Direct Regioselective [3 + 2]-Cyclization Reactions of Ambivalent Electrophilic/Nucleophilic β-Chlorovinyl Dithianes: Access to Cyclopentene Derivatives

Article abstract of DOI:10.1021/acs.orglett.6b02536

The highly regioselective and operationally straightforward [3 + 2] cyclizations of β-chlorovinyl dithianes with α,β-unsaturated carbonyl compounds have been developed. This protocol provides direct access to highly functionalized cyclopentenes with perfect chemo- and regioselectivities under extremely mild reaction conditions. In particular, the unprecedented cyclization allows for the selective preparation of hydroxylated cyclopentenes.

Full text of DOI:10.1021/acs.orglett.6b02536

Letter
pubs.acs.org/OrgLett
Direct Regioselective [3 + 2]-Cyclization Reactions of Ambivalent
Electrophilic/Nucleophilic β‑Chlorovinyl Dithianes: Access to
Cyclopentene Derivatives
Yongping Liang,† Junshan Lai,† Teng Liu,† and Shouchu Tang*,†,‡
‡
†School of Pharmacy and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
S
* Supporting Information
ABSTRACT: The highly regioselective and operationally straightfor-
ward [3 + 2] cyclizations of β-chlorovinyl dithianes with α,β-
unsaturated carbonyl compounds have been developed. This protocol
provides direct access to highly functionalized cyclopentenes with
perfect chemo- and regioselectivities under extremely mild reaction
conditions. In particular, the unprecedented cyclization allows for the
selective preparation of hydroxylated cyclopentenes.
ubstituted cyclopentenes and structurally related cyclo-
pentenones serve as key structural components in
unsaturated carbonyl compounds toward electrophiles as well
as nucleophiles with controlled stereoselectivity (Scheme 2).
S
numerous medicinally important compounds and biologically
active molecules.1,2 As a result, extensive development of
various strategies has been employed toward their prepara-
tion.3,4 Occurring as stepwise or concerted processes, [3 + 2]-
cycloaddition represents a particularly useful and direct
approach to prepare these five-membered carbon rings. The
most frequently applied method involves organic base-catalyzed
[3 + 2] cycloaddition of allenoates and enones (Scheme 1).5
Scheme 2. Nucleophilic and Electrophilic Reactivity Modes
of β-Chlorovinyl Ketone and β-Chlorovinyl Dithiane
Scheme 1. Formal [3 + 2] Cycloaddition of Allenoates and
Enones
These reactions are versatile, but mixtures of isomers are often
formed in most cases, and the [3 + 2] cyclization protocols can
only be of practical synthetic utility if one can control the
regioselectivity issues. Modifications of both substrates and
cyclization conditions have led to several versatile and efficient
protocols for the selective preparation of compounds
containing carbocycles.6 Nevertheless, methods for direct
formation of hydroxylated cyclopentenes are limited. Fur-
thermore, the search for new strategies that offer regioselective
methodology for the synthesis of a substantial variety of
cyclopententenes remains a topic of considerable interest.
In general, α,β-unsaturated carbonyl compounds constitute
one of the most frequently used electrophiles that readily
accommodate an appropriate nucleophile.7 Nevertheless,
alternative synthetic methods have been developed for the
successful formation of allenol/allenolates or vinyl carbanions
from α,β-unsaturated compounds.8 In terms of the reactivity
modes, these studies allow the ambivalent reactivity of α,β-
More recently, Oh and co-workers have disclosed the
nucleophilic and electrophilic reactivity modes of β-chlorovinyl
ketones in the development of novel C−C bond forming
reactions.9 It revealed a soft vinyl enolization strategy of β-
chlorovinyl ketones in which oxy-substituted [3]cumulene
derivatives as nucleophilic species reacted with a protic source9a
or aldehydes,9b and the ambivalent electrophilic reactivity mode
Received: August 24, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02536
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
with ketimine esters via a formal [3 + 2] cycloaddition
pathway.9c
Table 1. Optimization of the [3 + 2] Cyclization Reaction
between 1a and 2a
a
Among various types of organic reagents, dithiane com-
pounds serve as valuable umpolung intermediates in the field of
synthetic organic chemistry because of their unique properties
and reactivities.10−12 Recently, the demonstrated ambivalent
donor−acceptor (D−A) reactivities of β-chlorovinyl dithianes
in our group have opened up a novel class of reaction pathways
beyond the typical dithiane methods.13 The development of [3
+ 2]-cycloaddition has been achieved through identification of
reactive vinylidene dithiane species and aldehydes.13b Our
observation revealed that nucleophilic and electrophilic vinyl-
idene dithiane intermediates can be generated from a variety of
β-chlorovinyl dithianes under weakly basic conditions.
As a consequence of these observations, we therefore
envisioned that the union of a chlorovinyl dithiane and an
α,β-unsaturated carbonyl compound will greatly expand the [3
+ 2] cyclization scope and introduce a new regioselective
strategy that allows the direct construction of a highly
functionalized cyclopentene in only one step. However, such
ambivalent electrophilic and nucleophilic reactivity modes for
the regioselective two-component cyclization are uncommon.
Herein, we report our discovery of a highly regioselective,
operationally simple cyclization protocol that successfully leads
to highly functionalized cyclopentenes under extremely mild
reaction conditions. Remarkably, our design demonstrates the
first examples of the regioselective cyclization reaction of
vinylidene dithiane and enone species that displayed both
ambivalent electrophilic and nucleophilic reactivity modes.
Such a process would install a tertiary and secondary alcohols
with high chemselectivity and regioselectivity.
b
yield, (%) (3aa/
entry
1
solvent
DMSO
base (equiv)
additives
4aa)
c
t-BuOK
(3.0)
58/nd
2
3
4
5
DMSO
DMSO
DMSO
DMSO
t-BuOK
(3.0)
ZnCl2 (10 mol %)
60/8
t-BuOK
(3.0)
BF3·Et2O
(10 mol %)
57/trace
65/trace
56/trace
69/trace
t-BuOK
(3.0)
FeCl3 (10 mol %)
d
NaOH
(3.0)
d
6
7
8
DMSO
DMSO
DMSO
KOH (3.0)
e
Et3N (3.0)
nr
Cs2CO3
(3.0)
nr
Our investigations commenced with the cyclization of
equimolar amounts of β-chlorovinyl dithiane 1a and chalcone
2a under the previously reported conditions.13 As shown in
Table 1, we found promising results when employing an excess
amount of base (3.0 equiv t-BuOK in DMSO). Unexpectedly,
complete conversion was observed, and only the hydroxylated
cyclopentene 3aa was observed for a majority of product (entry
1). We next evaluated a series of Lewis acids,14 such as ZnCl2,
BF3·Et2O, and FeCl3, but they all did not significantly influence
reaction outcome and regioselectivities (entries 2−4). Interest-
ingly, we further found that the treatment of aqueous base (5 M
aq KOH)15 in a DMSO solution was slightly beneficial and
gave improved yields for the formation of the desired
cyclopentene 3aa, maintaining a good chemselectivity (entry
6). When a weak base such as Et3N or Cs2CO3 was used at
room temperature, no desired product was obtained (entries 7
and 8). Surprisingly, the solvent effect was found to directly
influence the selectivities of the cycloadducts.16 Gratifyingly,
changing the solvent from DMSO to CH3CN represented the
key to achieving high chemselectivity and did not produce any
observable amount of 3aa, and we were pleased to see that a
stoichiometric t-BuOK in CH3CN provided the single
regioisomer 4aa in 82% yield with excellent dr value (dr
>20:1) (entry 11). While the addition of Bronsted base such as
Ph3P did not facilitate the generation of 4aa (entry 13),5 the
relative configuration of the cyclopentenes was analyzed by
NMR spectroscopy, and the assignment of the structure Z-3aa
and trans-4aa was supported by X-ray crystallographic
analysis.17 In addition, it is worth noting that access to such
highly functionalized hydroxylated cyclopentene derivatives 3aa
is not readily accessible by other traditional nucleophilic
means.10
9
THF
t-BuOK
20/35
25/40
nd/82
(1.1)
10
11
toluene
t-BuOK
(1.1)
CH3CN t-BuOK
(1.1)
12
13
CH3CN KOH (1.1)
nr
CH3CN t-BuOK
Ph3P (10 mol %)
nd/80
(1.1)
a
Reaction conditions: 1a (28.2 mg, 0.11 mmol, 1.1 equiv), 2a (20.8
mg, 0.1 mmol, 1.0 equiv), and base (x equiv) dissolved in solvent (2
mL) at room temperature, 1 h. Isolated yields. No detected. 5 M
aqueous solution. No reaction.
b
c
d
e
Having established efficient conditions for regioselective
cyclization, we set out to explore the substrate scope of this
cyclization reaction for hydroxylated cyclopentenes 3. We first
examined the scope of β-chlorovinyl dithiane substrates for the
selective cyclization (Table 2). A variety of β-chlorovinyl
dithianes were tested, and the corresponding functionalized
products 3ba−ga were obtained under the optimized
conditions that make use of aq KOH. Substrates with an
electron-donating group, such as a methoxy and phenyl group,
or an electron-withdrawing group, such as a chloro or a bromo
group, at the para position of the benzene ring underwent the
reaction smoothly to provide products with excellent
regioselectivities. Next, the role of aryl substituent on α, β-
unsaturated carbonyl compounds was probed to investigate the
influence of electronic parameters on the transformation. All of
them gave the corresponding cyclopentenes in good to
excellent yields (entries 8−10). The electronic property of
aryl groups has little influence on the product yields.
Interestingly, reaction of an alkyl substrate such as benzal
B
DOI: 10.1021/acs.orglett.6b02536
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Organic Letters
Letter
Table 2. Scope of the [3 + 2] Cyclization for Hydroxylated
Cyclopentenes 3
Scheme 3. Substrate Scope of [3 + 2] Cyclization for
a
a
Cyclopentenes 4
b
entry
R
R1
R2
product yield, (%)
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3aa
3ba
3ca
3da
3ea
3fa
69
67
63
72
79
80
73
82
71
75
52
66
56
2
4-MeC6H4
4-MeOC6H4
4-EtOC6H4
biphenylyl
4-ClC6H4
4-BrC6H4
Ph
3
4
5
6
7
3ga
3ab
3ac
3ad
3af
8
4-ClC6H4
4-MeOC6H4
4-MeC6H4
Ph
4-ClC6H4
9
Ph
Ph
10
11
12
13
Ph
4-ClC6H4
Ph
Me
H
Ph
Ph
3ag
3ah
Ph
2-furyl
Ph
a
Reaction conditions: 1 (0.25 mmol, 1.1 equiv), 2 (0.225 mmol, 1.0
equiv), 5 M KOH (aq, 3.0 equiv), DMSO (2 mL), room temperature,
b
30 min to 1 h. Isolated yields.
a
Reaction conditions: 1 (0.25 mmol, 1.1 equiv), 2 (0.225 mmol, 1.0
equiv), t-BuOK (1.1 equiv), CH3CN (2 mL), room temperature, 1 h.
Isolated yields. Determined by H NMR spectroscopy.
b
c
1
acetone (entry 11) proceeded without any problems, whereas
the reaction resulted in moderate yield (3af). Most importantly,
treatment of cinnamaldehyde (entry 12) also gave the
hydroxylated cyclopentene 3ag in 66% yield with 21% of a
substituted furan compound.13b Additionally, heteroaromatic
enone was also applicable to this reaction (3ah).
Scheme 4. Gram-Scale and Synthesis of Hydroxylated
Cyclopentenone 5aa
As revealed in Scheme 3, a range of α,β-unsaturated
carbonyls were subsequently examined for the selective
formation of cyclopentenes 4 under our optimized conditions.
These reactions preceded smoothly to give the desired
cyclopentenes in good yields and with near-perfect regiose-
lectivities in most cases. In general, full and clean reaction
conversions were observed using CH3CN as an optimal solvent
within 1 h at ambient temperature without any isolable
byproducts. Electron-rich and electron-deficient chalcone
derivatives gave good yields of the corresponding cyclopentene
products (4aa−ah). Notably, the reaction of cyclopentenone
took place in 57% yield to give mainly a syn-cyclized product
4ai. Methyl acrylate and butyl acrylate also efficiently
underwent cyclization reactions and afforded the corresponding
products 4aj and 4ak, respectively, in acceptable yields. For the
subsitituted vinyl dithianes, both electron-rich and electron-
poor groups were compatible with these reaction conditions,
and no obvious substitution effect was observed (4ba−ja).
Overall, all substrates gave clean conversion to cyclopentenes
with high dr values, establishing that steric effects dominate the
perfect regioselectivity for the cyclization reaction.
To evaluate the scalability of these cyclizations, the
preparation of cyclopentene 3aa has been performed on 5
mmol scales. Comparable to the small-scale procedure,
hydroxylated cyclopentene 3aa was accessed in 59% yield
(Scheme 4). Most importantly, this method provides a
potential strategy for the construction of hydroxylated
cyclopentenone, an important structural motif that is found
among biologically active natural products such as the antibiotic
tylopilusins.18 At this stage, the utility of this methodology was
further realized by the synthesis of hydroxylated cyclo-
pentenone 5aa through a simple NCS-mediated desulfurization
process.19 However, the related desulfurization processe was
unsuitable for the cyclopentene 4aa to afford the corresponding
5-benzoylcyclopentenone compound.
To gain insight into the nature of the cyclization step, the
radical scvenger TEMPO was added into the reactions of 1a
and 2a. It was shown that the corresponding [3 + 2] cyclization
reactions were not prohibited, and products 3aa and 4aa were
obtained in good yields, indicating that the transformation
might not proceed via a free-radical pathway. To further
confirm the ambivalent electrophilic and nucleophilic behavior
of β-chlorovinyl dithianes, we found 1a underwent coupling
reaction in 32% yield to give a mixture of regioisomers in a 5:1
ratio with high stereoselectivities,20 resulting from the effect of
steric properties of vinylidene dithiane on the regioselectivity of
the vigronous process.
In summary, we have developed a regioselective protocol for
the [3 + 2] cyclization of β-chlorovinyl dithianes with α,β-
unsaturated carbonyl compounds under extremely mild
C
DOI: 10.1021/acs.orglett.6b02536
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Organic Letters
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reaction conditions that allow for the direct construction of
highly functionalized cyclopentenes. This method features
excellent chemselectivity and operational simplicity for efficient
formation of carbocycles. Further investigations into the
mechanistic details of these transformations, the asymmetric
induction, and application in natural product synthesis are
currently underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02536.
Experimental details, characterization data for the
products, NMR spectra (PDF)
Crystallographic data for 3aa (CIF)
Crystallographic data for 4aa (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: tangshch@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the Qiaohong Foundation,
PCSIRT (IRT15R28) and FRFCU (lzujbky-2016-ct02). We
thank Dr. Xiaolei Wang (Scripps Research Institute) and
Changgui Zhao (Tsinghua University), and Prof. Yingpeng Su
for helpful discussions. Dr. Yongliang Shao (SKLAOC) is
acknowledged for X-ray crystallographic data.
(11) For examples of the electrophilic reactions of 1,3-dithianes, see:
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Tetrahedron Lett. 1984, 25, 4199. (c) Kruse, C. G.; Wijsman, A.; Van
der Gen, A. J. Org. Chem. 1979, 44, 1847. (d) Picotin, G.; Miginiac, P.
J. Org. Chem. 1985, 50, 1299.
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(a) Du, W.; Lai, J.; Tian, L.; Xie, X.; She, X.; Tang, S. Chem. Commun.
2014, 50, 14017. (b) Du, W.; Tian, L.; Lai, J.; Huo, X.; Xie, X.; She, X.;
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(17) The crystallographic coordinates of 3aa and 4aa have been
deposited with the CCDC (nos. 1483969 and 1483976, respectively).
These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
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