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alkenes can be used for the construction of 2,3- centered radicals to C-, N- or O-tethered 1,7-enynes
dihydro-1H-cyclopenta[a]naphthalenes, which may leading to the formation of structurally diverse
[
16–18]
represent a powerful strategy, although just three scaffolds.
examples were given (Scheme 1a).
Nevertheless, to the best of our knowl-
[10]
Shortly after, edge, using the radical cascade reaction of 1,7-enynes
Paleo, Sardina and coworkers disclosed that dimethyl to construct cyclopentane-fused naphthalenes has not
naphthalene-1,2-dicarboxylate could be transformed been reported. Considering the great significance of
into 2,3-dihydro-1H-cyclopenta[a]naphthalene by a 2,3-dihydro-1H-cyclopenta[a]naphthalenes and com-
two-step procedure consisting of an initial reductive bining the basis of our preliminary research in the
alkylation of naphthalene diester promoted by alkaline synthesis of cyclic ring structures through radical
[19]
metals, followed by a decarbonilation reaction of the cascade processes, we would like to report a novel
[11]
resulting unsaturated diesters with Me SnLi. Alter- Ag-catalyzed radical cascade dicyclization/aromatiza-
3
natively, Lecornué, Thibaudeau and coworkers devel- tion of C-linked 1,7-enynes with 1,3-dicarbonyl com-
oped another two-step process to obtain 2,3-dihydro- pounds for the construction of 2,3-dihydro-1H-cyclo-
1H-cyclopenta[a]naphthalene by using a Pd-catalyzed penta[a]naphthalenes derivatives (Scheme 1c). This
SuzukiÀ Miyaura coupling reaction and a boron strategy provided a practical and straightforward
[12]
trifluoride etherate-catalyzed cycloaromatization.
method to access the important potentially bioactive
Recently, Chandrasekhar and coworkers reported an 2,3-dihydro-1H-cyclopenta[a]naphthalenes with high
elegant work to assemble cyclopentane-fused naphtha- efficiency and excellent regioselectivity.
lenes by using the Lewis-acid-catalyzed intramolecular
4+2] cycloaddition/aromatization of alkyne-, allene-, phenyl)but-3-en-1-ol 1a was selected as the bench-
and alkene-tethered aryne insertion adducts mark substrate to react with pentane-2,4-dione 2a in a
In our initial investigation, 1-(2-(phenylethynyl)
[
[13]
(
Scheme 1b). Whereas the above-mentioned proto- 1:2 mole ratio for the optimization of reaction
cols provide approaches to obtain 2,3-dihydro-1H- conditions (Table 1). After a series of trials, we were
cyclopenta[a]naphthalenes, they often suffer some pleased to find that the desired product 3a could be
limitations, such as the need for noble metal catalysts, achieved in 73% yield by employing 25 mol% AgNO3
or a preassembled naphthalene skeleton, or harsh as catalyst, 2.0 equivalent of K S O as the oxidant
2
2
8
reaction conditions, or a multi-step manipulation, etc. under N atmosphere after being heated in CH CN/
2
3
Therefore, to develop a straightforward and practical H O (v/v=3/1) at 90°C for 24 h (entry 1, Table 1).
2
synthetic methodology for the construction of structur- Subsequently, a number of other silver salts, such as
ally diverse 2,3-dihydro-1H-cyclopenta[a]naphthalenes AgNO , AgOTf, AgOAc, and AgI under the same
2
is highly desirable in organic synthesis and medicinal conditions were further explored, revealing that
chemistry.
AgNO was the best choice(entries 2–5). We next
3
Nowadays, the radical cascade reaction of 1,7- turned our attention to the screening of different
enynes has been proven to be a powerful strategy to solvent systems, and it was found that other solvents,
access highly functionalized polycyclic and heterocy- such as DMF, DMSO, DMA, or acetone, were less
clic compounds with the advantages of atom- and step- effective than CH CN (entries 6–9). Moreover, other
3
[14,15]
economy.
Significant advances have been made in oxidants, including Na S O , BPO, TBPB, TBHP,
2 2 8
radical cascade addition of various C-, N-, S-, or Si- DTBP were also carefully investigated, however, the
reaction was totally inhibited or the yield of 3a was
significantly decreased for these oxidants (entries 10–
14), indicating that the choice of oxidant played an
important role in the reaction. Unfortunately, increas-
ing the loading of AgNO3 to 50 mol% did not
significantly improve the yield of 3a and decreasing
the amount of K S O to 1.0 equivalent resulted in a
2
2
8
lower yield (entries 15 and 16). Having established
AgNO as the most efficient catalyst and K S O as the
3
2
2
8
best oxidant, the reaction temperature was then
examined, with the results that a lower or higher
temperature (70 or 110 °C) were inappropriate (en-
tries 17 and 18). Finally, the reaction could also
proceed under an air atmosphere with a slight decrease
in the yield of 3a (entry 19).
With the optimized reaction conditions in hand, the
scope of this Ag-catalyzed radical cascade bicycliza-
tion/aromatization of C-linked 1,7-enynes with 1,3-
dicarbonyl compounds was investigated, and the
Scheme 1. Representative Strategies for Constructing 2,3-Dihy-
dro-1H-cyclopenta[a]naphthalenes.
Adv. Synth. Catal. 2021, 363, 3750–3755
3751
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