Gold-catalyzed tandem annulations of pyridylhomopropargylic alcohols with propargyl alcohols

Article abstract of DOI:10.1021/acs.orglett.0c04070

A gold-catalyzed tandem annulation of propargylic alcohols and pyridylhomopropargylic alcohols is achieved, providing an atom-economical approach to a diverse set of polycyclic dihydrobenzofurans in good yields. The reaction proceeds via the 5-endo-dig cyclization/Meyer?Schuster rearrangement/Friedel?Crafts-type pathway. In this way, three C?C bonds and one C? O bond form to give a polycyclic skeleton in a one-pot process. Moreover, the products exhibit unique optical properties, which reveal their potential application value.

Full text of DOI:10.1021/acs.orglett.0c04070

pubs.acs.org/OrgLett
Letter
Gold-Catalyzed Tandem Annulations of Pyridylhomopropargylic
Alcohols with Propargyl Alcohols
Xue-Song Li, Dan-tong Xu, Zhi-Jie Niu, Ming Li, Wei-Yu Shi, Cui-Tian Wang, Wan-Xu Wei,
and Yong-Min Liang*
Cite This: Org. Lett. 2021, 23, 832−836
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sı Supporting Information
ABSTRACT: A gold-catalyzed tandem annulation of propargylic alcohols and pyridylhomopropargylic alcohols is achieved,
providing an atom-economical approach to a diverse set of polycyclic dihydrobenzofurans in good yields. The reaction proceeds via
the 5-endo-dig cyclization/Meyer−Schuster rearrangement/Friedel−Crafts-type pathway. In this way, three C−C bonds and one C−
O bond form to give a polycyclic skeleton in a one-pot process. Moreover, the products exhibit unique optical properties, which
reveal their potential application value.
omogeneous gold catalysis is an effective method to
synthesize various complex molecules based on the
H
selective activation of C−C multiple bonds in the presence of
1
hetero- and carbonucleophiles. In particular, gold-catalyzed
tandem cyclization is successfully utilized in the synthesis of
diverse polycyclic architectures. Pioneering works were
2
3
4
reported by the Hashmi, Toste, and Zhang groups. In
014, gold-catalyzed tandem cycloisomerization was demon-
2
Figure 1. Representative bioactive molecules.
strated by Ye’s group in the synthesis of functionalized indole
derivatives, whereby a single metal center served a dual-
catalytic role in two different catalytic cycles, which provided a
new type of concurrent tandem catalysis. There are several
challenging tasks in this field that need to be overcome, such as
cascade reaction leading to the practical synthesis of 2,3-
5
10
dihydronaphtho[1,2-b]furans. There are also some other
types of transformations that collaborate with the intra-
molecular cycloisomerization of alkynols to build tricyclic
matching the rates of each catalytic cycle and the compatibility
6
of the catalyst with substrates and intermediates. As part of
11
compounds, such as the Povarov reaction and Prins-type
our interest in this type of transformation and propargyl
alcohol chemistry, we envisioned that the combination of gold
and copper catalysis to synthesize polycyclic skeleton
compounds from propargyl alcohol could be possible.
It is well known that the polycyclic dihydrobenzofuran
skeletons exist in a wide range of pharmacologically relevant
natural products and biologically active molecules, such as
12
cyclization. However, a cascade reaction of alkynols that
constructs polycyclic compounds with tetracyclic or pentacy-
clic skeletons has rarely been explored. Therefore, the
development of a novel protocol for their preparation is highly
desirable. Herein we report an efficient synthesis of
dihydrobenzofuran-alkaloid-type polycycles from 3-(4,5-dihy-
13
cystibenetrimerol A, melapinol B, and carasiphenol C (Figure
7
Received: December 9, 2020
Published: January 28, 2021
1
). Therefore, great endeavors have been devoted to building
8
polycyclic dihydrobenzofuran skeletons. In 2014, Lu and
coworkers reported a intramolecular nucleophilic addition
between the carbon−palladium bond and cyanogen initiated
9
by the nucleopalladation of alkynes. Subsequently, the Liu
group described an endo-cycloisomerization/C−H activation
©
2021 American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c04070
8
32
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Letter
drofuran-2-yl)pyridine and propargyl alcohol through a
tandem 5-endo-dig cyclization/Meyer−Schuster rearrange-
ment/Friedel−Crafts reaction sequence.
hols. As illustrated in Scheme 1, electron-donating groups of
methoxyl and methyl on different positions of the aromatic
We initiated our study by using propargyl alcohol 1a and 5-
Scheme 1. Substrate Scope of Tertiary Propargylic
a
(
pyridin-3-yl)pent-4-yn-2-ol 2a as the model substrates to
Alcohols
optimize the reaction conditions. After screening a range of
reaction conditions, we isolated the desired polycyclic
dihydrobenzofuran derivative 3a in 76% yield using the
catalysts BrettphosAuCl/Cu(OTf) (10 mol %) in the first
2
step and the oxidant Cu(OTf) (0.8 equiv) in the second step
2
(Table 1, entry 1). When the reaction proceeded in one step
a
Table 1. Optimization of the Reactions Conditions
b
entry
variations from reaction conditions
yield (%)
1
2
3
4
5
6
7
8
9
76
10
27
38
14
58
51
40
46
45
53
36
41
64
32
42
74
only Cu(OTf) used
2
only Sc(OTf) used
3
only AgOTf used
only AuBrettPhosCl used
AuBrettPhosCl/AgOTf (10 mol %) used
AuBrettPhosCl/AgNTf (10 mol %) used
2
AuBrettPhosCl/AgSbF (10 mol %) used
6
Au(PPh) Cl instead of AuBrettphosCl
3
10
11
12
13
14
15
16
17
AuJohnPhosCl instead of AuBrettphosCl
Au(PPh) NTf instead of AuBrettphosCl
3
2
AuXPhosNTf instead of AuBrettphosCl
2
PhCl instead of 1,4-dioxane
PhCH instead of 1,4-dioxane
3
a
Unless otherwise specified, all reactions were performed on a 0.1
100 °C instead of 140 °C
120 °C instead of 140 °C
Ar instead of air
mmol scale under the standard conditions. Isolated yields.
a
3
Reaction conditions: A mixture of alkynol 2a (0.15 mmol, 1.5 equiv)
ring (R ) afforded the desired products (3a−3e) in moderate
and catalyst (10 mol %) was added to 1,4-dioxane (0.1 M) at 140 °C
under an air atmosphere. After 7 h, propargyl alcohol 2a (0.1 mmol,
to high yields ranging from 69 to 81%. The structure of 3b was
identified by X-ray crystal structure analysis. (See the
Supporting Information.) Propargylic alcohols substituted
with other electron-donating groups (Et, 3, 5-diMe, 1f, 1g)
and electron-withdrawing groups (CF , Ph, COOMe, NO ,
1
.0 equiv) and Cu(OTf) (0.8 equiv) were added, and the reaction
2
mixture was stirred at the reported temperature for another 12 h.
b
Isolated yields.
3
2
halogen, 1h−1o) at the para positions of the benzene ring
were tested, and the cyclization products 3h−3o were obtained
in moderate to good yields. The reaction also proceeded well
with symmetrical propargylic alcohols with different electronic
under Lewis acid Cu(OTf) , Sc(OTf) , AgOTf, and
2
3
BrettphosAuCl catalysts, the desired product was afforded in
low yields (entries 2−5). Afterward, several representative Ag
1
catalysts were employed in this system instead of Cu(OTf) ,
natures (Me, F, Cl, 1p, 1q, 1r,) on the aromatic rings of R and
2
2
and the desired products were obtained in 40−58% yields
R . Unsymmetrical propargylic alcohol substrates (1s−1x)
(
entries 6−8). Following this approach, several gold catalysts
containing either electron-donating (Me, OMe) or -with-
1
2
with different steric characteristics and electronic were also
tested, but they were all inferior to AuBrettphosCl (entries 9−
1
drawing (Ph, F, Cl) groups on the phenyl rings (R , R ) did
not affect the reaction (3s−3x). To our delight, the sensitive
substituent thiophene (1y) was well tolerated in this
transformation in 49% yield.
2). The influence of solvents on the reaction was then
investigated, but other solvents led to no improvement (entries
3 and 14). Moreover, decreasing the temperature resulted in a
1
Subsequently, pyridylhomopropargylic alcohols 2 bearing
electron-donating (Me) or electron-withdrawing groups (F
significant drop in yield (entries 15 and 16). Finally, replacing
air with argon had almost no effect on the yield of the reaction
2
and Cl) on the heterocyclic (R ) generated the expected
(
entry 17), which indicated that Cu(OTf) may play a major
products 3z−3ac in 14−22% yields, as illustrated in Scheme 2.
Compounds 2 bearing parent pyridine (Scheme 1) are much
better than those bearing functionalized pyridine (3z−3ac)
due to the substituents on pyridine (2z−2ac) that reduced the
electrophilicity of the four-position of pyridine and suppressed
2
role in the oxidation process of the reaction.
With the optimized reaction conditions, the applicability of
the tandem cycloaddition reaction was explored by employing
5
-(pyridin-3-yl)pent-4-yn-2-ol with various propargylic alco-
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pubs.acs.org/OrgLett
Letter
Scheme 2. Substrate Scope of Pyridylhomopropargylic
a
Alcohols
Figure 2. Fluorescence spectra of compounds 3.
a
Unless otherwise noted, all reactions were performed under standard
conditions. Isolated yields.
the intramolecular nucleophilic addition. Tertiary and primary
homopropargylic alcohols (1ad−1ah) also proceeded well,
giving the annulation products 3ad and 3ah in 33−72% yields.
The structures of 3ad and 3ae were confirmed by X-ray
crystallographic analysis. (See the Supporting Information.)
After diverse dihydrobenzofuran molecules were obtained,
we turned our interest to the photophysical properties of
synthesized dihydrobenzofurans 3. As we all know, most of the
conjugated frameworks are fluorescent at low concentrations.
Fluorescence will weaken or even cease in solid or aggregated
states due to the aggregation-caused quenching (ACQ) effect.
To our delight, when we concentrated on the fluorescence
properties of dihydrobenzofurans 3, we found that most of the
solid products 3 showed bright blue-green light emissions
Figure 3. Fluorescence spectra of compound 3b crystal and powder.
indicate a promising perspective on their practicality in
optoelectronic devices.
To gain insight into the reaction mechanism, some control
experiments were carried out (Scheme 3). In the reaction of 5-
Scheme 3. Mechanistic Study
(Table 2, Figures 2 and 3). Surprisingly, the crystal sample 3b
exhibited green luminescence centered at 501 nm, and after
grinding in a mortar, the luminescence presented a blue-shift
emission toward 469 nm and intensification of luminescence.
These properties of mechanochromic fluorescence may
Table 2. Photophysical Properties of Compounds of 3
entry
compound
λmax (nm)
λem (nm)
Φ (%)
F
1
2
3
4
5
6
8
9
3b (crystal)
3b (powder)
294
332
334
343
351
333
344
342
338
346
338
306
352
501
469
494
495
487
502
489
479
488
501
487
522
478
7.01
5.14
2.68
2.68
5.22
10.45
3.25
2.01
2.65
1.89
3.42
1.79
3.15
(
(
pyridin-3-yl)pent-4-yn-2-ols (2a) with BrettphosAuCl/Cu-
OTf) (10 mol %) in 1,4-dioxane, the 5-endo-dig cyclo-
2
3c
3d
3h
3j
3n
3o
3p
3q
3s
3t
isomerization product 3-(5-methyl-4,5-dihydrofuran-2-yl)-
pyridine C was isolated in 63% yield. When C and 1a were
allowed to conduct with Cu(OTf) (0.8 equiv), the product 3a
was obtained in 14% yield. When C and 1a proceeded under
the standard conditions, the product 3a could be isolated in
2
4
7% yield, which may indicate that the gold catalyst plays an
10
11
12
13
14
important role in two different catalytic cycles, whereby a
nearly stoichiometric amount of Cu(OTf) may act as the
oxidant during the formation of 3.
2
On the basis of the aforementioned results and the reported
3y
14
literature, a plausible mechanistic hypothesis for this cascade
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Organic Letters
pubs.acs.org/OrgLett
Letter
annulation reaction is put forward in Scheme 4. The
mechanism is initiated by coordination of the Au complex to
orcid.org/0000-0001-8280-8211; Email: liangym@
lzu.edu.cn
Authors
Scheme 4. Plausible Catalytic Cycles
Xue-Song Li − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Dan-tong Xu − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Zhi-Jie Niu − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Ming Li − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Wei-Yu Shi − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Cui-Tian Wang − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Wan-Xu Wei − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04070
the triple bond of alkynol 2 to form intermediate B via 5-endo-
dig cyclization, followed by protodemetalation to give 2,3-
dihydrofuran C and release of the catalytic Au(I) complex.
Propargyl alcohol 1 undergoes a Meyer−Schuster rearrange-
Notes
ment in the presence of the Au(I) complex and Cu(OTf) and
2
The authors declare no competing financial interest.
affords intermediate E via an intermolecular nucleophilic attack
of 2,3-dihydrofuran C. This intermediate E undergoes
intramolecular nucleophilic addition to yield the intermediate
F, which undergoes Friedel−Crafts alkylation and the release
of a proton to furnish intermediate H. Finally, the oxidation of
intermediate H generates the product 3.
In summary, we have introduced a gold/copper-promoted
concurrent tandem catalysis system for the construction of
structurally diversified polycyclic dihydrobenzofurans from
easily prepared alkynol substrates. The sequences involve 5-
endo-dig cycloisomerization, Meyer−Schuster rearrangement,
and Friedel−Crafts-type pathways. The products act as
valuable building skeletons in many biologically active
compounds and optical materials. The photophysical proper-
ties of synthesized compounds were also investigated, and the
interesting properties of the mechanochromic fluorescence
indicate its potential application value. Further properties and
applications of the polycyclic dihydrobenzofuran skeletons will
be further pursued by our work team.
ACKNOWLEDGMENTS
■
We acknowledge support grants from the National Natural
Science Foundation of China (NSF 21772075 and 21532001).
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ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04070.
Experimental procedures, crystallographic data, com-
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AUTHOR INFORMATION
Corresponding Author
■
Yong-Min Liang − State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, China;
J. W.; Sauter, D.; Kram
̈
er, P.; Rominger, F.; Mulks, F. F.; Rudolph,
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