Furfuryl Cation Induced Cascade Formal [3 + 2] Cycloaddition/Double Ring-Opening/Chlorination: An Approach to Chlorine-Containing Complex Triazoles

Article abstract of DOI:10.1021/acs.orglett.8b03121

A TiCl₄-promoted cascade formal [3 + 2] cycloaddition/double ring-opening/chlorination of 2-furylcyclobutanols with alkyl or aryl azides is described. This highly efficient transformation involves the formation/cleavage of several C-N, C-Cl, C-

Full text of DOI:10.1021/acs.orglett.8b03121

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Furfuryl Cation Induced Cascade Formal [3 + 2] Cycloaddition/
Double Ring-Opening/Chlorination: An Approach to Chlorine-
Containing Complex Triazoles
Jiawei Guo,† Xiaoming Xu,† Qingzhao Xing,† Ziwei Gao,† Jing Gou,*,‡ and Binxun Yu*,†
†Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry & Chemical Engineering,
Key Laboratory of the Ministry of Education for Medicinal Resources and Natural Pharmaceutical, Shaanxi Normal University,
Xi’an 710062, China
‡Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Normal University, Xi’an 710062, China
S
* Supporting Information
ABSTRACT: A TiCl4-promoted cascade formal [3 + 2] cycloaddition/double ring-opening/chlorination of 2-
furylcyclobutanols with alkyl or aryl azides is described. This highly efficient transformation involves the formation/cleavage
of several C−N, C−Cl, C−C, and C−O bonds in a single operation. It enables the quick construction of trisubstituted 1,2,3-
triazoles with an (E)-enone moiety and a 3-chloropropyl unit. The chlorinated products are readily transformed into other
structurally diverse analogues.
Biomass-derived furan is a well-known chemical scaffold
large number of natural products contain chlorine and
Amany of them show significant bioactivity. Additionally, present in many bioactive molecules.10 It also represents a
1
versatile synthetic tool, and quite a few transformations have
been documented for it.11 Owing to its low aromaticity, a furan
ring can serve as masked alkenes, enol ethers,12 1,4-
diketones,13 and carboxylic acids.14 Meanwhile, it usually can
allow for facile ring transformation reactions into different
carbo-15,16,18 and heterocycles.17 Toward this topic, we have
explored the intermolecular cascade formal [3 + 2] cyclo-
addition/furan ring-opening of 2-furylcarbinols with organo-
azides by using the Lewis acid promoters as well as the
corresponding three-component reactions (3-CR) to synthe-
size the complex triazoles (Scheme 1a).19 The primary and
secondary 2-furylcarbinols were applied as starting materials in
these furan recyclizations. Encouraged by these results, our
efforts turned to developing more efficient furfuryl cation-
based cascade reactions to construct structurally more complex
and easily derived triazole architectures. We assumed that the
reactive tertiary 2-furylcarbinols could form more stable
furfuryl cations, which could be more favorable for the formal
[3 + 2] cycloaddition with azides. Moreover, the strained
cyclobutyl linked with 2-furan could be opened via C−C
cleavage after furan-dearomatized ring opening. Hence, this
reaction could realize a conversion from a bicyclic system to a
trisubstituted triazole (Scheme 1b).
more than 70% of all pharmaceutical products possess chlorine
or are produced using chlorine.2 Chlorine-containing mole-
cules have gained considerable attention owing to their
widespread applications in organic synthesis as versatile
building blocks and intermediates. The importance of the
development of synthetic methods for C(sp3)−Cl bond
construction has been continually pursued using atom-
economy and step-economy principles. Many existing
approaches to synthesis of alkyl chlorides start from alcohols,3
carboxylic acids,4 and diazo compounds.5 Also, olefin6 and
alkyne7 precursors can be converted into corresponding alkyl
chlorides easily via chlorination.
As an alternative strategy, direct chlorination of unreactive
alkanes can be a more attractive strategy to generate alkyl
chlorides. Recently, radical ring-opening of cycloalkanols via
C−C bond cleavage to form a C(sp3)−halide bond has been
widely reported.8 By contrast, carbon cation initiated ring-
opening of strained ring systems could barely achieve the
formation of carbon−halogen bonds during the C−C single-
bond cleavage, and the ring expansion and subsequent
rearrangements commonly were observed.9 Herein, we report
an unprecedented cascade formal [3 + 2] cycloaddition/
double ring-opening/chlorination of 2-furylcyclobutanols with
azides in which Lewis acid TiCl4 is employed not only as a
cationic promoter but also as a chlorine source for
chlorination.
Received: September 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03121
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Furfuryl Cation Induced Cascade for
Construction of Chlorine-Containing Triazoles
Scheme 2. Azide 2 Variations
With this guideline, we initially attempted the double ring-
opening cascade reaction toward constructing the trisubsti-
tuted triazoles (Table 1). Thus, 5-methyl-2-furylcyclobutanol
a
Table 1. Optimization of the Cascade Reaction Conditions
a
b
entry
conditions (equiv)
AlCl3 (1.1)/CH2Cl2
FeCl3 (1.1)/CH2Cl2
InCl3 (1.1)/CH2Cl2
ZnCl2 (1.1)/CH2Cl2
TiCl4 (1.1)/CH2Cl2
TiCl4 (1.1)/Et3N (1.5)/CH2Cl2
SnCl4 (1.1)/CH2Cl2
t(°C)/time (min) yield (%)
1
2
3
4
5
6
7
−20 to rt/20
−20 to rt/20
rt to 80/120
rt to 80/120
−20 to rt/20
−20 to rt/20
−20 to rt/20
trace
trace
0
a
Conditions: 5-methyl-2-furylcyclobutanol 1a (1.0 mmol), azide 2
(1.1 mmol), TiCl4 (1.1 mmol), CH2Cl2 (10 mL), −20 °C to rt; only
1
the E-isomer was observed by H NMR in each example; isolated
0
purified yield (average of two runs).
c
68
0
relatively low yields might be caused by the conjugation effects
of phenyl, which could weaken the nucleophilicity of azide in
the formal [3 + 2] stage. Although both primary and secondary
benzylic azides commonly are considered as unstable when
treated with Lewis acids, they were effective in affording
trisubstituted triazole products 3g−j in 43−63% yields under
the standard conditions. When the allylic azides 2k and 2l was
employed, the allyl-substituted products 3k and 3l was
obtained in 66% and 74% yields, respectively. The structures
of 3a and 3k were determined by single-crystal X-ray
diffraction analysis (Figure 1). The trans-stereospecificity of
olefins for the other products was assigned by analogy to 3a
c
30
a
Conditions: 5-methyl-2-furylcyclobutanol 1a (1.0 mmol), cyclohexyl
azide 2a (1.1 mmol), CH2Cl2 (10 mL). Isolated yield. Only the E
isomer was observed as determined by H NMR spectroscopy.
b
c
1
1a and cyclohexyl azide 2a were selected as model substrates
and subjected to our previously reported reaction conditions
using stoichiometric TiCl4 as cationic promoter in CH2Cl2.
Fortunately, the 1,2,3-triazole 3a with an (E)-enone moiety
was observed in 68% yield at ambient temperature (entry 5).
Importantly, 3a possesses a primary 3-chloropropyl side chain,
which proved the concept that tertiary furylcyclobutanols
could react with azides to achieve a formal [3 + 2]
cycloaddition/double ring-opening/chlorination. Apparently,
the Lewis acid TiCl4 serves as a Cl source in this
transformation as well. The desired product 3a was not
observed when base Et3N was added, although it is a crucial
additive in our previous 3-CR reaction conditions19b,c (entry
6). AlCl3 and FeCl3 could afford 3a in very low yields (entries
1 and 2). Similarly, weaker Lewis acids such as InCl3 and
ZnCl2 could not promote the cascade reaction (entry 3 and 4).
Additionally, SnCl4 was found to give a relatively worse result
with 30% yield of 3a (entry 7).
With the optimized conditions in hand, we studied the scope
of the reaction with different azides 2. As shown in Scheme 2,
5-methyl-2-furylcyclobutanol 1a could react well with both
secondary cyclopentyl and primary n-butyl azides, affording the
corresponding trisubstituted products 3b and 3c in 63% and
79% yields, respectively. As indicated by examples with phenyl
azides 2d−f, substituents including bromo and methyl groups
gave the corresponding triazoles in 40−45% yields. The
1
and 3k and also with H NMR.
Figure 1. ORTEP drawing of 3a (left, CCDC no. 1870537) and 3k
(right, CCDC no. 1870538).
We then applied this one-pot protocol to exam a range of
different 2-furylcyclobutanols under the optimal conditions
with cyclohexyl azide 2a (Scheme 3). Pleasingly, this series of
substrates also produced the desired 3-chloropropyl products
in moderate yields. When the substrate 1m without methyl at
the 5-position of furan was subjected to the standard
conditions, the reaction afforded the desired aldehyde 3m
with a trans double bond in a yield of 56%. 4,5-Dimethylfuran
substrate 1n was investigated and gave 3n, which has a
B
DOI: 10.1021/acs.orglett.8b03121
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. 2-Furylcyclobutanol 1 Variations
Scheme 4. Transformations of 3-Chloropropyl Triazole
respectively. It indicated that our well-established chlorine-
contained triazole synthesis via a biomass furan-based cascade
reaction could provide many opportunities for the late-stage
diversification to drug and bioactive molecule synthesis.
On the basis of the experimental results above, a plausible
reaction mechanism has been proposed (Scheme 5). The
a
Conditions: 2-furylcyclobutanols 1 (1.0 mmol), cyclohexyl azide 2a
(1.1 mmol), TiCl4 (1.1 mmol), CH2Cl2 (10 mL), −20 °C to rt; only
1
the E-isomer was observed by H NMR in each example; isolated
purified yield (average of 2 runs); Cy = cyclohexyl. The reaction was
prolonged to 60 min. The reaction was prolonged to 120 min.
b
c
Scheme 5. Proposed Reaction Mechanism
trisubstituted double bond with E-configuration. 3-Aryl-
substituted cyclobutanols containing electron-withdrawing or
-donating substituents at the para position of the benzene ring
were good substrates to afford the desired products 3o−r in
moderate yields (44−54%). Naphthyl-substituted cyclobutanol
1s was found to be compatible under optimized conditions,
leading to the desired product 3s in 48% yield. Generally, the
reaction rate of the cyclobutanol bearing a substituent was
slower than the reaction rate of the substrates without a
substituent, which could be ascribed to the more steric effects
of the former to hinder the approaching of azide. The
introduction of a more flexible n-hexyl at the 3-position of
cylobutanol of the substrate dramatically reduced the reaction
rate but did not interrupt the reactivity (63% yield).
Chlorine-contained compounds are usually useful precursors
for further functionalization and easily provide other valuable
synthetic intermediates. To demonstrate their utility, a variety
of transformations were developed by using 3-chloropropyl
triazole 3a or 3g as starting materials (Scheme 4). The chlorine
of 3a could be attacked selectively by morpholine to form C−
N bond and give 5 in excellent yield, while the existence of an
enone moiety does not affect the reaction. The iodinations of
3a and 3g occurred smoothly to convert Cl into I of the propyl
group and provided 4a and 4g in 88% and 92% yields
individually, which serve as the common intermediates for
derivatizations. The subsequent radical ring-closure reactions
with nBu3SnH/AIBN rapidly prepared the cyclohexyl-fused
triazoles 6a and 6g by forming C−C bonds. In particular,
biologically interesting molecules, such as estrone and
sclareolide derivative, can be coupled with 4a and 4g directly
to give the corresponding conjugated compounds 7 and 8,
reaction is initiated by coordination of the tertiary α-hydroxyl
with strong Lewis acid TiCl4, followed by dehydroxylation to
produce bicyclo-conjugated oxocarbenium II and a Cl anion.
Then the more electron-poor α-C, rather than C2 of furan,
undergoes nucleophilic attack of the azides to form the
aminodiazonium III. The subsequent intramolecular Friedel−
Crafts like reaction gives a formal [3 + 2] cycloaddition spiro-
intermediate with exclusive regioselectivity. Furthermore, a
synergic driving force from the aromatization of triazoles and
the nucleophilic chlorination of Cl− to strained cyclobutyl
leads to a double ring-opening of furan and cyclobutyl to
deliver zwitterions V. Finally, tautomerization of enolates leads
to the formation of the corresponding enones in an exclusive
trans-configuration, which is thermodynamically favored in this
single-step transformation. Further studies are underway to
help elucidate the mechanism of this cascade transformation.
In summary, we have developed a novel intermolecular
formal [3 + 2] cycloaddition/double ring-opening/chlorina-
tion cascade reaction with high step-economy of 2-
furylcyclobutanols and organoazides. A variety of trisubstituted
complex triazoles with a 3-chloropropyl unit and an (E)-enone
C
DOI: 10.1021/acs.orglett.8b03121
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03121.
■
S
Experimental procedures, product characterization,
copies of NMR spectra, and crystallographic data for
3a and 3k (PDF)
(7) D’Oyley, J. M.; Aliev, A. E.; Sheppard, T. D. Angew. Chem., Int.
Ed. 2014, 53, 10747.
Accession Codes
CCDC 1870537−1870538 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: goujing@snnu.edu.cn.
*E-mail: yubx@snnu.edu.cn.
ORCID
Binxun Yu: 0000-0001-8091-0669
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was financially supported by the National Key
Research Program of China (2016YFA0202403) and the
National Natural Science Foundation of China (21772117 and
21603140). We are also grateful to Ms. Xin-Ai Guo and Mr.
Min-Zhen Wang for NMR analysis, Dr. Hua-Min Sun for X-ray
crystallographic analysis of compound 3a and 3k, and Ms. Juan
Fan for mass spectrometric analysis (Shaanxi Normal
University).
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