Ytterbium-Catalyzed Intramolecular [3 + 2] Cycloaddition based on Furan Dearomatization to Construct Fused Triazoles

Article abstract of DOI:10.1021/acs.orglett.0c01780

The 1,2,3-triazole-containing polycyclic architecture widely exists in a broad spectrum of synthetic bioactive molecules, and the development of expeditious methods to synthesize these skeletons remains a challenging task. In this work, the catalytic cyclization of biomass-derived 2-furylcarbinols with an azide to form fused triazoles is described. This approach takes advantage of a single catalyst Yb(OTf)3 and operates via a furfuryl-cation-induced intramolecular [3 + 2] cycloaddition/furan ring-opening cascade.

Full text of DOI:10.1021/acs.orglett.0c01780

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Letter
Ytterbium-Catalyzed Intramolecular [3 + 2] Cycloaddition based on
Furan Dearomatization to Construct Fused Triazoles
Xiaoming Xu, Ying Zhong, Qingzhao Xing, Ziwei Gao, Jing Gou,* and Binxun Yu*
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ABSTRACT: The 1,2,3-triazole-containing polycyclic architecture
widely exists in a broad spectrum of synthetic bioactive molecules,
and the development of expeditious methods to synthesize these
skeletons remains a challenging task. In this work, the catalytic
cyclization of biomass-derived 2-furylcarbinols with an azide to
form fused triazoles is described. This approach takes advantage of
a single catalyst Yb(OTf)3 and operates via a furfuryl-cation-
induced intramolecular [3 + 2] cycloaddition/furan ring-opening cascade.
intramolecular Cu(I) or other transition-metal-catalyzed [3 +
2] cycloaddition of azides with alkynes, which is often called a
“click reaction”, is capable of synthesizing the fused 1,2,3-
triazoles under mild conditions.7 Several modifications have
been disclosed to give the fused 1,2,3-triazoles via the metal-
catalyzed intramolecular C−C coupling or the C−H activation
strategies of the triazole moiety, which must be primarily
constructed by means of the click reaction.8 However, the
cytotoxicity of the transition metal could represent a crucial
drawback for pharmaceutical applications, and the develop-
ment of transition-metal-free protocols remains a continuous
challenge for synthetic chemists. In fact, organocatalytic
methods have been developed, which basically involve a
base-mediated cycloaddition/elimination cascade between a
polar methylene adduct with an azide to access fused 1,2,3-
triazoles.9
he fused 1,2,3-triazole represents one of the privileged
1−5 The preparation and
Tscaffolds in medicinal chemistry.
creation of such skeletally related triazoles is of high synthetic
and biological interest. Herein we described a novel catalytic
synthesis of a fused triazole from the azido furylcarbinol
adducts, employing a Yb(OTf)3-catalyzed intramolecular [3 +
2] cycloaddition/furan ring-opening cascade reaction.
The most straightforward protocol to access the fused 1,2,3-
triazoles is via the intramolecular 1,3-dipolar cycloaddition of
azides and alkynes, which typically involves a high temperature
of >100 °C under metal-free conditions (Scheme 1a).6 The
Scheme 1. Representative Methods for Fused Triazole
Synthesis in Comparison with the Present Work
Biomass-derived furan is a vital synthetic tool due to its low
aromaticity.10 Not only can it perform as latent alkenes, enol
ethers,111,4-diketones,12 and carboxylic acids13 but also it
normally enables facile recyclizations into various carbo-14−16
and heterocycles.17,18 Recently, a complementary triazole
synthesis directly from readily available furan derivatives was
developed by our group (Scheme 1b).19 This methodology
involves an intermolecular [3 + 2] cycloaddition between a 2-
furylcarbinol and an organo azide to form triazoles and a
subsequent furan ring-opening to give enone motif. However,
the major issue is that stoichiometric aggressive Lewis acids,
such as TiCl4 or SnCl4, were applied. They are environmentally
Received: May 27, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01780
© XXXX American Chemical Society
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A

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Table 1. Catalyst Screen for Intramolecular [3 + 2] Cycloaddition Reaction
a
b
b
entry
catalyst (mol %)
solvent/time (h)/temp (°C)
yield of 2a (%)
yield of 3a (%)
1
2
3
4
5
6
7
8
In(OTf)3 (5)
Dy(OTf)3 (5)
Sc(OTf)3 (5)
Yb(OTf)3 (5)
La(OTf)3 (5)
In(OTf)3 (5)
Dy(OTf)3 (5)
Sc(OTf)3 (5)
La(OTf)3 (5)
Yb(OTf)3 (5)
Yb(OTf)3 (2)
CH2Cl2/5/40
CH2Cl2/5/40
CH2Cl2/5/40
CH2Cl2/5/40
CH2Cl2/5/40
HFIP/1/0−rt
HFIP/1/0−rt
HFIP/1/0−rt
HFIP/1/0−rt
HFIP/1/0−rt
86
82
89
95
80
87
85
90
84
98
91
9
10
11
HFIP/1/0−rt
a
b
1
Conditions: azido furfurylcarbinol 1a (0.2 mmol), solvent (1.0 mL). Isolated yield; Z/E configuration were determined by H NMR. HFIP,
hexafluoroisopropanol.
a
unfriendly, owing to their extreme moisture/air sensitivity and
generation of metal-salt waste. The reaction often poses
significant handling risks and workup dilemmas and suffers
from poor yields when scaled up. Hence, our efforts focused on
exploring a stable and environmentally friendly catalytic
condition to realize the intramolecular [3 + 2] cycloaddition
of 2-furylcarbinols and azides while avoiding the use of a
stoichiometric transition-metal reagent to efficiently establish
the fused triazole systems (Scheme 1c).
Scheme 2. Benzylic Alcohol Scope
In our initial trials, the azido furfurylcarbinol adduct 1a was
employed to optimize the reaction conditions, and the results
are summarized in Table 1. To our delight, the intended fused-
triazole product 3a was obtained in good yield in CH2Cl2 with
the use of 5 mol % In(OTf)3, Dy(OTf)3, and La(OTf)3 in 5 h
at 40 °C (entries 1, 2, and 5). The configuration of enone was
detected to be E. Sc(OTf)3 provided a little improvement in
yield (entry 3). Markedly optimized results were obtained with
catalytic Yb(OTf)3 in 95% yield (entry 4). Using the strong
hydrogen-bond-donating solvent hexafluoro-2-propanol
(HFIP) effectively accelerated this furan recyclization, in
which the reaction was completed in 1 h at room temperature
by utilizing 5 mol % catalysts. Importantly, the resulting
product was 2a, which furnished a Z-enone. The E-isomer was
not observed in this reaction. Yb(OTf)3 was identified as a
superior agent and afforded a 98% yield (entry 10). When the
catalyst loading was decreased to 2 mol %, the yield was
slightly reduced (entry 11).
The optimized conditions with Yb(OTf)3 in CH2Cl2 and
HFIP, respectively (Table 1, entries 4 and 10), were then
applied to the intramolecular [3 + 2] cyclization (Scheme 2).
Substrates containing aromatic substituents with electron-rich
(1b) or electron-poor (1c−e) character were tolerated and
underwent the desired cascade reaction to give [6,6,5]-fused
tricyclic products 2b−e and 3b−e in 90−95% yields. With the
acid-sensitive substrate 1f containing an acetal group, which
was undisturbed, the yields of 2f and 3f were excellent. The
reaction of naphthyl adduct 1g occurred to afford 2g and 3g in
brilliant yields. Gratifyingly, the exclusive Z- or E-config-
urations were observed for these products.
a
Conditions: azido furfurylcarbinol 1 (0.2 mmol), 10 mL sealed tube.
Yb(OTf)3 (0.01 mmol), HFIP (1 mL), 0 °C−rt, 1 h. Yb(OTf)3
b
c
(0.01 mmol), CH2Cl2 (1 mL), 40 °C, 5 h.
1i gave trisubstituted enones 2i in 92% yield under the
conditions of Yb(OTf)3 in HFIP. However, a 1:1 mixture of 3i
and 2i was generated in 90% total yield when the condition
was changed to CH2Cl2. Meanwhile, the reaction needed to be
heated to 100 °C. Interestingly, adduct 1j without any
substituent on the C-5 position of furan afforded the trans
product 2j in both CH2Cl2 and HFIP, which might be caused
by the higher reactivity of the resulting conjugated aldehyde
compared with the previously mentioned ketones.
Encouraged by the success of with the synthesis of fused
benzo triazoles, aliphatic azide adducts were then investigated,
giving the results summarized in Scheme 4. The reactions of a
variety of aliphatic azides containing furfurylcarbinols
proceeded smoothly under the standard conditions to afford
Subsequently, we studied the furan scaffold scope (Scheme
3). n-Pentyl-substituted furan 1h was found to work well and
afforded the corresponding products 2h and 3h in high yields
with exclusive Z/E selectivities. 4,5-Dimethyl furfurylcarbinol
B
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a
2p, and 3p were obtained. X-ray diffraction analysis further
confirmed the structures of 3n (CCDC no.1965895) and 3o
(CCDC no.1965896). The cyclo substrates 1q−s containing a
quaternary azide side chain can be used to carry out the
reactions and afforded the desired spiro systems 2q, 3q, 2r, 3r,
2s, and 3s in decent yields. Spiro ketal compound 1t was also
found to be compatible with the reaction conditions, giving the
cycloaddition products 2t and 3t in 90 and 86% yields,
respectively. It is worth noting that most of the previously
mentioned reaction systems can provide pure enough NMR
spectra of desired products after regular workup without
further purification, indicating the excellent conversion of this
new transformation. The transannular [3 + 2] cycloaddition of
furfurylcarbinol with a secondary azide reacted, delivering the
bridge triazole 3u with great Z/E selectivities in CH2Cl2, but it
gave a low yield of 19%. Unfortunately, the reaction in HFIP
generated a complicated unidentified mixture, and the desired
Z-product could not be observed.
Scheme 3. Furan Scaffold Scope
a
Conditions: azido furfurylcarbinol 1 (0.2 mmol), 10 mL sealed tube.
To gain a deeper insight into the Z/E stereoselectivities in
two different solvents, we monitored the reaction of azido
b
c
Yb(OTf)3 (0.01 mmol), HFIP (1 mL), 0 °C−rt, 1 h. Yb(OTf)3
(0.01 mmol), CH2Cl2 (1 mL), 40 °C, 5 h. Reaction needed to be
heated to 100 °C and afforded a mixture of 2i and 3i (1:1).
d
1
furfurylcarbinol 1a in CD2Cl2 (0.1 M) by in situ H NMR
(Figure 1). The conversion of 1a could not be found at room
temperature in 5 min. When the sample in NMR tube was
heated to 40 °C, the 1H NMR signals of Z-β-triazole-enone 2a
were observed, along with the consumption of 1a within 30
min and 60% conversion within 50 min. The formation of the
appreciable E-product 3a was observed within 110 min; then,
1a totally disappeared within 130 min. The isomerization of 2a
the corresponding [5,5], [5,6], and [5,7] bicyclo triazoles 2k,
3k, 2l, 3l, 2m, and 3m in excellent yields with great Z/E
selectivities. When the secondary azides were fixed on the
cyclopentane or cyclohexane (1n−p), the more complex
[5,6,6], [5,6,5], and [5,5,6] tricyclo triazoles 2n, 3n, 2o, 3o,
a
Scheme 4. Various Fused Triazoles
a
Conditions: azido furfurylcarbinol 1 (0.2 mmol), Yb(OTf)3 (0.01 mmol) for HFIP (1 mL) under 0 °C in 1 h, or Yb(OTf)3 (0.01 mmol) for
b
c
d
CH2Cl2 (1 mL) under 40 °C in 5 h, 10 mL sealed tube. Reaction time was prolonged to 8 h. Under 60 °C. Yb(OTf)3 (0.03 mmol) was used.
e
Reaction afforded an unidentified mixture in HFIP, and no desired Z-product was observed.
C
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Scheme 5. Proposed Reaction Mechanism
of B generates triazole, and the subsequent ring-opening of
furan delivers the enolate C, which has been observed in in situ
NMR studies. A further tautomerization gives D. Of note, in
CH2Cl2, the ytterbium catalyst coordinated with the oxygen
atom of the Z-enone group could activate the conjugated
double bond and lead to the occurrence of the isomerization
from the Z-configuration to the thermodynamically more
favorable E-configuration. The intermediate E undergoes the
dissociation of Yb(OTf)3 to afford desired E-product 3. The
high dielectric constant (ε = 15.7) and low nucleophilicity
mean that HFIP is an ideal solvent in which to generate and
study cations.20 The strong effects of HFIP on the stabilization
of allyl21 and benzylic22 cations with nucleophiles have been
proven. When the Yb-catalyzed reaction is run in HFIP
medium, it is feasible that furfuryl cation A benefits from the
stabilized effects, and a significant acceleration has been
observed compared with the reaction in CH2Cl2. Moreover,
after generating D, a fast exchange of the catalyst with HFIP
could occur, finally giving Z-product 2. The relatively low
acidity of HFIP could not result in the further isomerization of
double bonds.23
To test the practicality of this methodology, the gram-scale
reactions using 1a as a substrate were carried out (Scheme 6).
Figure 1. In situ 1H NMR study of the reaction of 1a (0.1 M, CD2Cl2,
and HFIP-D2).
Scheme 6. Further Transformations
continually occurred, and full conversion was observed after
1
200 min, affording pure H NMR spectra of E-product 3a.
Then, we probed the solvent effects of the reaction in HFIP-
1
D2 through in situ H NMR and found that the conversion of
1a into the desired 2a was quickly accomplished within 15 min
at 25 °C. Although the reaction temperature was successively
elevated to 40 and 60 °C for 60 min, Z-product 2a was still
1
stable, and the H NMR signals of 3a could not be observed.
The isomerization of 2a was detected until the temperature
was elevated to 80 °C, and only a 50% conversion of 2a into 3a
was reached at 180 min. Therefore, the results indicated that E-
products are produced by the thermodynamic isomerization of
Z-products under the condition of CH2Cl2 rather than being
directly generated from adducts. Furthermore, HFIP is capable
of stabilizing the Z-configuration of β-triazole-enone, inhibiting
its isomerization into the thermodynamically more favored E-
configuration at ambient temperature.
On the basis of the above experimental findings, a plausible
mechanism is proposed in Scheme 5. The initial step involves
the Yb(OTf)3-mediated dehydroxylation of azido furfurylcar-
binol 1 to furnish furfuryl cation A,24 which could react with
the azide group via an intramolecular formal [3 + 2]
cycloaddition to give the spiro cation B. The aromatization
Pleasingly, the catalyst Yb(OTf)3 can be reduced to 1 mol %,
providing 2a and 3a in 92 and 90% yields in CH2Cl2 and
HFIP, respectively. Additionally, to demonstrate the utility of
the products, further manipulation reactions were conducted.
For example, the Diels−Alder reaction of 3a with 2-methyl
furan under BF3·Et2O as the Lewis acid proceeded smoothly to
provide 4 in 88% yield, whereas the Corey−Chaykovsky
reaction of 3a was performed to give cyclopropane 5 in 87%
D
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yield. Interestingly, we found that the oxidative cleavage of the
double bond of 3a can be achieved with a mild condition of
H2O2/NaOH, offering the fused triazole aldehyde 6 in 92%
yield.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01780
Notes
In conclusion, we have developed a Yb(OTf)3-catalyzed
intramolecular [3 + 2] cycloaddition of furfurylcarbinols with
azides to devise complex enone triazoles with diverse fused
cyclic patterns. The excellent yields of products can be
achieved by low catalyst loading, even reducing to 1 mol % to
accomplish the full conversion of a gram-scaled reaction.
Notably, the catalytic reaction is capable of providing the high
Z/E selectivity of enones by the judicious choice of CH2Cl2 or
HFIP solvents. Studies focused on the detailed mechanistic
aspects, the synthetic scope, and the applications of this
reaction are underway.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financially supported by the National
Natural Science Foundation of China (21772117) and the
Natural Science Basic Research Plan in Shaanxi Province of
China (2018JM2050). We are also grateful to Ms. Xin-Ai Guo
and Mr. Min-Zhen Wang for the NMR analysis, Dr. Hua-Min
Sun for the X-ray crystallographic analysis of compound 3n
and 3o, and Ms. Juan Fan for the mass spectrometric analysis
(Shaanxi Normal University).
ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Authors
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Ying Zhong − Key Laboratory of Applied Surface and Colloid
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Ziwei Gao − Key Laboratory of Applied Surface and Colloid
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710062, China; orcid.org/0000-0002-2547-9375
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