Asymmetric Phosphine-Catalyzed [4+1] Annulations of o-Quinone Methides with MBH Carbonates

Article abstract of DOI:10.1002/adsc.201801152

Chiral dihydrobenzofuran units are frequently present in molecules with significant biological and pharmaceutical activities. Herein, we present the first enantioselective formal [4+1] annulation of Morita-Baylis-Hillman carbonates with o-quinone methides (o-QMs) catalyzed by a newly designed chiral phosphine catalyst. Under the mild and eco-friendly conditions, a wide range of polysubstituted dihydrobenzofurans were obtained in good yields with excellent enantioselectivities. (Figure presented.).

Full text of DOI:10.1002/adsc.201801152

Title: Asymmetric Phosphine-Catalyzed [4 + 1] Annulations of o-
Quinone Methides with MBH Carbonates
Authors: Tao Zhou, Tong Xia, Zhenli Liu, Lu Liu, and Junliang Zhang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801152
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10.1002/adsc.201801152
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Asymmetric Phosphine-Catalyzed [4 + 1] Annulations of o-
Quinone Methides with MBH Carbonates
Tao Zhou,a Tong Xia,a Zhenli Liu,a Lu Liua* and Junliang Zhanga*
a
School of Chemistry and Molecular Engineering, East China Normal University, 500 Dongchuan Road, Shanghai
200241, People’s Republic of China
E-mail: jlzhang@chem.ecnu.edu.cn; lliu@chem.ecnu.edu.cn.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Chiral dihydrobenzofuran units are frequently
present in molecules with significant biological and
pharmaceutical activities. Herein, we present the first
enantioselective formal [4
+
1] annulation of
Morita−Baylis−Hillman carbonates with o-quinone
methides (o-QMs) catalyzed by a newly designed chiral
phosphine catalyst. Under the mild and eco-friendly
conditions,
a
wide range of polysubstituted
dihydrobenzofurans were obtained in good yields with
excellent enantioselectivities.
Keywords:[4+1] Annulation; chiral phosphines;
asymmetric catalysis; MBH carbonates; ortho-quinone
methides
Scheme 1. Dihydrobenzofuran synthesis via [4 + 1]
cyclizations of o-QMs
Chiral dihydrobenzofuran scaffolds, which represent
one class of the most important heterocycle, are
important building blocks in organic synthesis and
widely exist in natural products and bioactive
molecules, i.e. anti-inflammatory, CB2 receptor
agonist, oral antioxidant (Figure 1).[1] Thus, the
development of new methods, especially asymmetric
ones, for the construction of these important target
motifs has recently attracted considerable interest.
Recently, quinone methides (QMs)[2] have emerged
as common-used building blocks or versatile
intermediates in organic synthesis. p-Quinone
methides (p-QMs) have been normally used as the
acceptor for 1,6-addition or 1,8-addition reactions,[3]
and o-quinone methides (o-QMs)[4,5] were very
successfully employed for various [4
+
n]
cyclizations.[6-9] However, most of the annulations of
o-QMs were [4 + 2] with electron-rich alkenes,
delivering the chroman skeletons.[6] The [4 + 1]
annulation of o-QMs with C1 synthons is the
straightforward
approach
to
construct
dihydrobenzofuran scaffolds, however, only a few
examples of [4 + 1] cyclizations are reported.[9] For
example, the carbon nucleophile with leaving group,
e.g. sulfur ylides,[9a-d] pyridinium methylides,[9e]
ammonium
ylides,[9f]
α-halogen
carbonyl
compounds,[9h-i] were successfully utilized as C1
synthons to perform [4 + 1] annulations with o-QMs
(Scheme 1a). Very recently, the formal [4 + 1]
reaction of o-QMs and α-branched allenoates as C1
synthons has been developed (Scheme 1b).[9g] In spite
of these elegant achievements, the asymmetric
Figure 1. Dihydrobenzofuran motif in natural products and
bioactive molecules.
1
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10.1002/adsc.201801152
Advanced Synthesis & Catalysis
catalytic version of [4+1] cyclizations of o-QMs with
C1 synthons are still rare and the leaving groups are
limited to halogen.[9h-i]
carbonates could undergo the similar pathway with
acyclic enones. If so, the asymmetric phosphine-
catalyzed [4 + 1] annulation of o-QMs with MBH
carbonates may be realized to afford optically active
dihydrobenzofuran scaffolds.
Table 1. Optimization of Reaction Conditionsa).
With this hypothesis in mind, o-QMs 1a and MBH
carbonate 2a were chosen as model substrates for this
[4
+
1] annulation reaction. Various chiral
bifunctional phosphine catalysts 4a-4h, which are
easily prepared from chiral sulfonamide in a few
steps, were tested (Table 1). The Xiao-Phos 4a with a
chiral sulfonamide as the H-bonding donor could not
catalyze this reaction (Table 1, entry 1). To our
delight, when the catalyst 4b with 3,5-
bistrifluoromethyl benzoyl amide instead of
sulfonamide was used, the reaction proceeds
smoothly to afford the desired [4 + 1] product 3aa in
moderate yield with 1:1.4 dr and 99%/29% ee (Table
1, entry 2). This result indicates that the catalyst with
stronger H-bonding was beneficial to the yield and
stereoselectivity. We then turned to examine the
effect of alkyl substituents on α–position of nitrogen
(4c−4d), but the yield and stereoselectivity cannot be
improved (Table 1, entries 3-4). Another phosphine
catalyst developed by us Wei-Phos 4e was also tested,
which gave only trace amount of product (Table 1,
entry 5). Gratifyingly, 83% yield with 99%/68% ee
and 1:1.5 dr could be obtained when the modified
Wei-Phos 4g using 3,5-bis(trifluoromethyl) benzoyl
amide instead of sulfonamide was used (Table 1,
entry 7). Other H-bonding donors cannot improve the
results. [16] After solvent screening (Table 1, entries 9-
14), the best result was obtained when the reaction
was carried out in DCE (Table 1, entry 11). The
absolute configuration of the minor isomer of 3aa
was determined by single-crystal X-ray diffraction
analysis. [17]
yield
(%)b)
NR
56
52
56
trace
72
83
72
56
dr
ee
entry cat
solvent
(%)c)
-
(%)d)
-
1
2
3
4
5
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
THF
4a
4b
4c
4d
4e
4f
1:1.4
1:1.4
1:1
99/29
89/60
60/39
-
94/62
99/68
90/68
92/89
98/90
99/90
90/79
-
6
1.3:1
1:1.5
1:2.3
1:2.5
1:3
7
8
9
10
11
12
4g
4h
4g
4g
4g
4g
DCM
DCE
Et2O
23
88 (85)
43
1:2.1
1:1.2
1,4-
dioxane
CHCl3
13
14
trace
-
-
4g
With the optimal conditions in hand, we next
examined the MBH carbonate scope of this
asymmetric [4 + 1] annulation reaction. As shown in
Table 2, o-QMs 1a reacted with a variety of MBH
carbonates 2 with electron-donating groups and
electron-withdrawing groups (R2), including alkyl,
alkoxyl, halogen and cyano etc., on the para-position
of phenyl ring, leading to the corresponding
dihydrobenzofuran 3ab-3ai in good yields with good
enantioselectivities (Table 2). The substituents on the
meta-position of phenyl ring had no effect on the
reactivity and enantioselectivity, either (Table 2, 3aj-
3an). It was noteworthy that the reaction of MBH
carbonates 2o-2p with the substituents on the ortho-
position of phenyl ring showed a slightly lower yields
4g
80
1:1
98/82
a)
Unless otherwise stated, all reactions were carried out
using 1a (0.2 mmol), 2a (0.24 mmol), 4 (10 mol%) in
solvent (2 mL) at 20 oC for 6 hours.
Yields were
b)
determined by 1H-NMR, using CH2Br2 as internal standard.
c)
Numbers given in parathesis are isolated yield.
Determined by crude 1H-NMR. d) Determined by HPLC.
Meanwhile, asymmetric phosphine-catalyzed
annulation reaction provides a highly powerful and
efficient approach for the construction of highly
functionalized
carbocyclic
In
or
this
heterocyclic
context,
compounds.[10,11]
Morita−Baylis−Hillman (MBH) carbonates[12,13] are
readily available and versatile reactants, which can
act as C1 synthons in annulations promoted by
phosphine catalysts. In 2010, we developed the first
phosphine-catalyzed [4 + 1] annulation of α,β-
unsaturated ketones (enones) with MBH carbonates,
leading to the constuction of 2,3-dihydrofurans in
racemic manner.[14] As part of our ongoing interest in
the development of novel chiral phosphine catalysts
and the new reactions catalyzed by phosphine,[15] we
wondered whether the reaction of o-QMs with MBH
and
enantioselectivity
but
much
higher
diastereoselectivities (up to 1:16) than that of 2a
(Table 2, 3ao-3ap). The steric hindrance effects of
ortho-substituents might account for this result. The
3,5-bis(trifluoromethyl) group exhibited the similar
effect with ortho-substituents on this transformation
(Table 2, 3aq). 2-Naphthyl was also compatible with
this [4 + 1] cyclization (Table 2, 3ar). Furthermore,
we turned to examine the scope of o-QMs for this
transformation. Gratifyingly, the reaction of MBH
2
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10.1002/adsc.201801152
Advanced Synthesis & Catalysis
carbonates 2i with various o-QMs 1b-1e, including
substituted phenyl ring and heterocycle, worked well,
furnishing the desired dihydrobenzofurans 3bi-3ei in
good yields with good enantioselectivities (Table 2).
intermediate C via direct or indirect 1,3-H shift.
Finally, the intramolecular O anion attacked the
double bond and eliminated the catalyst to obtain the
target product and realize the catalytic cycle.
Table 2. Phosphine-Catalyzed Asymmetric [4+1]
Annulation of o-QMs with MBH Carbonates a).
Scheme 2. Proposed catalytic cycle
In summary, we have developed the first
asymmetric [4 + 1] annulation reaction of o-QMs and
MBH carbonates catalyzed by a novel chiral
phosphine catalyst. This method provides an efficient
and concise strategy for the construction of chiral 2,3-
dihydrobenzofuran derivatives. The salient features
of this reaction include broad substrate scope, mild
reaction conditions, high enantioselectivity and
convenient operation. This reaction will not only
enrich asymmetric cyclizations of o-QMs, but also
broaden the utilization of MBH adducts. Furthermore,
this reaction encourage us to develop novel
phosphine catalysts based on chiral sulfonamide.
Experimental Section
A stirred solution of 1a (50.5 mg, 0.2 mmol), 2a (89.0 mg,
1.2 equiv) and 4g (14.5 mg, 0.02 mmol) in DCE (2 mL) at
a)
Unless noted, all reactions were carried out using 1 (0.2
o
20 C. The mixture was stirred at this temperature for 6
mmol), 2 (0.24 mmol), 4g (10 mol %) in DCE (2 mL) at 20
hours. After completion of the reaction, the reaction
mixture was directly purified by silica gel chromatography
using petroleum ether/EtOAc = 20/1 as the eluent to afford
two isolated isomers of the desired products 3aa.
oC.
Total yields of two isolated isomers. Yield of
b)
c)
isolated single isomer. d) ee of the major isomer.
On the basis of the above results and previous
relevant studies, a plausible catalytic cycle for this
phosphine-catalyzed [4 + 1] cyclization is outlined in
Scheme 2. Initially, chiral phosphine catalyst 4 acted
as an organocatalyst to activate MBH carbonates 2,
affording an allylic phosphorus ylide A. Then,
intermediate A would attack o-QMs 1 via Michael
addition, in which step there are two possible
reigoselectivities (α- or γ- selective). In this case,
ylide A attacked o-QMs 1 with a γ-selectivity to form
intermediate B, which could transform to
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Nos. 21772042, 21572065, 21425205), the Science and
Technology
Commission
of
Shanghai
Municipality
(18JC1412300) and the Changjiang Scholars and Innovative
Research Team in University (IRT_16R25) for financial support.
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10.1002/adsc.201801152
Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201801152
Advanced Synthesis & Catalysis
COMMUNICATION
Asymmetric Phosphine-Catalyzed [4 +
1] Annulations of o-Quinone Methides with MBH
Carbonates
Adv. Synth. Catal. 2018, Volume, Page – Page
Tao Zhou, Tong Xia, Zhenli Liu, Lu Liu* and
Junliang Zhang*
6
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