Diastereo- And enantioselective catalytic tandem michael addition/ mannich reaction: Access to chiral isoindolinones and azetidines with multiple stereocenters

Article abstract of DOI:10.1002/anie.200907320

(Figure Presented) Michael and Mannich cooperate: A rapid and reliable approach to highly functionalized chiral isoindolindone and azetidine derivatives, possessing multiple contiguous stereogenic centers, has been achieved (see scheme).

Full text of DOI:10.1002/anie.200907320

Communications
DOI: 10.1002/anie.200907320
Tandem Reactions
Diastereo- and Enantioselective Catalytic Tandem Michael Addition/
Mannich Reaction: Access to Chiral Isoindolinones and Azetidines
with Multiple Stereocenters**
Shengmei Guo, Yinjun Xie, Xinquan Hu, Chungu Xia, and Hanmin Huang*
Construction of multiple contiguous stereogenic centers in
acyclic compounds by asymmetric catalysis represents a
particularly difficult challenge.[1] For the success of such a
catalytic process, carbon–carbon bond-forming tandem reac-
tions using well-known reactions as key transformation steps
that give high diastereo- and enantiocontrol are a prereq-
uisite.[2] The asymmetric Mannich-type reaction is a popular
method for preparing optically active b-aminocarbonyl
frameworks, which are interesting structures found in many
useful biologically active compounds.[3] This type of reaction
has been frequently used as a fundamental step for establish-
ing tandem protocols,[2] and in those processes, the catalytic
generation of chiral enolates is crucial for achieving high
activity and selectivity.
It is widely appreciated that copper-catalyzed conjugate
addition of R2Zn to a,b-unsaturated carbonyl compounds is
one of the most attractive way for constructing a carbon–
carbon bond,[4] in which the chiral zinc enolate is generated
in situ. In the case when an appropriate electrophile is added
to the reaction system, the tandem conjugate addition/
electrophilic trapping reaction might be realized. Based on
this concept, various electrophiles such as aldehydes, ketones,
esters, nitriles, oxocarbenium ions, carboxylates, alkyl halides,
nitrosos, and tosylates have been used in the intermolecular
or intramolecular conjugate addition/electrophilic trapping
reactions for the construction of complex molecular frame-
works through tandem processes.[5] As with the carbon
electrophile, imines might be another attractive substrate
for the conjugate addition/electrophilic trapping reaction.
However, to the best of our knowledge, the use of this type of
synthetically versatile aldimine as the terminal electrophile
for this kind of reaction remains unexplored—presumably
owing to their relatively lower electrophilicity in comparison
with aldehydes and other carbonyl-containing electrophiles.[5]
Moreover, most of the stereoselective tandem reactions of
this kind install the stereogenic centers on cyclic compounds.
This limitation is probably due to difficulty in the stereose-
lective construction of contiguous stereogenic centers in
acyclic compounds as compared to cyclic systems. Herein we
report a highly diastereo- and enantioselective construction of
three contiguous acyclic stereogenic centers through a
copper-catalyzed tandem conjugate addition/Mannich reac-
tion of organozinc reagents and acyclic a,b-unsaturated
ketones in the presence of imines to afford b-aminocarbonyl
derivatives (Scheme 1). Thus, allowing the asymmetric syn-
thesis of chiral azetidines in high enantioselectivity. Notably,
in a one-pot manipulation, chiral isoindolinones can also be
obtained with high levels of relative and absolute stereo-
chemical control by using this method.
First, we investigated the reactivity of several types of
imines as electrophiles toward the chalcone 1 and Et2Zn in
the presence of Cu(CH3CN)4BF4 (1 mol%) and with (S,S)-L1
(1.2 mol%) as a chiral ligand in toluene at À108C for
12 hours—these are the same reaction conditions that pre-
viously led to good results for the copper-catalyzed conjugate
Michael addition protocol.[6] The desired tandem reactions
did not proceed with either N-arylimine or N-alkylimine, and
only the 1,4-conjugate addition products were obtained.
However, the use of N-tosyl aldimine 2a as an electrophile
under the same reaction conditions gave the desired tandem
adduct in 72% yield with good enantioselectivity (76% ee)
for the major diastereomer (Table 1, entry 1). Also, only two
of the four possible diastereomers were detected. This
exceptionally high reactivity may be attributed to the higher
electrophilicity of the N-tosyl aldimine or the strong coordi-
nation of the copper ion to the N-tosyl aldimine through a 1,3-
binding mode of the nitrogen atom of the imine moiety and
the oxygen atom of the sulfonyl group.[7]
[*] S. Guo, Y. Xie, Prof. Dr. X. Hu, Prof. Dr. C. Xia, Prof. Dr. H. Huang
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, Lanzhou, 730000 (China)
Fax: (+86)931-496-8129
E-mail: hmhuang@licp.cas.cn
We then focused our attention on this type of imine as the
electrophilic tapping reagent and examined several catalytic
systems. We used chalcone 1a, N-tosyl aldimine 2a, and
Et2Zn as the reactants, and the reaction was performed at
À108C in toluene for 12 hours. With L1 as a ligand, we
screened a variety of copper sources including copper(I) and
copper(II) species, and found the stereoselectivity was
significantly affected by the nature of the counterion of the
copper salts (Table 1, entries 1–8). The more coordinating
counterions proved to be more suitable, and among those
tested, CuBr led to the best results with respect to reactivity
and stereoselectivity (Table 1, entry 3). Among the solvents
Y. Xie, Prof. Dr. X. Hu
College of Chemical Engineering and Materials Science
Zhejiang University of Technology (China)
S. Guo
Graduate School of Chinese Academy of Sciences (China)
[**] This work was supported by the Chinese Academy of Sciences and
the National Natural Science Foundation of China (20802085,
20625308).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200907320.
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Chemie
To extend the application of the
reaction, a variety of N-sulfonyl aldi-
mines were subjected to the optimal
reaction conditions using chalcone 1a
and Et2Zn. As summarized in Table 2,
the desired optically active tandem
adducts—with three contiguous acyclic
stereogenic centers—were obtained in
up to 99% yield, 95% ee, and 95:5 d.r..
It appeared that the electronic nature of
the para and meta substituents on the
phenyl ring of the imine did not have a
strong influence on the reactivity and
stereoselectivity. Substrates with both
electron-rich methyl and methoxyl sub-
stituents as well as electron-deficient
Scheme 1. Copper-catalyzed tandem conjugate addition/Mannich reaction. L*=chiral ligand.
Table 1: Optimized reaction conditions for the tandem conjugate
Table 2: Copper-catalyzed tandem conjugate addition/Mannich reaction
of N-sulfonyl aldimines.[a]
addition/Mannich reaction.[a]
d.r.[c]
ee
Entry
1
2
Yield of d.r.[c]
ee
Entry
CuX
Solvent
T
[8C]
Yield
[%][b]
(Ar1, Ar2)
(R1, R2)
3 [%][b]
[%][d]
[%][d]
1
2
3
4
5
6
7
8
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
Ph, Ph
4-CH3C6H4, Ph
Ph, Ts
3a, 96
3b, 92
3c, 89
3d, 66
3e, 92
3 f, 99
3g, 83
3h, 90
3i, 72
3j, 77
3k, 84
3l, 80
91:9
91:9
91:9
88:12 95
86:14 93
82:18 91
91:9
93:7
83:17 92
67:33 95
92
91
93
1
2
3
4
5
6
7
8
Cu(CH3CN)4BF4
Cu(OTf)2
CuBr
CuBr2
CuCl
CuCl2
CuI
Cu(OAc)2
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
À10
À10
À10
À10
À10
À10
À10
À10
À10
À10
À10
À10
À10
À10
À20
À20
72
62
85
77
81
89
56
84
44
23
98
78
84
80
97
96
75:25
70:30
81:19
70:30
82:18
72:28
65:35
75:25
54:46
81:19
90:10
85:15
86:14
83:17
91:9
76
66
90
84
90
90
92
88
68
À4
84
92
86
84
87
92
4-ClC6H4, Ts
4-BrC6H4, Ts
4-FC6H4, Ts
4-CH3C6H4, Ts
4-MeOC6H4, Ts
3-ClC6H4, Ts
1-naphthyl, Ts
c-C6H11, Ts
92
94
9
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2-furyl, Ts
10
11
12
13
14
15
16[e]
4-CH3OC6H4, Ns
4-CH3C6H4, Ns
Ph, Ts
93:7
92:8
92
93
94
92
Et2O
nBu2O
MTBE
CPME
Et2O
3m, 95 92:8
4-MeOC6H4, Ph Ph, Ts
3n, 94
3o, 93
3p, 85
3q, 90
3r, 76
3s, 95
95:5
4-ClC6H4, Ph
4-BrC6H4, Ph
4-FC6H4, Ph
3-ClC6H4, Ph
Ph, 4-ClC6H4
4-BrC6H4, Ph
4-ClC6H4, Ph
Ph, 4-ClC6H4
Ph, Ts
Ph, Ts
Ph, Ts
Ph, Ts
Ph, Ts
86:14 93
89:11 94
88:12 89
81:19 87
88:12 88
70:30 95
71:29 93
78:22 90
Et2O
91:9
[a] Reaction conditions: chalcone 1a (0.5 mmol), Et2Zn (0.75 mmol),
imine 2a (0.6 mmol), CuX (0.005 mmol), L1 (0.006 mmol), solvent
(2.0 mL), 12 h. [b] Yield of isolated product. [c] Determined by H NMR
spectroscopy. [d] The ee value was determined by HPLC on a chiral
stationary phase and is quotes only for the main product. [e] L2 was used
as the chiral ligand. CPME=cyclopentyl methyl ether, MTBE=methyl
tert-butyl ether, Tf=trifluoromethanesulfonyl, THF=tetrahydrofuran,
Ts =4-toluenesulfonyl.
1
2-CO2MeC6H4, Ts 3t, 88
2-CO2MeC6H4, Ts 3u, 86
2-CO2MeC6H4, Ts 3v, 85
[a] Reaction conditions: chalcone 1 (0.5 mmol), Et2Zn (0.75 mmol),
imine (0.6 mmol), CuBr (0.005 mmol), L2 (0.006 mmol), Et2O
2
(2.0 mL), À208C, 12 h. [b] Yield of isolated product. [c] Determined by
1H NMR spectroscopy, other diastereomers less than 1%. [d] Deter-
mined by HPLC on a chiral stationary phase. Ns=2-nitrobenzenesul-
fonyl.
examined, Et2O gave the highest yield and diastereoselectiv-
ity (Table 1, entry 11). Although we observed a little erosion
in asymmetric induction with Et2O compared to the other
solvents tested. The temperature also influenced the diaste-
reoselectivity and enantioselectivity of the reaction, but not
the reactivity (Table 1, entry 11 vs. 15). Furthermore, the
product was formed in higher yield with a higher ee value
when L2 instead of L1 was used as the chiral ligand (Table 1,
entry 15 vs. 16).
halogen substitutents all reacted smoothly and gave moderate
to high yield, 91–95% ee, and 82:18–91:9 d.r. (Table 2,
entries 1–7). Besides the substituted phenyl N-sulfonyl aldi-
mine, the naphthyl N-sulfonyl aldimine also gave the tandem
product in high yield and good stereoselectivity (Table 2,
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Communications
entry 8). Aliphatic and heteroaromatic aldehyde-derived
imines were also tested, and these gave high ee values but
relatively lower yields and diastereoselectivities (Table 2,
entries 9 and 10). When the protecting group on the nitrogen
center of 2 was changed from Ts to Ns, then high enantio-
selectivity and diastereoselectivity could also be realized
(Table 2, entries 11 and 12).
Further broadening of the substrate scope indicated that
the tandem reaction of N-tosyl aldimine 2a with a series of
substituted chalcones and Et2Zn could also be achieved to
produce the desired tandem adducts under the optimal
reaction conditions. As shown in Table 2, high yields and
enantioselectivities of up to 95% ee were obtained regardless
of the electronic and steric nature of the substituted chalcones
examined here (Table 2, entries 13–19). Notably, the syntheti-
cally more versatile, but less reactive aldimine 2t (see
Scheme 2 for structure) was also compatible with this
tandem reaction. Under the optimized reaction conditions,
2t was subjected to the tandem reaction with different
chalcones and the corresponding tandem adducts were
obtained in good yields with high stereoselectivities (up to
95% ee; Table 2, entries 20–22). The configuration of the
major isomer of 3t was determined to be 1S,2R,3S by single-
crystal X-ray analysis,[8] and the other products were tenta-
tively assigned by analogy.
Scheme 2. Catalytic synthesis of chiral isoindolinone derivatives by a
tandem reaction.
Given the importance of chiral isoindolinone and isoindo-
line derivatives in biological science and medicinal chemis-
try,[9] the synthesis of these compounds is of great interest.[10]
The substrate 2t could also be used for the synthesis of chiral
isoindolinones. Under the optimized reaction conditions, 2t
underwent the tandem reaction with different chalcones and
Et2Zn. The corresponding formation of the tandem adducts
and subsequent in situ lactamization at 508C gave the chiral 3-
substituted N-tosylphthalimidines 4a–c in good yields and
with high enantioselectivities (90–95% ee; Scheme 2). Crys-
tals of enantiopure 4a were obtained and single-crystal X-ray
analysis also revealed the configuration to be 1S,2R,3S
(Figure 1, left).[8]
Figure 1. X-ray structures of the enantiomerically pure isoindolinone
4a (left) and azetidine 5a (right). The thermal ellipsoids are drawn at
the 50% probability level. Blue N, green Br, red O, yellow S.
It was anticipated that the tandem adducts 3 could be
employed for the efficient preparation of chiral 2,3,4-trisub-
stituted N-tosylazetidines (Scheme 3). Azetidines are four-
membered nitrogen-containing heterocyclic compounds, and
they possess various important biological properties such as
activity against influenza virus A as well as anti-HIV, anti-
HSV-1, and anti-HSV-2 activity.[11] Despite their indisputable
importance as bioactive compounds and pharmaceutical
building blocks, the methods for the preparation of chiral
azetidines are currently limited which is probably due to their
strained nature and the difficulty of formation of the four-
membered ring.[12] Thus, this transformation was carried out
as follows: product 3a was obtained on gram scale through
the newly developed tandem reaction, and the crude com-
pound was recrystallized to afford the enantiomerically pure
product with 98% ee and more than 95:5 d.r. The purified 3a
was converted into the b-amino alcohol 6a as one diastereo-
mer with four contiguous stereogenic centers by Pd/C-
catalyzed hydrogenation, and was subsequently treated with
TsCl/KOH under reflux in THF[12e,f] to furnish the desired N-
tosylazetidine 5a as a single isomer with 62% overall yield in
three steps. By using a similar method, 3e was converted into
the corresponding N-tosylazetidine 5b as a single isomer with
65% overall yield. The structure of 5a was confirmed
unambiguously by single-crystal X-ray analysis, and the
newly formed chiral center was determined to have R confi-
guration (Figure 1, right).[8]
In summary, we have developed an efficient asymmetric
tandem conjugate addition/Mannich reaction for the con-
struction of multiple contiguous acyclic stereogenic centers in
product with high diastereo- and enantioselectivity. This
protocol provides a reliable and rapid approach for the
synthesis of chiral N-tosyl isoindolinones and 2,3,4-trisubsti-
tuted N-tosylazetidines. Further investigation to extend the
reaction scope and applications of this process in organic
synthesis are underway.
Received: December 29, 2009
Published online: March 9, 2010
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Scheme 3. Synthesis of chiral N-tosylazetidine derivatives.
Keywords: asymmetric catalysis · Mannich reaction ·
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