Catalytic enantioselective addition of alcohols to isatin-derived N-Boc ketimines

Article abstract of DOI:10.1016/j.tet.2013.06.076

The enantioselective addition of alcohols to isatin-derived N-Boc ketimines was investigated for the first time. Isatin-derived N,O-aminals were obtained in excellent yields with moderate-to-good enantioselectivities (up to 78% ee) in the presence of a quinine-based bifunctional catalyst.

Full text of DOI:10.1016/j.tet.2013.06.076

Tetrahedron 69 (2013) 7314e7319
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Catalytic enantioselective addition of alcohols to isatin-derived
N-Boc ketimines
*
*
Tian-Ze Li, Xi-Bo Wang, Feng Sha , Xin-Yan Wu
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 April 2013
Received in revised form 14 June 2013
Accepted 21 June 2013
Available online 28 June 2013
The enantioselective addition of alcohols to isatin-derived N-Boc ketimines was investigated for the first
time. Isatin-derived N,O-aminals were obtained in excellent yields with moderate-to-good enantiose-
lectivities (up to 78% ee) in the presence of a quinine-based bifunctional catalyst.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Isatin-derived N-Boc ketimines
Alcoholysis
N,O-Aminals
3-Alkoxy-3-amiooxindoles
1. Introduction
asymmetric synthesis of 3-substituted-3-amino-2-oxindoles re-
mains far less explored,8,9 and the established methods narrowly
N,O-Aminals are important structural motifs widely found in
natural products and pharmaceuticals.1 They are also important
building blocks in organic synthesis.2 Despite the much effort made
to construct N,O-aminals,3 elegant examples that produce enan-
tiomerically pure aminals have been rare.4 The asymmetric addi-
tion of alcohols to imines represents a straightforward route to
chiral N,O-aminal. In 2008, Antilla et al. reported a highly enan-
tioselective alcoholysis of aldimines catalyzed by phosphoric acid.4a
Later, List et al. extended this methodology to imines generated in
situ from aldehydes and hydroxy amides, affording cyclic N,O-
aminals in a high enantioselectivity.4b However, these approaches
are limited to aldimines. To our knowledge, asymmetric nucleo-
philic additions of alcohols to ketimines are unprecedented due
largely to the fact that both alcohols and ketimines are unreactive
substrates while the stereoselectivity of the resulting adducts is
hard to be controlled.
The asymmetric construction of a quaternary stereocenter is
a challenge in organic synthesis.5 The oxindole motifs bearing
C3-quaternary structures are widely distributed in many bioactive
natural products and pharmaceutically active compounds.6
With the advancement of asymmetric organometallic and organic
catalysis, rapid progress has been made in the construction of
optically pure 3-substituted-hydroxyl-2-oxindoles.7 However,
focus on the addition of 3-substituted-2-oxindoles to nitroso or
diazo compounds. Recently, isatin-derived N-Boc ketimines have
been widely used in the asymmetric synthesis of various chiral 3-
aminooxindoles.10 We envision that these ketimines might be ca-
pable of interacting with alcohols to afford optically pure 3-alkoxy-
3-amiooxindoles. Herein we report the first enantioselective ad-
dition of alcohols to N-Boc ketimines to afford isatin-derived N,O-
aminals.
2. Results and discussion
The choice of a suitable catalyst that efficiently activates alcohols
and ketimines may practically address their low reactivity. Tertiary
amine-based bifunctional organocatalysts are widely used in
asymmetric synthesis.11 We recognize that the tertiary amine group
could activate alcohol and the hydrogen bond donors activate imine
through hydrogen bonding, and these synergistic interactions
would help control the stereoselectivity.
Initially, the reaction of isatin-derived N-Boc ketimine 1a with
ethanol was chosen as a model reaction to screen the chiral orga-
nocatalysts (Fig. 1). As illustrated in Table 1, the reaction was per-
formed smoothly in a mixed solvent of CH2Cl2 and EtOH with
10 mol % Takemoto catalyst A1 at room temperature, affording the
desired product in 98% yield and 39% ee (entry 1). Cinchonine-
based tertiary amine-thiourea A2 (46% ee, entry 2) provided bet-
ter enantioselectivity than did A1. The structural difference be-
tween A2 and A3 seems to result in the obvious difference in their
* Corresponding authors. Tel.: þ86 21 64252011; fax: þ86 21 64252758; e-mail
address: xinyanwu@ecust.edu.cn (X.-Y. Wu).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.06.076

T.-Z. Li et al. / Tetrahedron 69 (2013) 7314e7319
7315
Table 2
Optimization of the reaction conditionsa
Entry
Solvent
Temp (ꢀC)
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
Toluene
CH2Cl2
THF
TBME
Ether
EtOAc
CH3CN
DMF
25
25
25
25
25
25
25
25
25
25
25
25
0
6
16
6
6
6
93
99
97
99
91
90
99
60
96
99
98
94
97
93
99
97
60
56
67
65
68
66
69
62
66
68
69
67
76
79
75
74
6
12
24
6
13
12
24
60
120
48
96
Fig. 1. Structures of organocatalysts A1eA8.
9
EtOH
10d
11e
12f
13
14
15g
16h
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Table 1
Screening of the chiral organocatalystsa
ꢁ20
0
0
a
Unless stated otherwise, the reactions were carried out with 1a (0.2 mmol),
EtOH (0.5 mL), A3 (0.02 mmol), and solvent (0.5 mL).
b
Isolated yield.
c
The ee values were determined by chiral HPLC analysis.
Using 1 mL EtOH/CH3CN (1/2).
Using 1 mL EtOH/CH3CN (2/1).
Using 5 equiv EtOH and 1 mL CH3CN.
Using 0.5 mL EtOH/CH3CN (1/1).
Using 2.0 mL EtOH/CH3CN (1/1).
d
e
Entry
Catalyst
Time (h)
Yieldb (%)
eec (%)
f
1
2
3
4
5
6
7
8
A1
A2
A3
A4
A5
A6
A7
A8
48
48
16
84
84
12
120
8
98
98
99
93
97
99
78
99
ꢁ39
ꢁ46
56
2
g
h
4
basis of the above-results, the optimal reaction condition was
established to be the presence of 0.2 M ketimine 1 in CH3CN/EtOH
(1/1) with 10 mol % catalyst A3 at 0 ꢀC.
With the optimal reaction condition in hand, we examined the
scope of the method using different alcohols as the nucleophile
(Table 3). The steric properties of alcohol are determined to have
a remarkable effect on the reaction. A series of primary alcohols are
suitable substrates for this reaction (entries 1e6). Isopropanol
exhibited a significantly low reactivity (80% yield, 74% ee, entry 7).
ꢁ14
9
0
a
The reactions were carried out with isatin-derived ketimines (0.2 mmol), EtOH
(0.5 mL), and catalyst (0.02 mmol) in CH2Cl2 (0.5 mL) at room temperature.
b
Isolated yield.
c
The ee values were determined by chiral HPLC analysis.
catalytic behaviors. Quinine-derived aliphatic thioureas A4 and A5
gave the product in poor enantioselectivities. Bifunctional amine-
squaramide A6 improved the reaction rate, but gave an inferior
enantioselectivity (entry 3). Using amide A7 with a single H-
bonding donor, the reaction became sluggish and resulted in
a lower yield. Multifunctional organocatalyst A8 produced the ra-
cemic adduct. These results suggest that the thiourea moiety with
a strong hydrogen-bonding ability plays an important role in the
asymmetric addition.
Table 3
Asymmetric addition of different alcohols to isatin-derived ketimine 1aa
Next, the effect of the reaction solvents was investigated using
10 mol % of A3 as chiral catalyst (Table 2). In all the solvents ex-
amined except DMF, product 3a was obtained in excellent yields. In
DMF, however, the reaction proceeded slowly and resulted in
moderate yield and stereoselectivity (entry 8). Non-polar solvents,
such as toluene and CH2Cl2 led to inferior enantioselectivities
(entries 1 and 2). CH3CN turns out to be the most suitable solvent,
which afforded 3a in nearly quantitative yield and the highest ee,
though required a longer reaction time (99% yield, 69% ee, entry 7).
When the reaction was carried out in EtOH, compound 3a was
obtained in 96% yield with 66% ee (entry 9). The ratio of EtOH to
CH3CN has no significant effect on the enantioselectivity (entries 10
and 11). The amount of EtOH could be reduced down to 5 equiv, but
with an extended reaction time of 24 h (entry 12). Low reaction
temperatures provided better enantioselectivity (entries 14 and 15
vs 9). Varying the substrate concentration affected the reaction
time but not the enantioselectivity (entries 15 and 16 vs 13). On the
Entry
ROH
Product
Yieldb (%)
eec (%)
1
2
3
4
MeOH
EtOH
n-PrOH
n-BuOH
C12H25OH
C6H5CH2CH2OH
i-PrOH
3b
3a
3c
3d
3e
3f
3g
3h
3i
98
97
95
94
96
95
80
Trace
79
67
76
75
77
73
71
74
n.d.
76
5d
6e
7
8
t-BuOH
9e
10e
11e
a
3j
94
91
74
56
3k
Unless stated otherwise, the reactions were carried out with 1a (0.2 mmol),
alcohol (0.5 mL), and A3 (0.02 mmol) in CH3CN (0.5 mL) at 0 ꢀC.
b
Isolated yields.
The ee values were determined by chiral HPLC analysis.
Using 5 equiv of alcohol and 1 mL ether.
Using 5 equiv of alcohol and 1 mL CH3CN.
c
d
e

7316
T.-Z. Li et al. / Tetrahedron 69 (2013) 7314e7319
Tertiary alcohols, such as tert-butanol could not react with 1a under
the typical reaction condition (entry 8). Due to its solubility, when
lauryl alcohol was used as nucleophile, the reaction was performed
in ether. Unsaturated alcohols, such as allyl alcohol and trans-cin-
namyl alcohol were also well tolerated, leading to a good selectivity
(entries 9e11). However, a relatively lower yield was obtained
when allyl alcohol was used as nucleophile (entry 9). The N,O-
aminal decorated with a propargyl alcohol was provided in high
yield but with the lowest ee (entry 11).
Further exploration of the reaction scope was focused on vary-
ing the substituents on the isatin-derived ketimines (Table 4). Ex-
cellent yields were obtained in all the examined cases (90e98%
yields), whereas the enantioselectivity varied in terms of the lo-
cation of the substituents. Introduction of substituents at 6-
position had a slightly positive effect (entries 5 and 6). The enan-
tioselectivity decreased dramatically when substituents were in-
troduced to the 7-position (entries 7e9).
Fig. 3. Proposed transition state.
3. Conclusion
In summary, we have described the first example of enantio-
selective addition of alcohols to isatin-derived ketimines. In the
presence of 10 mol % of a quinine-based bifunctional catalyst, chiral
N,O-aminals were achieved in excellent yields and with moderate-
to-good enantioselectivities. The method established here repre-
sents a new protocol for the asymmetric construction of isatin-
derived N,O-aminals.
Table 4
Asymmetric addition of ethanol to different isatin-derived ketimines 1a
4. Experimental section
4.1. General methods
Entry
R
Product
Yieldb (%)
eec (%)
Melting points were taken without correction. Optical rotations
were measured on a WZZ-2A digital polarimeter at the wavelength
of the sodium D-line (589 nm). 1H NMR and 13C NMR spectra were
recorded on Bruker 400 spectrometer and the chemical shifts were
1
2
3
4
5
6
7
8
9
5-Me
5-F
3l
90
98
97
95
98
92
93
96
94
77
77
76
74
78
77
69
63
65
3m
3n
3o
3p
3q
3r
5-Cl
5-Br
6-Cl
6-Br
7-F
referenced to tetramethylsilane (d 0.00 ppm) using acetone-d6 as
solvent. IR spectra were recorded on Nicolet Magna-I 550 spec-
trometer. High Resolution Mass spectra (HRMS) were recorded on
Micromass GCT with Electron Spray Ionization (ESI) resource. HPLC
analysis was performed on Waters equipment using Chiralpak AS-H
or AD-H column.
Toluene, THF, TBME, and ether were freshly distilled from
sodiumebenzophenone. CH2Cl2, ethyl acetate, and CH3CN were
freshly distilled from CaH2. All alcohols were commercially avail-
able and were purified by standard methods. Thin-layer chroma-
7-Cl
7-Me
3s
3t
a
The reactions were carried out with 1 (0.2 mmol), EtOH (0.5 mL), and A3
(0.02 mmol) in CH3CN (0.5 mL) at 0 ꢀC.
b
Isolated yields.
The ee values were determined by chiral HPLC analysis.
c
The absolute configuration of product 3q was determined to be
R by X-ray analysis (Fig. 2). And the configurations of other N,O-
aminals were tentatively assigned by referring to that of 3q.
According to the above-mentioned experimental results and the
related reports,10a,12 a probable transition-state structure was pro-
posed. As shown in Fig. 3, the thiourea moiety activates the keti-
mine through hydrogen-bonding interactions while the alcohol
activated by the tertiary amine then attacks the ketimine from the
re-face to generate the product with an (R)-configuration.
tography (TLC) was performed on 10e40
mm silica gel plates.
Column chromatography was performed, using silica gel
(300e400 mesh) eluted with ethyl acetate and CH2Cl2.
All substituted N-Boc ketimines were synthesized according to
literature.10a Catalyst A1 was commercially available; A2 (cincho-
nine-thiourea), A3, A4, A5 (quinine-thiourea), A6 (cinchonine-
squaramide), A7 (quinine-amide), and A8 were prepared according
to literature procedures.13
4.2. Typical experimental procedure for the asymmetric
reactions
To a solution of catalyst A3 (0.02 mmol, 11.9 mg) in 1.0 mL
CH3CN/EtOH (1/1) was added isatin-derived N-Boc ketimine
(0.2 mmol) at 0 ꢀC, and the resulting mixture was stirred at this
temperature until the reaction completed (monitored by TLC). The
solvent was removed under reduced pressure and the residue was
purified by column chromatography (20/1 CH2Cl2/acetate) to give
the desired product 3.
4.2.1. (R)-tert-Butyl (3-ethoxy-1-methyl-2-oxoindolin-3-yl) carba-
20
mate (3a). White solid, 97% yield, 76% ee, mp 121.0e121.2 ꢀC; [
a]
D
þ17.8 (c 2.61, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d 7.52 (d,
J¼7.2 Hz, 1H), 7.38 (td, J¼7.6, 1.2 Hz, 1H), 7.11e7.07 (m, 1H), 6.99 (d,
J¼7.6 Hz, 1H), 3.62e3.50 (m, 1H), 3.18 (s, 1H), 3.11 (s, 1H), 1.24 (s,
Fig. 2. X-ray crystal structure of N,O-aminal 3q.
1H), 1.07 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d 172.8,

T.-Z. Li et al. / Tetrahedron 69 (2013) 7314e7319
7317
154.3, 145.2, 131.1, 128.4, 125.3, 123.3, 109.3, 85.3, 80.1, 60.1, 28.3,
26.4, 15.6; IR (KBr, cmꢁ1):
3261, 2976, 2931, 1728, 1614, 1471, 1369,
2.79 (t, J¼7.2 Hz, 2H), 1.24 (s, 9H); 13C NMR (100 MHz, acetone-d6):
n
d 172.6, 154.2, 145.1, 139.5, 131.2, 129.9, 129.1, 128.2, 127.1, 125.4,
1250, 1157, 1061, 752; HRMS (ESI) calcd for
C
16H22N2O4Na
123.3, 109.3, 85.2, 80.2, 65.5, 36.9, 28.4, 26.4; IR (KBr, cmꢁ1):
n 3321,
([MþNa]þ): 329.1477, found: 329.1477; HPLC analysis (AS-H col-
2924, 2872, 1738, 1711, 1614, 1471, 1367, 1252, 1101, 760, 706; HRMS
umn,
l¼254 nm, eluent: hexane/2-propanol¼95/5, flow rate:
(ESI) calcd for
405.1785; HPLC analysis (AD-H column,
C
22H26N2O4Na ([MþNa]þ): 405.1790, found:
0.5 mL/min): tR¼31.77 min (minor), 34.63 min (major).
l
¼254 nm, eluent: hexane/
2-propanol¼90/10, flow rate: 0.9 mL/min): tR¼18.63 min (minor),
4.2.2. (R)-tert-Butyl (3-methoxy-1-methyl-2-oxoindolin-3-yl) car-
39.76 min (major).
bamate (3b). White solid, 98% yield, 67% ee, mp 105.2e106.4 ꢀC;
[
a
]
20 þ28.4 (c 2.93, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.54
4.2.7. (R)-tert-Butyl (3-isopropoxy-1-methyl-2-oxoindolin-3-yl) car-
D
20
(d, J¼7.2 Hz, 1H), 7.39 (td, J¼8.0, 1.2 Hz, 1H), 7.18e7.08 (m, 1H), 7.00
bamate (3g). White solid, 80% yield, 74% ee, mp 109.7e110.0 ꢀC; [
a]
D
(d, J¼8.0 Hz, 1H), 3.30 (s, 3H), 3.19 (s, 3H), 3.13 (s, 1H), 1.26 (s, 9H);
þ6.14 (c 2.49, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d 7.61 (d,
13C NMR (100 MHz, acetone-d6):
d
172.5, 154.2, 145.2, 131.2, 127.9,
J¼7.2 Hz, 1H), 7.40 (td, J¼8.0, 1.2 Hz, 1H), 7.10e7.07 (m, 1H), 6.99 (d,
J¼8.0 Hz,1H), 4.16e4.05 (m,1H), 3.20 (s, 3H), 3.08 (s,1H),1.26 (s, 9H),
1.07 (d, J¼6.4 Hz, 3H), 0.97 (d, J¼6 Hz, 3H); 13C NMR (100 MHz, ac-
125.5,123.3,109.3, 85.5, 80.1, 51.6, 28.3, 26.3; IR (KBr, cmꢁ1):
n 3442,
2981,1736,1616,1495,1473,1373,1250,1167,1093, 756; HRMS (ESI)
calcd for C15H20N2O4K ([MþK]þ): 331.1060, found: 331.1063; HPLC
etone-d6): d 173.3, 154.1, 145.0, 131.0, 128.9, 125.8, 123.3, 109.3, 84.7,
analysis (AD-H column,
l
¼254 nm, eluent: hexane/2-propanol¼90/
80.1, 67.5, 28.4, 26.4, 24.2, 24.0; IR (KBr, cmꢁ1):
n 3329, 2979, 2935,
10, flow rate: 0.9 mL/min): tR¼25.06 min (minor), 38.13 min
1736, 1616, 1495, 1473, 1371, 1248, 1165, 1092, 754; HRMS (ESI) calcd
(major).
for C17H24N2O4Na ([MþNa]þ): 343.1634, found: 343.1638; HPLC
analysis (AD-H column,
20, flow rate: 0.5 mL/min): tR¼7.65 min (minor), 10.95 min (major).
l
¼254 nm, eluent: hexane/2-propanol¼80/
4.2.3. (R)-tert-Butyl (1-methyl-2-oxo-3-propoxyindolin-3-yl) carba-
20
mate (3c). Colorless oil, 95% yield, 75% ee; [
a]
þ15.2 (c 2.70,
D
CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.53 (d, J¼7.2 Hz, 1H),
4.2.8. (R)-tert-Butyl (3-(allyloxy)-1-methyl-2-oxoindolin-3-yl) car-
20
7.38 (dd, J¼7.6, 1.2 Hz), 7.11e7.07 (m, 1H), 6.99 (d, J¼7.6 Hz, 1H),
3.48e3.39 (m, 1H), 3.19 (s, 3H), 3.11 (s, 1H), 1.52e1.43 (m, 2H), 1.25
(s, 9H), 0.82 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
bamate (3i). Colorless oil, 79% yield, 76% ee; [
a
]
þ17.8 (c 2.40,
D
CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.57 (d, J¼7.2 Hz, 1H),
7.39 (td, J¼8.0, 1.2 Hz, 1H), 7.10 (td, J¼7.6, 0.8 Hz, 1H), 7.00 (d,
J¼7.6 Hz, 1H), 5.87e5.77 (m, 1H), 5.22e5.17 (m, 1H), 5.06e5.03 (m,
1H), 4.15e4.05 (m, 2H), 3.19 (s, 3H), 3.09 (s, 1H), 1.26 (s, 9H); 13C
d
172.6, 154.1, 145.1, 131.0, 128.2, 125.3, 123.2, 109.2, 85.2, 80.0, 65.8,
28.2, 26.3, 23.5, 10.7; IR (KBr, cmꢁ1):
n 3425, 2974, 1736, 1318, 1495,
1398,1248,1165,1092,1057, 754; HRMS (ESI) calcd for C17H24N2O4K
NMR (100 MHz, acetone-d6):
d 172.5, 154.1, 145.0, 135.3, 131.1, 128.0,
([MþK]þ): 359.1373, found: 359.1370; HPLC analysis (AS-H column,
125.4, 123.3, 116.6, 109.3, 85.0, 80.1, 65.4, 28.3, 26.3; IR (KBr, cmꢁ1):
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate: 0.5 mL/
n 3425, 2981, 2370, 1724, 1616, 1473, 1373, 1250, 1163, 1092, 1057,
min): tR¼21.48 min (major), 23.28 min (minor).
754; HRMS (ESI) calcd for C17H22N2O4Na ([MþNa]þ): 341.1477,
found: 341.1475; HPLC analysis (AD-H column,
l¼254 nm, eluent:
4.2.4. (R)-tert-Butyl (3-butoxy-1-methyl-2-oxoindolin-3-yl) carba-
hexane/2-propanol¼80/20, flow rate: 0.9 mL/min): tR¼11.41 min
20
mate (3d). Colorless oil, 94% yield, 77% ee; [
a
]
þ13.2 (c 2.65,
(minor), 18.44 min (major).
D
CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.54 (d, J¼7.2 Hz, 1H),
7.38 (td, J¼8.0, 1.2 Hz, 1H), 7.11e7.07 (m, 1H), 6.99 (d, J¼8.0 Hz, 1H),
3.51e3.43 (m, 2H), 3.19 (s, 3H), 3.11 (s, 1H), 1.48e1.41 (m, 3H), 3.19
(s, 3H), 3.11 (s, 1H), 1.48e1.41 (m, 2H), 1.34e1.20 (m, 11H), 0.82 (t,
4.2.9. (R,E)-tert-Butyl (3-(cinnamyloxy)-1-methyl-2-oxoindolin-3-
20
yl) carbamate (3j). Semi-solid, 94% yield, 74% ee; [
a]
þ25.7 (c
D
2.62, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.61 (d, J¼7.2 Hz,
J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d
172.7, 154.1, 145.1,
1H), 7.42e7.37 (m, 3H), 7.32e7.29 (m, 2H), 7.24e7.21 (m, 1H),
7.13e7.09 (m,1H), 7.00 (d, J¼8.0 Hz,1H), 6.55 (d, J¼12.0 Hz,1H), 6.25
(dt, J¼12.0, 6.0 Hz, 1H), 4.33e4.23 (m, 2H), 3.18 (s, 9H), 3.13 (s, 1H),
131.0,128.2,125.3,123.2,109.2, 85.1, 80.0, 63.8, 32.4, 28.3, 26.3,19.6,
14.0; IR (KBr, cmꢁ1):
n 3311, 2960, 2933, 2874,1732,1616,1471,1369,
1248, 1161, 1092, 752; HRMS (ESI) calcd for
C
18H26N2O4Na
1.27 (s, 9H); 13C NMR (100 MHz, acetone-d6):
d 172.7, 154.3, 145.2,
([MþNa]þ): 357.1790, found: 357.1787; HPLC analysis (AS-H col-
137.7,132.8,131.3,129.5,128.5,128.1,127.3,126.5,125.6,123.4,109.4,
umn,
l¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate:
85.1, 80.2, 65.4, 28.4, 26.4; IR (KBr, cmꢁ1):
n
3232, 3120, 2982, 1730,
0.5 mL/min): tR¼11.66 min (minor), 14.13 min (major).
2378, 1703, 1616, 1473, 1373, 1248, 1055, 752, 692; HRMS (ESI) calcd
for C23H26N2O4Na ([MþNa]þ): 395.1971, found: 395.1920; HPLC
4.2.5. (R)-tert-Butyl
(3-(dodecyloxy)-1-methyl-2-oxoindolin-3-yl)
analysis (AS-H column,
10, flow rate: 0.9 mL/min): tR¼10.87 min (minor),12.13 min (major).
l
¼254 nm, eluent: hexane/2-propanol¼90/
carbamate (3e). Colorless oil, 96% yield, 73% ee; [
a]
20 þ8.38 (c 3.25,
D
CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.40 (d, J¼7.2 Hz, 1H),
7.25 (dd, J¼7.6, 1.2 Hz,1H), 6.98e6.94 (m,1H), 6.86 (d, J¼8.0 Hz,1H),
3.78e3.30 (m, 2H), 3.08 (s, 1H), 3.06 (s, 3H), 1.36e1.29 (m, 2H),
1.13e1.12 (m, 26H), 0.74 (t, J¼6.8 Hz, 3H); 13C NMR (100 MHz, ac-
4.2.10. (R)-tert-Butyl (1-methyl-2-oxo-3-(prop-2-yn-1-yloxy) in-
dolin-3-yl)carbamate (3k). White solid, 91% yield, 56% ee, mp
20
92.2e93.5 ꢀC; [
a]
þ25.4 (c 2.77, CH2Cl2); 1H NMR (400 MHz, ac-
D
etone-d6):
d
172.8, 154.2, 145.2, 131.1, 128.3, 125.4, 123.3, 109.3, 85.2,
etone-d6):
d
7.62 (d, J¼7.2 Hz, 1H), 7.40 (td, J¼8.0, 1.2 Hz, 1H),
80.1, 64.3, 32.7, 30.5, 30.4, 30.3, 30.1, 30.1, 30.0, 30.0, 28.3, 26.6, 26.4,
7.12e7.08 (m, 1H), 7.01 (d, J¼8.0 Hz, 1H), 4.46 (dd, J¼15.2, 2.4 Hz,
23.4, 14.4; IR (KBr, cmꢁ1):
n 3396, 2926, 2855, 1727, 1717, 1376, 1249,
1H), 7.35 (dd, J¼15.2, 2.4 Hz, 1H), 3.19 (s, 3H), 3.14 (s, 1H), 2.93 (t,
1090, 1051, 880, 752; HRMS (ESI) calcd for C26H42N2O4 ([MþH]þ):
J¼2.4 Hz, 1H), 1.28 (s, 9H); 13C NMR (100 MHz, acetone-d6):
d 172.0,
447.3223, found: 447.3224; HPLC analysis (AD-H column,
154.2, 144.9, 131.4, 127.6, 125.7, 123.4, 109.4, 84.6, 80.3, 76.0, 52.5,
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate: 0.9 mL/
30.6, 28.2, 26.3; IR (KBr, cmꢁ1):
n 3294, 2979, 2937, 2125, 1724,1616,
min): tR¼6.92 min (minor), 17.71 min (major).
1473, 1373, 1250, 1161, 1092, 1055, 754; HRMS (ESI) calcd for
C
17H20N2O4Na ([MþNa]þ): 339.1321, found: 339.1317; HPLC anal-
4.2.6. (R)-tert-Butyl (1-methyl-2-oxo-3-phenethoxyindolin-3-yl)car-
ysis (AD-H column,
flow rate: 0.9 mL/min): tR¼34.67 min (minor), 37.55 min (major).
l
¼254 nm, eluent: hexane/2-propanol¼80/20,
20
bamate (3f). Semi-solid, 95% yield, 71% ee; [
a
]
D
þ6.28 (c 3.25,
CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.47 (d, J¼7.2 Hz, 1H),
7.36 (td, J¼8.0, 1.2 Hz, 1H), 7.26e7.22 (m, 2H), 7.19e7.15 (m, 3H),
7.06 (d, J¼7.6 Hz, 1H), 3.75 (t, J¼7.2 Hz, 2H), 3.16 (s, 3H), 3.13 (s, 1H),
4.2.11. (R)-tert-Butyl (3-ethoxy-1,5-dimethyl-2-oxoindolin-3-yl) car-
bamate (3l). White solid, 90% yield, 77% ee, mp 109.0e111.6 ꢀC;

7318
T.-Z. Li et al. / Tetrahedron 69 (2013) 7314e7319
20
[
a
]
20 þ4.21 (c 1.95, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d
7.21
[
a
]
þ11.8 (c 2.42, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d 7.44
D
D
(s, 1H), 7.05 (d, J¼7.6 Hz, 1H), 6.73 (d, J¼7.6 Hz, 1H), 3.47e3.35 (m,
(d, J¼8.0 Hz, 1H), 7.27 (dd, J¼8.0, 1.6 Hz, 1H), 7.21 (d, J¼1.6 Hz, 1H),
2H), 3.02 (s, 3H), 2.94 (s, 1H), 2.18 (s, 3H), 1.11 (s, 9H), 0.93 (d,
3.64e3.51 (m, 2H), 3.21 (s, 3H), 3.12 (s, 1H), 1.27 (s, 9H), 1.07 (t,
J¼6.8 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d
172.7, 154.2, 142.8,
J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d 172.6, 154.2, 146.7,
132.6, 131.23, 128.4, 126.0, 109.0, 85.3, 80.0, 60.0, 28.3, 26.3, 21.1,
127.6, 126.8, 126.0, 124.2, 112.7, 84.8, 80.3, 60.2, 28.3, 26.5, 15.5; IR
15.5; IR (KBr, cmꢁ1):
n
3425, 2979, 1732, 1624, 1504, 1398, 1369,
(KBr, cmꢁ1):
n 3329, 2978, 1739, 1682, 1608, 1496, 1371, 1290, 1111,
1250, 1167, 1101, 422; HRMS (ESI) calcd for
C
17H24N2O4Na
1066,1020, 895, 717; HRMS (ESI) calcd for C16H21BrN2O4K ([MþK]þ):
([MþNa]þ): 343.1634, found: 343.1637; HPLC analysis (AD-H col-
423.0322, found: 423.0320; HPLC analysis (AD-H column,
umn,
l¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate:
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate: 0.9 mL/
0.9 mL/min): tR¼13.06 min (minor), 36.96 min (major).
min): tR¼13.10 min (minor), 16.53 min (major).
4.2.12. (R)-tert-Butyl (3-ethoxy-5-fluoro-1-methyl-2-oxoindolin-3-
4.2.17. (R)-tert-Butyl (3-ethoxy-7-fluoro-1-methyl-2-oxoindolin-3-
yl)carbamate (3m). White solid, 98% yield, 77% ee, mp
yl)carbamate (3r). White solid, 93% yield, 69% ee, mp 80.1e81.2
20
20
108.5e109.7 ꢀC; [
a
]
þ26.8 (c 2.77, CH2Cl2); 1H NMR (400 MHz,
ꢀC; [
a]
þ16.8 (c 2.64, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
D
D
acetone-d6):
d
7.38 (d, J¼7.6 Hz, 1H), 7.17 (td, J¼8.4, 2.4 Hz, 1H),
d
7.18 (d. J¼7.2 Hz, 1H), 7.08e7.03 (m, 1H), 6.98e6.93 (m, 1H),
7.03e7.00 (m, 1H), 3.65e3.53 (m, 2H), 3.17 (s, 3H), 3.10 (s, 1H), 1.28
3.49e3.38 (m, 2H), 3.24 (d, J¼2.8 Hz, 3H), 2.93 (s, 1H), 1.22 (s, 1H),
(s, 9H), 1.09 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
0.93 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d 172.6,
d
172.5, 161.1, 158.8, 154.3, 141.2, 130.1 (d, J¼7.5 Hz), 117.1 (d,
154.2, 149.7, 147.3, 131.6 (t, J¼10.5 Hz), 124.2 (d, J¼6.2 Hz), 121.1,
J¼23.4 Hz), 113.4 (d, J¼25.9 Hz), 110.3 (d, J¼7.9 Hz), 85.1, 80.3, 60.2,
118.9 (d, J¼19.5 Hz), 85.1, 80.3, 60.2, 28.9 (d, J¼5.6 Hz), 28.3, 15.5; IR
28.3, 26.5, 15.5; IR (KBr, cmꢁ1):
n
3300, 2982, 2931, 2461, 1728, 1678,
(KBr, cmꢁ1):
n 3333, 2976, 1736, 1633, 1481, 1369, 1292, 1240, 1053,
1497, 1271, 1111, 868, 808, 554; HRMS (ESI) calcd for C16H21FN2O4Na
879, 781, 731, 561; HRMS (ESI) calcd for C16H21FN2O4K ([MþK]þ):
([MþNa]þ): 347.1383, found: 347.1387; HPLC analysis (AD-H col-
363.1122, found: 363.1118; HPLC analysis (AD-H column,
umn,
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate:
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate: 0.9 mL/
0.9 mL/min): tR¼11.69 min (minor), 29.21 min (major).
min): tR¼13.11 min (minor), 28.47 min (major).
4.2.13. (R)-tert-Butyl (5-chloro-3-ethoxy-1-methyl-2-oxoindolin-3-
4.2.18. (R)-tert-Butyl (7-chloro-3-ethoxy-1-methyl-2-oxoindolin-3-
yl)carbamate (3n). White solid, 97% yield, 76% ee, mp
yl)carbamate (3s). White solid, 96% yield, 63% ee, mp
20
117.7e119.2 ꢀC; [
a
]
þ3.90 (c 3.23, CH2Cl2); 1H NMR (400 MHz,
103.1e104.2 ꢀC; [
a
]
20 þ28.0 (c 3.15, CH2Cl2); 1H NMR (400 MHz, d6-
D
D
acetone-d6):
d
7.42 (s, 1H), 7.27 (dd, J¼8.4, 2.4 Hz, 1H), 6.90 (d,
acetone):
d
7.28 (d, J¼7.2 Hz, 1H), 7.21 (dd, J¼8.0, 1.2 Hz, 1H),
J¼8.4 Hz,1H), 3.46 (m, 2H), 3.06 (s, 3H), 2.97 (s,1H),1.14 (s, 9H), 0.95
6.97e6.93 (m, 1H), 3.46e3.40 (m, 5H), 2.94 (s, 1H), 1.11 (s, 9H), 0.93
(t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d
172.3, 154.3,
(t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d 173.1, 154.1,
143.9, 130.9, 130.4, 128.2, 125.6, 110.8, 85.0, 80.4, 60.2, 28.3, 26.5,
140.9, 133.1, 124.6, 123.7, 115.9, 84.6, 80.3, 60.3, 29.8, 28.3, 15.5; IR
15.5; IR (KBr, cmꢁ1):
n
3431, 2981, 2939,1736, 1614,1493,1367,1261,
(KBr, cmꢁ1):
n 3271, 2978, 2931, 2887, 1722, 1608, 1466, 1165, 1134,
1165,1105,1057, 689; HRMS (ESI) calcd for C16H22ClN2O4 ([MþH]þ):
1053, 878, 789, 627; HRMS (ESI) calcd for C16H21ClN2O4Na
341.1268, found: 341.1266; HPLC analysis (AD-H column,
([MþNa]þ): 363.1088, found: 363.1089; HPLC analysis (AD-H col-
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate: 0.9 mL/
umn,
l
¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate:
min): tR¼14.75 min (minor), 28.43 min (major).
0.9 mL/min): tR¼13.05 min (minor), 32.95 min (major).
4.2.14. (R)-tert-Butyl (5-bromo-3-ethoxy-1-methyl-2-oxoindolin-3-
yl)carbamate (3o). White solid, 95% yield, 74% ee, mp
4.2.19. (R)-tert-Butyl (3-ethoxy-1,7-dimethyl-2-oxoindolin-3-yl) carb
20
amate (3t). White solid, 94% yield, 65% ee, mp 104.5e105.7 ꢀC; [
a]
D
20
100.6e101.3 ꢀC; [
a]
þ3.33 (c 3.15, CH2Cl2); 1H NMR (400 MHz,
þ15.8 (c 2.99, CH2Cl2); 1H NMR (400 MHz, acetone-d6):
d 7.31 (d,
D
acetone-d6):
d
7.68 (s, 1H), 7.56 (dd, J¼8.4, 2.4 Hz, 1H), 6.99 (d,
J¼7.2 Hz, 1H), 7.11 (d, J¼7.6 Hz, 1H), 6.98e6.94 (m, 1H), 3.56e3.49
J¼8.4 Hz,1H), 3.66e3.54 (m, 2H), 3.19 (s, 3H), 3.13 (s,1H),1.28 (s, 9H),
(m, 2H), 3.46 (s, 3H), 3.13 (s, 1H), 2.58 (s, 3H), 1.25 (s, 9H), 1.05 (t,
1.09 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d
172.2,154.2,
J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d 173.5, 154.2, 142.8,
134.7,129.2,123.3,122.9,120.8, 84.6, 80.0, 59.9, 29.8, 28.3,19.1,15.5;
144.3,133.7,130.6,128.2,115.3,111.2, 84.8, 80.3, 60.2, 28.3, 26.4,15.4;
IR (KBr, cmꢁ1):
n
3284, 2978, 2931,1740,1686,1608,1491,1367,1115,
IR (KBr, cmꢁ1):
n 3306, 2978, 2465, 1682, 1606, 1504, 1363, 1296,
1020, 878, 812, 532; HRMS (ESI) calcd for
C
16H21BrN2O4Na
1176, 1122, 858, 771, 764; HRMS (ESI) calcd for C17H24N2O4Na
([MþNa]þ): 407.0582, found: 407.0581; HPLC analysis (AD-H col-
([MþNa]þ): 343.1634, found: 343.1637; HPLC analysis (AD-H col-
umn,
l¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate:
umn,
l¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate:
0.9 mL/min): tR¼15.18 min (minor), 27.68 min (major).
0.9 mL/min): tR¼18.25 min (minor), 50.32 min (major).
4.2.15. (R)-tert-Butyl (6-chloro-3-ethoxy-1-methyl-2-oxoindolin-3-
yl)carbamate (3p). White solid, 98% yield, 78% ee, mp
Acknowledgements
20
131.4e131.7 ꢀC; [
a
]
þ29.9 (c 3.15, CH2Cl2); 1H NMR (400 MHz,
D
We are grateful for the financial support from National Natural
Science Foundation of China (21242007) and the Fundamental
Research Funds for the Central Universities.
acetone-d6):
d
7.37 (d, J¼8.0 Hz,1H), 6.97 (dd, J¼8.0,1.6 Hz,1H), 6.94
(d, J¼1.6 Hz, 1H), 3.50e3.38 (m, 2H), 3.07 (s, 3H), 2.92 (s, 1H), 1.13 (s,
9H), 0.94 (t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, acetone-d6):
d 172.7,
154.2, 146.7, 136.2, 127.2, 126.5, 122.9, 109.9, 84.8, 80.3, 60.3, 28.3,
Supplementary data
26.5, 15.5; IR (KBr, cmꢁ1):
n 3336, 2981, 2935, 1741, 1614, 1496, 1371,
1242, 1074, 905, 820, 733; HRMS (ESI) calcd for C16H22ClN2O4
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.06.076.
([MþH]þ): 341.1268, found: 341.1270; HPLC analysis (AD-H column,
l¼254 nm, eluent: hexane/2-propanol¼90/10, flow rate: 0.9 mL/
min): tR¼13.31 min (minor), 16.79 min (major).
References and notes
4.2.16. (R)-tert-Butyl (6-bromo-3-ethoxy-1-methyl-2-oxoindolin-3-yl)
carbamate (3q). White solid, 92% yield, 77% ee, mp 135.6e127.7 ꢀC;
1. For examples, see: (a) Perry, N. B.; Blunt, J. W.; Munro, M. H. G.; Pannell, L. K. J.
Am. Chem. Soc. 1988, 110, 4850e4851; (b) Sakemi, S.; Ichiba, T.; Kohmoto, S.;

T.-Z. Li et al. / Tetrahedron 69 (2013) 7314e7319
7319
Saucy, G.; Higa, T. J. Am. Chem. Soc. 1988, 110, 4851e4853; (c) Fusetani, N.;
Sugawara, T.; Matsunaga, S. J. Org. Chem. 1992, 57, 3828e3832; (d) Tanaka, J.-I.;
Higa, T. Tetrahedron Lett. 1996, 37, 5535e5538; (e) Cichewicz, R. H.; Valeriote, F.
A.; Crews, P. Org. Lett. 2004, 6, 1951e1954; (f) Pettit, G. R.; Xu, J.-P.; Chapuis, J.-
C.; Pettit, R. K.; Tackett, L. P.; Doubek, D. L.; Hooper, J. N. A.; Schmidt, J. M. J. Med.
Chem. 2004, 47, 1149e1152; (g) Gao, H.; Liu, W.; Zhu, T.; Mo, X.; Mandi, A.;
Kurtan, T.; Li, J.; Ai, J.; Gu, Q.; Li, D. Org. Biomol. Chem. 2012, 10, 9501e9506.
2. For a review on the application of N,O-aminals, see: Warriner, S. Category 4:
Compounds with Two Carbon-heteroatom Bonds in Science of Synthesis: Houben-
Weyl Methods of Molecular Transformations; Georg Thieme: Stuttguart, 2007;
Vol. 30, pp 7e46.
3. For selected references of the non-stereoselective synthesis of N,O-aminals, see:
(a) Huang, X.; Shao, N.; Palani, A.; Aslanian, R. Tetrahedron Lett. 2007, 48,
1967e1971; (b) Kiren, S.; Shangguan, N.; Williams, L. J. Tetrahedron Lett. 2007, 48,
7456e7459; (c) Wan, S.; Green, M. E.; Park, J.-H.; Floreancig, P. E. Org. Lett. 2007, 9,
5385e5388; (d) Fujioka, H.; Kubo, O.; Senami, K.; Okamoto, K.; Okitsu, T.; Kita, Y.
Heterocycles 2009, 79, 1113e1120; (e) Yin, P.; Ma, W.-B.; Chen, Y.; Huang, W.-C.;
Deng, Y.; He, L. Org. Lett. 2009, 11, 5482e5485; (f) Downey, C. W.; Fleisher, A. S.;
Rague, J. T.; Safran, C. L.; Venable, M. E.; Pike, R. D. Tetrahedron Lett. 2011, 52,
4756e4759; (g) Naud, S.; Macnaughton, S. J.; Dyson, B. S.; Woollaston, D. J.;
Dallimore, J. W. P.; Robertson, J. Org. Biomol. Chem. 2012, 10, 3506e3518.
4. For the catalytic asymmetric synthesis of chiral N,O-aminals, see: (a) Li, G.;
Fronczek, F. R.; Antilla, J. C. J. Am. Chem. Soc. 2008, 130, 12216e12217; (b) Vel-
lalath, S.; Coric, I.; List, B. Angew. Chem., Int. Ed. 2010, 49, 9749e9752; (c) Kim,
H.; Rhee, Y. H. J. Am. Chem. Soc. 2012, 134, 4011e4014; (d) Zhou, F.; Zeng, X.-P.;
Wang, C.; Zhao, X.-L.; Zhou, J. Chem. Commun. 2013, 49, 2022e2024.
5. Christoffers, J.; Baro, A. Quaternary Stereocentres: Challenges and Solutions for
Organic Synthesis; Wiley-VCH: Weinheim. 2005.
7. For reviews, see: (a) Zhou, F.; Liu, Y.-L.; Zhou, J. Adv. Synth. Catal. 2010, 352,
1381e1407; (b) Klein, J.; Taylor, R. J. K. Eur. J. Org. Chem. 2011, 6821e6841; (c)
Dalpozzo, R.; Bartoli, G.; Bencivenni, G. Chem. Soc. Rev. 2012, 41, 7247e7290; (d)
Shen, K.; Liu, X.; Lin, L.; Feng, X. Chem. Sci. 2012, 3, 327e334.
8. For diastereoselective approaches using chiral auxiliaries, see: (a) Lesma, G.;
Landoni, N.; Pilati, T.; Sacchetti, A.; Silvani, A. J. Org. Chem. 2009, 74,
4537e4541; (b) Jung, H. H.; Buesking, A. W.; Ellman, J. A. Org. Lett. 2011, 13,
3912e3915; (c) Yan, W.; Wang, D.; Feng, J.; Li, P.; Wang, R. J. Org. Chem. 2012, 77,
3311e3317.
9. For selected catalytic asymmetric a-amination, see: (a) Cheng, L.; Liu, L.; Wang,
D.; Chen, Y.-J. Org. Lett. 2009, 11, 3874e3877; (b) Qian, Z.-Q.; Zhou, F.; Du, T.-P.;
Wang, B.-L.; Ding, M.; Zhao, X.-L.; Zhou, J. Chem. Commun. 2009, 6753e6755;
(c) Bui, T.; Hernandez-Torres, G.; Milite, C.; Barbas, C. F. Org. Lett. 2010, 12,
5696e5699; (d) Mouri, S.; Chen, Z.; Mitsunuma, H.; Furutachi, M.; Matsunaga,
S.; Shibasaki, M. J. Am. Chem. Soc. 2010, 132, 1255e1257; (e) Yang, Z.; Wang, Z.;
Bai, S.; Shen, K.; Chen, D.; Liu, X.; Lin, L.; Feng, X. Chem.dEur. J. 2010, 16,
6632e6637; (f) Shen, K.; Liu, X.; Wang, G.; Lin, L.; Feng, X. Angew. Chem., Int. Ed.
2011, 50, 4684e4688.
10. (a) Yan, W.; Wang, D.; Feng, J.; Li, P.; Zhao, D.; Wang, R. Org. Lett. 2012, 14,
2512e2515; (b) Feng, J.; Yan, W.; Wang, D.; Li, P.; Sun, Q.; Wang, R. Chem.
Commun. 2012, 48, 8003e8005; (c) Hara, N.; Nakamura, S.; Sano, M.; Tamura,
R.; Funahashi, Y.; Shibata, N. Chem.dEur. J. 2012, 18, 9276e9280; (d) Lv, H.;
Tiwari, B.; Mo, J.; Xing, C.; Chi, Y. R. Org. Lett. 2012, 14, 5412e5415; (e) Hu, F.-L.;
Wei, Y.; Shi, M.; Pindi, S.; Li, G. Org. Biomol. Chem. 2013, 11, 1921e1924; (f)
Chauhan, P.; Chimni, S. S. Tetrahedron: Asymmetry 2013, 24, 343e356.
11. For selected reviews on bifunctional catalysis, see: (a) Connon, S. J. Chem.
Commun. 2008, 2499e2510; (b) Takemoto, Y. Chem. Pharm. Bull. 2010, 58,
593e601; (c) Lu, L. Q.; An, X. L.; Chen, J. R.; Xiao, W. J. Synlett 2012, 490e508.
12. Wang, S.-X.; Chen, F.-E. Adv. Synth. Catal. 2009, 351, 547e552.
6. For reviews, see: (a) Hibino, S.; Choshi, T. Nat. Prod. Rep. 2001, 18, 66e87; (b)
Lin, H.; Danishefsky, S. J. Angew. Chem., Int. Ed. 2003, 42, 36e51; (c) Marti, C.;
Carreira, E. M. Eur. J. Org. Chem. 2003, 2209e2219; (d) Galliford, C. V.; Scheidt,
K. A. Angew. Chem., Int. Ed. 2007, 46, 8748e8758; (e) Peddibhotla, S. Curr. Bioact.
Compd. 2009, 5, 20e38.
ꢀ
ꢀ
13. (a) Vakulya, B.; Varga, S.; Csampai, A.; Soos, T. Org. Lett. 2005, 7, 1967e1969; (b)
Shi, M.; Lei, Z.-Y.; Zhao, M.-X.; Shi, J.-W. Tetrahedron Lett. 2007, 48, 5743e5746;
(c) Yang, W.; Du, D.-M. Org. Lett. 2010, 12, 5450e5453; (d) Li, W.; Wu, W.; Yu, F.;
Huang, H.; Liang, X.; Ye, J. Org. Biomol. Chem. 2011, 9, 2505e2511.
́