Asymmetric organocatalytic cascade Michael/Michael/Henry reaction sequence: Control of all stereocenters in one cyclohexane skeleton

Article abstract of DOI:10.1002/adsc.201200008

The highly diastereo- and enantioselective relay cascade Michael/Michael/Henry reaction catalyzed by combination of readily available diphenylprolinol silyl ether and the quinine thiourea in a one-pot fashion has been developed. Up to 70% yield and up to >99% enantioselectivity of the single major isomer were obtained from the cascade reactions. Copyright

Full text of DOI:10.1002/adsc.201200008

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DOI: 10.1002/adsc.201200008
Asymmetric Organocatalytic Cascade Michael/Michael/Henry
Reaction Sequence: Control of All Stereocenters in One
Cyclohexane Skeleton
Zhifeng Mao,a,b,c Yaomei Jia,a,c Zhaoqing Xu,b and Rui Wanga,*
a
Key Laboratory of Preclinical Study for New Drugs of Gansu Province, School of Basic Medical Sciences, Institute of
Biochemistry and Molecular Biology, School of Life Sciences, and State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, Lanzhou 730000, Peopleꢀs Republic of China
E-mail: wangrui@lzu.edu.cn
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Peopleꢀs Republic of China
These authors contributed equally to this work.
b
c
Received: January 4, 2012; Revised: March 2, 2012; Published online: May 15, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200008.
Abstract: The highly diastereo- and enantioselec-
tive relay cascade Michael/Michael/Henry reaction
catalyzed by combination of readily available di-
phenylprolinol silyl ether and the quinine thiourea
in a one-pot fashion has been developed. Up to
70% yield and up to >99% enantioselectivity of
the single major isomer were obtained from the cas-
cade reactions.
lysts.[4d] The synthesis of hexasubstituted cyclohexane
with control of all six stereocenters by asymmetric
tandem or cascade reaction still remains a challenge.
To the best of our knowledge, the generation of six
contiguous stereocenters in one cyclohexane skeleton
from two simple compounds by an asymmetric
domino reaction has been merely reported.[5] Herein
we described an example of such a reaction catalyzed
by the combination of two different organocatalysts
in one-pot.[6]
Our proposed three-step asymmetric organocatalyt-
ic relay cascade to hexasubstituted cyclohexanes is
shown in Scheme 1. In the first step, the catalyst (S)-
4a activates aldehyde 1 by enamine formation, which
then adds to the nitroalkene 2 in a Michael-type reac-
tion.[7] The following hydrolysis liberates the catalyst
Keywords: asymmetric catalysis; cyclohexanes;
domino reactions; Henry reaction; Michael addi-
tion
Asymmetric synthesis of complex molecules with con- (S)-4a and provides adduct A. In the subsequent step,
trol of multiple stereocenters in a one-pot fashion by the bifunctional base/Brønsted acid catalyst of the
asymmetric catalysis is very useful for the preparation type QT-5a would activate the adduct A and the ni-
of natural products and other molecules of interest.[1] troalkene 2 again, thus promoting a second stereose-
Asymmetric organocatalytic tandem or cascade reac- lective Michael addition.[8] The Michael adduct B,
tions are a very powerful way for achieving this with its suitably positioned aldehyde and nitroalkane
goal.[2] Polysubstituted chiral cyclohexanes construct- functionalities, would then undergo a base-promoted
ed through organocatalytic cascade approaches have Henry reaction to generate the desired cyclohexane 3.
attracted a great deal of attention, owing to the prev- As a result, three new bonds and six contiguous ste-
alence of such building blocks in pharmaceutical com- reogenic centers could be assembled, incorporating
pounds and complex natural products.[3] Up to five multiple functional groups in a simple-to-perform,
new contiguous stereocenters in one reaction have single-operation cascade sequence. In the course of
been made by the Jørgensen group.[3f] Recently, the the reaction, nitroalkene 2 is employed twice, which
relay catalysis has been reported as an efficient route means at least two equivalents of nitroalkene are re-
in asymmetric catalysis by combination of two orga- quired to complete the transformation.
nocatalysts or metal/organocatalyst in one-pot reac-
We initiated our investigation by using propanal 1a
tions.[4] Xu and Dixon et al. reported an excellent and nitroalkene 2a as the substrates combined with
asymmetric organocatalytic relay cascade reaction for two catalysts, (R)-4a (10 mol%) and QT-5a
the synthesis of substituted cyclohexanes with four (20 mol%). To our delight, by performing the reaction
stereocenters by using two different kinds of cata- in toluene, we were able to isolate the desired product
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Scheme 1. Organocascade Michael/Michael/Henry reaction promoted by a combination of two organocatalysts.
and another two minor isomers in 22% yield (Table 1, shown to be compatible in the reaction and provided
entry 1). In order to improve the yield, various cata- the corresponding cyclohexanol derivatives 3n and 3o
lyst combinations and different solvents were with moderate yields, good diastereoselectivities and
screened. Compared to (R)-4a, the combinations of excellent enantioselectivities (Table 2, entries 14 and
(S)-4a and QT-5a improved the yield and the enantio- 15). When we use aliphatic nitroalkenes 2, only traces
selectivity of the product effectively, however the op- of the cyclohexane 3 can be obtained. We treated the
posite enantiomer 3a was formed (Table 1, entry 2). major diastereoisomer 3a with DBU (1 equiv.) at
These results indicated that (S)-4a and QT-5a were room temperature. After 48 h, we determined the
matched catalyst pairs. (S)-4b and (S)-4c gave poor product by 1H NMR, and found that only small
results (Table 1, entries 3 and 4). With (S)-4a as the amounts of the minor diastereoisomer were observed
first catalyst, both 5b and 5c showed inferior perfor- (4:1:0 dr). In addition, we also stopped the reaction
mance than QT-5a, indicating that QT-5a has an ex- after 24 h, and checked the diastereomer ratio (13:4:1
cellent asymmetric induction ability and catalytic ac- dr) at that stage, which has no pronounced change
tivity (Table 1, entries 5 and 6). The combination of compared to the result after 72 h. Furthermore, the
(R)-4a and 5c led to the formation of the opposite final product 3a which was precipitated when purifica-
enantiomer of 3a (Table 1, entry 7). The highest yields tion of the reaction mixture by flash chromatography
were obtained using toluene as the solvent (Table 1, was performed at room temperature, no other diaste-
entries 2, 8, and 9).
reoisomers were observed even after a few months.
After optimizing the reaction conditions, we studied So the major final product must then be the thermo-
the possibility of using different aldehydes as well as dynamic product.
different nitroalkenes to synthesize various hexasub-
To study the mechanism of the reaction, we con-
stituted cyclohexane derivatives. As shown in Table 2, ducted a series of control experiments (Scheme 2). In-
linear aliphatic aldehydes 1a, 1b, 1c could be used as itially, reactions were carried out using 1a and 2a in
the nucleophiles in reaction with nitrostyrene giving the presence of either (S)-4a or QT-5a as catalyst.
high diastereoselectivities. In all cases, the major iso- However, the product 3a was not detected when
mers were isolated in moderate to good yields with either catalyst was used. The adduct 6 was afforded
excellent enantioselectivities (Table 2, entries 1–3).
by the Michael reaction of propanal (1a) and (E)-ni-
Other nitroalkenes were also examined under our trostyrene (2a) in the presence of (S)-4a overnight in
standard conditions. Aromatic and heteroaromatic good yield (97%, the yield given is for the two isolat-
substituted electrophiles 2b–j could be used well in ed stereoisomers) with low diastereoselectivity (syn/
this reaction (Table 2, entries 4–15). Excellent enan- anti=3:1) but with high enantioselectivity (96%/
tioselectivities (up to 99%) were observed, irrespec- 92%).[7] The low diastereoselectivity was due to iso-
tive of the electronic nature or position of the sub- merization before 6 underwent the next step. Having
stituents on the phenyl ring. Furthermore, two thio- obtained the pure adduct 6, we next examined the
phenyl-substituted electrophiles 2i and 2j were also second stage of the cascade reaction. We conducted
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Adv. Synth. Catal. 2012, 354, 1401 – 1406

Asymmetric Organocatalytic Cascade Michael/Michael/Henry Reaction Sequence
Table 1. Representative screening results for the reaction of the reaction propanal 1a with nitroalkene 2a by using different
catalyst combinations.[a]
Entry
Cat.4/Cat.5
Solvent
Yield [%][b]
dr[c]
ee [%][d]
1
2
3
4
5
6
7
8
9
(R)-4a/QT-5a
(S)-4a/QT-5a
(S)-4b/QT-5a
(S)-4c/QT-5a
(S)-4a/5b
(S)-4a/5c
(R)-4a/5c
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
22
60
21
52
27
39
61
42
29
4:1:1
15:4:1
9:1:1
14:4:1
6:1:1
7:1:1
17:5:1
9:3:1
16:4:1
À40
>99
95
99
88
63
À93
(S)-4a/QT-5a
(S)-4a/QT-5a
99
99
[a]
The reaction was carried out with propanal 1a (0.5 mmol), nitroalkene 2a (1.5 mmol), catalyst 4 (10 mol%), catalyst 5
(20 mol%) in solvent (1.0 mL) at room temperature for 72 h.
[b]
[c]
[d]
Combined yield of the three stereoisomers.
1
Determined by H NMR analysis of the crude reaction mixture.
The ee values for the isolated major product were determined by HPLC on a chiral stationary phase.
several reactions of 6 with 2a (2 equiv.) in the pres- minor stereoisomers were determined by analysis of
1
ence of one or more catalysts. After 24 h, 3a was not nOe and H-1H COSY experiments (for details, see
formed when 6 reacted with (S)-4a (Scheme 2, path the Supporting Information). This cascade reaction
A). The presence of QT-5a (20 mol%) led to the for- generates six stereogenic centers, and theoretically
mation of 3a in good diastereoselectivity (4:1:1 dr) could give rise to 26 =64 different stereoisomers. In
and excellent enantioselectivity (>99% ee) and in fact, we note that this asymmetric cascade reaction
27% yield (the yield given is for the major isolated forms just three diastereomers, and the least isomers
stereoisomer 3a) with respect to adduct 6 (Scheme 2, were generally obtained at <10:1 (compared to the
path B, 24 h). Actually, the final base-promoted intra- major isomers). We rationalize the high stereoselec-
molecular Henry reaction was so quick that inter- tivity for this cascade reaction as follows: the first Mi-
mediate 7 could not be detected in either pathway. In chael addition catalyzed by (S)-4a is known to pro-
the presence of (S)-4a (10 mol%) and QT-5a ceed with high enantioselectivity; in the second step,
(20 mol%), similar results were obtained (Scheme 2, the bifunctional base/Brønsted acid catalyst QT-5a
path C, 24 h). This indicated that the effect of both or- could activate both the intermediate A and the nitro-
ganocatalysts may be relatively independent.
alkene 2, and promote a highly stereoselective Mi-
The absolute configuration of the major stereoiso- chael addition again .
mer was assigned by single-crystal X-ray analysis of
In summary, we have developed a highly diastereo-
the 3k (Figure 1).[9] The stereochemistries of the and enantioselective relay cascade Michael/Michael/
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Table 2. Scope of the asymmetric cascade Michael/Michael/Henry reactions.[a]
Entry
1 (R1)
2 (R2)
Product: Yield [%][b]
dr[c]
ee [%][d]
1
2
3
4
5
6
7
8
1a (Me)
1b (Et)
1c (n-Bu)
1b (Et)
1b (Et)
1b (Et)
1b (Et)
1b (Et)
1b (Et)
1b (Et)
1a (Me)
1a (Me)
1a (Me)
1a (Me)
1a (Me)
2a (Ph)
2a (Ph)
2a (Ph)
3a:[f] 60
3b: 67
3c: 57
3d: 70
3e: 60
3f: 62
3g: 55
3h: 47
3i: 65
3j: 59
3k: 53
3l: 45
3m: 33
3n: 35
3o: 34
15:4:1
9:2:1
>99
>99
96
99
>99
98
>99
>99
>99
98
>99
>99
>99
99
>99
26:8:1
16:4:1
18:4:1
11:4: 1
12:4: 1
17:6: 1
14:4:1
29:7:1
16:4: 1
10:3:1
2b (4-Br-C6H4)
2c (4-Cl-C6H4)
2d (4-CH3-C6H4)
2e (3-MeO-C6H4)
2f (4-MeO-C6H4)
2g (4-F-C6H4)
2h (2-naphthyl)
2b (4-Br-C6H4)
2d (4-CH3-C6H4)
2f (4-MeO-C6H4)
2i (thiophen-2-yl)
2j (5-bromothiophen-2-yl)
9
10
11
12
13
14
15[e]
49:13: 1
15:3:1
12:2:1
[a]
The reaction was carried out with aldehyde 1 (0.5 mmol), nitroalkene 2 (1.5 mmol), catalyst (S)-4a (10 mol%), catalyst
QT-5a (20 mol%) in toluene (1.0 mL) at room temperature for 72 h.
[b]
[c]
[d]
[e]
[f]
Combined yield of the three stereoisomers.
1
Determined by H NMR analysis of the crude reaction mixture.
The ee values for the isolated major products were determined by HPLC on a chiral stationary phase.
The reaction time is 96 h.
The reaction was carried out on an 8-mmol scale with slightly lower yield (54%); 1.15 g of the major stereoisomer 3a
were isolated.
Scheme 2. Probing the mechanism.
Henry reaction catalyzed by combination of readily could furnish hexasubstituted cyclohexane derivatives
available diphenylprolinol silyl ether and the quinine from very simple starting materials with excellent
thiourea. Under optimal conditions, this reaction control of all six contiguous stereocenters in a one-
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Asymmetric Organocatalytic Cascade Michael/Michael/Henry Reaction Sequence
ment” of the Ministry of Science and Technology
(2012ZX09504001-003).
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Experimental Section
Typical Procedure for the Cascade Reaction of Alde-
hyde 1a and Nitroalkene 2a with Catalyst 4a and
Catalyst 5a
To a mixture of catalyst 4a (0.05 mmol, 10 mol%), catalyst
5a (0.10 mmol, 20 mol%) in toluene (1.0 mL) was added al-
dehyde 1a (0.5 mmol) and nitroalkene 2a (1.5 mmol). After
72 h of stirring at room temperature, the reaction mixture
was directly purified by flash chromatography (petroleum
ether/ethyl acetate, 10:1) to afford the product 3a as a white
solid. 1H NMR (300 MHz, CDCl3): d=0.99 (d, J=6.6 Hz,
3H), 2.56 (s, 1H), 3.05–3.16 (m, 1H), 3.52 (dd, J=12.3,
4.5 Hz, 1H), 4.36 (dd, J=12.3, 4.8 Hz, 1H), 4.70 (s, 1H),
5.00 (t, J=4.8 Hz, 1H), 6.10 (dd, J=12.3, 2.4 Hz, 1H), 7.12–
7.22 (m, 4H), 7.28–7.35 (m, 6H); 13C NMR (75 MHz,
CDCl3): d=16.2, 33.2, 42.0, 45.4, 72.3, 86.2, 95.1, 127.0,
127.8, 128.2, 128.7, 129.2, 129.4, 134.0, 136.2; IR (CHCl3):
n=3536, 3328, 2973, 2928, 1550, 1497, 1454, 1374, 1337,
1037, 747, 701 cmÀ1; HR-MS (ESI): m/z=374.1704, calcd.
for C19H20N2O5 +NH4: 374.1710; [a]2D8:1: +47 (c 1.0 in
CHCl3); The ee value was determined by HPLC on a chiral
phase (Chiralpak AD-H column, n-hexane/2-propanol
85:15) relative to the racemic sample: major isomer
12.8 min, minor isomer 18.2 min, ee >99%.
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