Ag-Catalyzed Oxidative ipso-Cyclization via Decarboxylative Acylation/Alkylation: Access to 3-Acyl/Alkyl-spiro[4.5]trienones

Article abstract of DOI:10.1021/acs.orglett.0c01588

A strategy to functionalized spiro[4.5]trienones, by domino silver-catalyzed decarboxylative acylation or alkylation/ ipso-cyclization of N-arylpropiolamides with α-keto acids/alkyl carboxylic acids, is presented. This transformation offers a wide range of substituted 3-acyl/alkyl-spiro[4.5]trienones in high yields with a broad substrate scope. The approach was further extended to access fused tricyclic frameworks, 6,7-dihydro-3H-pyrrolo[2,1-j]quinoline-3,9(5H)-diones.

Full text of DOI:10.1021/acs.orglett.0c01588

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Letter
Ag-Catalyzed Oxidative ipso-Cyclization via Decarboxylative
Acylation/Alkylation: Access to 3‑Acyl/Alkyl-spiro[4.5]trienones
Chada Raji Reddy,* Dattahari H. Kolgave, Muppidi Subbarao, Mounika Aila,
and Santosh Kumar Prajapti*
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ABSTRACT: A strategy to functionalized spiro[4.5]trienones, by
domino silver-catalyzed decarboxylative acylation or alkylation/
ipso-cyclization of N-arylpropiolamides with α-keto acids/alkyl
carboxylic acids, is presented. This transformation offers a wide
range of substituted 3-acyl/alkyl-spiro[4.5]trienones in high yields
with a broad substrate scope. The approach was further extended
to access fused tricyclic frameworks, 6,7-dihydro-3H-pyrrolo[2,1-
j]quinoline-3,9(5H)-diones.
zaspirocyclohexadienones are an important molecular
the synthesis of 3-alkyl spiro[4,5]trienones using the TBHP as
an oxidant.7 Regardless of their merits, the current strategies
suffer from disadvantages such as high reaction temperature,
limited substrate scope, and/or regioselectivity. Therefore,
more general and mild approaches for the construction of
diversely functionalized spiro[4,5]trienones are still of great
interest.
A
motif widely present in natural products (I) and
biologically active compounds (II, Figure 1).1 Indeed,
On the other hand, silver-catalyzed decarboxylative
acylation/alkylation reactions are gaining prominence for the
introduction of the acyl or alkyl groups, starting from α-keto
acids3a or alkyl carboxylic acids producing CO2 as the
waste.9−12 Among them, the incorporation of an in situ
generated acyl or alkyl radical on to a functional group and
subsequent radical cyclizations represent the best direct
synthesis of corresponding cyclic frameworks.11,12 Nonethe-
less, the decarboxylative cascade addition of an acyl or alkyl
radical on the alkyne followed by ipso-cyclization is unexplored.
Our interest toward construction of valuable heterocyclic
compounds via difunctionalizaion of alkyne13 provoked us to
explore the above approach for the synthesis of corresponding
spiro[4.5]trienones. Herein, we disclose silver-catalyzed
decarboxylative ipso-carboacylation/alkylation of N-aryl pro-
piolamide with α-keto acids/alkyl carboxylic acids to
synthesize 3-acyl/alkyl spiro[4,5]trienones. The reaction
strategy involves the generation of radical via decarboxylation
of acids, which initiate the cascade reactions such as acylation/
Figure 1. Representative azaspirocyclohexadienones
azaspiro[4,5]dienones have been broadly utilized as a versatile
intermediate (III) for constructing important alkaloids and
related products.2 Consequently, considerable efforts have
been dedicated to construct azaspiro[4,5]trienones and several
methods thereby developed.3−6 The electrophilic or radical
cascade ipso-cyclization has emerged as a powerful tool for
difunctionalization and dearomatization of the alkyne based
aromatic compounds to provide diverse azaspiro[4,5]-
trienones.4,5 To date, two acylative ipso-cyclizations6 and one
alkylative ipso-cyclization7 are known in the literature along
with a few other ipso-carbocyclizations.8 In 2014, Li’s group
reported an oxidative ipso-carboacylation of N-arylpropiola-
mides with aldehydes for the preparation of 3-acylspiro[4,5]-
trienones at 110 °C.6a Recently, Liu and Tang et al. have
developed visible-light-mediated ipso-carboacylation of alkynes
with aroyl chlorides to access 3-aroylspiro[4,5]trienones at 100
°C.6b In 2016, Li and co-workers reported a copper-catalyzed
oxidative coupling followed by ipso-cyclization of N-arylpro-
piolamides with unactivated alkanes (mostly cycloalkanes) for
Received: May 9, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01588
© XXXX American Chemical Society
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alkylation, ipso-annulation, dearomatization, and dehydration
in a single operation.
Scheme 1. Scope of N-Arylpropiolamides
At the onset of our investigations, the decarboxylative ipso-
annulation of N-(4-methoxyphenyl)-N-methyl-3-phenyl-pro-
piolamide (1a) with pyruvic acid (2a) was chosen as the
model reaction to find the optimal reaction conditions (Table
1). To our delight, the initial experiment using AgNO3 (10 mol
a
Table 1. Optimization of Reaction Conditions
b
entry catalyst (mol %)
oxidant
K2S2O8
Na2S2O8
(NH4)2S2O8
TBHP
K2S2O8
K2S2O8
K2S2O8
K252O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
solvent
yield (%)
1
2
3
4
5
6
7
8
AgNO3 (10)
AgNO3 (10)
AgNO3 (10)
AgNO3 (10)
AgOAc (10)
AgOTf (10)
Ag2CO3 (10)
Ag2CO3 (5)
Ag2CO3 (10)
None
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN/H2O
CH3CN
74
63
38
trace
46
42
94
81
52
0
c
9
10
11
12
Ag2CO3 (10)
Ag2CO3 (10)
23
0
d
CH3CN/H2O
a
Unless otherwise stated, all the reactions were performed using 1a
(0.3 mmol) with 2a (0.6 mmol). K2S208 (1.5 mmol) and catalyst in 3
b
c
mL of solvent mixture (9:1). Isolated yield. 0.9 mmol of K2S208
d
used. Reaction in the absence of 2a.
4-chloro (1e) and 4-fluoro (1f), as well as electron-with-
drawing groups, such as 4-cyano (1g) and 4-COMe (1h), on
the alkyne moiety were well tolerated under the typical
reaction conditions to give 3e to 3h in very good yields. In
addition, the reaction of (3-(trifluoromethyl)phenyl)-
propiolamide 1i with 2a furnished 3i in 91% yield.
Interestingly, naphthyl substituted propiolamide 1j and
thiophenyl propiolamide 1k were also found to be suitable
to provide the 3-acylspiro[4,5]trienones 3j and 3k in 85% and
78% yield, respectively. However, the reactions of propiola-
mides having a terminal alkyne (1l) and methyl substituted
alkyne (1m) were futile under the optimal conditions. Notably,
the substrates having substitutions on the N-(4-methoxyphen-
yl) ring, such as 3-Me (1n), 3-F (1o), and 2-Me (1p) groups,
were found to be reliable in reaction with 2a to afford the
products 3l (83%), 3m (86%), and 3n (78%), respectively.
The ipso-cyclization of N-arylpropiolamides having various N-
protecting groups such as benzyl (1q), tosyl (1r), and 2-iodo-
benzyl (1t) were productive in giving the corresponding
spirocycles 3o (85%), 3p (72%), and 3q (80%), respectively.
But, N-acetylpropiolamide 1s failed to give the desired
product. A gram-scale reaction of 1b (3.4 mmol) with 2a
proceeded effectively to deliver the corresponding spiro[4,5]-
trienone 3b in 81% yield, which showcases the scalability of the
method.
%) as a catalyst and K2S2O8 as the oxidant in CH3CN:H2O
(9:1) led to the desired product 3a in 74% yield (Table 1,
entry 1). Other oxidants, including Na2S2O8, (NH4)2S2O8, and
TBHP, gave a reduced yield (Table 1, entries 2 to 4) of 3a and
recovery of 1a. This might be due to the superior solubility of
potassium salt in organic solvent compared to the other
oxidants used.3b The employment of other catalysts such as
AgOAc and AgOTf resulted in the formation of product in low
yields (Table 1, entries 5 and 6) along with the unreacted 1a.
Interestingly, the yield of the desired product 3a was
dramatically improved to 94%, when we switched the catalyst
to 10 mol % of Ag2CO3 (entry 7). Diminishing yield of the
product was observed with either lowering the catalyst loading
to 5 mol % of Ag2CO3 (entry 8) or reducing the oxidant molar
ratio to 3 equiv (entry 9). Notably, the spirocyclization did not
happen without silver catalyst, demonstrating the significant
role of the catalyst in this transformation (entry 10). In the
absence of water, only 23% of 3a was isolated, revealing that
water is essential (entry 11). The failure of ipso-annulation of
1a in the absence of 2a suggests that the acyl radical is
initiating the reaction (entry 12).
With the optimized reaction conditions in hand, the scope of
this Ag-catalyzed decarboxylative dearomatization reaction was
investigated using a series of substituted N-arylpropiolamides
with 2a (Scheme 1). First, the effect of the substituents on the
alkynyl moiety was studied. N-(4-Methoxyphenyl)-
propiolamides bearing the electron-donating groups such as
3,5-dimethyl (1b), OMe (1c), and Me (1d) led to spiro[4,5]-
trienones 3b to 3d in 83−88% yields. The reactions of
propiolamides containing electron-deactivating groups, such as
Motivated by the above results, we next set out to explore
the scope of various α-keto acids in the current decarboxylative
ipso-carbocyclization strategy to install diverse carbonyl groups
onto the azaspiro[4.5]trienone framework (Scheme 2). To our
delight, different aliphatic carbonyls could be successfully
introduced by employing the reaction of 1a with aliphatic α-
B
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Scheme 2. Scope of α-Keto Acids
Scheme 4. Synthesis of Pyrrolo-[2,1-j]quinolones
keto acids 2b−2d to obtain the spirocycles 3r to 3t in 85−90%
yield. Gratifyingly, the reaction has expanded to phenylketo
acid 2e to attain the 3-benzoylspiro[4,5]trienone 3u in 76%
yield.
To expand the scope of this domino approach, we have
verified the possibility of using carboxylic acids as an alkyl
radical source with respect to the formation of a novel class of
spiro[4,5]trienones (Scheme 3). Delightfully, the reaction of
propiolamide 6a with 2b ensued efficiently, giving the acylated
pyrrolo-[2,1-j]-quinolone 7a in 74% yield. Other α-keto acids
2d and 2e smoothly underwent reaction with 6a, furnishing
tricyclic products 7b (78%) and 7c (73%), respectively.
Replacing the phenyl group on the alkyne with 4-Cl-Ph (6b)
and 4-Ac-Ph (6c) had no deleterious electronic effect on the
reaction, providing the acyl-trienones 7d to 7i. The alkylative
ipso-cyclization of 6a with 4d was also equally progressed to
afford the tert-butyl-pyrrolo-[2,1-j]-quinolone 7j in 85% yield.
To shed light on the possible reaction mechanism, a couple
of control experiments was performed. The addition of a
radical scavenger (TEMPO) into the reaction suppressed the
ipso-annulation (by forming TEMPO adduct 8),14 indicating
that the reaction seemingly proceeds via a radical pathway
(Scheme 5a). Besides, to know the source of oxygen, 1a was
Scheme 3. Synthesis of 3-Alkylspiro[4,5]trienones
Scheme 5. Control Experiments
1a with acetic acid (4a) under the optimized conditions (see
Supporting Information) led to 3-methylspiro[4,5]trienone 5a
via domino decarboxylative alkylation/ipso-cyclization. Like-
wise, other carboxylic acids such as propionic acid (4b),
isobutyric acid (4c), pivalic acid (4d), and 2-phenylacetic acid
(4e) are well suited to provide 5b (78%), 5c (83%), 5d (87%),
and 5e (72%), respectively. The cyclic secondary alkylation
was also realized using cyclohexyl (4f) carboxylic acids,
affording the product 5f in 78% yield. Sterically hindered 1-
adamantane-carboxylic acid (4g) and 3-hydroxyadamantane-1-
carboxylic acid (4h) were also effective in reacting with 1a to
produce tertiary alkylated products 5g and 5h in 82% and 80%
yields, respectively.
treated in CH3CN/H2O18, which provided the 18O-integrated
spiro[4,5]trienone 18O-3a (Scheme 5b). This supports the
source of oxygen is H2O during the oxidative dearomatization.
Based on the aforesaid outcomes and relevant literature
precedents,6b,15 a possible reaction pathway is proposed
(Scheme 6). Initially, Ag(I) is oxidized with stoichiometric
K2S2O8 to give Ag(II), which engages the α-keto acid 2a to
generate the acyl radical A involving decarboxylation. Next,
addition of the acyl radical A onto alkyne 1a produces the vinyl
radical B, which undergoes intramolecular ipso-cyclization and
dearomatization to give the intermediate C. Afterward,
oxidation of C to oxocarbonium D followed by nucleophilic
attack of hydroxide (generated from H2O) provides the
intermediate E. Finally, hydrolysis of hemiacetal E delivers the
product 3a.
To further accentuate the compatibility of this approach, we
examined the tolerance of N-(alkynoyl)-6-methoxytetrahydro-
quinolines to obtain pyrrolo-[2,1-j]-quinolone, a challenging
framework found in tricyclic marine alkaloids3c (Scheme 4).
The domino acylative ipso-cyclization of quinoline-based
In summary, an unprecedented approach, for the synthesis
of 3-acyl/alkyl-spiro[4.5]trienones from N-arylpropiolamides,
has been developed involving silver-catalyzed decarboxylative
C
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01588
Scheme 6. Plausible Mechanism
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to Dr. J. S. Yadav on the occasion of his 70th
birthday. M.S. and M.A. thank Council of Scientific and
Industrial Research (CSIR), New Delhi for research funding
and fellowships. D.H.K. thanks the University of Grants
Commission (UGC), New Delhi for a fellowship. S.K.P. thanks
the Science and Engineering Research Board (SERB), New
Delhi for funding (PDF/2017/000509) [CSIR-IICT Commu-
nication No. IICT/Pubs./2020/117].
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acylation/alkylation followed by oxidative ipso-cyclization.
Readily available α-keto acids and alkyl carboxylic acids were
used as acyl and alkyl agents, respectively. Additionally, the
reaction has been extended for the first time to access unique
acyl/alkyl tricyclic pyrrolo-[2,1-j]-quinolones. The ease of
operation, wide substrate scope, and readily accessible starting
materials are the salient features to make the current method
valuable.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01588.
Experimental procedures, characterization details, and
1H and 13C NMR spectra of new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Chada Raji Reddy − Department of Organic Synthesis &
Process Chemistry, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, India; Academy of Scientific
and Innovative Research (AcSIR), Ghaziabad 201 002, India;
orcid.org/0000-0003-1491-7381; Email: rajireddy@
iict.res.in
Authors
Dattahari H. Kolgave − Department of Organic Synthesis &
Process Chemistry, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, India; Academy of Scientific
and Innovative Research (AcSIR), Ghaziabad 201 002, India
Muppidi Subbarao − Department of Organic Synthesis &
Process Chemistry, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, India; Academy of Scientific
and Innovative Research (AcSIR), Ghaziabad 201 002, India
Mounika Aila − Department of Organic Synthesis & Process
Chemistry, CSIR-Indian Institute of Chemical Technology,
Hyderabad 500007, India; Academy of Scientific and
Innovative Research (AcSIR), Ghaziabad 201 002, India
Santosh Kumar Prajapti − Department of Organic Synthesis &
Process Chemistry, CSIR-Indian Institute of Chemical
Technology, Hyderabad 500007, India
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