Synthesis of 2 H-Chromenes via Unexpected [4 + 2] Annulation of Alkynyl Thioethers with o-Hydroxybenzyl Alcohols

Article abstract of DOI:10.1021/acs.orglett.9b04421

A novel Br?nsted acid-catalyzed reaction of alkynyl thioethers with o-hydroxybenzyl alcohols via an unexpected formal [4 + 2] annulation has been developed. This metal-free protocol leads to the facile and practical synthesis of valuable polysubstituted 2

Full text of DOI:10.1021/acs.orglett.9b04421

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of 2H‑Chromenes via Unexpected [4 + 2] Annulation of
Alkynyl Thioethers with o‑Hydroxybenzyl Alcohols
Hao-Zhen Bu,‡ Hang-Hao Li,‡ Wen-Feng Luo,† Chen Luo,‡ Peng-Cheng Qian,*,†
and Long-Wu Ye*,‡,§
†Institute of New Materials & Industry Technology, College of Chemistry & Materials Engineering, Wenzhou University, Wenzhou
325035, China
‡State Key Laboratory of Physical Chemistry of Solid Surfaces and Key Laboratory for Chemical Biology of Fujian Province, College
of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
§State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A novel Brønsted acid-catalyzed reaction of
alkynyl thioethers with o-hydroxybenzyl alcohols via an
unexpected formal [4 + 2] annulation has been developed.
This metal-free protocol leads to the facile and practical
synthesis of valuable polysubstituted 2H-chromenes in mostly
good to excellent yields under mild reaction conditions and
features a wide substrate scope and excellent functional group
tolerance.
2H-Chromenes, especially the 3-substituted 2H-chromenes,
are important structural motifs which have been found in an
array of bioactive molecules and natural products (Figure 1).1,2
cycles.3,4 These intermediates serve as electron-deficient
oxobutadienes to participate in a broad variety of formal [4
+ 2] annulation reactions in the presence of organocatalysts or
transition-metal catalysts. Despite these significant achieve-
ments, these [4 + 2] annulations are generally limited to
electronically rich olefins such as vinyl ethers, vinyl sulfides,
enaminones, and styrenes (Scheme 1a). To date, no direct
methods have been developed based on using alkynes as 2-π
partners.5
Electronically rich heterosubstituted alkynes are versatile
building blocks in organic synthesis and have attracted much
Scheme 1. Catalytic Reaction of o-Hydroxybenzyl Alcohols
with Unsaturated CC Bonds
Figure 1. 2H-Chromenes in bioactive molecules and natural products.
Although many impressive methods have been established for
their synthesis in the past decades, they still suffer from
drawbacks such as multistep synthesis, limited substrate scope,
harsh reaction conditions, and low yields.2 Thus, the
development of novel strategies for the construction of 2H-
chromene scaffolds is still highly desirable, especially those
with high flexibility, efficiency, and good modularity.
In recent years, ortho-quinone methides (o-QMs), a type of
highly reactive intermediates which can be easily accessible
from the corresponding o-hydroxybenzyl alcohols, have
attracted much attention and have been widely used in the
construction of a diverse range of synthetically useful oxo
Received: December 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04421
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Organic Letters
Letter
attention in the past decade. However, the chemistry of
thioynol ethers has traditionally been far less explored6−8
compared with the well-established ynamide chemistry.9
Inspired by the above results and by our recent studies on
developing heterosubstituted alkynes for heterocycle syn-
thesis,10 we envisioned that the catalytic reaction of o-
hydroxybenzyl alcohols with electron-rich alkynyl thioethers
might lead to the formation of 4H-chromenes via the above
similar [4 + 2] annulation.4d Interestingly, the corresponding
2H-chromene motifs were obtained instead in this case.
Herein, we would like to communicate this unexpected formal
[4 + 2] annulation by Brønsted acid-catalyzed reaction of
alkynyl thioethers with o-hydroxybenzyl alcohols (Scheme 1b).
This metal-free protocol leads to the facile and practical
synthesis of valuable polysubstituted 2H-chromenes in mostly
good to excellent yields under mild reaction conditions and
features a wide substrate scope and excellent functional group
tolerance.
2H-chromenes 3 were afforded in mostly good to excellent
yields. As shown in Scheme 2, alkynyl thioethers bearing
Scheme 2. Brønsted Acid-Catalyzed Formal [4 + 2]
Annulation of Different Alkynyl Thioethers 1 with o-
a
Hydroxybenzyl Alcohol 2a
At the outset, alkynyl thioether 1a and o-hydroxybenzyl
alcohol 2a were chosen as the model substrates, and some of
the results are summarized in Table 1. Initially, various typical
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
conditions
yield (%)
c
1
TsOH
MsOH
HOTf
HNTf2
Cu(OTf)2
Zn(OTf)3
Sc(OTf)3
FeCl3
DCE, rt, 24 h
DCE, rt, 24 h
DCE, rt, 15 min
DCE, rt, 15 min
DCE, rt, 18 h
DCE, rt, 12 h
DCE, rt, 12 h
DCE, rt, 15 min
THF, rt, 24 h
toluene, rt, 3 h
<5
30
73
92
75
83
79
76
83
53
d
2
3
4
5
6
7
8
9
10
a
Reaction conditions: 1 (0.2 mmol), 2a (0.24 mmol), HNTf2 (0.02
HNTf2
HNTf2
mmol), DCE (4 mL), rt, 15 min, in vials; isolated yields are reported.
a
Reaction conditions: [1a] = 0.05 M. DCE: 1,2-dichloroethane.
b
Measured by 1H NMR using diethyl phthalate as the internal
electron-withdrawing substituents such as F, Cl, Br, and even
CF3 (entries 2−4, 8, and 9) and electron-donating substituents
such as Me and MeO (entries 5−7) on the aromatic ring were
compatible with this annulation to produce the corresponding
2H-chromenes 3a−3i in 75−98% yields. In addition, the
reaction also proceeded smoothly with the vinyl- and
cyclopropyl-substituted thioynol ethers, leading to the
expected products 3j and 3k/3l in 66 and 47−60% yields,
respectively (entries 10−12). This annulation was also
extended to other thioether-substituted substrates, and the
desired 3m−3o were formed in 79−97% yields (entries 13−
15). The molecular structure of 3f was further confirmed by X-
ray crystallography (Figure 2). Attempts to extend the reaction
to the ynamide substrates failed to produce the corresponding
2H-chromenes, and further studies in this direction are
currently ongoing.
Then, a variety of substituted o-hydroxybenzyl alcohols 2
were also explored, as outlined in Scheme 3. Typical aryl-
substituted o-hydroxybenzyl alcohols 2 (R2 = Ar) bearing both
electron-withdrawing and electron-donating groups were well
tolerated in this reaction, leading to the desired 2H-chromenes
3p−3r in 66−84% yields (entries 1−3). Of note, a longer
reaction time (4 h) was needed in the case of OMe-substituted
c
d
standard. 90% of 1a remained unreacted. 50% of 1a remained
unreacted.
Brønsted acid catalysts were investigated in the presence of
DCE as solvent at room temperature (Table 1, entries 1−4).
Although TsOH failed to catalyze this reaction (Table 1, entry
1), it was found that the use of MsOH as catalyst led to the
formation of the corresponding unexpected [4 + 2] annulation
adduct 3a in 30% yield (Table 1, entry 2). Pleasingly, further
screening of stronger Brønsted acids (Table 1, entries 3 and 4)
revealed that 92% yield of 2H-chromene 3a could be achieved
by employing HNTf2 as catalyst (Table 1, entry 4).11 In
addition, the use of other non-noble metals, including
Cu(OTf)2, Zn(OTf)2, Sc(OTf)3, and FeCl3, also produced
the desired 3a in 75−83% yields (Table 1, entries 5−8).
Finally, other solvents such as THF and toluene were also
examined but failed to improve the yield (Table 1, entries 9
and 10).
With the optimal reaction conditions in hand, the scope of
this metal-free [4 + 2] annulation of alkynyl thioethers 1 with
o-hydroxybenzyl alcohols 2 was examined. First, an array of
different alkynyl thioethers 1 were screened, and the desired
B
DOI: 10.1021/acs.orglett.9b04421
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Organic Letters
Letter
readily oxidized into the sulfonyl group by m-CPBA, followed
by hydrogenation of the double bond, leading to the final
highly functionalized chroman 3ab in excellent yield with
excellent cis diastereoselectivity (d.r. > 50:1; determined by
1
crude H NMR spectroscopy).
On the basis of the above experimental observations, a
plausible mechanism to rationalize the formation of 2H-
chromenes 3 is proposed (Scheme 5). Initially, the o-
Scheme 5. Plausible Reaction Mechanism
Figure 2. Structure of compound 3f in its crystal. Thermal ellipsoids
are shown at 50% probability.
Scheme 3. Brønsted Acid-Catalyzed Formal [4 + 2]
Annulation of Alkynyl Thioether 1a with Different o-
a
Hydroxybenzyl Alcohols 2
hydroxybenzyl alcohols 2 are activated by the Brønsted acid
to generate o-QM intermediates A. Then, intermolecular
Michael addition of electron-rich thioynol ethers 1, serving as
nucleophiles, to the intermediates A affords the sulfonium
intermediates B, which are further transformed into the four-
membered spiro intermediates C. Formally, this process can be
regarded as a [2 + 2] cycloaddition12 and is kind of similar to
Baire’s recent work on the formal [3 + 2] annulation between
alkynes and 3-indolylmethoanols involving the formation of
spirocyclobutene intermediates.5a Here, it should be men-
tioned that the carbon anion of the enolate moiety selectively
attacks the sulfonium moiety probably due to the fact that the
sulfur is softer, compared to the nitrogen atom of ynamides,
and prefers to be attacked by the carbon nucleophile but not
the oxygen nucleophile. Subsequent ring opening of four-
membered intermediates C, followed by another intra-
molecular Michael addition, leads to the target 2H-chromenes
3. Thus, this Michael addition should be the driving force for
this unexpected formal [4 + 2] annulation, as the use of vinyl
sulfides as substrates only led to the normal [4 + 2]
annulation.4d
a
Reaction conditions: 1 (0.2 mmol), 2a (0.24 mmol), HNTf2 (0.02
mmol), DCE (4 mL), rt, 15 min, in vials; isolated yields are reported.
b
Reaction time: 4 h.
substrate (entry 2). Different substituents such as Cl, Br, and
Me on the phenyl ring were also investigated, and the expected
products 3s−3u could be obtained in 75−81% yields (entries
4−6). Finally, it is notable that no enantioselectivity was
observed when the reaction was performed in the presence of
chiral Brønsted acids.
In summary, we have developed a novel Brønsted acid-
catalyzed reaction of alkynyl thioethers with o-hydroxybenzyl
alcohols via an unexpected formal [4 + 2] annulation. This
metal-free method leads to the facile and practical synthesis of
valuable polysubstituted 2H-chromenes in mostly good to
excellent yields under mild reaction conditions and features a
wide substrate scope and excellent functional group tolerance.
Importantly, this protocol represents the first example of direct
formal [4 + 2] annulation of o-hydroxybenzyl alcohols with
alkynes to our best knowledge, which significantly enriches the
chemistry of o-QMs. Further investigations into the synthetic
applications of the current protocol are in progress in our lab.
To further test the practicality of the current protocol, a
gram-scale reaction of 0.81 g of 1a with 1.20 g of 2a was
carried out with a lower catalyst loading (5 mol %), and 1.21 g
of the desired 2H-chromene 3a was formed in 75% yield,
highlighting the synthetic utility of this chemistry (Scheme 4).
Subsequently, it was found that the thioether of 3a could be
Scheme 4. Gram-Scale Synthesis and Further
Transformations of the Product 3a
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04421.
Experimental procedures and spectral data for all new
compounds (PDF)
C
DOI: 10.1021/acs.orglett.9b04421
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
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CCDC 1967706 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: qpc@wzu.edu.cn.
*E-mail: longwuye@xmu.edu.cn.
ORCID
Long-Wu Ye: 0000-0003-3108-2611
Author Contributions
H.-Z.B. and H.-H.L. contributed equally.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (21622204, 21772161, and
21828102), the Natural Science Foundation of Fujian Province
of China (2019J02001), the President Research Funds from
Xiamen University (20720180036), the Foundation of
Wenzhou Science & Technology Brueau (No. W20170003),
NFFTBS (No. J1310024), PCSIRT, and Science & Technol-
ogy Cooperation Program of Xiamen (3502Z20183015). We
thank Mr. Zanbin Wei from Xiamen University (College of
Chemistry and Chemical Engineering) for assistance with X-
ray crystallographic analysis.
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