A chiral squaramide-catalyzed asymmetric dearomative tandem annulation reaction through a kinetic resolution of MBH alcohols: Highly enantioselective synthesis of three-dimensional heterocyclic compounds

Article abstract of DOI:10.1039/c9cc05483a

In this communication, a chiral squaramide-catalyzed asymmetric dearomative tandem annulation reaction of unmodified Morita-Baylis-Hillman alcohols with azomethine imines has been achieved through a kinetic resolution of MBH alcohols under mild conditions

Full text of DOI:10.1039/c9cc05483a

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Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/C9CC05483A
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Chiral squaramide-catalyzed asymmetric dearomative tandem
annulation reaction through kinetic resolution of MBH alcohols:
highly enantioselective synthesis of three-dimensional heterocyclic
compounds
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Leijie Zhou,a Yuan Zeng,a Xing Gao,a Qijun Wang,a Chang Wang,a Bo Wang,a Wei Wang,b Yongjun
Wu,b Bing Zheng,*a and Hongchao Guo*a
www.rsc.org/
constructed through CADA strategies have been much less
investigated.5
In this paper,
a chiral squaramide-catalyzed asymmetric
dearomative tandem annulation reaction of unmodified Morita–
Baylis–Hillman alcohols with azomethine imines has been achieved
through a kinetic resolution of MBH alcohols under mild conditions,
giving pharmaceutically interesting functionalized hetero-
bicyclo[3.3.1]nonane derivatives in good to excellent yields with
excellent enantioselectivities.
R
N
H
N
H
Me
OH
N
N
OH
H
HO
O
R
O
HO
R
CH3
R
CH3
I
Morphine( )
IV
II
Euphococcinine (
)
Sandwicine ( )
Bicyclic or polycyclic heterocyclic compounds are hot topics in
synthetic chemistry and drug discovery, and often possess intriguing
and broad range of biological activities, owing to their complicated,
diverse, unparalleled, specific three-dimensional structures.1 Among
them, the aza-bicyclo[3.3.1]nonane family is commonly found as a
basic skeleton in many bioactive natural products and drugs, and
V
Adaline ( )
III
Norsandwicine (
)
H
n-Pentyl
Ts
8
N
H
N
 N
3
N
HO
HO
O
O
2
6
CO2R2
R1


O
HN
O
N
H
N
1
O
NH
H
5 H
7
HN
4

9
2
therefore have attracted much attention. As shown in Figure 1,
R3
morphine (I) has a rigid pentacyclic structure based on the aza-
bicyclo[3.3.1]nonane core, which is a widely used powerful pain
reliever.2a Many classical natural products/alkaloids bearing the
skeleton such as sanwicine (II),2b norsandwicine (III),2b
Euphococcinine (IV),2c Adaline (V),2c Cephathrindole A (VI) and B
(VII)2d were also shown in Figure 1. They share the characteristics
such as rigid polycyclic structure, multi-chiral centers, functional
tertiary amine and oxygen-containing, which contribute a lot to their
biological activities. Due to their distinctive biological activities,
synthesis of natural product-like aza-bicyclo[3.3.1]nonane family
from simple materials is very meaningful and significant.3
our target product
VII
)
VI
cephathrindole B (
cephathrindole A (
)
Scheme 1 Several bioactive natural products and drugs having aza-
bicyclo [3.3.1] nonane scaffold.
a. Albrecht and Jørgensen's [4+2] cycloaddition and Chen's [5+3] cycloaddition
EtO2C
O
R'
N
O
EtO2C
N
O
O
O
N
OH
R
N


N

R'
Boc
Catalytic asymmetric dearomatization (CADA) reactions,4a provide
an assembly of useful strategies that transforming planar aromatic
compounds into enantioenriched chiral cyclic molecules, and
become one of the most attractive research topics in recent years.4
Since many fruitful achievements have been made by chemists
around the world, CADA reactions for many types of aromatic
compounds and their derivatives such as benzene, pyridine, pyrrole,
indole and quinoline were well established.4 However, as one of the
most common aromatic heterocycles, quinazoline and its derivatives
N


+
thiol
O
R
R
O
NH2
Boc
NH2
[4+2] cycloaddition
[5+3] cycloaddition
Ts
b. Current work
N
 N
OH
N
CO2R2
R1


HN
chiral squaramide
R3
R1
N
Ts
+
O

N
CO2R2

R3
Scheme 2 Previous strategies on direct activation of unmodified MBH
alcohols and our activation mode.
a
Department of Applied Chemistry, China Agricultural University, Beijing 100193,
China. E-mail: hchguo@cau.edu.cn, 2014026@cau.edu.cn
The Morita–Baylis–Hillman (MBH) alcohols attracted lots of
interests as a typical category of abundant feedstocks.6,7 Although
many elegant activation strategies have been developed, the direct
b College of Public Health, Zhengzhou University, Zhengzhou 450001, China
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2017, 00, 1-3 | 1
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activation mode of unmodified MBH alcohols, especially its
In our previous work, various chiral phosphine failed to give
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application in annulation reactions, has been much less explored.8 acceptable enantioselectivity.9a We thus examined various tertiary
DOI: 10.1039/C9CC05483A
There are only very limited examples. As outlined in Scheme 1a, amine catalysts (Table 1). The quinine C1 showed no
Albrecht and Jørgensen developed [4+2] formal cycloaddition of enantioselectivity (entry 1). Chiral urea C2, thiourea C3, squaramide
unmodified MBH alcohols and 3-olefinic oxindoles.8a Using the same C4−C9 were then successively evaluated in CH2Cl2 at −20 °C (entries
MBH alcohols from 2-cyclopentenone and aldehydes, Chen disclosed 2−9). Most chiral catalysts provided high yields of product (entries
a formal [5+3] cycloaddition reaction of cyclic azomethine imines.8b 2−8), except the squaramide C9, which has no tertiary amine moiety
These two elegant works indicated the capability of unmodified and only provided trace amount of product (entry 9). It indicated that
MBH alcohols in cycloadditions as a powerful way to build molecular tertiary amine moiety plays a key role in catalytic cycle. To our
complexity. Nevertheless, with this activation mode, elimination of delight, the squaramide C5 displayed both excellent reactivity and
hydroxy group leads to loss of atom-economy and failing to form enantioselectivity (90% yield, 91% ee, entry 5). Decreasing the
oxygen heterocycles. For making full use of the MBH adducts, we catalyst loading worsened reactivity and enantioselectivity (entries
studied direct activation of MBH alcohols without dehydroxylation 10−12). With the use of 1% of C5, the product was synthesized with
and developed phosphine-catalyzed dearomative tandem 81% ee, albeit in only 20% yield (entry 12). Further optimization
annulation of MBH alcohols with azomethine imines, forming 3D through screening solvents such as 1,2-dichloroethane (DCE), CHCl3,
heterocyclic compounds.9a Herein, we present its asymmetric THF and toluene were not successful (entries 13−16). According to
variant achieved through chiral squaramide-catalyzed kinetic the above results, the optimal reaction conditions were determined
resolution of MBH alcohols10 (Scheme 1b).
as follow: using the squaramide C5 as catalyst in CH2Cl2 at −20 °C.
Table 2 Scope of MBH alcohols 2a
Table 1 Optimization of the reaction conditionsa
Ts
N
Ts
N
OH
OH
N
N
N
N
C5
(20 mol%)
CH2Cl2, 20 o
H
H
Cat.
solvent
+
R1
CO2R2
R1
Ph
+
Ts
HN
CO2Me
Ph
N
Ts
HN
N
C
N
CO2R2
N
CO2Me
2a
O
O
H
H
1a
1a
2
3aa
3
O
Yield
(%)b
90
98
86
94
98
85
82
98
84
83
85
97
93
98
95
80
95
96
61
55
79
29
ee
Entry
R1/R2
C6H5/Me (2a)
2-FC6H4/n-Pr (2b)
3-FC6H4/n-Pr (2c)
4-FC6H4/Me (2d)
3
t (h)
(%)c
91
93
90
90
88
91
92
84
92
91
89
91
92
86
95
94
82
94
−36
46
80
83
O
O
OMe
N
1
2
3
4
5
6
7
8
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
65
60
78
60
65
60
72
65
69
72
57
72
72
72
69
59
72
48
48
82
72
48
N
N
N
N
N
NH
OH
NH
NH
O
N
N
N
2-ClC6H4/n-Pr (2e)
3-ClC6H4/n-Pr (2f)
4-ClC6H4/n-Pr (2g)
2-BrC6H4/n-Pr (2h)
3-BrC6H4/n-Pr (2i)
4-BrC6H4/n-Pr (2j)
2-MeC6H4/n-Pr (2k)
3-MeC6H4/n-Pr (2l)
4-MeC6H4/n-Pr (2m)
2-MeOC6H4/n-Pr (2n)
2-MeOC6H4/n-Pr (2o)
2-MeOC6H4/n-Pr (2p)
2-naphthyl/Me (2q)
2-thiophenyl/Me (2r)
Et/n-Pr (2s)
O
NHAr
S
NH2Ar
N
NHAr
O
C1
C4
C2
C3
CF3
N
N
9
10
11
12
13
14
15
16
17
18
19
20
21
22
3aj
3ak
3al
HN
HN
HN
HN
O
O
NH
H
O
NH
O
NH
Ar
O
O
NHAr
N
NH
Ar
3am
3an
3ao
3ap
3aq
3ar
3as
3at
O
O
HN
C9
O
O
CF3
C5
Ar = 3,5-(CF3)2C6H3
C6
C7
C8
Entry
Catalyst
T (°C)
Solvent
t (h)
Yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9
10d
11e
12f
13
14
15
16
C1
C2
C3
C4
C5
C6
C7
C8
C9
C5
C5
C5
C5
C5
C5
C5
0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
18
84
72
84
65
84
84
84
84
78
84
84
84
84
96
96
50
80
93
76
90
36
60
40
trace
88
54
20
90
85
trace
54
0
35
33
−56
91
84
74
72
-
89
86
81
87
80
-
i-Pr/Me (2t)
N-Boc-3-indolyl/n-Pr (2u)
styryl/n-Pr (2v)
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
3au
3av
a Unless indicated otherwise, reactions of 1a (0.10 mmol) and 2 (0.25 mmol), were
carried out at −20 °C in the presence of catalyst (0.02 mmol) in 1.5 mL of solvent. b
Isolated yield. ee was determined by chiral HPLC analysis. The dr is >20:1,
c
determined by 1H NMR analysis.
With the use of squaramide C5 as chiral catalyst, asymmetric
variant of the current reaction has successfully been achieved, as
revealed in Table 2. In the presence of C5, MBH adducts with
aromatic, fused aromatic, and hetero-aromatic group performed
smoothly the reaction to furnish the multicyclic heterocycles in good
to excellent yields with good to excellent enantioselectivities and
excellent diastereoselectivity (80−98% yields, 82−95% ee, entries 1–
18). The alkyl substituted MBH adducts were not very reactive,
providing the corresponding products with moderate yields and
CHCl3
THF
toluene
20
a Unless indicated otherwise, reactions of 1a (0.10 mmol) and 2a (0.25 mmol), were
carried out in the presence of catalyst (0.02 mmol) in 1.5 mL of the solvent.
Isolated yield. ee was determined by chiral HPLC analysis. The dr is >20:1,
determined by 1H NMR analysis. d 10 mol% catalyst was used. e 5 mol% catalyst was
used. f 1 mol% catalyst was used.
b
c
2 | J. Name., 2012, 00, 1-3
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moderate enantioselectivities (entries 19–20). N-Boc-3-indole
As displayed in Scheme 3a, protection of the amino-group of chiral
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substituted MBH adduct 2u yielded the product 3au in good yield and product 3aa was achieved in presence of Boc O and DMAP at rt for
DOI: 10.1039/C9CC05483A
2
ee (entry 21), while styryl substituted substrate 2v exhibited poor 12 h, giving the derivative 5 in 82% yield with 91% ee. Further
reactivity but good enantioselectivity (29% yield, 83% ee, entry 22). treatment of chiral product 5 with LiAlH4 provided the reduction
The absolute configuration of product 3aa was unambiguously product 6 in 65% yield with 91% ee, while the direct reduction of
determined by X-ray analysis and those of others were assigned by chiral product 3aa failed (Scheme 3a). With the use of aza o-QM
analogy (detailed in SI).11
precursor 7 and Na2CO3, the product 3aa performed a 1,4-addition
As demonstrated in Table 3, substituted azomethine imines also to deliver the functionalized product 8 in 86% yield and 90% ee
acted satisfactorily (entries 1−2). 6-Fluoro-substituted azomethine (Scheme 3a). It is worth mentioning that kinetic resolution
imine delivered the cycloadduct 3ba in 96% yield with 89% ee (entry experiment of 2a also worked when using 0.1 mmol of 1a and 0.1
1), while 6-methyl-substituted azomethine imine afforded 85% yield mmol of 2a in the model reaction (Scheme 3b).12
and 96% ee of the cycloadduct 3cw (entry 2). Different sulphonyl
According to previous reports,8 our work9a and the results in
protecting groups in azomethine imines had not remarkable Scheme 3b, a reaction mechanism was presented in the Scheme 4.
influence on reactivity and stereoselectivities, the corresponding Initially, the catalyst C5 stereoselectively attacks MBH alcohol (R)-2a
cycloadducts were obtained in high yields and excellent to generate the zwitterionic intermediate I, and simultaneously leads
enantioselectivities (83–98% yields, 91–94% ee, entries 3–6).
to the kinetic resolution of MBH alcohol. Subsequent proton transfer
gives the intermediate II. The squaramide motif in the intermediate
II fixes the substrate 1a through hydrogen bond. In this environment,
oxygen anion undergoes stereoselective addition to the Re-face of
electron-deficient carbon-nitrogen double bond of 1a to generate
intermediate III, which then finishes a proton transfer to afford the
intermediate IV. Consequent intramolecular Mannich type reaction
via Si-face attack accomplishes annulation to give the intermediate
V. Successive proton transfer and nucleophilic substitution delivered
the product 3aa with simultaneous regeneration of the catalyst.
Table 3 Scope of azomethine imines 1a
R2
O2S
N
OH
N
N
C5
(20 mol%)
CH2Cl2, 20 o
H
R1
O2
S
CO2R3
Ph
Ph
+
HN
N
N
R2
C
CO2R3
O
H
1
2
R1
3
Entry
R1/R2/R3
3
t (h)
Yieldb (%)
eec (%)
O
3aa
N
-2a
(R)
1
2
3
4
5
6
6-F/4-MeC6H4/Me
6-Me/4-MeC6H4/n-Pr
H/C6H5/n-Pr
H/1-naphthyl/n-Pr
H/4-t-BuC6H4/n-Pr
H/2-MeC6H4/n-Pr
3ba
3cw
3dw
3ew
3fw
69
52
72
72
28
35
96
85
83
96
95
98
89
96
94
91
93
93
N
C5
OH
H
O
Ts
N
Ph
Cat
CO2Me
N
N
H
I
CO2Me
Ph
HN
O
3gw
H
VI
a Unless indicated otherwise, reactions of 1 (0.10 mmol) and 2 (0.25 mmol), were
carried out at −20 °C in the presence of catalyst (0.02 mmol) in 1.5 mL of the
N
O
O
b
solvent. Isolated yield. c ee was determined by chiral HPLC analysis. The dr is >
Ph
O
20:1, determined by 1H NMR analysis.
O
N
II
N
H
N
H
O
Ts
N
1a
N
H
3aa
a) Further transformation of
O
Ts
N
Ts
N
N
CO2Me
Ph
N
N
H
O
H
N
N
O
H
V
H
H
NTs
Boc
N
N
O
N
Boc2O, DMAP
CO2Me
CH2OH
Ph
N
HN
LiAlH4
THF, 0 oC, 1 h
3aa
N
H
O
O
CO2Me
Ph
THF/CH3CN
N
91%ee
O
25 oC, overnight
Ph
NHTs
O
H
H
N
N
H
III
H
5
6
82% yield, 91% ee
CO2Me
Ph
65% yield, 91% ee
N
O
H
Ts
N
NHTs
IV
N
NHTs
Cl
H
Na2CO3
CH2Cl2, 25 o
overnight
3aa
+
CO2Me
Ph
N
C
Scheme 4 A proposed mechanism.
91%ee
O
7
H
8
86% yield, 90% ee
Conclusions
2a
Ts
N
b) Kinetic resolution of
N
N
In conclusion, we have disclosed the first enantioselective tandem
annulation reaction of Morita–Baylis–Hillman alcohols with
azomethine imines via chiral squaramide catalysis. Asymmetric
induction was dependent on a kinetic resolution of MBH alcohols. A
variety of MBH alcohols and azomethine imines were well tolerated
under the mild reaction conditions to give diversified
H
OH
OH
C5
(20 mol%)
CO2Me
+ Ph
Ph
HN
+
CH2Cl2, 20 o
C
Ph
O
N
Ts
N
CO2Me
70 h
CO2Me
2a
H
1a
3aa
48% yield, 84% ee
(S)-2a
47% yield, 83% ee
(epoxymethano)pyrazolo[5,1-b]quinazoline
biologically interesting three-dimensional multicyclic compounds
derivatives.
These
Scheme 3 Elaboration of the product 3aa and kinetic resolution
experiment.
This journal is © The Royal Society of Chemistry 20xx
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Rios, Catal. Sci. Technol., 2012, 2, 267; (m) Y. Wei and M. Shi,
were easily accessed in high yields with excellent
diastereoselectivities and enantioselectivities. The current work
demonstrated that the unmodified MBH adducts will probably have
more interesting applications in asymmetric catalysis.
This work is supported by the Natural Science Foundation of China
(Nos. 21372256, 21572264, 21871293) and Chinese Universities
Scientific Fund (No. 2018TC052, 2018TC055 and 2019TC085).
View Article Online
Chem. Rev., 2013, 113, 6659; (n) P. Xie and Y. Huang, Org.
DOI: 10.1039/C9CC05483A
Biomol. Chem., 2015, 13, 8578.
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Conflicts of interest
There are no conflicts to declare.
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4 | J. Name., 2012, 00, 1-3
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