Bi(OTf)3-mediated cycloisomerization of γ-alkynyl arylketones: Application to the synthesis of substituted furans

Article abstract of DOI:10.1021/acs.orglett.5b00246

A novel Bi(OTf)₃-mediated cycloisomerization of γ-alkynyl arylketones 4, 7, or 10 with molecular sieve (MS) in MeNO₂ affords 3-substituted furans 3, 8, or 11 at rt for 3 h in moderate to good yields. The method provides mild, less-to

Full text of DOI:10.1021/acs.orglett.5b00246

Letter
pubs.acs.org/OrgLett
Bi(OTf)3‑Mediated Cycloisomerization of γ‑Alkynyl Arylketones:
Application to the Synthesis of Substituted Furans
Meng-Yang Chang,* Yu-Chieh Cheng, and Yi-Ju Lu
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan
S
* Supporting Information
ABSTRACT: A novel Bi(OTf)3-mediated cycloisomerization of γ-
alkynyl arylketones 4, 7, or 10 with molecular sieve (MS) in MeNO2
affords 3-substituted furans 3, 8, or 11 at rt for 3 h in moderate to good
yields. The method provides mild, less-toxic, atom-economic and
efficient conditions. The mechanism has been studied and proposed.
Moreover, this route can be enlarged to gram scale.
unctionalized furan is a versatile synthetic building block in
numerous biologically active molecules, pharmaceuticals,
structure of 3-sufonyl furans 3, as shown in Scheme 2. A number
of articles have highlighted fascinating developments based on
F
and natural products.1 Development of new routes to substituted
furans (1) from readily available starting materials still represents
a continuing need in the organic synthetic field. Among these
protocols, transition metals (Pd2+, Ag+/Au3+, In3+, or Zn2+)
promoting cycloisomerization of diversified γ-alkynones (or in
situ generated from coupling of alkynes and 1,3-dicarbonyl
synthons) is a major pathway (see Scheme 1).2−4 Routes to base
Scheme 2. Synthetic Route to 3-Sulfonyl Furans 3
the skeleton of α-sulfonyl ketones (β-ketosulfones)9 and γ-
alkynones are important intermediates in organic transforma-
tions, which can be easily constructed to the other functionalized
frameworks such as dihydropyrroles or dihydrofurans.10 Sulfonyl
furans exhibit some potent biological activities, such as an
endothelial lipase inhibitor or EP1 receptor antagonist.11
Scheme 1. Metal-Mediated Syntheses of Substituted Furans
(1)
After further comparison of literature on the metal-catalyzed
benzannulation of γ-alkynones, we chose model substrate 4a (R
= Tol, Ar = Ph, Y = H) to initiate our studies with the screening of
metal triflate-mediated reaction conditions. Alkylation of 2a with
propargyl bromide afforded 4a in a 90% yield in boiling acetone
for 8 h. Initially, the use of various metal triflates as catalysts was
investigated for the formation of the 3-sulfonyl furan 3a.
Catalytic amounts of AgOTf, Mg(OTf)2, Cu(OTf)2, Ni(OTf)2,
Sn(OTf)2, Zn(OTf)2, Hg(OTf)2, Yb(OTf)3, Sm(OTf)3, Ga-
(OTf)3, In(OTf)3, La(OTf)3, Sc(OTf)3, Fe(OTf)3, and Bi-
(OTf)3 provided different results in MeNO2 at 25 °C for 3 h.
Among the catalysts screened, AgOTf, Hg(OTf)2, or In(OTf)3
provided 5a as the sole product in 81%, 86%, or 88% yields,
respectively (see Table 1, entries 1−3).4a−d Bi(OTf)3 produced
the desired product 3a in an 80% yield along with a 10% yield of
5a (entry 4). Under these conditions, the recovery of the starting
material 4a was only observed for the other 11 metal triflates.
Furthermore, the use of other Bi(III) salts was examined for
synthesizing 3a and treatment of 4a with BiCl3, BiBr3, Bi(OAc)3,
or Bi2(SO4)3 produced 5a in good yields (70%, 75%, 68%, and
66%) at 25 °C for 3 h with no isolation of 3a (entries 5−8). In
(DBU or NaH) mediated cyclization of γ-alkynones have been
documented.5 However, the preparation of furan derivatives
often presents drawbacks (e.g., multistep operations and harsh
conditions), and this has encouraged organic researchers to
explore more efficient synthetic protocols. To the best of our
knowledge, for the synthesis of furans, no literature on Bi(III)-
mediated annulation of γ-alkynones has been reported. Bi(III)
compounds with nontoxic and environmentally friendly proper-
ties has been documented for a wide variety of organic
reactions.6,7 Bi(III)-promoted reactions of γ-alkenyl,7a,b γ-
allenyl,7c or γ-alkynyl7d carbonyl synthons provide diversified
skeletons.
In continuation of our investigation of the application of α-
sulfonyl ketones 2,8 an easy and atom-economic Bi(OTf)3-
mediated intramolecular 5-exo-dig cycloisomerization of α-
sulfonyl-γ-alkynyl arylketones 4 (derived from alkylation of 2
with propargylic bromides) is employed to construct the
Received: January 26, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00246
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Conditions for the Construction of 3a
Scheme 3. Possible Mechanism
b
yield (%)
entry
catalyst (mol %), solvent (mL), temp (°C)
3a 5a
1
AgOTf (2), MeNO2 (5), 25
Hg(OTf)2 (2), MeNO2 (5), 25
In(OTf)3 (2), MeNO2 (5), 25
Bi(OTf)3 (2), MeNO2 (5), 25
BiCl3 (2), MeNO2 (5), 25
81
86
88
10
70
75
68
66
8
2
3
4
80
5
6
BiBr3 (2), MeNO2 (5), 25
7
Bi(OAc)3 (2), MeNO2 (5), 25
Bi2(SO4)3 (2), MeNO2 (5), 25
Bi(OTf)3 (5), MeNO2 (5), 25
Bi(OTf)3 (2), MeNO2 (10), 25
Bi(OTf)3 (2), MeNO2 (5), 100
Bi(OTf)3 (2), MeCN (5), 25
Bi(OTf)3 (2), (CH2Cl)2 (5), 25
Bi(OTf)3 (2), benzene (5), 25
Bi(OTf)3 (2), 1,4-dioxane (5), 25
8
9
76
72
70
8
10
11
12
13
14
15
16
17
10
12
80
c
d
d
15 (86)
27
60
With optimized conditions in hand (Table 1, entry 16), we
further explored the substrate scope of the reaction, and the
results are shown in Table 2. For the Ar and R substituents of 3
(Ar = Ph, 4-FC6H4, 4-MeOC6H4, 4-MeC6H4, 4-CF3C6H4, 4-
PhC6H4, 2-naphthalene, and 2-furan; R = 4-MeC6H4, Ph, Me, 3-
MeC6H4, 4-tBuC6H4, 4-MeOC6H4, and 4-FC6H4), the aromatic
ring, with both electron-withdrawing and electron-donating
c
26 (85)
e
Bi(OTf)3 (2), MeNO2 (5, dry), 25
86
42
f
Bi(OTf)3 (2), MeNO2 (5, wet), 25
39
a
b
Reaction was run on a 1.0 mmol scale with 4a, 3 h. Isolated yield.
c
d
72% (for entry 13) or 60% (for entry 15) of 4a was recovered. 60 h
e
(for entry 13); 45 h (for entry 15). 4 Å MS (100 mg) was added.
f
H2O (0.1 mL) was added.
a b
−
Table 2. Synthesis of 3
comparison with this Bi(III) complex, Bi(OTf)3 is an optimal
catalyst for the generation of 3a. Controlling the Bi(OTf)3 as the
catalyst, the factors of equivalence, concentration, and temper-
ature were studied next. When using 5 mol % of Bi(OTf)3, the
isolated yield was similar to 2 mol % (entry 9). No obvious
changes occurred when decreasing the reaction concentration (5
→ 10) or elevating the temperature (25 → 100) as shown in
entries 10−11, respectively. After changing the solvents (from
MeNO2 to MeCN, dichloroethane, benzene, or 1,4-dioxane),
different results were observed (entries 12−15). Entry 16
showed that removal of water decreased the yield of 3a in the
presence of molecular sieves (MS). On the contrary, the
involvement of water increased the ratio of 4a and 3a (entry 17).
The results show that the introduction of water caused
competition between intermolecular hydration (for the methyl
ketone skeleton) and intramolecular 5-exo-dig addition (for furan
skeleton) during the Bi(OTf)3-mediated process. Overall, we
believed that the Bi(OTf)3/MeNO2/MS system should be an
optimal combination for generating 3a.
On the basis of the results, a possible reaction mechanism is
shown in Scheme 3. How are 3a and 5a produced? The
mechanism should be initiated to form A by complexation of an
alkynyl motif of 4a with Bi(OTf)3, and participation of a carbonyl
group could lead to the oxonium cation B via intramolecular 5-
exo-dig annulation. Deprotonation of B should give vinyl bismuth
intermediate C. Then, protonation by in situ generated HOTf
achieves to alternative oxonium cation D, which following
debismuthation is able to provide E and recover Bi(OTf)3.
Isomerization from E forms 3a. However, F should be proposed
via the involvement of A with water. After deprotonation of F,
tautomerization of G, and then HOTf-promoted debismuthation
of β-ketobismuth intermediate H, the removal of Bi(OTf)3
affords I. Subsequently, the tautomerization of H generates 5a.
c
yield (%)
entry
2, Ar, R; Y
4
3
1
2a, Ph, 4-MeC6H4; H
4a, 90
4b, 92
4c, 88
4d, 90
4e, 85
4f, 90
4g, 86
4h, 89
4i, 90
4j, 90
4k, 86
4l, 89
4m, 86
4n, 85
4o, 87
4p, 88
4q, 85
4r, 84
4s, 83
4t, 80
3a, 86
3b, 76
3c, 75
3d, 78
3e, 80
3f, 82
3g, 78
3h, 73
3i, 75
3j, 78
3k, 76
3l, 80
3m, 82
3n, 78
3o, 75
3p, 77
3q, 78
3r, 62
3s, 60
2
2b, 4-FC6H4, 4-MeC6H4; H
2c, 4-MeOC6H4, 4-MeC6H4; H
2d, 4-MeC6H4, 4-MeC6H4; H
2e, 4-CF3C6H4, 4-MeC6H4; H
2f, 4-PhC6H4, 4-MeC6H4; H
2g, 2-naphthalene, 4-MeC6H4; H
2h, 4-FC6H4, Ph; H
3
4
5
6
7
8
9
2i, 4-MeOC6H4, Ph; H
2j, 4-PhC6H4, Ph; H
10
11
12
13
14
15
16
17
18
19
20
2k, 4-MeC6H4, Me; H
2l, 4-PhC6H4, Me; H
2m, Ph, 3-MeC6H4; H
2n, Ph, 4-tBuC6H4; H
2o, Ph, 4-MeOC6H4; H
2p, Ph, 4-FC6H4; H
2q, 2-thiophene, 4-MeC6H4; H
2r, Ph, 4-MeC6H4; Me
2s, Ph, 4-MeC6H4; Et
d
2t, Ph, 4-MeC6H4; Ph
3t
a
α-Alkylation was run on a 1.0 mmol scale with 2, K2CO3 (2.9 mmol),
propargylic bromides (1.05 mmol), acetone (10 mL), reflux, 8 h.
Cycloisomerization was run on a 1.0 mmol scale with 4, Bi(OTf)3
(13 mg, 0.02 mmol), MeNO2 (5 mL), rt, 3 h. Isolated yield.
b
c
d
Compound 5b was isolated in 78% yield.
B
DOI: 10.1021/acs.orglett.5b00246
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
substituents, was well tolerated, providing the desired products 4
and 3 in moderate to good yields. First, by controlling the use of
1.05 equiv of propargylic bromides (Y = H, Me, Et, and Ph), 4a−t
with an α-propargyl group was yielded in good yields (entries 1−
20). The cycloisomerization reaction of 4 with the combination
of Bi(OTf)3/MeNO2/MS was next examined. All entries showed
that 3a−t were isolated in a range of 60%−86% yields when Ar
and R were the alkyl or aryl groups. For 3a−q, the involvement of
different substituents did not affect the reaction procedure and
the isolated yield was maintained (entries 1−17). When Y was
changed to Me and Et groups, the isolated yields of 3r−s were
decreased to 60% and 62%, respectively (entries 18−19).
Furthermore, Y was adjusted to the Ph group, 5b was isolated
in a 78% yield, and no desired 3t was formed (entry 20). The
structures of sulfonyl 2-arylfurans 3e−f, 3h, and 3k were
determined by single-crystal X-ray crystallography.12
phenyl chalcone intermediate of 13 bearing a full conjugation
was generated easily.14
In summary, Bi(OTf)3-mediated cycloisomerization of α-
substituted γ-alkynyl arylketones is developed for synthesizing a
series of 2-arylfurans in good yields. The structures of the key
products were confirmed by X-ray crystallography. This atom-
economic route can be enlarged to gram scale. Further
investigation regarding synthetic applications of α-sulfonyl
ketones will be conducted and published in due course.
ASSOCIATED CONTENT
* Supporting Information
Experimental data and scanned photocopies of 1H and 13C NMR
spectral data. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
Moreover, when 1.56 g of 4a (5.0 mmol) was treated with the
combination, 1.17 g of 3a was isolated in a 75% yield. This route
can be enlarged to gram scale. To examine the limitation of this
route (see Scheme 4), the sulfonyl group was first changed to the
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mychang@kmu.edu.tw.
Notes
Scheme 4. Synthetic Routes to 8, 11, and 14a−b
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology of the
Republic of China for financial support (NSC 102-2113-M-037-
005-MY2).
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D
DOI: 10.1021/acs.orglett.5b00246
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