Doubly stereocontrolled asymmetric aza-henry reaction with in situ generation of n-boc-imines catalyzed by novel rosin-derived amine thiourea catalysts

Article abstract of DOI:10.1002/adsc.200900413

The doubly stereocontrolled organocatalytic aza-Henry reaction of nitroalkanes to N-Bocimines generated in situ from a variety of substituted α-amido sulfones was investigated for the first time, in general, affording the corresponding products with high

Full text of DOI:10.1002/adsc.200900413

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DOI: 10.1002/adsc.200900413
Doubly Stereocontrolled Asymmetric Aza-Henry Reaction with
in situ Generation of N-Boc-Imines Catalyzed by Novel Rosin-
Derived Amine Thiourea Catalysts
Xianxing Jiang,a,b,c Yifu Zhang,a,c Lipeng Wu,a Gen Zhang,a Xing Liu,a
Hailong Zhang,a Dan Fu,a and Rui Wanga,b,*
a
State Key Laboratory of Applied Organic Chemistry, Institute of Biochemistry and Molecular Biology, Lanzhou
University, Lanzhou 730000, Peopleꢀs Republic of China
Fax : (+86)-931-8911255; e-mail: wangrui@lzu.edu.cn or bcrwang@polyu.edu.hk
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Kowloon, Hong Kong
These authors contributed equally to this work.
b
c
Received: June 17, 2009; Published online: September 10, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900413.
Abstract: The doubly stereocontrolled organocata-
lytic aza-Henry reaction of nitroalkanes to N-Boc-
imines generated in situ from a variety of substitut-
ed a-amido sulfones was investigated for the first
time, in general, affording the corresponding prod-
ucts with high to excellent yields (up to 93% yield)
and enantioselectivities (up to 98% ee), and satis-
factory diastereoselectivies (anti/syn up to 98:2).
Furthermore, these organocatalysts based on rosin
have been proved to be the very effective promot-
ers for this catalytic asymmetric process along side
the Cinchona alkaloid-derived catalysts.
imines; b: starting from a-amido sulfones, Scheme 1).
Since Shibasaki reported the first catalytic asymmetric
aza-Henry reaction of nitromethane to N-protected
imines,[4] impressive progress has been made on the
development of more selective and efficient catalytic
systems involving metallic[5] and organic[6] catalysts
for this asymmetric version. However, the preparation
of these imines, such as N-carbamoylimines requires
harsh conditions and their purification and storage
are rather troublesome because of their inherent in-
stability. To the best of our knowledge, only the two
groups of Palomo[7a] and Ricci[7b] have independently
reported catalytic asymmetric aza-Henry reactions
with in situ generation of carbamate-protected imines
from a-amido sulfones using the same chiral quater-
nary ammonium salt catalyst based on a Cinchona al-
kaloid by phase-transfer catalysis (PTC). Neverthe-
less, this more convenient and useful catalytic version
remains a much less developed field and a practical
Keywords: a-amido sulfones; asymmetric synthesis;
aza-Henry reaction; imines; rosin-derived thiourea
The catalytic asymmetric aza-Henry reaction[1] consti- asymmetric aza-Henry reaction with in situ generation
tutes one of the most versatile and attractive ap- of carbamate-protected imines catalyzed by other cat-
proaches for accessing optically active chiral b-nitro- alysts based on new chiral scaffolds has not yet been
ACHTUNGTRENNUNGamines, which can be readily converted into valuable reported. Furthermore, except for a recently reported
synthetic building blocks or biologically active vicinal example,[8] Cinchona alkaloid derivatives were em-
diamines[2] and a-amino carbonyl compounds,[3] and ployed as exclusive efficient catalysts in all of the re-
could effectively be obtained by only two asymmetric lated asymmetric Mannich reactions with in situ gen-
catalytic ways to date (a: starting from N-protected eration of carbamate-protected imines from a-amido
sulfones.[9]
We recently reported[10] a new class of thiourea-de-
rived bifunctional catalysts[11,12] based on rosin, which
have successfully been applied to the doubly stereo-
controlled synthesis of g-nitro heteroaromatic ke-
tones. Herein, we describe our contribution to the
progress of the catalytic asymmetric aza-Henry reac-
Scheme 1. Two asymmetric catalytic ways.
tion and report the first doubly stereocontrolled
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Table 1. Screening of bases and thiourea catalysts in reaction
of a-amido sulfone 2a with nitromethane.[a]
Entry
Catalyst
Base[b]
Yield [%][c]
ee [%][d]
1[e]
2[e]
3[e]
4
5
6
7
8
9
10
11
12
13
N
KOH
76
72
70
85
92
90
81
86
88
n.d.
n.d.
70
72
68 (S)
0
0
69 (S)
72 (S)
80 (S)
78 (S)
93 (S)
87 (R)
n.d.
K2CO3
Cs2CO3
CsOH
NaOH
KOH
Na2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Figure 1. Structure of thiourea catalysts.
n.d.
78 (S)
80 (R)
asymmetric aza-Henry reaction with in situ generation
of N-carbamate imines catalyzed by novel organoca-
talysts based on rosin. Undoubtedly, the scope of cat-
alytic asymmetric Mannich reactions with in situ gen-
eration of N-carbamate imines (especially, the catalyt-
ic aza-Henry version) and the application of rosin de-
rivatives as well as thiourea will be considerably ex-
panded by our work. In background studies, novel
[a]
Unless noted, the reaction was conducted with 2a
(0.1 mmol) and nitromethane (0.3 mmol) in CHCl3
(1.0 mL) and H2O (1.0 mL) at 08C for 48 h.
Base (1.0 equiv.).
[b]
[c]
[d]
Isolated yield.
The ee values were determined by HPLC, and the config-
uration was assigned by comparison of HPLC date and
specific rotation with literature data.[6b]
In CHCl3 (1.0 mL).
[e]
thiourea catalysts based on rosin
ACHTUNGTREN(NUGN 1R,2R)-L2, L3 and
ACHTUNGTRENNUNG(1S,2S)-L2, L3 were designed and synthesized
(Figure 1). Subsequently, the effects of the catalysts
were investigated in comparison with other thiourea
catalysts and N-Boc protected a-amido sulfones K2CO3 in the biphasic solvent (86% yield and 93% ee,
(Boc=tert-butyloxycarbonyl) were employed as sub- entry 8). Gratifyingly, (1R,2R)-L2 also exhibited supe-
strates considering the easy removal of the N-Boc rior catalytic activity with an opposite sense of asym-
protecting group. metric induction (88% yield and 87% ee, entry 9).
AHCTUNGTRENNUNG
A model reaction of nitromethane with a-amido The effects of other catalysts were tested next (en-
sulfone 2a was performed in the presence of 15 mol% tries 10–13). Thiourea catalyst L4 also gave the de-
of thiourea catalysts at 08C under different conditions sired products with good yields and enantioselectivi-
(Table 1). We initially screened a range of inorganic ties (entries 12 and 13). The reaction also showed a
bases in the presence of 15 mol% ACTHNUTRGNEU(GN 1S,2S)-L2 in dramatic ligand effect and those thiourea catalysts L1
CHCl3 (entries 1–8). We found that inorganic base and L3 proved to be essentially inactive for this trans-
and the solvent were crucial to obtain high reaction formation (entries 10 and 11). In contrast, thiourea
efficiency. While the desirable product could be ob- catalyst L2 is found to be the best one for this doubly
tained in the absence of an amount of water, only stereocontrolled organocatalytic asymmetric process
moderate enantioselectivity was observed in the reac- under these mild conditions. We have disclosed that
tion employing 1.0 equivalent of KOH (entry 1) and the stereochemical control of the reaction is mainly
other cases afforded the racemic products (entries 2 provided by the 1,2-diaminocyclohexane moiety of
and 3). In comparison, the reactions furnished the ad- the thiourea in the heterogeneous catalytic reac-
ducts with moderate to excellent enantioselectivities tion.[10] In this biphasic catalytic system, the above re-
(69–93%, entries 4–8) and high to excellent yields sults suggest that the cyclohexyl scaffold of the thiour-
(81–92%) in the biphasic solvent. These results indi- ea catalyst would also play a crucial role on the ste-
cated that a substantial change of inorganic bases did reochemical control of the reaction (nitroalkane after
not have a significant effect on the yield, however, deprotonation as the nitronate anion could coordinate
the impact of these bases appeared to be different for to a tertiary amino group of cyclohexyl scaffold). The
stereoselection. The best result was observed when observed absolute configuration (S) or (R) of the
the reaction was conducted with 1.0 equivalent of adduct was explained by the attack of the nitronate
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Xianxing Jiang et al.
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Table 2. Aza-Henry reaction of nitroalkanes to N-Boc-imines generated in situ from a-amido sulfones[a]
Entry
R
R’
Yield [%] (1S/1R)[b]
ee [%] (1S/1R)[c]
dr (anti/syn) (1S/1R)[d]
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
86/88
82/80
83/82
89/87
83/86
92/86
85/83
85/90
81/81
80/82
81/85
15/10
85/82
93/83
86/85
90/87
82/80
82/83
93/87
93/82
84/82
95/91
98/93
92/84
91/97
89/81
95/89
93/93
81/94
n.d.
85/81
95/87
85/78
92/81
91/80
88/85
1-naphthyl
2-naphthyl
4-ClC6H4
4-FC6H4
2-FC6H4
2-ClC6H4
3-ClC6H4
2-MeOC6H4
3-MeOC6H4
2-furyl
9
10
11[e]
12[e]
13
14
15
16
17
18[e]
cyclohexyl
Ph
88:12/85:15
78:22/93:7
98:2/98:2
86:14/90:10
85:15/96:4
98:2/88:12
4-ClC6H4
2-ClC6H4
2-FC6H4
4-MeOC6H4
2-furyl
[a]
The reaction was conducted with a-amido sulfone (0.1 mmol), nitroalkane (0.3 mmol) and K2CO3 (1.0 equiv.) in CHCl3
(1.0 mL) and H2O (1.0 mL) for 48 h.
Isolated yield.
[b]
[c]
The ee values were determined by HPLC, and the configuration was assigned by comparison of HPLC date and specific
rotation with literature data.[6b]
[d]
[e]
1
Determined by 400 MHz H NMR.
The reaction was stirred for 56 h.
anion to the corresponding si-face or re-face of the ac-
tivated amido sulfones.
In conclusion, we have developed a new class of
rosin-derived thiourea catalysts, which has been suc-
The results of experiments under the optimized cessfully applied to the doubly stereocontrolled asym-
conditions that probe the scope of the reaction are metric aza-Henry reaction with in situ generation of
summarized in Table 2. The doubly stereocontrolled N-carbamate imines firstly. These organocatalysts
catalytic aza-Henry reaction of nitroalkanes to N- based on rosin have been proved to be the very effec-
Boc-imines generated in situ from a variety of substi- tive promoters for this catalytic asymmetric process
tuted a-amido sulfones was examined. The results along side the Cinchona alkaloid-derived catalysts.
showed that, except for the cyclohexyl-substituted a- Especially, the reaction could be achieved under the
amido sulfone (entry 12), all reactions with nitrome- mild conditions in a doubly stereocontrolled manner.
thane and aromatic/heterocyclic amido sulfones pro- Further investigations and mechanistic studies are on-
ceeded cleanly affording the desired products of the going in our laboratories.
(S) or (R) configuration with high to excellent enan-
tioselectivities [(1S)-adducts, 81–98% ee; (1R)-ad-
ducts, 81–97% ee, entries 1–11] and yields [(1S)-ad-
Experimental Section
ducts, 80–92%; (1R)-adducts, 81–90%, entries 1-11].
As expected, nitroethane has proven to be an excel-
lent substrate for the generation of two contiguous ni-
trogen-bearing stereogenic centers. The reactions af-
forded anti-diastereoselective adducts (anti/syn up to
98:2) with high to excellent enantioselectivities and
yields in a doubly stereocontrolled manner for all of
the a-amido sulfones tested (entries 13–18).
Procedure for the Synthesis of Rosin-Derived Amine
Thiourea Catalysts
Using the reported procedures,[13] the pure dehydroabietic
amine as a white solid was obtained in 45% yield. Carbon
disulfide (4.0 mL) and N,N’-dicyclohexylcarbodiimide
(10 mmol) were added to a solution of dehydroabietic
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amine (10 mmol) in dry ether (35 mL) at 08C. The reaction References
mixture was allowed to warm slowly to room temperature
over a period of 3 h and then was stirred for a further 12 h
at room temperature. After separation of the precipitated
thiourea by filtration, the solvent was removed under re-
duced pressure. After column chromatography on silica gel
eluted with 25% ethyl acetate in hexanes, the corresponding
isothiocyanate as a white solid was isolated in 92% yield.
Under an argon atmosphere, to a solution of the above
isothiocyanate (9.17 mmol) in dry dichloromethane (80 mL)
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was
added
N,N-dimethyl-trans-diaminocyclohexane
(10 mmol). The reaction mixture was stirred for 12 h at
room temperature and was concentrated under vacuum.
After column chromatography on silica gel (ethyl acetate/
hexane=2/1 as eluent), the thiourea L1 as a white solid was
isolated in 83% yield.
1-[(1R,2R)-2-(Dimethylamino)cyclohexyl]-3-
{[(1R,4aS,10aR)-7-isopropyl-1,4a-dimethyl-1,2,3,4,4a,9,
10,10a-octahydrophenanthren-1-yl]methyl}thiourea (1R,2R)-
1
L2: [a]20: +2 (c 2.0, CHCl3); mp 828C. H NMR (300 MHz,
CDCl3)D: d=7.14–7.17 (d, J=8.1 Hz, 1H), 6.97–6.99 (m, 1H),
6.88 (s, 1H), 6.45 (br, 1H), 3.68–3.70 (m, 1H), 3.35 (m, 1H),
2.79–2.91 (m, 3H), 2.39–2.45 (m, 1H), 2.20–2.34 (m, 7H),
1.67–1.88 (m, 8H), 1.43–1.50 (m, 2H), 1.21–1.38 (m, 16H),
0.85–1.64 (m, 5H); 13C NMR (75 MHz, CDCl3): d=147.1,
145.7, 134.7, 126.8, 124.1, 123.8, 67.1, 56.3, 55.4, 45.6, 40.3,
38.3, 37.8, 37.4, 36.6, 33.4, 33.1, 30.0, 25.2, 25.1, 24.5, 24.0,
23.9, 22.1, 19.1, 18.6, 18.5; IR: n=3437, 3259, 3066, 2928,
2248, 2122, 1550, 1452, 1380, 1058, 1029, 913, 822, 759,
624 cmÀ1; ESI-MS: m/z=470 [M+]; HR-MS-ESI: m/z=
470.3573, calcd. for C29H47N3S [M+H]+: 470.3563.
Typical Procedure for the Asymmetric Aza-Henry
Reaction of Nitroalkanes to N-Boc-Imines Generated
in situ from a-Amido Sulfones
To
a
stirred solution of
G
ACHTUNGTRENUN(NG 1S,2S)-L2
(0.015 mmol, 7 mg), a-amido sulfone 2a (0.1 mmol, 34.7 mg)
in the biphasic solvent of CHCl3 (1.0 mL) and H2O
(1.0 mL), K2CO3 (0.1 mmol, 13.8 mg) was added. Subse-
quently, nitromethane (0.3 mmol, 16.1 mL) was added under
argon. The solution was stirred at 08C. After the reaction
was completed (the course of the reaction was monitored by
means of thin-layer chromatography, TLC), the mixture was
extracted with CHCl3 (10 mL ꢂ 4) and dried with sodium
sulfate. Concentration and flash chromatography on silica
gel (eluent, ethyl acetate/hexane 1:8) afforded the optical
active product (S)-3a; yield: 22.8 mg (86%) or (R)-3a; yield:
23.4 mg (88%). The enantiomeric purity of the product was
determined by using HPLC (Chiralpak AD column, l=
210 nm, i-PrOH/hexane 10:90, 0.6 mLminÀ1): (S)-3a ee=
93%, tmajor =16.88 min, tminor =17.91 min; (R)-3a ee=87%,
tminor =17.07 min, tmajor =17.91 min.
Acknowledgements
We are grateful for the grants from the National Natural Sci-
ence Foundation of China (Nos. 20525206, 20772052 and
20621091), the Chang Jiang Program of the Ministry of Edu-
cation of China for financial support.
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Xianxing Jiang et al.
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