Enantioselective Nitroso Aldol Reaction Catalyzed by a Chiral Phosphine–Silver Complex

Article abstract of DOI:10.1002/ejoc.201601143

A catalytic asymmetric O-nitroso aldol reaction of alkenyl trifluoroacetates with nitrosoarenes was achieved by using QuinoxP*·AgOAc [(R,R)-QuinoxP* = (–)-(R,R)-2,3-bis(tert-butylmethylphosphino)quinoxaline] as the chiral precatalyst and N,N-diisopropylethylamine as the base precatalyst in the presence of methanol. Optically active α-aminooxy ketones with up to 99 % ee were regioselectively obtained in moderate to high yields by the in situ generated chiral silver enolates.

Full text of DOI:10.1002/ejoc.201601143

A Journal of
Accepted Article
Title: Enantioselective Nitroso Aldol Reaction Catalyzed by Chiral Phosphine-Silver
Complex
Authors: Akira Yanagisawa; Yuqin Lin; Akihiro Takeishi; Kazuhiro Yoshida
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601143
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601143
Supported by

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COMMUNICATION
Enantioselective Nitroso Aldol Reaction Catalyzed by Chiral
Phosphine-Silver Complex
Akira Yanagisawa*,[a] Yuqin Lin,[a] Akihiro Takeishi,[a] and Kazuhiro Yoshida[a]
Abstract: A catalytic asymmetric O-nitroso aldol reaction of alkenyl
the aforementioned asymmetric O-nitroso aldol reaction would
also occur in the absence of the organotin catalyst. Thus, we
tested the catalytic activity of the QuinoxP*·AgOAc complex in
the alternative O-nitroso aldol reaction. When a 2:1 mixture of
alkenyl trifluoroacetate 1a[7] and nitrosobenzene (2a) was
treated with (R,R)-QuinoxP* (5 mol%), AgOAc (5 mol%), and
N,N-diisopropylethylamine (40 mol%) in the presence of MeOH
(5 equiv) in toluene at -78 ˚C for 2 h, -aminooxyketone 3aa (O-
adduct) was obtained in 94% yield, whereas -
hydroxyaminoketone 4aa (N-adduct) was not observed at all.
The O-adduct had 98% ee (Table 1, entry 1). Then, we
examined (R)-DTBM-SEGPHOS as another candidate for the
chiral phosphine ligand; however, it did not give better results
trifluoroacetates with nitrosoarenes was achieved using
QuinoxP*·AgOAc
as the chiral
pre-catalyst and N,N-
diisopropylethylamine as the base pre-catalyst in the presence of
methanol. Optically active -aminooxyketones with up to 99% ee
were regioselectively obtained in moderate to high yields via the in
situ generated chiral silver enolates.
Introduction
The asymmetric nitroso aldol reaction is a beneficial method for
the preparation of optically active -aminooxy and/or -
hydroxyamino carbonyl compounds.[1-5]
Although various
(entries
2
and 3).
Reducing the amount of (R,R)-
organocatalysts[2,3] and chiral Lewis acid catalysts[4,5] have been
developed for the asymmetric transformation, to the best of our
knowledge, there is only one example of the catalytic process
that advances through a chiral metal alkoxide.[4d] We report here
a novel example of the enantioselective O-nitroso aldol reaction
of alkenyl trifluoroacetates with nitrosoarenes via a chiral silver
methoxide using the QuinoxP*·AgOAc complex as the chiral
pre-catalyst and N,N-diisopropylethylamine as the base pre-
catalyst in the presence of methanol (Scheme 1).
QuinoxP*·AgOAc to 3 mol% essentially did not affect the
chemical yield, the O/N ratio, or the enantioselectivity (entry 4 vs.
entry 1). Raising the reaction temperature to -60 ˚C resulted in
an almost quantitative yield (entry 6). Further improvement in
the enantiomeric excess of O-adduct 3aa was achieved when
the amount of the tertiary amine was decreased to 20 mol%
(entry 7).
Table 1. Optimization of the reaction conditions for the chiral silver-catalyzed
O-nitroso aldol reaction of alkenyl trifluoroacetate 1a with nitrosobenzene 2a.[a]
cat. (R,R)-QuinoxP*
Chiral phosphine (x mol% )
AgOAc (y mol% )
OCOCF3
cat. AgOAc
O
O
N
OCOCF3
+
O
NHPh
O
OH
N
i-Pr2NEt (z mol% )
cat. (i-Pr)2NEt
O
N
O
MeOH (5 equiv)
R1
O
+
+
*
R1
NHAr
*
*
Ph
solvent, temp., 2
h
MeOH
Toluene, -78 oC
Ph
2a
Ar
R2
R2
1a (2 equiv)
3aa (O -adduct)
4aa (N-adduct)
entry chiral phosphine (x)
y
5
z
solvent tem p, oC yield, % [b] O/N
ee, % (O-adduct)[c]
Scheme 1. Catalytic asymmetric O-nitroso aldol reaction of alkenyl
trifluoroacetates.
1
2
(R,R)-Q uinoxP* (5)
40 toluene
-78
-78
-40
-78
-78
-60
-78
-78
94
78
>99/1
26/74
41/59
>99/1
95/5
98
(R)-DTBM -SEGPHOS (4) 10 40
THF
THF
1[d]
38[d]
98
3[e] (R)-DTBM -SEGPHOS (4) 10 40
88
4
5
6
7
8
(R,R)-Q uinoxP* (3)
(R,R)-Q uinoxP* (2)
(R,R)-Q uinoxP* (3)
(R,R)-Q uinoxP* (3)
(R,R)-Q uinoxP* (3)
3
2
3
3
3
40 toluene
40 toluene
40 toluene
20 toluene
10 toluene
91
Results and Discussion
97
96
>99
98
>99/1
99/1
97
We have previously reported that dibutyltin dimethoxide and the
QuinoxP*·AgOAc complex catalyze the asymmetric O-nitroso
aldol reaction between alkenyl trichloroacetates and
nitrosoarenes in the presence of methanol.[5c] The reaction
proceeds via a tin enolate, and tin dimethoxide is regenerated in
the presence of MeOH. Later, we have found that a chiral
phosphine·silver(I) complex-catalyzed asymmetric Mannich-type
99
92
>99/1
96
O
t-Bu
Me
O
O
N
N
P
PAr2
PAr2
P
Me t-Bu
O
(R)-DTBM -SEGPHOS (Ar
=
3,5-tBu2-4-MeOC6H2
)
(R,R)-QuinoxP*
[a] Unless otherwise specified, the reaction was carried out using chiral
phosphine (2–5 mol%), silver acetate (2–10 mol%), alkenyl trifluoroacetate 1a
(2 equiv), nitrosobenzene (2a, 1 equiv), diisopropylethylamine (10–40 mol%),
and methanol (5 equiv) in the specified solvent at -40~-78 oC for 2 h. [b]
Isolated yield. [c] The value is for the O-adduct and was determined by HPLC
analysis. [d] The value is for the N-adduct and was determined by HPLC
analysis. [e] The reaction time was 1 h.
[a]
Molecular Chirality Research Center, Department of Chemistry,
Graduate School of Science, Chiba University
Inage, Chiba 263-8522, Japan
E-mail: ayanagi@faculty.chiba-u.jp
Homepage: http://moon.gmobb.jp/ohba/yanagisawa/index.html
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/ejoc.xxxxxxxxx.
reaction of alkenyl trichloroacetates with imines takes place in
the presence of substoichiometric amount of N,N-
diisopropylethylamine without the tin dimethoxide.[6]
We
envisaged that if a similar tertiary amine was used as an additive,
With the optimum reaction conditions (Table 1, entry 7) in
hand, we carried out the chiral silver-catalyzed asymmetric O-
nitroso aldol reaction of alkenyl trifluoroacetate 1a with diverse
a
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European Journal of Organic Chemistry
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COMMUNICATION
nitrosoarenes 2a–h (Table 2). Not only nitrosobenzene (2a) but
also 2-nitrosotoluene (2b), 4-nitrosotoluene (2c), and 1-butyl-2-
nitrosobenzene (2d) showed exclusive O-selectivity and
extremely high enantioselectivity (entries 1-4). Introduction of a
bulkier alkyl group to the 2-position of nitrosobenzene was also
effective in gaining higher O-selectivity and enantioselectivity,
but the reactivity of nitrosoarene 2e was decreased because of
steric bulkiness (entry 5). The 2-CF3 substituent enhanced the
electrophilicity of nitrosoarene 2f (entry 6). An electron-rich
nitrosoarene also indicated remarkable reactivity as well as
superior O-selectivity and enantioselectivity (entry 9).
Table 3. Enantioselective O-nitroso aldol reaction of various alkenyl esters
1b–h with nitrosoarenes 2a and 2b catalyzed by (R,R)-QuinoxP*·AgOAc.[a]
(R,R)-QuinoxP* (3 mol% )
R3
AgOAc (3 mol% )
OCOCF3
O
N
O
HN
O
O
OH
N
i-Pr2NEt (20 m ol% )
MeOH (5 equiv)
R1
*
*
R1
R1
+
+
toluene, -78 oC, 2
h
R3
R3
R2
1b–h (2 equiv)
R2
3ba–ha (O-adduct)
R2
2a (R3
2b (R3
=
=
H)
4ba–ha (N-adduct)
2-Me)
entry
1
alkenyl ester
nitrosoarene
product yield, % [b] O/N
ee, % (O-adduct)[c]
OCOCF3
2a
3ba+4ba
84
90/10
96
1b
MeO
Table 2. Enantioselective O-nitroso aldol reaction of alkenyl trifluoroacetate 1a
with various nitrosoarenes 2a–i catalyzed by (R,R)-QuinoxP*·AgOAc.[a]
OCOCF3
1c
2
3
2a
2a
3ca+4ca
3da+4da
>99
99
93/7
95
99
(R,R)-QuinoxP* (3 m ol% )
R
AgOAc (3 mol%)
OCOCF3
OCOCF3
+
O
N
O
HN
O
O
OH
N
i-Pr2NEt (20 mol% )
MeOH (5 equiv)
88/12
*
*
+
1d
toluene, -78 oC,
2 h
R
R
OCOCF3
1a (2 equiv)
2a–i
3aa–ai (O -adduct)
4aa–ai (N-adduct)
ee, % (O-adduct)[c]
1e
4
5
2a
2b
3ea+4ea
3eb+4eb
94
84
99/1
93/7
99
99
entry
nitrosoarene
R
product
yield, % [b]
O/N
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
H
3aa+4aa
3ab+4ab
3ac+4ac
3ad+4ad
3ae+4ae
3af+4af
3ag+4ag
3ah+4ah
3ai+4ai
98
67
99/1
>99/1
>99/1
>99/1
>99/1
99
OCOCF3
2-M e
4-M e
2-Bu
2-iPr
2-CF3
4-CF3
4-Br
99
35/65[d]
92
6
2a
3fa+4fa
>99
1f (E/Z
= 1/13)
91
99
OCOCF3
>99
67
96 (S)[d]
99 (S)[d]
86
40/60[e]
4/96[f]
80
7
8
2a
2b
3ga+4ga
3gb+4gb
86
93
1g (E/Z
= 1/22)
–
>99
54
57/43[e]
OCOCCl3
9[g]
2a
3ha+4ha
10
<1/99[h]
–
2g
2h
2i
52/48[f]
93/7
72 (S)[d]
95 (S)[d]
97
1h (E/Z
= 1/4)
76
3-OM e
95
95/5
[a] Unless otherwise specified, the reaction was carried out using (R,R)-
QuinoxP* (3 mol%) , silver acetate (3 mol%), alkenyl ester 1b–h (2 equiv),
nitrosoarene (2a or 2b,
1 equiv), diisopropylethylamine (20 mol%), and
[a] Unless otherwise specified, the reaction was carried out using (R,R)-
QuinoxP* (3 mol%), silver acetate (3 mol%), alkenyl trifluoroacetate 1a (2
equiv), nitrosoarene (2a–i, 1 equiv), diisopropylethylamine (20 mol%), and
methanol (5 equiv) in toluene at -78 oC for 2 h. [b] Isolated yield. [c] The value
is for the O-adduct 3aa–3ai and was determined by HPLC analysis. [d] The
absolute configulation is shown in parentheses. [e] The N-adduct 4af had 30%
ee. [f] The N-adduct 4ag had 6% ee.
methanol (5 equiv) in toluene at -78 oC for 2 h. [b] Isolated yield. [c] The
value is for the O-adduct and was determined by HPLC analysis. [d] The N-
adduct 4fa had 7% ee. [e] The N-adduct 4ga had 18% ee. [f] The N-adduct
4gb had 43% ee. [g] The reaction was performed at -40 oC for 4 h. [h] The N-
adduct 4ha had 11% ee.
Thus obtained -aminooxyketone 3ea (O-adduct) was
further transformed into corresponding -hydroxyketone 5e[9] in
44% yield without decreasing its enantiomeric excess by
treatment with 30 mol% CuSO4 in MeOH at 0 ˚C for 16 h
(Scheme 2).
The above-mentioned results further motivated us to use
alkenyl trifluoroacetates 1b-h of other cyclic and acyclic ketones
in the asymmetric -aminooxylation (Table 3). Consequently, 1-
tetralone derivatives 1b and 1c, 1-indanone derivative 1d, and
1-benzosuberone derivative 1e gave optical purities that were
similar to the optical purity of alkenyl ester 1a (compare entries
1-4 in Table 3 with entry 1 in Table 2). Noteworthy was the fact
that 2-methyl-1-tetralone derivative 1c also showed high
reactivity toward nitrosobenzene (2a) regardless of the steric
bulkiness at 2-position (entry 2). In contrast, acyclic alkenyl
trifluoroacetates 1f-h furnished the opposite N-selectivity (entries
6-9). In the reaction of butyl phenyl ketone derivative 1g with 2-
nitrosotoluene (2b), N-adduct 4gb was formed highly selectively
in 93% combined yield (entry 8). Alkenyl trichloroacetate 1h[8] of
3-pentanone underwent hydroxyamination exclusively with
nitrosobenzene (2a), but the chemical yield of 4ha was low even
at -40 ˚C (entry 9).
O
O
O
Ph
OH
N
H
CuSO4 (30 mol% )
MeOH, 0 oC, 16 h
3ea, 99% ee (S)
5e, 44% yield
97% ee (S)
Scheme 2. N-O bond cleavage of -aminooxyketone 3ea.
A plausible catalytic mechanism is indicated in Figure 1.
Initially, the (R,R)-QuinoxP*·AgOAc complex reacts with
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European Journal of Organic Chemistry
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COMMUNICATION
Figure 2. Proposed transition state structures for the asymmetric O-nitroso
aldol reaction.
methanol in the presence of N,N-diisopropylethylamine to yield
the corresponding (R,R)-QuinoxP*·AgOMe complex, which is
the true catalyst in the present asymmetric O-nitroso aldol
reaction. Next, the thus formed chiral silver methoxide is
allowed to attack alkenyl trifluoroacetate 1 to yield chiral silver
enolate 6. The subsequent addition reaction of chiral silver
enolate 6 with nitrosoarene 2 provides chiral silver amide of -
aminooxyketone 7. Finally, protonation of chiral silver amide 7
with MeOH results in the formation of optically active -
aminooxyketone 3 and the regeneration of the chiral silver
Conclusions
In conclusion, we have developed a novel catalytic asymmetric
O-nitroso aldol reaction. The use of in situ generated chiral
silver methoxide as the chiral catalyst allows the synthesis of
various
nonracemic
-aminooxyketones
having
enantioselectivities of up to 99% ee. The catalytic cycle via a
chiral silver enolate provides an alternative asymmetric -
aminooxylation process. To the best of our knowledge, this is
the first example of the asymmetric nitroso aldol reaction via a
chiral silver enolate catalyzed by a chiral silver alkoxide.[10]
Further studies on the extension of the present silver
catalytic system to other asymmetric reactions are underway.
methoxide.
accelerates the catalytic cycle.
The rapid methanolysis of silver amide 7
O
N
P
P
R3
R3
Ag
*
P
P
Ag
R2
O
O
N
O
2
*
R1
R1
Experimental Section
R2
6
7
General Experimental Procedure for Asymmetric O-Nitroso Aldol
Reaction Catalyzed by (R,R)-QuinoxP*·AgOAc Complex and i-Pr2NEt
(Tables 2 and 3): A mixture of AgOAc (2.5 mg, 0.015 mmol) and (R,R)-
QuinoxP* (5.0 mg, 0.015 mmol) was dissolved in dry toluene (2 mL) under
an argon atmosphere and with direct light excluded, and stirred at room
temperature for 10 min. To the resulting solution were added CH3OH (100
L, 2.5 mmol) and i-Pr2NEt (17 L, 0.10 mmol) successively at -78 °C and
the mixture was stirred at this temperature for 5 min. Then, a mixture of
alkenyl trifluoroacetate (1.0 mmol) and nitrosoarene (0.5 mmol) in dry
toluene (2 mL) was added dropwise to the resulting solution at -78 °C.
After stirring for 2 h at this temperature, the mixture was treated with
MeOH (2 mL). Then, the mixture was filtered off with a glass filter funnel
filled with Celite® and washed with ether, and the combined filtrate and
washes were concentrated in vacuo. The residual crude product was
purified by column chromatography on silica gel to give corresponding -
aminooxy ketone (O-adduct) and -hydroxyamino ketone (N-adduct). The
enantiomeric ratios of the O-adduct and/or the N-adduct were determined
by HPLC analysis.
M eOCOCF3
OCOCF3
MeOH
O
R3
HN
O
P
P
R1
AgOMe
*
*
R1
R2
1
R2
(i-Pr)2EtHN+ AcO–
3
t-Bu
Me
M eOH, i-Pr2NEt
AgOAc
N
P
P
P
P
=
*
*
P
N
P
Me
t-Bu
(R,R)-QuinoxP*
Figure 1. Plausible catalytic cycle for the asymmetric O-nitroso aldol reaction
catalyzed by chiral silver methoxide.
The proposed transition state structures of the present
asymmetric O-nitroso aldol reaction are exhibited in Figure 2. A
chiral silver enolate complexed with (R,R)-QuinoxP* approaches
the oxygen atom of a nitrosoarene while avoiding steric
repulsion from a tert-butyl group of the chiral phosphine ligand.
Thus, -aminooxylation takes place selectively at the Si face of
the silver enolate to provide the (S)--aminooxyketone.
Supporting Information (see footnote on the first page of this article):
Experimental procedures, characterization data, and copies of the 1H NMR
and 13C NMR spectra and HPLC traces.
Acknowledgments
O
R3
R3
This work was supported by MEXT/JSPS KAKENHI Grant
Number 25410107, The Naito Foundation, and The COE
Program in Chiba University headed by Professor Takayoshi
Arai. We gratefully acknowledge the generous gift of (R,R)-
QuinoxP* from Nippon Chemical Industrial Co., Ltd. and (R)-
DTBM-SEGPHOS from Takasago International Corporation.
R
O
O
O
N
N
R1
N
H
R2
R2
R1
Me
P
disfavored
t-Bu
Ag
t-Bu
O
N
O
P
R3
Keywords: asymmetric catalysis • enolate • nitroso aldol
reaction • nitrosoarene • silver
S
O
N
Me
R1
N
H
R3
R2
favored
[1]
For reviews of catalytic asymmetric nitroso aldol reactions, see: a) P.
Merino, T. Tejero, Angew. Chem. Int. Ed. 2004, 43, 2995; b) J. M.
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European Journal of Organic Chemistry
10.1002/ejoc.201601143
COMMUNICATION
Janey, Angew. Chem. Int. Ed. 2005, 44, 4292; c) H. Yamamoto, N.
Momiyama, Chem. Commun. 2005, 3514; d) H. Yamamoto, M.
Kawasaki, Bull. Chem. Soc. Jpn. 2007, 80, 595; e) P. Kumar, N.
Dwivedi, Acc. Chem. Res. 2013, 46, 289; f) L. I. Palmer, C. P. Frazier, J.
Read de Alaniz, Synthesis 2014, 46, 269; g) F. Zhou, F.-M. Liao, J.-S.
Yu, J. Zhou, Synthesis 2014, 46, 2983; h) B. Maji, H. Yamamoto, Bull.
Chem. Soc. Jpn. 2015, 88, 753; i) P. Merino, T. Tejero, I. Delso, R.
Matute, Synthesis 2016, 48, 653.
H. Lin, X.-W. Sun, G.-Q. Lin, Org. Lett. 2014, 16, 752. Also see refs 2a
and 2i.
[4]
For examples of N-nitroso aldol reactions catalyzed by chiral Lewis
acids (Ag, Sc, Sn, Cu, and Mg), see: a) N. Momiyama, H. Yamamoto,
J. Am. Chem. Soc. 2004, 126, 5360; b) Y. Yamamoto, N. Momiyama, H.
Yamamoto, J. Am. Chem. Soc. 2004, 126, 5962; c) K. Shen, X. Liu, G.
Wang, L. Lin, X. Feng, Angew. Chem. Int. Ed. 2011, 50, 4684; d) A.
Yanagisawa, T. Fujinami, Y. Oyokawa, T. Sugita, K. Yoshida, Org. Lett.
2012, 14, 2434; e) B. Maji, M. Baidya, H. Yamamoto, Chem. Sci. 2014,
5, 3941.
[2]
For recent notable examples of N-nitroso aldol reactions promoted by
organocatalysts, see: a) N. Momiyama, H. Yamamoto, J. Am. Chem.
Soc. 2005, 127, 1080; b) T. Kano, M. Ueda, J. Takai, K. Maruoka, J.
Am. Chem. Soc. 2006, 128, 6046; c) C. Palomo, S. Vera, I. Velilla, A.
Mielgo, E. Gómez-Bengoa, Angew. Chem. Int. Ed. 2007, 46, 8054; d) T.
Zhang, L. Cheng, L. Liu, D. Wang, Y.-J. Chen, Tetrahedron: Asymmetry
2010, 21, 2800; e) H.-J. Yang, L. Dai, S.-Q. Yang, F.-E. Chen, Synlett
2012, 23, 948; f) T. Kano, F. Shirozu, K. Maruoka, J. Am. Chem. Soc.
2013, 135, 18036; g) B. Maji, H. Yamamoto, Angew. Chem. Int. Ed.
2014, 53, 8714; h) T. Kano, F. Shirozu, K. Maruoka, Org. Lett. 2014, 16,
1530; i) C. Xu, L. Zhang, S. Luo, Angew. Chem. Int. Ed. 2014, 53,
4149; j) C. Xu, L. Zhang, S. Luo, Org. Lett. 2015, 17, 4392.
[5]
For examples of O-nitroso aldol reactions catalyzed by chiral Lewis
acids (Ag and Cu), see: a) N. Momiyama, H. Yamamoto, J. Am. Chem.
Soc. 2003, 125, 6038; N. Momiyama, H. Yamamoto, J. Am. Chem. Soc.
2004, 126, 6498 (correction); b) M. Kawasaki, P. Li, H. Yamamoto,
Angew. Chem. Int. Ed. 2008, 47, 3795; c) A. Yanagisawa, S. Takeshita,
Y. Izumi, K. Yoshida, J. Am. Chem. Soc. 2010, 132, 5328; d) M. Baidya,
K. A. Griffin, H. Yamamoto, J. Am. Chem. Soc. 2012, 134, 18566; e) C.
P. Frazier, D. Sandoval, L. I. Palmer, J. Read de Alaniz, Chem. Sci.
2013, 4, 3857; f) B. Maji, H. Yamamoto, Angew. Chem. Int. Ed. 2014,
53, 14472; g) B. Maji, H. Yamamoto, Synlett 2015, 26, 1528. Also see
refs 4a and 4b.
[3]
For recent notable examples of O-nitroso aldol reactions promoted by
organocatalysts, see: a) G. Zhong, Angew. Chem. Int. Ed. 2003, 42,
4247; b) S. P. Brown, M. P. Brochu, C. J. Sinz, D. W. C. MacMillan, J.
Am. Chem. Soc. 2003, 125, 10808; c) A. Bøgevig, H. Sundén, A.
Córdova, Angew. Chem. Int. Ed. 2004, 43, 1109; d) Y. Hayashi, J.
Yamaguchi, T. Sumiya, M. Shoji, Angew. Chem. Int. Ed. 2004, 43,
1112; e) M. Lu, D. Zhu, Y. Lu, Y. Hou, B. Tan, G. Zhong, Angew. Chem.
Int. Ed. 2008, 47, 10187; f) M. Lu, D. Zhu, Y. Lu, X. Zeng, B. Tan, Z. Xu,
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European Journal of Organic Chemistry
10.1002/ejoc.201601143
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Asymmetric Catalysis
OCOCF3
cat. (R,R)-QuinoxP*·AgOAc
O
O
OH
N
O
cat. (i-Pr)2NEt
O
R1
+
+
*
*
R1
NHAr
R1
Ar
N
M eOH
Ar
Akira Yanagisawa*, Yuqin Lin, Akihiro
Takeishi and Kazuhiro Yoshida
R2
Toluene, -78 oC
R2
R2
O-adduct (up to 99% ee)
N-adduct
O /N = up to 99/1
Page No. – Page No.
An efficient catalytic enantioselective nitroso aldol reaction between alkenyl
Enantioselective Nitroso Aldol
Reaction Catalyzed by a Chiral
Phosphine-Silver Complex
trifluoroacetates and nitrosoarenes is described. The use of in situ generated chiral
silver methoxide as the chiral catalyst allows the synthesis of various nonracemic -
aminooxyketones with enantioselectivities of up to 99% ee.
This article is protected by copyright. All rights reserved