To further explore the scope of our method, various
N-aryl-substituted glycine esters were subjected to the reaction.
To our delight, methyl, methoxyl, and chloro groups on the
phenyl ring had no influence on the reaction. However, when
N-(4-nitrophenyl) glycine ester was subjected to the reaction,
only a trace of product 5ad was formed.
Due to the existence of an active imine CN bond, we
further studied the transformation of this kind of 4-pyrrolin-2-
one 5 (Scheme 5). The reaction of indole, pyrrole, or 2,4-
dimethylpyrrole with 5aa catalyzed by NH2SO3H afforded the
addition products 6−8 in 50−83% yields. When CH3NO2 and
diethyl malonate were used as the nuceophiles, basic catalyst
DBU was found to catalyze the reaction efficiently to afford 9
and 10 in 57% and 59% yield, respectively. Furthermore, the
reaction of 5aa with NIS provided the iodinated product 11.
However, when NIS was used instead of NBS, the product 12
was obtained in 50% yield via further addition with
hypobromous acid.
a
b
Recrystallization yield with 5 mmol of 2a. 1.5 equiv of 1w was used.
Scheme 3. Reaction of Acetophenone Oxime Acetate with
Ethyl p-Tolylglycinate 4a
Other amino acid ester derivatives were also evaluated in the
two kinds of transformations (Scheme 5). For the reaction of
1a with α-benzamido malonate 2g, besides the desired product
13 (30%), compound 14 was also generated in 25% yield.
When one of the ester groups of 2a was replaced by a methyl
group, no corresponding product 15 was formed, which
indicated the significance of coexistence of the two ester
groups. In the case of substrate 4, changing the N-aryl group to
an alkyl, tosyl, or benzamido group led to the failure of the
reaction.
A possible mechanism is described in Scheme 6. Initially, the
oxidative addition of the CuI to the N−O bond of the acetyl
oxime 1 produces iminium radical A followed by the removal
of CuII, which oxidizes 2a and 4 to the corresponding imine
intermediates C and F, respectively. Radical A reacts with CuI
complex mixture was obtained. When K2CO3 was added to the
reaction, a main product was formed in 11% yield, and most of
the starting material remained unreacted. H NMR analysis
showed it was not the anticipated product P but the further
oxidized product 5aa (entry 2). It was worth noting that the
CN double bonds had a sole E-configuration through the
NOESY analysis. This was quite different from the results
previously reported by Chen’s group, and therein, pyridines
were obtained with the C−N bond cleavage of amines.30
Obviously, an oxidative process was involved in the conversion
1
C
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Scheme 4. CuI-Catalyzed Reaction of Oxime Acetates with
N-Aryl Glycine Esters and the Further Transformation of
5aa
Scheme 6. Plausible Mechanism
intramolecular cyclization generates E, which is tautomerized
to 3aa. In path b, addition of enamine-CuII B to F occurs to
generate intermediate G. Subsequent intramolecular cycliza-
tion gives H, which equals I. The intermediate I is further
oxidized by Cu(II) to furnish 5 with the release of Cu(I),
which can be oxidized to Cu(II) by DTBP to enter the next
cycle.
In summary, a novel convenient procedure for the synthesis
of 3-sulfonamido/imino 4-pyrrolin-2-ones has been developed.
In the presence of a catalytic amount of CuI, oxime esters act as
an internal oxidant to oxidize α-amino acid esters to 1,2-
dielectrophilic species, and a 1,3-dinucleophilic species was
generated simultaneously. The following intermolecular
nucleophilic cyclization affords 4-pyrrolin-2-ones. Readily
available starting materials, mild conditions, and good
functional compatibility make this method very attractive.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00870.
a
Column chromatographic yield with 5 mmol of 4a.
1
Experimental procedures, characterization data, and H
NMR and 13C NMR spectra for all new products (PDF)
Scheme 5. Reaction of 1a with Other Amino Acid Ester
Derivatives
AUTHOR INFORMATION
Corresponding Authors
■
Chun-Bao Miao − Jiangsu Key Laboratory of Advanced
Catalytic Materials and Technology, School of Petrochemical
Engineering, Changzhou University, Changzhou, Jiangsu
213164, P. R. China; orcid.org/0000-0003-4666-2619;
Email: estally@yahoo.com
Hai-Tao Yang − Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology, School of Petrochemical Engineering,
Changzhou University, Changzhou, Jiangsu 213164, P. R.
China; orcid.org/0000-0001-9803-5452; Email: yht898@
yahoo.com
Authors
to form an enamino-CuII intermediate B. In path a,
nucleophilic addition of enamino-copper B to C followed by
An-Qi Zheng − Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology, School of Petrochemical Engineering,
D
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pubs.acs.org/OrgLett
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Li-Jin Zhou − Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology, School of Petrochemical Engineering,
Changzhou University, Changzhou, Jiangsu 213164, P. R.
China
Xinyu Lyu − Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology, School of Petrochemical Engineering,
Changzhou University, Changzhou, Jiangsu 213164, P. R.
China
Complete contact information is available at:
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Author Contributions
‡A.-Q.Z. and L.-J.Z. contributed equally. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the Natural
Science Foundation of Jiangsu Province (BK20181462), the
Jiangsu Key Laboratory of Advanced Catalytic Materials and
Technology (BM2012110), and the Advanced Catalysis and
Green Manufacturing Collaborative Innovation Center.
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