A Re2O7catalyzed cycloetherification of monoallylic diols

Article abstract of DOI:10.1016/j.tetlet.2017.01.066

A Re₂O₇catalyzed cycloetherification of monoallylic diols is described. The reaction features short reaction time, mild reaction conditions and exclusive E selectivity. A wide range of monoallylic alcohols with alkyl or aryl substituents on olefin smoothly undergo ring closure to deliver corresponding oxa-heterocycles. The reaction is also operationally simple and not sensitive to air and moisture.

Full text of DOI:10.1016/j.tetlet.2017.01.066

Tetrahedron Letters xxx (2017) xxx–xxx
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Tetrahedron Letters
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A Re2O7 catalyzed cycloetherification of monoallylic diols
Xiaolong Wan a,c,1, Jiadong Hu b,e,1, Dongyang Xu d, Yang Shang d, Yanxia Zhen b, Chenchen Hu b, Fan Xiao b,
e,
b,c,
Yu-Peng He d, , Yisheng Lai , Weiqing Xie
⇑
⇑
⇑
a School of Materials Science and Engineering, Shanghai University, No. 99, Shangda Road, Shanghai 200444, China
b Shaanxi Key Laboratory of Natural Products & Chemical Biology, School of Chemistry & Pharmacy, Northwest A&F University, 22 Xinong Road, Yangling 712100, Shaanxi, China
c State Key Laboratory of Bioorganic & Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
d College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Dandong Lu West 1, Fushun 113001, China
e State Key Laboratory of Natural Medicines, Center of Drug Discovery, China Pharmaceutical University, 24 Tongjiaxiang, Nanjing 210009, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Re2O7 catalyzed cycloetherification of monoallylic diols is described. The reaction features short reac-
tion time, mild reaction conditions and exclusive E selectivity. A wide range of monoallylic alcohols with
alkyl or aryl substituents on olefin smoothly undergo ring closure to deliver corresponding oxa-hetero-
cycles. The reaction is also operationally simple and not sensitive to air and moisture.
Ó 2017 Elsevier Ltd. All rights reserved.
Received 26 December 2016
Revised 17 January 2017
Accepted 20 January 2017
Available online xxxx
Keywords:
Cycloetherification
Rhenium catalyst
Monoallylic alcohol
Oxa-heterocycles
Lewis acid
Oxa-heterocycles are privileged scaffold presented in natural
products (Fig. 1, e.g. gambierol1 and laurefurenyne2) and pharma-
ceutical molecules.3 Therefore, enormous protocols for construc-
tion of oxa-heterocycles have been developed to enable the rapid
and stereospecific access to such moiety.3 In this regard, direct
cycloetherification of monoallylic diols via nucleophilic substitu-
tion of allylic alcohol is a competent tool for synthesis of oxa-hete-
rocycles due to the ready availability of starting materials and
environmentally benign of this reaction (water as the only
waste).4–10 Additionally, it also provided a versatile handle (alkene)
for further manipulations of the resulting oxa-heterocycles. Since
the first Pd(0) catalyzed cycloetherification of monoallylic diols
described by Stork,4 extensive studies has been devoted to this
stereospecific cycloetherification reactions, especially using Pd(II)
catalyst.5 Subsequent experimental observations and mechanistic
studies revealed a ‘‘syn coordination, syn oxypalladation, and syn
elimination” reaction pathway.5d However, other reaction
pathways could not be totally ruled out, which depend on the
substrate structure and reaction conditions.5g Despite of the easy
availability of Pd catalysts, the high catalyst loading retards its
synthetic applications.5
Other transition metals such as Au,6 Ru7 exhibit excellent effi-
ciency on promoting diastereoselective or stereoselective
cycloetherification of monoallylic diols. However, those catalysts
suffer from high price and non-commercial availability. Lewis acid
such as FeCl3,8a BF3ÅOEt2 are also capable of catalyzing this
8b
reaction, while those reactions are limited to narrow substrate
scopes. Strong Br/nsted acids (such as HCl,9a Amberlyst-159b
)
have long been known to be efficient promoters for the
dehydration of monoallylic alcohol. However, only tertiary or
arene substituted alcohols are suitable for these reaction
conditions.
Recently,
Hall
and
co-worker
developed
intramolecular oxy-cyclization of allylic alcohol by using boronic
acid as mild catalyst.10 More recently, Qu and coworkers found
that intramolecular direct substitution of allylic alcohol by hydro-
xyl group could be realized in hot water.11 However, both Hall and
Qu’s protocols need phenyl present on olefin carbon to generate
the stable benzyl carbocation intermediate.
It’s well documented that Re(VII)-oxo complexes are potent cat-
alysts for the isomerization of allylic alcohols via concerted or
anionic reaction pathway under very mild reaction conditions.12
On the other hand, Re(VII)-oxo complexes also exhibit Lewis acid
property, which have been used as effective promoters for acetal-
ization, Prins cyclization and dienone-Phenol rearrangement.13
⇑
Corresponding authors at: Shaanxi Key Laboratory of Natural Products
Chemical Biology, School of Chemistry & Pharmacy, Northwest A&F University, 22
Xinong Road, Yangling 712100, Shaanxi, China (W. Xie).
&
E-mail addresses: yupenghe2014@163.com (Y.-P. He), yslai@cpu.edu.cn (Y. Lai),
xiewq@nwafu.edu.cn (W. Xie).
1
Both authors contributed equally to this work.
http://dx.doi.org/10.1016/j.tetlet.2017.01.066
0040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
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2
X. Wan et al. / Tetrahedron Letters xxx (2017) xxx–xxx
only resulted in full recovery of diol 1a (Table 1 entries 2–4). To
our delight, Ph3SiO-ReO3 and Re2O7 were both competent
promoters for this reaction (Table 1, entries 5 and 6). It was also
noteworthy that the reactions were completed in 5 min,
producing vinyl tetrahydrofuran 2a in excellent yields. As Re2O7
was commercially available, Re2O7 was selected as the optimal
catalyst for this reaction. Subsequent screening of various
solvents revealed that CH2Cl2 was the best reaction medium
(Table 1, entries 7–12) and coordinated solvents such as ethyl
acetate THF and CH3CN only led to partial conversion of diol 1a
even after longer reaction time (Table 1, entries 10–12).
Furthermore, catalyst loading could be reduced to 2.5 mol%
without deteriorating the isolated yield and slowing down the
reaction rate (Table 1, entries 14).
After establishing the optimal reaction conditions, the substrate
scope was subsequently examined. Placing alkyl groups, electron-
rich or electron-deficient aromatic substituents on C-1 of the allyl
alcohol had no reverse effects on the reaction rate and led to the
formation of cyclized products in very high yields
(Table 2, 2a to 2j). This could be ascribed to the very high
oxo-affinity of Re(VII)-oxo complex to allylic alcohol, enabling
the facile formation of allylic carbocation intermediate. Gram scale
synthesis of 2d was also successively implemented to showcase
the synthetic utility of this reaction. Substrate with inner allylic
alcohol (Table 2, 1k) was also reactive toward the reaction
conditions, producing corresponding tetrahydrofuran 2k in good
yields. Next, the formation of tetrahydropyran ring via this
protocol was investigated. Fast and clean reactions was also
observed for cyclization of monoallylic diols 1l to 1m, producing
tetrahydropyran 2l to 2m in good to excellent yields. Putting
substituents on C-3 slightly reduced the yields, displaying that
tetra-substituted carbon center could be constructed via this
reaction (Table 2, 2m and 2n). Allylic alcohol with appendage
secondary hydroxyl group also smoothly underwent cycloetherifi-
cation, delivering tetrahydropyran 2o in 88% yield with moderate
diastereoselectivities (dr 4:1). It should be pointed out that only
E-olefin was detected in all these reactions, suggesting a thermo-
dynamically favored reaction pathway.
Fig. 1. Selected natural products incorporated with oxa-heterocycles and Re(VII)-
oxo promoted cycloetherification of monoallylic diols.
Consequently, tandem allylic isomerization/nucleophilic cycliza-
tion catalyzed by Re(VII)-oxo has emerged as a novel strategy for
constructing oxa-heterocycles in recent years.14 As a continuation
of our work on the Re(VII) chemistry,13h,14d we envisioned that
when monoallylic diol is subjected to the action of Re2O7, an
allyic carbocation intermediate I could be generated (Fig. 1),
which would be intercepted by the appendage alcohol to
produce 2-vinyl substituted oxa-heterocycles. Herein, we would
like to report that Re2O7 catalyzes a very fast cycloetherification
of monoallylic diols under mild reaction conditions.
Our studies commenced with the cycloetherification of diol 1a
under various V(V) or Re(VII)-oxo complexes based on the facts
that those catalysts could efficiently produce allylic carbocation
intermediate from allylic alcohol.12a,12b Initially, O = VSO4, POVO
and MTO were found to be ineffective for this reaction, which
Table 1
Reaction condition optimization.a
Entry
Catalystd
Solvent
t
1 (%)b
Yield (%)b
1
2
3
4
5
6
7
8
–
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
Toluene
CH2ClCH2Cl
EtOAc
2 h
2 h
2 h
2 h
5 min
5 min
5 min
5 min
5 min
2 h
100
100
100
100
0
0
0
0
0
21
38
22
0
0
0
0
0
O=VSO4
POVO
MTO
Ph3SiO-ReO3
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
Re2O7
95
95
87
83
86
48c
36c
63c
94
95
9
10
11
12
13e
14f
THF
2 h
2 h
5 min
5 min
CH3CN
CH2Cl2
CH2Cl2
0
a
b
c
d
e
f
1 (0.1 mmol) in 0.5 mL solvent was added to a solution of catalyst (x mol%) in 0.5 mL solvent at rt.
Isolated yields.
Determined by 1H NMR using 1,4-dimethoxybenzene as inner standard.
10 mol% catalyst loading.
5 mol% catalyst loading.
2.5 mol% catalyst loading.
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X. Wan et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
Table 2
Substrate scope.a,b
Substrate
Product
Yield (%)c
2a: R3 = Pr
96
96
97
93d
98
97
98
97
98
97
83
2b: R3 = n-Bu
2c: R3 = Bn
2d: R3 = phenethyl
2e: R3 = Ph
2f: R3 = 3-MePh
2g: R3 = 3,5-Me2Ph
2h: R3 = 4-MeOPh
2i: R3 = 4-ClPh
2j: R3 = 4-BrPh
2k
2m: R1 = R2 = R3 = H, R4 = Ph
98
88
85
88
2n: R1 = R2 = H, R2 = Me, R4 = phenylethyl
2o: R1 = R3 = H, R2 = Me, R4 = Ph
2p: R1 = Cy, R2 = R3 = H, R4 = Ph
dr 4:1
a
Reaction conditions: 1 (0.2 mmol) in 1.0 mL CH2Cl2 was added to a solution of Re2O7 (2.5 mol%) in 1.0 mL CH2Cl2. The solution was stirred at rt until all
starting material was consumed as judged by TLC.
b
Reaction time was 5 min unless otherwise stated.
Isolated yields.
8 mmol scale.
c
d
Acknowledgments
We are grateful for financial support from the National Natural
Science Foundation of China (grants 21672171, 21372239 to W. X.
and grant 21202077 to Y. H.), the Scientific Research Foundation of
Northwest A&F University (grants Z111021501, Z109021600).
Financial support from the Open Fund of State Key Laboratory of
Bioorganic & Natural Products is also acknowledged.
A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2017.01.
066.
Scheme 1. Other substrates.
To explore whether the reaction proceeded stereospecifically,
the enantioenriched allyl alcohol 1c was prepared via standard
procedure using chiral allyl alcohol as starting material (see SI).
Unfortunately, the cycloetherification of 1c only produced racemic
tetrahydrofuran 2c, which implied that the allylic carbocation I
may be the reaction intermediate (Scheme 1). Based on this obser-
vation, we postulated that when monobenzyl diol 1p was
employed as starting material, cycloetherification would also take
place via formation of stable benzyl carbocation intermediate
under the action of Re2O7. Indeed, when diol 1p was exposed to
Re2O7 in CH2Cl2, the cycloetherification reaction smoothly took
place with tetrahydrofuran 2p being isolated in 73% yield.
In conclusion, Re2O7 is capable of catalyzing a rapid cycloether-
ification of monoallylic diols. The reaction runs in open flask with-
out cautious extrusion of moisture and oxygen. The formation of
allylic carbocation intermediate is also proposed as enantioen-
riched allylic alcohol only leads to racemic product under the reac-
tion conditions. Further application of this property of Re(VII)-oxo
on other type of reactions is currently under investigation in our
laboratory.
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