A streamlined synthesis of 2,3-dihydrobenzofurans via the ortho-quinone methides generated from 2-alkyl-substituted phenols

Article abstract of DOI:10.1002/adsc.201300830

A facile method for the stereoselective synthesis of trans-2,3- dihydrobenzofurans from ortho-quinone methides in situ generated from readily available 2-alkyl-substituted phenols using silver oxide-mediated oxidation has been developed. Additionally, the 2,3-dihydrobenzofurans can be further transformed into the aromatized 2,3-di-substituted benzofurans in the presence of DDQ.

Full text of DOI:10.1002/adsc.201300830

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DOI: 10.1002/adsc.201300830
A Streamlined Synthesis of 2,3-Dihydrobenzofurans via the
ortho-Quinone Methides Generated from 2-Alkyl-Substituted
Phenols
Bo Wu,a Mu-Wang Chen,a Zhi-Shi Ye,a Chang-Bin Yu,a and Yong-Gui Zhoua,*
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
Road, Dalian 116023, Peopleꢀs Republic of China
Fax : (+86)-411-84379220; phone: (+86)-411-84379220; e-mail: ygzhou@dicp.ac.cn
Received: September 11, 2013; Published online: January 22, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300830.
Abstract: A facile method for the stereoselective
synthesis of trans-2,3-dihydrobenzofurans from
ortho-quinone methides in situ generated from
readily available 2-alkyl-substituted phenols using
silver oxide-mediated oxidation has been devel-
oped. Additionally, the 2,3-dihydrobenzofurans can
be further transformed into the aromatized 2,3-di-
cesses and harsh reaction conditions, prohibiting their
wider application. Therefore, the development of
a convenient, efficient and atom-economical method
for the rapid synthesis of these derivatives is still
highly desirable.
ortho-Quinone methides (o-QMs) are emerging as
versatile intermediates in a large number of chemical
and biological processes.[11] Consequently, several ap-
proaches have been successfully developed for gener-
ating o-QMs, including tautomerization, oxidation,
acid or base catalysis, thermolysis, photolysis and ole-
fination of o-quinones which have been described in
the literature.[12,13] Recently, we reported an efficient
approach for the generation of o-QMs intermediates
under the mild basic conditions which further under-
ACHTUNGTRENNUNGsubstituted benzofurans in the presence of DDQ.
Keywords: 2,3-dihydrobenzofurans; ortho-quinone
methides; synthesis
2,3-Dihydrobenzofuran derivatives are an important went reaction with sulfur ylides to afford the trans-
class of compounds due to their remarkable signifi- 2,3-dihydrobenzofurans with high yields
cance in various biological active molecules, synthetic (Scheme 1).[14a] Considering that the o-QMs inter-
drugs and natural products.[1] For instance, (+)-cono- mediates could be conveniently generated by the oxi-
carpan which was first isolated from the wood of Con- dation of easily available 2-alkyl-substituted phenol
ocarpus erectus[2] acts as an insecticidal,[1a] antifun- derivatives,[13a–c,15] we speculated that the generated o-
gal[1b] and antitrypanosomal agent.[1c] Obtusafuran is QMs intermediates could be rapidly trapped by sulfur
a quinone reductase[1d] and propafenone-derived dihy- ylides to allow the construction of 2,3-dihydrobenzo-
drobenzofurans possess anti-multidrug resistance
properties[1e] (Figure 1). Additionally, 2,3-dihydroben-
zofurans have also been developed for the treatment
of traumatic and ischemic central nervous system
injury.[3] Thus, the synthesis of 2,3-dihydrobenzofurans
has received extensive attention in the past decades
and various efficient methodologies for the construc-
tion of 2,3-dihydrobenzofurans have been developed,
including electrocyclization,[4] radical cyclization,[5]
anionic cyclization,[6] transition metal-catalyzed cycli-
zation,[7] cycloaddition,[8] biomimetic coupling,[9]
Lewis acid-promoted reactions.[10] Although signifi-
cant progress has been made in the synthesis of 2,3-di-
hydrobenzofurans, most of these existing routes have
several drawbacks involving poor chemo- and/or dia-
stereoselectivities, unsatisfactory yields, tedious pro- Figure 1. Bioactive 2,3-dihydrobenzofuran derivatives.
Adv. Synth. Catal. 2014, 356, 383 – 387
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383

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Bo Wu et al.
salt 2a as model substrates at room temperature. The
oxidant played a crucial role in the reaction and trig-
gered the generation of o-QMs intermediates, there-
fore the oxidant was firstly thoroughly evaluated.
To our delight, the reaction proceeded smoothly to
provide the desired product 3a in 92% yield and ex-
cellent an diastereomeric ratio when Ag2CO3 was em-
ployed as oxidant (entry 1, Table 1). However, the
other oxidants, AgNO3, DDQ, BPO and PhI
displayed lower reactivity (entries 2–5)
ACHTUNGTRENNUNG
K3[Fe(CN)6] gave only moderate yields albeit with
perfect diastereoslectivity (entry 6). Additionally,
good yields and diastereoslectivity were also obtained
in the presence of Ag2O as oxidant (entry 7). Subse-
quently, different solvents were extensively examined,
and it was found that the solvent effect displayed a sig-
nificant influence on the reactivity (entries 8–12).
CH2Cl2 was proven to be the most favorable solvent
Scheme 1. The new strategy for the synthesis of 2,3-dihydro-
benzofurans.
furans, which is more atom-economical and simple in with respect to excellent yields and diastereoselectiv-
comparison to our previous work. Herein, we disclose ity. Finally, the influence of base was explored. Sever-
a straightforward and atom-economical method for al common anionic bases, such as Na2CO3, NaOH,
the rapid synthesis of 2,3-dihydrobenzofurans via the KO-t-Bu and K2CO3, all afforded the desired product
o-QMs generated from 2-alkyl-substituted phenols in in satisfactory yields (entries 12–17). For the reason
the presence of an oxidant (Scheme 1).
that K2CO3 is cheaper and easy to handle it was
Our initial investigation was conducted with 2- chosen as the best base.
alkyl-substituted phenol 1a (1.2 equiv) and sulfonium
Table 1. Optimization for the reaction of 2-alkyl-substituted phenol 1a with sulfonium salt 2a.[a]
Entry
Solvent
Base
Oxidant
Yield [%][b]
1
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Et2O
THF
toluene
DMF
EtOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Na2CO3
NaOH
Ag2CO3
AgNO3
DDQ
BPO
PhIACTHNGURETNNU(G OAc)2
K3[Fe(CN)6]
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
92
36
31
20
39
75
92
55
71
45
11
87
89
91
88
97
95
9
10
11
12
13
14
15
16
17
K3PO4
KO-t-Bu
K2CO3
[a]
Reaction conditions: 1a (0.24 mmol), 2a (0.20 mmol), base (1.2 equiv.), oxidant (2.0 equiv.), solvent (3 mL), room temper-
ature, 12 h. DDQ=2,3-dichloro-5,6-dicyanobenzoquinone; BPO=benzoyl peroxide.
Isolated yields.
[b]
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A Streamlined Synthesis of 2,3-Dihydrobenzofurans
Scheme 3. Synthesis of aromatic 2,3-disubstituted benzofur-
ans 4.
thylsulfoxonium iodide could react with 1a to deliver
the desired 2,3-dihydrobenofuran product (3m) in
moderate yield when KO-t-Bu was employed, possi-
bly owing to the fact that the strong base KO-t-Bu ac-
celerated the rate of the reaction and inhibited the di-
merization or polymerization of o-QMs.
The structure and stereochemistry of 3 were deter-
mined by the combination of NMR, HR-mass spec-
troscopy and single-crystal X-ray diffraction analysis
{for the structure of trans-N,N-diethyl-7-(4-methoxy-
phenyl)-6,7-dihydrobenzofuroACHTNUTRGENN[UG 6,5-d]ACHUTNTGREN[NUGN 1,3]dioxole-6-car-
boxamide 3l, see the Supporting Information}.[17]
The product 2,3-dihydrobenzofurans could be fur-
ther converted into the corresponding aromatic ben-
zofurans, which is a privileged scaffold in various im-
portant natural products and show a wide range of
biological activities.[18] Using DDQ as the oxidizing
agent, 2,3-dihydrobenzofuran products 3a and 3b can
be transformed smoothly to the desired aromatized
products 4a and 4b with good yields according to the
known literature method (Scheme 3).[19]
Scheme 2. Scope of the reaction of 2-alkyl-substituted phe-
nols 1 with sulfonium salts 2.
In conclusion, we have developed a facile method
for the synthesis of 2,3-dihydrobenzofurans via the o-
QMs in situ generated from readily available 2-alkyl-
With the aforementioned reaction conditions in substituted phenols using silver oxide-mediated oxida-
hand, we explored the reaction scope using a variety tion in good yields and excellent diastereoselectivity.
of 2-alkyl-substituted phenols 1 and sulfonium salts In addition, 2,3-dihydrobenzofurans were convenient-
2[16] (Scheme 2). In general, the transformations per- ly converted to the aromatized 2,3-disubstituted ben-
formed very well and moderate to excellent yields zofurans using DDQ as oxidant.
were obtained. Notably, for aryl substituents R1, the
electronic property had little influence on the yield
and diastereoselectivity. For example, the reactions
furnished the desired product 3a and 3e in the 95%
Experimental Section
and 89% yield, respectively. Interestingly, vinyl sub-
strate 1g was a suitable reaction partner and provided
the product in the good yield which could then be ap-
plied to further transformations. Nevertheless, re-
placement of substrate 1a by 4,5-dimethoxy-2-(4-me-
thoxybenzyl)phenol resulted in a decrease of yield.
Futhermore, with a series of sulfur ylides the reac-
tions proceeded smoothly, and moderate to good
yields as well as excellent diastereoselectivities were
General Procedure for the Synthesis of 2,3-
Dihydrobenzofurans
A
reaction mixture of 2-alkyl-substituted phenol
1 (0.24 mmol), sulfonium salt 2 (0.20 mmol), Ag2O (111 mg,
0.48 mmol) and K2CO3 (33 mg, 0.24 mmol) in dichlorome-
thane (3 mL) was stirred at room temperature for 12 h.
Then water (20 mL) was added to the mixture. The organic
layer was separated and the aqueous layer was extracted
with dichloromethane (30 mLꢂ3). The combined organic
observed. Moreover, the unstable sulfur ylide trime- layer was dried by anhydrous sodium sulfate, concentrated
Adv. Synth. Catal. 2014, 356, 383 – 387
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asc.wiley-vch.de
385

COMMUNICATIONS
Bo Wu et al.
under vacuum. The crude product was purified by flash
chromatography on silica gel using petroleum ether and
ethyl acetate to give the corresponding trans-2,3-dihyroben-
zofuran products 3.
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trans-{7-(4-Methoxyphenyl)-6,7-dihydrobenzofuro
ACHTUNGTRENN[UNG 6,5-d]-
N
ACHTUNGTRENNUNG
1H NMR (400 MHz, CDCl3): d=7.97–7.87 (m, 2H), 7.59 (t,
J=7.4 Hz, 1H), 7.45 (t, J=7.7 Hz, 2H), 7.14 (d, J=8.7 Hz,
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5.94–5.85 (m, 2H), 5.76 (d, J=6.2 Hz, 1H), 4.78 (d, J=
6.2 Hz, 1H), 3.81 (s, 3H); 13C NMR (100 MHz, CDCl3): d=
195.0, 159.2, 153.8, 148.2, 142.7, 134.6, 134.0, 129.4, 129.2,
128.9, 120.6, 114.5, 105.2, 101.6, 93.4, 91.7, 55.5, 50.6; HR-
MS: m/z=397.1031, calculated for C23H18NaO5 [M+Na]+:
397.1052.
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Acknowledgements
Financial support from National Natural Science Foundation
of China (21125208 & 20921092) and the National Basic Re-
search Program of China (2010CB833300) is gratefully ac-
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