Gold-Catalyzed Oxidative Cyclization Involving Nucleophilic Attack to the Keto Group of α,α′-Dioxo Gold Carbene and 1,2-Alkynyl Migration: Synthesis of Furan-3-carboxylates

Article abstract of DOI:10.1021/acs.orglett.1c02389

A multicomponent strategy for the synthesis of functionalized furan-3-carboxylates based on gold-catalyzed oxidative cyclization of diynones with alcohols or water has been developed. Mechanistic studies revealed that a rare nucleophilic attack to the carbonyl group of the α,α′-dioxo gold carbene instead of the carbene center and 1,2-alkynyl group migration were involved in this transformation. This method offers several advantages such as mild conditions, high regio- and chemoselectivity, and wide functional group compatibility.

Full text of DOI:10.1021/acs.orglett.1c02389

pubs.acs.org/OrgLett
Letter
Gold-Catalyzed Oxidative Cyclization Involving Nucleophilic Attack
to the Keto Group of α,α′-Dioxo Gold Carbene and 1,2-Alkynyl
Migration: Synthesis of Furan-3-carboxylates
Ali Wang, Mingduo Lu, and Yuanhong Liu*
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ABSTRACT: A multicomponent strategy for the synthesis of functionalized furan-3-carboxylates based on gold-catalyzed oxidative
cyclization of diynones with alcohols or water has been developed. Mechanistic studies revealed that a rare nucleophilic attack to the
carbonyl group of the α,α′-dioxo gold carbene instead of the carbene center and 1,2-alkynyl group migration were involved in this
transformation. This method offers several advantages such as mild conditions, high regio- and chemoselectivity, and wide functional
group compatibility.
convenient protocols,5a−h such as gold-catalyzed cycloisome-
he furan rings widely occur as key structural subunits in
rization of ester-bearing enynes5a or propargyl vinyl ethers,5b
Tnumerous natural products, pharmaceuticals, and flavor
and fragrance compounds,1 and they are also useful and
gold-catalyzed reactions of 1,3-dicarbonyl sulfonium ylides and
alkynes,5c Pd-catalyzed oxidative cyclizations,5d Cu- or Co-
catalyzed [3 + 2] cycloaddition of α-diazocarbonyls with
versatile synthetic intermediates for access to heterocyclic and
acyclic compounds.2 Especially, substances containing a furan-
enamines5e or alkynes,5f,g silver-mediated oxidative cyclization
3-carboxylate core display a broad range of pharmacological
of 1,3-dicarbonyl compounds with terminal alkynes,5h etc.5
However, these protocols usually suffer from the drawbacks
such as narrow substrate scope, utilization of prefunctionalized
acyclic compounds as the substrates, limited structure diversity
of the products etc. Therefore, the development of novel and
efficient approaches to furan-3-carboxylates with wide
substrate scope and diverse substitution patterns from easily
available building blocks is highly desirable.
In recent years, gold-catalyzed oxygen transfer reactions
using pyridine/quinoline N-oxides, sulfoxides, or nitrones as
the oxidants have emerged as efficient methodologies for the
construction of carbo- or heterocycles.6 In most cases, the
highly electrophilic α-oxo gold carbene intermediates are
activities.3 For example, dihydroxypyrrolidine-linked furan is a
selective β-galactosidase inhibitor;3a S-linked fucosides show
affinity toward E- and P-selectins;3b providencin displayed
modest in vitro cytotoxicity against MCF7 breast cancer;3c and
(+)-wortmannin is a potent PI3K inhibitor3d (Figure 1). While
numerous strategies for the synthesis of furans have been
developed, the efficient routes to furan-3-carboxylates are still
limited. Frequently used methods for these compounds are
direct functionalization of furan-3-carboxylates.4 Recently, a
variety of transition-metal-catalyzed reactions have emerged as
Received: July 17, 2021
Figure 1. Typical examples of biologically active furan-3-carboxylates.
https://doi.org/10.1021/acs.orglett.1c02389
© XXXX American Chemical Society
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A

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formed, which can be captured by nucleophiles to initiate the
cascade reactions. A particularly attractive strategy for the
generation of α,α′-dioxo gold carbenes is based on the use of
ynones or propiolaldehydes as the substrates due to the
enhanced electrophilicity and regioselectivity arising from the
polarized triple bond (Scheme 1a). In principle, both of the
Scheme 2. Optimization Studies
Scheme 1. Gold-Catalyzed Oxidative Reactions of Ynones
a
The yields were determined by 1H NMR using 1,3,5-trimethox-
b
ybenzene as an internal standard. 1.0 equiv of 2a, and 1.0 equiv of 3a
were used. Isolated yields. 2 mol% of IPrAuNTf2 was used, and the
reaction was stirred for 20 h.
c
d
two keto groups and gold-carbene moiety can serve as an
electrophilic center and would be attacked by a nucleophile.
However, most of the reactions involve nucleophilic attack to
gold carbenes (path a)7 (Scheme 1b), and the selective attack
to the keto groups remains a great challenge. During our
ongoing project on gold-catalyzed reactions of ynones,8,9 we
envisioned that the use of diynones may have an important
impact on the reaction pathways, which may allow the efficient
attack of the nucleophile to the C-3 keto group (path b) due to
the high reactivity and less steric hindrance of this site
(Scheme 1c). Herein, we disclosed that the expected reactivity
could be achieved using diynones as the substrates, enabling
efficient access to furan-3-carboxylates or furan-3-carboxylic
acids with wide structural diversity through gold-catalyzed
oxidative cyclization of conjugated diynones with alcohols or
water. Interestingly, a selective 1,2-alkynyl vs 1,2-Nu shift to
gold carbene was also observed (Scheme 1c). It is noted that
the intermolecular reactions of the gold carbene species with
external nucleophiles are quite rare.10
only 20% yield of 4a (entry 3). AuBr3 was significantly less
efficient (entry 4). Among the screened gold catalysts, the N-
heterocyclic carbene gold(I) complex showed higher activity,
and 77% of 4a could be achieved using IPrAuNTf2 as the
catalyst (entry 5). When the reaction was carried out with 5
mol % of IPrAuCl and 10 mol % of AgNTf2, the yield of 4a
decreased to 58% (entry 6). Activation of IPrAuCl by other
silver salts such as OTs or BF4 was less effective (entries 7 and
8). To our delight, 82% of 4a was obtained through decreasing
the reaction temperature to 60 °C (entry 10). Other N-oxides
showed lower reactivity (entries 11−13). The solvent screen-
ing (MeCN, toluene, and THF) indicated that toluene was
also suitable for this reaction (entries 14−16). In the absence
of a gold catalyst, no desired product was formed (entry 17).
The use of AgNTf2 also failed to give the desired product
(entry 18).
With the optimized reaction conditions in hand, we next
examined the substrate scope. A broad range of diynones with
different R1 or R2 groups were compatible for this reaction.
First, we checked the effects of the R1 group at the alkyne
terminus on this reaction. For aryl alkynes, whenever it bears
electron-donating groups (p-tBu, p-OMe) or electron-with-
drawing groups (p-F, p-Br, and p-CO2Me), all worked very
well to afford 4b−4f in 67−81% yields (Scheme 3). Notably,
sterically encumbered o-Me-aryl alkyne transformed into
product 4g in excellent yield within 10 h. 2-Naphthyl-
substituted diynone proceeded efficiently (4h). The reaction
with thienyl-substituted diynone was also suitable (4i).
Alkenyl-substituted substrate afforded 4j in moderate yield. A
The requisite diynones can be easily prepared by Cadiot−
Chodkiewicz cross-coupling of propargyl alcohols with alkynyl
bromide11 followed by oxidation. To study the feasibility of the
hypothesis, we initially investigated the gold-catalyzed
oxidative reaction of 1,5-diphenylpenta-2,4-diyn-1-one 1a
using cinnamyl alcohol 3a as the nucleophile in the presence
of 3,5-dichloropyridine N-oxide 2a. Gratifyingly, furan-3-
carboxylate 4a with blue fluorescence could be formed in
23% yield using 5 mol % of PPh3AuNTf2 as the catalyst in
DCE at 80 °C for 6 h (Scheme 2, entry 1). The use of
Johnphos Au(MeCN)SbF6 (catalyst A) improved the yield of
4a to 63% (entry 2). However, the use of a gold complex with
t
a more crowded ligand such as BuXphos (catalyst B) led to
B
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Scheme 3. Substrate Scope of Diynones
Scheme 4. Substrate Scope of Alcohols
presence of 5 equiv of alcohols afforded the best results. Under
these reaction conditions, a large variety of alcohols could be
used as effective nucleophiles for this reaction. For example,
primary alcohols such as methanol, cyclopropylmethanol, or 2-
ethoxyethan-1-ol reacted with 1a smoothly to afford 5a−5c in
good to high yields. Secondary alcohols were found to also be
perfect substrates. For example, isopropanol, cyclododecanol,
and 2-adamantanol transformed to 5d−5f successfully. In
addition, a natural product of cholesterol was proved to be a
suitable substrate (5g). The reaction proceeded well with
tertiary alcohol such as 2-methylpropan-2-ol (5h). Benzyl
alcohol delivered the desired product 5i in 76% yield. The use
of phenol or 4-iodophenol as a nucleophile delivered 5j−5k in
52−71% yield. Alkyl allylic alcohols such as crotonyl alcohol
and geraniol could be efficiently incorporated into the products
(5l−5m). In addition, water could also be used as the
nucleophile, and 2,5-diphenylfuran-3-carboxylic acid 6 was
formed in 56% yield. Phenylhydroxamic acid reacted smoothly
with 1a to give 7 in 65% yield. These results demonstrate the
synthetic utility of this methodology.
To understand the reaction mechanism, various control
experiments were carried out. It was known that ynones could
undergo 1,3-oxygen transposition under gold catalysis.12
Treatment of 1c or 1l with 5 mol % of IPrAuNTf2 resulted
in no formation of the transposed products via mono or double
1,3-oxygen transposition. Thus, it is likely that 1,3-oxygen
transposition is not involved in our reaction (Scheme 5, eq 1).
To learn if the α,α′-dioxo gold carbene intermediate is
variety of alkyl-substituted diynones cyclized smoothly. For
example, normal alkyl, phenylethyl, tert-butyl, and cyclopropyl
groups were all compatible to give 4k−4n in 45−80% yields.
Next, we examined the effects of the R2 group on the keto
moiety. The electronic properties of the R2 group were
explored. Aryl ketones with p-Me, p-Cl, p-Ph, m-Br, and o-F
groups on the aryl rings all could be used as effective substrates
for this reaction (4o−4s). It is noted that when m-Br- or o-F-
substituted ketones were used as the substrates higher yields of
4r−4s (80−94%) were obtained, possibly due to the enhanced
reactivity of diynones by these electron-withdrawing groups. 2-
Naphthyl-substituted ketone reacted smoothly (4t). However,
with a 9-phenanthrenyl substituent, the desired 4u was formed
in only 46% yield. This is possibly due to the steric effect of the
9-phenanthrenyl group, and the substrate bearing the 2-furanyl
group led to 2,2′-difuran 4v in 65% yield. The reaction was
also applicable to alkenyl- or dienyl-substituted substrates,
which provided 4w−4x in 62% and 40% yields, respectively.
The alkyl ketone could also be used, leading to 4y in 45% yield.
The structure of 4i was confirmed by X-ray crystallographic
analysis.
We next turned our attention to explore the scope of
alcohols using 1a as the reaction partner (Scheme 4). We
found that in the case of common alcohols the use of 5 mol %
of IPrAuCl and 10 mol % of AgSbF6 as the catalyst in the
C
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Scheme 5. Control Experiments
Scheme 6. Plausible Reaction Mechanism
In summary, we have developed a new and multicomponent
strategy for the synthesis of functionalized furan-3-carboxylates
based on gold-catalyzed oxidative cyclization of diynones with
alcohols or water. Mechanistic studies revealed that a rare
nucleophilic attack to the carbonyl group of the α,α′-dioxo
gold carbene and 1,2-alkynyl group migration were involved as
the key steps for this transformation. This method offers
several advantages such as mild reaction conditions, high
regioselectivity, wide functional group compatibility, and easily
accessible starting materials. Further applications of this
chemistry with a wide range of nucleophiles are in progress.
involved or not in this reaction, we synthesized 2-diazo-1,3-
dione 8. The reaction of 8 with allylic alcohol 3a under the
standard conditions afforded the same product 4a in 23% yield,
indicating that possibly the reaction proceeds via an α,α′-dioxo
gold carbene (Scheme 5, eq 2). When ynone 9 was used
instead of diynone 1a, the product 10 was formed via
nucleophilic attack of MeOH to gold carbene followed by 1,2-
aryl migration (Scheme 5, eq 3). The results suggest that in the
reactions of diynones with alcohols a 1,2-alkynyl migration
might also be involved. To understand the deauration process,
the cyclization of 1a with 5.0 equiv of CD3OD was performed.
Furan 4a-d with significant deuterium incorporation at the C-4
position was observed (Scheme 5, eq 4). These results strongly
support the formation of a C-4-aurated furan intermediate. An
18O-labeling experiment with 18O-labeled substrate 18O-1a was
also performed. It converted to the corresponding furan 18O-4a
in 75% yield, in which the 18O label is located at oxygen of the
furan ring, indicating that this oxygen atom comes from the
diynone (Scheme 5, eq 5).
ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02389.
Experimental details and spectroscopic characterization
of all new compounds, (PDF)
Accession Codes
CCDC 2091384−2091385 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
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AUTHOR INFORMATION
Corresponding Author
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Based on the above results, a plausible reaction mechanism
is given in Scheme 6. Initially, alkyne activation by gold occurs
to afford a π-alkyne gold complex, which is attacked by N-
oxide regioselectively to give an alkenyl gold intermediate 11.
11 fragmentizes into the α,α′-dioxo gold carbene species 12 via
N−O bond cleavage. Subsequent nucleophilic attack of the
alcohol to the highly electrophilic carbonyl group adjacent to
the alkyne moiety generates intermediate 13. This is followed
by 1,2-alkynyl migration (pinacol type) to give intermediate
14, which might be in equilibrium with 15 due to the proton
shuttle between two carbonyls. Deauration of 15 affords enol
intermediate 16. Then Z/E isomerization followed by
nucleophilic attack of the oxygen to alkyne and protodeaura-
tion lead to the furan products and regenerates the gold
catalyst.
Yuanhong Liu − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy of Sciences, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China; orcid.org/
0000-0003-1153-5695; Email: yhliu@sioc.ac.cn
Authors
Ali Wang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy of Sciences, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
Mingduo Lu − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
D
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Shanghai Institute of Organic Chemistry, University of
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Chinese Academy of Sciences, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02389
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Key Research and Development
Program (2016YFA0202900), the Science and Technology
Commission of Shanghai Municipality (18XD1405000), and
the Strategic Priority Research Program of the Chinese
Academy of Sciences (XDB20000000) for financial support.
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