Highly enantioselective Michael-aldol-dehydration reaction for the synthesis of chiral 3,5-diaryl-cyclohexenones catalyzed by primary amine

Article abstract of DOI:10.1016/j.tet.2017.01.004

A simple organocatalytic Michael-aldol-dehydration domino approach to chiral 3,5-diaryl-cyclohexenones from acetone and α,β-unsaturated ketones was developed for the first time using a simple chiral primary amine as a catalyst. Moderate to good yields (up to 85%) and excellent enantioselectivities (88–98% ee) were obtained.

Full text of DOI:10.1016/j.tet.2017.01.004

Accepted Manuscript
Highly enantioselective Michael-aldol-dehydration reaction for the synthesis of chiral
3,5-diaryl-cyclohexenones catalyzed by primary amine
Li Tang, Yuan Luo, Jing-Wen Xue, Yan-Hong He, Zhi Guan
PII:
S0040-4020(17)30004-2
10.1016/j.tet.2017.01.004
TET 28374
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 22 October 2016
Revised Date: 2 January 2017
Accepted Date: 4 January 2017
Please cite this article as: Tang L, Luo Y, Xue J-W, He Y-H, Guan Z, Highly enantioselective Michael-
aldol-dehydration reaction for the synthesis of chiral 3,5-diaryl-cyclohexenones catalyzed by primary
amine, Tetrahedron (2017), doi: 10.1016/j.tet.2017.01.004.
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ACCEPTED MANUSCRIPT
Graphical Abstract

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Highly Enantioselective Michael-Aldol-Dehydration Reaction for the Synthesis
of Chiral 3,5-Diaryl-Cyclohexenones Catalyzed by Primary Amine
Li Tang, Yuan Luo, Jing-Wen Xue, Yan-Hong He* and Zhi Guan*
Key Laboratory of Applied Chemistry of Chongqing Municipality, School of Chemistry and Chemical
Engineering, Southwest University, Chongqing, 400715, P. R. China
E-mails: heyh@swu.edu.cn (for Y.-H. He); guanzhi@swu.edu.cn (for Z. Guan)
Abstract: A simple organocatalytic Michael-aldol-dehydration domino approach to chiral
3,5-diaryl-cyclohexenones from acetone and α,β-unsaturated ketones was developed for the first
time using a simple chiral primary amine as a catalyst. Moderate to good yields (up to 85%) and
excellent enantioselectivities (88-98% ee) were obtained.
Keywords: organocatalysis; domino reaction; 3,5-diaryl-cyclohexenones; enantioselective
synthesis
Introduction
Chiral cyclohexenones are important chiral scaffolds which have been widely used in the
synthesis of asymmetric natural products and a broad spectrum of biologically active molecules.1
Therefore, many useful strategies have been developed for the synthesis of this kind of
compounds, including various multistep synthesis,2 intramolecular aldol condensation reactions,3
kinetic resolution of racemic substituted cyclohexenones by asymmetric catalytic reactions,4 and
enantioselective Robinson annulation, which consists of three consecutive processes: (I) the
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asymmetric Michael addition of a carbonyl compound to an α, β-unsaturated ketone/aldehyde, (II)
an intramolecular aldol reaction, and (III) dehydration. Low cost starting materials are advantages
for Robinson annulation strategy.5
In recent 20 years, asymmetric organocatalysis has emerged as a versatile strategy for the
stereoselective preparation of valuable chiral compounds. Pure organic molecules are utilized as
chiral catalysts providing a valuable complement to the traditional organometallic and biological
approaches to asymmetric catalysis.6 These stereo-controlled methods offer a practical pathway for
the construction of a variety of enantio-enriched cyclohex-2-enones.5 For instance, Jorgensen et
al.7 and Hayashi et al.8 successfully used diarylprolinol silyl ether as an organocatalyst, and
α,β-unsaturated aldehydes as the Michael acceptors to prepare various chiral cyclohexenones with
excellent enantioselectivities. Deng and coworkers reported an asymmetric Michael addition for
the synthesis of chiral cyclohexenones catalyzed by 9-amino-9-deoxyepiquinine.9 Zhao et al.
applied chiral primary-secondary diamine catalysts to catalyze the Michael-aldol-dehydration
reaction between benzoylacetates, β-ketoamides and α,β-unsaturated ketones to form chiral
cyclohexenones in high enantioselectivities with excellent yields.10 However, the reports about
synthesis of chiral 3,5-diaryl-cyclohexenones are very rare. In 2000, the Corey group reported the
preparation of (S)-3,5-diaryl-cyclohexenones via a five step synthesis.2c Two straightforward
procedures for the preparation of non-chiral 3,5-diaryl-cyclohexenones via Robinson annulation
reaction of chalcones and acetone have been revealed. In 1998, Inanaga et al. synthesized
non-chiral 3,5-diaryl-cyclohexenones via the reaction of chalcones and acetone catalyzed by
lanthanoid salts.11 In 2014, the Ghosh group reported the pyrrolidine-catalyzed direct synthesis of
non-chiral 3,5-diaryl-cyclohexenones from acetone and chalcones.12 However, to the best of our
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knowledge, the asymmetric version of the Robinson annulation of chalcones with acetone for the
preparation of chiral 3,5-diaryl-cyclohexenones has not been reported yet.
In asymmetric catalysis, chiral secondary amines have been widely studied as highly versatile
and extremely powerful catalysts. However, chiral primary amines as organocatalysts are
relatively underutilized. In consideration of the advantage of primary amine catalysis for
successfully dealing with major challenges in various sterically hindered carbonyl compounds,
which many approaches hardly handle,13 it is greatly desirable to develop more chiral primary
amine catalysts in asymmetric organocatalysis. Moreover, for the choice of chiral catalysts for
asymmetric synthesis, a combination of efficiency, availability, and economy is the core
consideration. Enantiomerically pure trans-1,2-diaminocyclohexane is a structurally simple
molecule which fits the requirements very well. As described in a review by Hanessian and
coworker:14 trans-1,2-diaminocyclohexane was first reported by Wieland et al. in 1926,15 who
prepared it from hexahydrophthalic acid through conversion to the hydrazide followed by a
Curtius reaction. Nowadays, this diamine is readily available because it is a component in a
byproduct amine stream produced during the purification of 1,6-hexanediamine, which is one of
the materials for the manufacture of Nylon 66.16 Its optical resolution can be easily done in
aqueous medium by utilization of D- or L-tartaric acid to obtain the (R,R)- or the (S,S)-
enantiomer in enantiopure form, respectively.17 Fascinated by its features of ready availability and
structural simplicity, our group has been paying much attention on development of new
trans-1,2-diaminocyclohexane derived chiral catalysts for asymmetric organocatalysis.18
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Scheme 1 Robinson annulation reaction
Although there are numerous asymmetric aldol reactions reported, aliphatic ketones and
aldehydes are mainly concerned; to the best of our knowledge, there is scarcely any reported
example for the aromatic ketones without any other assisted functional group participating
asymmetric (domino) aldol reactions with other aliphatic ketones. For the Robinson annulation
reaction (the sequential asymmetric Michael addition/intramolecular aldol reaction/dehydration)
as shown in Scheme 1, generally when R1 and R2 are both aliphatic groups or R1 aromatic while
R2 aliphatic, the initial (metal free organocatalytic) Michael reaction and subsequent aldol
reactions may readily take place, while when both R1 and R2 are aromatic substituted groups such
as chalcone, the enantioselectivity and reactivity of the first Michael addition is difficultly to be
controlled due to steric hindrance and the minor differences between the two groups. To the best
of our knowledge, there has been only one example on the asymmetric Michael addition of
acetone to chalcone which was reported in 2014 and a chiral benzoylthiourea–pyrrolidine was
used as a catalyst; only 53% yield with 50% ee was obtained without subsequent aldol reaction.19
Based on double catalysis: enamine/imine-hydrogen bonding mechanism such as Noyori’s
chiral ligand N-[(1R,2R)-2-amino-1,2-diphenylethyl]-4-methylbenzenesulfonamide (Ts-DPEN), 20
a chiral primary amine bifunctional catalyst, which has been successfully used for the highly
enantioselective Michael addition of acetone to nitroalkenes,21 (1S,2S)-diaminocyclohexane and
its benzoyl derivatives were chosen as very simple bifunctional organocatalysts. In this work, a
highly enantioselective Robinson annulation of chalcones with acetone was developed using a
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chiral primary amine as a catalyst to construct (R)-3,5-diaryl-cyclohexenones.
Results and discussion
Initially, the domino reaction of chalcone 1a and acetone 2 was used as a model reaction at
30 oC in PhMe, and a variety of optically active primary amines shown in Figure 1 were screened
as catalysts. As can be seen from the results in Table 1, when 3a was used in the reaction, only a
trace amount of product 4a was detected (Table 1, entry 1). Catalysts 3b-3f could promote the
model reaction, giving the desired product with high enantioselectivities (92-96% ee, the absolute
configuration of 3a was determined as R by comparison of HPLC with literature.3e, 10a) but in low
yields (10-31%) (Table 1, entries 2-6). Taking into consideration of both yield and enantiomeric
excess, 3f was chosen as the catalyst for the cascade reaction. We speculated that the low yields
may be due to the evaporation of acetone. Thus, we attempted to improve the yield by adding
acetone (20 eq.) in two portions (10 eq. each portion) at the beginning and after 2 days into the
reaction mixture, which gave the product in a better yield of 42% with 96% ee (Table 1, entry 7).
Therefore, the amount of acetone (10 equiv. + 10 equiv./2d) was adopted in the following
investigation.
Figure 1. Structures of the catalysts
Table 1. Screening of chiral primary amine catalysts 3a-3f.a
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Entry
Catalyst
3a
Yield (%)b
ee (%)c (R)
1
2
trace
15
n.d
92
94
94
92
96
96
3b
3
27
3c
4
24
3d
5
10
3e
6
7d
31
3f
42
3f
a The reactions were carried out using 1a (0.20 mmol), 2 (10 equiv., 0.15 mL), and catalyst (20 mol %) in
PhMe (1.0 mL) at 30 oC for 4 days.
b Isolated yield after column chromatography on silica gel.
c Determined by chiral HPLC analysis (AS-H).
d Acetone 2 (10 equiv., 0.15 mL
) was used when setting the reaction, and another portion of acetone 2 (10
equiv., 0.15 mL) was added into the reaction mixture after 2 days.
Different solvents were tested for the model reaction catalyzed by 3f at 30 oC (Table 2). The
results revealed that the solvent had a significant effect on the rate and the enantioselectivity of the
reaction. In polar solvents, such as DMF, MeCN, MeOH, and EtOH, the reaction resulted in poor
yields (Table 2, entries 1-4). By contrast, in non-polar solvents or solvents with low polarity, such
as PhMe, PhBr, and xylene, the reaction gave better yields (42-56%) and better
enantioselectivities (90-97% ee, Table 2, entries 10-12). Among them, the reaction in xylene
provided product with an excellent enantioselectivity (96% ee) in a relatively good yield (52%)
(Table 2, entry 12). Thus, xylene was selected for further optimization. Next, the influence of
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o
temperature on the model reaction in xylene was investigated at 20, 40, and 50 C, respectively.
The results indicated that the yield improved from 36% to 60% when the temperature was
increased from 20 to 50 oC (Table 2, entries 12-15), however the enantioselectivity decreased from
96% ee to 90% ee. Therefore, in view of both yield and stereoselectivity, 30 oC was selected as the
suitable temperature for the domino reaction.
Table 2. Effect of solvents and temperature on the reaction.a
Entry
1
Solvent
DMF
Temp. (o C)
Time (d)
Yield (%)b
trace
trace
18
ee (%)c (R)
30
30
30
30
30
30
30
30
30
30
30
30
20
40
50
6
6
6
6
4
6
6
4
4
4
4
4
4
4
4
n.d.
n.d.
86
84
88
86
84
84
82
97
90
96
96
94
90
2
MeCN
MeOH
EtOH
3
4
25
5
Ethyl acetate
MTBE
Acetone
CHCl3
CH2Cl2
PhMe
48
6
28
7
38
8
48
9
42
10
11
12
13
14
15
42
PhBr
56
xylene
xylene
xylene
xylene
52
36
54
60
a The reactions were carried out using 1a (0.20 mmol), 2 (10 equiv. + 10 equiv./2d), and 3f (20 mol %) in
solvent (1.0 mL).
b Isolated yield after column chromatography on silica gel.
c Determined by chiral HPLC analysis (AS-H)
.
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Other parameters, including the amount of acetone, the catalyst loading and the volume of
solvent, were also investigated (Table 3). No superior results were obtained by screening of the
amount of acetone and catalyst loading (Table 3, entries 1-7). However, when the volume of the
solvent (xylene) was reduced from 1.0 mL to 0.50 mL, the reaction gave a better yield of 62%
with high stereoselectivity of 94% ee after 4 d (Table 3, entry 9). Through these screenings, the
optimized reaction conditions were found to be a combination of 3f (20 mol %), acetone (10 equiv.
+ 10 equiv./2d), and 0.5 mL of xylene as the solvent (for 0.20 mmol scale of chalcone 1a) at 30 oC.
In addition, the effect of some additives, such as AcOH, CF3CO2H and PhCO2H etc, was also
investigated, but no positive results were obtained.
Table 3. Effect of the amount of acetone, catalyst loading and volume of solvent on the reaction. a
Entry
2 (x equiv.)
3f (mol %)
Xylene (mL)
Yield (%)b
ee (%)c (R)
1
2
3
4
5
6
7
8
9
5
20
20
20
20
10
15
30
20
20
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
0.5
41
52
54
54
36
45
53
42
62
96
96
90
88
96
96
95
96
94
10
15
20
10
10
10
10
10
a The reactions were carried out using 1a (0.20 mmol), 2 (x equiv. + x equiv./2d), and catalyst 3f in xylene at
30 oC for 4 d.
b Isolated yield after column chromatography on silica gel.
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c Determined by chiral HPLC analysis (AS-H)
.
Under the optimized reaction conditions, the scope and the limitations of this domino
reaction were examined with different chalcones 1 (Table 4). As can be seen from the table,
catalyst 3f showed good catalytic activity for the cascade reaction. The product yields of up to
85% and enantioselectivities of up to 98% ee were achieved with various chalcones bearing either
electron-withdrawing or electron-donating substituents in the ortho, meta or para position of the
aromatic ring (Table 4, entries 1-18). The position of substituent in the aromatic ring had obvious
effect on the yields. For instance, the reaction with 2-Cl substituted chalcone gave better yield
than that with 3- or 4-Cl substituted chalcone (Table 4, entries 2-4). Besides chalcones,
4-phenylbut-3-en-2-one was also tested, giving the product in 80% yield and 88% ee (Table 4,
entry 19). In addition, some ketones other than acetone, were also surveyed, such as butanone,
3-methylbutanone,
3-pentanone,
1-chloroacetone,
ethyl
acetoacetate
and
1,3-diphenylpropan-2-one, under the present reaction conditions, but no desired products were
detected.
Table 4. Substrate scope.a
Entry
R1
R2
Ph
Ph
Ph
Ph
Product
4a
Time (d)
Yield (%)b
ee (%)c (R)
1
2
3
4
Ph
4
2
4
6
62
82
62
54
94
96
94
94
2-ClC6H4
3-ClC6H4
4-ClC6H4
4b
4c
4d
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5
3-BrC6H4
4-BrC6H4
2,3-Cl2C6H3
2,4-Cl2C6H3
2,6-Cl2C6H3
4-NO2C6H4
4-CH3C6H4
4-CH3OC6H4
Ph
Ph
Ph
4
4
4
2
2
6
6
6
6
4
6
2
4
2
4
60
56
74
85
72
52
41
32
48
67
50
76
71
64
80
95
95
95
96
98
92
91
96
97
96
94
96
94
95
88
4e
4f
6
7
Ph
4g
4h
4i
8
Ph
9
Ph
10
11
12
13
14
15
16
17
18
19
Ph
4j
Ph
4k
4l
Ph
4-CH3C6H4
4-CH3C6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-NO2C6H4
CH3
4m
4n
4o
4p
4q
4r
4s
2-ClC6H4
Ph
2-ClC6H4
4-ClC6H4
Ph
Ph
a The reactions were carried out using 1a (0.20 mmol), 2 (10 equiv. + 10 equiv./2d, except for entries 2, 8, 9,
16 and 18, in which only 10 equiv. of 2 was used), and catalyst 3f (20 mol %) in xylene (0.50 mL) at 30 oC.
b Isolated yield after column chromatography on silica gel.
c Determined by chiral HPLC analysis (AS-H, AD-H)
.
Finally, the mechanism of the primary amine catalyzed tandem Michael-aldol-dehydration
reaction was explored. To investigate the interplay between catalyst 3f and substrates, some NMR
control experiments were conducted. Chalcone (1a) and acetone (2) were separately mixed with
1
1
catalyst 3f to test their H NMR in DMSO-d6. It was found that the H NMR for the mixture of
acetone+3f showed some interesting changes: The signals of aromatic protons and methoxyl
groups of 3f were obviously split, revealing that acetone must have certain interaction with the
amide part which directly connects to the benzene ring in catalyst 3f. However, in the mixture of
chalcone+3f, the chemical shifts for the aromatic protons and methoxyl groups of 3f didn’t show
obvious changes, but the chemical shifts of protons from the cyclohexyl of 3f moved to downfield
slightly; on the contrary, the chemical shifts of protons from chalcone moved to upfield slightly.
This suggested that chalcone did not affect the amide part of 3f, and instead it may have some
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interaction with NH2 of catalyst 3f. This is probably due to the steric hindrance of chalcone. (For
the 1H NMR spectra of these NMR control experiments, please see the Supporting information).
Based on the above control experiments and previous reports,3e,9,10a,13 a possible mechanism
was proposed (Scheme 2): Firstly, chalcone 1 is activated by catalyst 3f via the formation of imine,
and acetone 2 is activated by the amide of 3f through the formation of enol and hydrogen bond
with amide carbonyl oxygen. The enol attacks the α,β-unsaturated imine from Si face to generate
the Michael adduct. Secondly, catalyst 3f further activates the Michael adduct via the formation of
chiral enamine intermediate, and an intramolecular aldol addition occurs which gave the
cyclohexane ring 5. Finally, product chiral cyclohexenone 4 is generated by the dehydration of 5.
Scheme 2 Possible mechanism for the primary amine catalyzed domino reaction.
Conclusion
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In conclusion, a highly enantioselective organocatalytic domino Michael-aldol-dehydration
reaction for the synthesis of 3,5-disubstituted chiral cyclohexenones was developed for the first
time. Easily available acetone and chalcones were used as starting materials, and a simple chiral
primary amine was used as a catalyst. The various products were obtained in moderate to good
yields of up to 85% with excellent enantioselectivities of 91-98% ee. This procedure provided a
simple and straightforward method to construct 3,5-disubstituted chiral cyclohexenones.
Experimental
General
1
NMR data were obtained for H at 300 MHz, 600 MHz and for 13C at 75 MHz, 150 MHz.
Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance as the
internal standard in CDCl3 or (CD3)2SO solution. In each case, the enantiomeric excess was
determined by chiral HPLC analysis on chiralpak AD-H and AS-H in comparison with authentic
racemates. All optical rotations [α] D20 were obtained from a polarimeter (WZZ-2S 2SS).
High-resolution mass spectra were obtained by using ESI ionization sources (Varian 7.0T
FTICR-MS). All chemical reagents (include 3a) and solvents were purchased from commercial
vendors, and used without any further purification unless otherwise stated. All the reactions were
monitored by thin-layer chromatography (TLC) with Haiyang GF254 silica gel plates. Flash
column chromatography was carried out using 200–300 mesh silica gel at increased pressure.
General procedure for the synthesis of catalysts 3b-3f.
Catalysts 3a was purchased from commercial vendors; 3b-3f were synthesized following previous
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procedures.21
General procedure for the Michael-aldol-dehydration reactions.
A mixture of catalyst 3f (12 mg, 20 mol %), chalcone 1 (0.20 mmol) and acetone 2 (10 equiv. +
o
10 equiv./2d) in xylene (0.50 mL) was stirred at 30 C. Upon completion of the reaction
(monitored by TLC), the solvent was evaporated under reduced pressure. The residue was
purification by flash column chromatography (EtOAc/hexane = 1/3-1/10, v/v) to afford the
corresponding products.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(No. 21472152 and No. 21672174), and the Basic and Frontier Research Project of Chongqing
（cstc2015jcyjBX0106）
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