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Direct asymmetric Michael addition of ketones to chalcones catalyzed by a hydroxyphthalimide derived triazole–pyrrolidine
Tetrahedron: Asymmetry 24 (2013) 1615–1619
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Tetrahedron: Asymmetry journal homepage: www.elsevier.com/locate/tetasy
Direct asymmetric Michael addition of ketones to chalcones catalyzed by a hydroxyphthalimide derived triazole–pyrrolidine Togapur Pavan Kumar ⇑, Mohammad Abdul Sattar, Vanka Uma Maheshwara Sarma Division of Natural Products Chemistry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 3 October 2013 Accepted 1 November 2013
a b s t r a c t An efficient protocol for the enantioselective Michael additions of ketones to chalcones catalyzed by a hydroxyphthalimide linked triazole–pyrrolidine derivative has been developed. The corresponding products, 1,5-dicarbonyl compounds, were obtained in good yields with high levels of stereoselectivities under mild reaction conditions employing benzoic acid as an additive. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction N
Organocatalysis has become an area of intense investigation over the past few years.1 One of the most significant organocatalytic transformations is the asymmetric Michael addition reaction, which results in the formation of functionalized products with multiple stereogenic centers in a single step. Due to its importance, the asymmetric Michael addition reaction has gained much attention from the scientific community.2 A large number of designed organocatalysts bearing a pyrrolidine moiety were developed and investigated for a wide range of organic transformations and were found to be efficient with various levels of success.3,4 The most commonly used Michael acceptors thus far are nitroolefins5 and sulfones,6 while other Michael acceptor substrates have been less well explored. We noticed that enantioselective organocatalytic Michael addition reactions of ketones to chalcones have remained significantly less explored, probably due to the steric and reactivity barriers of the substrates.7 Therefore, an investigation into new catalytic methods for this relatively underexplored transformation is highly desirable. In a continuation of our interest on organocatalysis,8 we recently developed the hydroxyphthalimide derived triazole–pyrrolidine 1 (Fig. 1) as an efficient catalyst for asymmetric Michael addition reactions of ketones to nitroolefins via an enamine mechanism using water as the reaction medium.8g Having been encouraged by this study and considering the consistency of catalytic mechanisms, wherein the privileged chiral pyrrolidine backbone acts as the catalytically active site and the triazole ring with a phthalimide group as the selectivity director,9 we therefore started our investigations by testing whether catalyst 1 would also catalyze asymmetric Michael addition reactions of ketones to chalcones. Herein we report on the use of a hydroxyph-
⇑ Corresponding author. Tel.: +91 40 27191727; fax: +91 40 27160512. E-mail address: [email protected] (T.P. Kumar). 0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetasy.2013.11.002
O
N
N
O
N H
N O
1
Figure 1. Hydroxyphthalimide linked triazole–pyrrolidine.
thalimide linked triazole–pyrrolidine in asymmetric Michael addition reaction of ketones with activated chalcones. 2. Results and discussion In order to evaluate the efficacy of catalyst 1 for asymmetric Michael additions of ketones to chalcones, we initiated our experiments with a model reaction using cyclohexanone 2a as a donor and chalcone 3a as an acceptor (Scheme 1) and the reaction optimization involved variation of solvent, additive, and template. Firstly, the reaction was examined in several solvents using 20 mol % of the catalyst at room temperature and the results are summarized in Table 1. As evident from the survey, the conjugate addition reaction proceeded well in polar solvents such as DMF, MeOH, i-PrOH, and t-BuOH to afford the product 4a in good yields and stereoselectivities (Table 1, entries 3, 7–9), while in other solvents the reaction is relatively less productive. To our delight the
O
O
O
O
1 (10 mol%)
2a
3a
solvent (0.5 mL) rt
4a
Scheme 1. Michael addition of cyclohexanone to chalcone.
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T. P. Kumar et al. / Tetrahedron: Asymmetry 24 (2013) 1615–1619
Table 1 Screening of solventsa
a b c d
Entry
Solvent
Time (d)
Yieldb (%)
1 2 3 4 5 6 7 8 9 10
CHC13 Toluene DMF CH3CN THF Neat MeOH i PrOH t BuOH H2O
6 6 4 5 5 3 4 3 3 5
58 65 73 70 66 78 73 71 69 45
syn/antic
eed (%)
88:12 85:15 91:9 93:7 91:9 94:6 9:1 92:8 95:5 8:2
63 61 70 65 68 81 72 76 74 67
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol). Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
reaction performed under solvent free conditions has resulted in the product with best yield (78%) and stereoselectivities (94:6 syn/anti and 81% ee) among the conditions tested (Table 1, entry 6). Encouraged by solvent screening survey, we then directed our investigations to test the effect of acid additive in the above transformation. As evident from the literature, the presence of an acid additive enhances the efficiency of catalytic cycle by accelerating enamine formation. In anticipation, we examined the effect of various acid additives such as TFA, HCOOH, PhCOOH, PhCH2COOH, PhOH, pTSA, CSA, and HCl. As illustrated in Table 2, from these experiments we found PhCOOH as the best additive in combination with catalyst 1 (89% yield, 97:3 syn/anti, and 86% ee, Table 2, entry 3) and it was selected as the additive for further investigation.
Table 2 Screening of additivesa Entry
Additive
1 2 3 4 5 6 7 8
TFA HCOOH PhCOOH PhCH2COOH PhOH pTSA HCl CSA
Time (d)
Yieldb (%)
5 3 3 3 3 3 5 3
Trace 75 89 82 78 72 Trace 76
syn/antic
eed (%)
— 93:7 97:3 94:6 91:9 93:7 — 86:14
— 77 86 83 80 74 — 63
a Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol), additive (10 mol %). b Isolated yields. c Determined by the 1H NMR of the crude product. d Determined by chiral HPLC.
loading or reaction temperature resulted in longer reaction time with no substantial effect on the yield and selectivity (Table 3, entries 1 and 3). Having established the optimal reaction conditions for Michael addition of cyclohexanone 2a with chalcone 3a, we then examined the scope and limitation of this reaction with different ketones and chalcones under solvent free reaction conditions using catalyst 1 (20 mol %) in combination with benzoic acid (10 mol %) at room temperature. As shown in Tables 4 and 5, chalcones 3b–k reacted smoothly with cyclohexanone 2a (Table 4, entries 1–10) and other six-membered ring ketones 2b–d (Table 5, entries 1–3) under the established reaction conditions and the corresponding Michael products were obtained in good yields with high levels of diastereoselectivities and enantioselectivities regardless of the nature of substitution pattern in chalcones. However, reactions with cyclopentanone and acetone were less compatible, and the resultant products are obtained in moderate yields and selectivities (Table 5, entries 4 and 5). Overall, there is a good substrate scope for the conjugate addition of ketones to chalcones using hydroxyphthalimide derived triazole–pyrrolidine catalyst, providing access to a variety of 1,5-dicarbonyl compounds in high selectivities. The absolute stereochemical outcome of this transformation can be explained by considering the possible transition state model as shown in Figure 2. The pyrrolidine moiety of the catalyst acts as an activation site leading to the formation of enamine on reaction with ketone and the planar triazole ring as a steric controller, which effectively shields the Si face of the enamine and allows the reaction to occur via Re–Re approach.10 While the phthalimide group might participate in H-bonding interaction with the carbonyl group by the intermediacy of benzoic acid resulting in a cavity like arrangement, thus bringing about a tighter transition state and leading to the formation of desired products with high stereoselectivities. 3. Conclusions In conclusion, we have demonstrated the application of hydroxyphthalimide derived triazole–pyrrolidine as an effective organocatalyst for the asymmetric Michael addition of ketones to chalcones. The process was found to be productive in terms of yield and stereoselectivities, when performed under solvent-free reaction conditions using benzoic acid as an additive. Further investigations to extend the scope of this valuable transformation and development of new organocatalysts are underway in our laboratory. 4. Experimental section 4.1. General
We then studied the effect of temperature and catalyst loading to further optimize the reaction conditions and however none of these variations could further improve the reaction either in terms of yield or selectivity. As shown in Table 3, a decrease in catalyst
All solvents and reagents were purified by standard techniques. Crude products were purified by column chromatography on silica gel of 60–120 mesh. IR spectra were recorded on a Perkin–Elmer
Table 3 Effect of temperature and catalyst loadinga
a b c d
Entry
Catalyst (mol %)
1 2 3 4
20 20 10 30
Temp. (°C)
Time (d)
Yieldb (%)
0 50 RT RT
6 2 5 3
85 71 83 90
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol), PhCOOH (10 mol %), neat. Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
syn/antic
eed (%)
94:6 8:2 91:9 97:3
84 58 81 86
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T. P. Kumar et al. / Tetrahedron: Asymmetry 24 (2013) 1615–1619 Table 4 Substrate scope of chalconesa O Ar1 2a
Entry
O
O Ar2
Ar1
1 (20 mol%) PhCOOH (10 mol%)
2
Ar
neat, rt 4b-k
3b-k
Product
O
Time (d)
Yieldb (%)
syn/antic
eed (%)
3
86
92:8
84
3
89
96:4
91
3
90
93:7
87
3
85
97:3
91
3
87
91:9
85
3.5
89
93:7
90
3
92
95:5
88
3.5
86
92:8
83
3
85
97:3
86
3
87
94:6
85
Cl
1
O
O Ph
4b Me
2
O
O Ph
4c NO2
3
O
O Ph 4d
O
Ph
4
O
4e NO2 O
Ph
O
5 4f O
Ph
Cl O
6 4g O
Ph
OMe O
7 4h O
Ph
Me O
8 4i O
Ph
O Cl
9 4j O
Ph
O
Cl
10 4k a b c d
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol). Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
683 spectrometer. Optical rotations were obtained on a Jasco Dip 360 digital polarimeter. 1H and 13C NMR spectra were recorded in CDCl3 solution on a Varian Gemini 200 and Brucker Avance 300. Chemical shifts were reported in parts per million with respect to internal TMS. Coupling constants (J) are quoted in Hz. Mass spectra were obtained on an Agilent Technologies LC/MSD Trap SL. Chiral HPLC analysis was carried out on chiral pak OD-H, IC, or IA columns using a mixture of isopropanol and hexanes as the eluent.
4.1.1. General procedure for the Michael addition of cyclohexa none to chalcones To a mixture of catalyst 1 (20 mol %), and cyclohexanone (5 mmol) was added PhCOOH (10 mol %) and stirred for 20 min. at room temperature. Then, chalcone (1 mmol) was added to the resulting mixture and stirred for appropriate time (Table 3) at room temperature. After completion of the reaction (monitored by TLC), the mixture was purified by silica-gel column chromatography to afford the desired product. Relative and absolute
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T. P. Kumar et al. / Tetrahedron: Asymmetry 24 (2013) 1615–1619
Table 5 Substrate scope of ketonesa O
Ph 4l-p
Product O
Ph
O
neat, rt
3a
2b-f
Ph
1 (20 mol%) PhCOOH (10 mol%)
Ph
Ph
Entry
O
O
Time (d)
Yieldb (%)
syn/antic
eed (%)
3.
O
1
Ph
4l
4
84
92:8
81
Ph
4m
4
82
91:9
79
4.5
83
93:7
80
7:3
62
O O
Ph
O
2
O
Ph
O
3
Ph
4n
Ph
4o
5
79
4p
5
38
N O
Ph
O
4 O
Ph
O
5 a b c d
4. Ph
—
47
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol). Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
N
N O
N N
O O
H Ar2
5.
O
N
O O
Ar1
Figure 2. Proposed transition state.
configuration of the products was determined by comparison of 1H NMR, 13C NMR, and specific rotation values with those reported in the literature.11 Acknowledgements
6.
T.P.K. thank DST New Delhi for INSPIRE Faculty Award (IFA12CH-30). M.A.S. thank UGC, New Delhi for the award of research fellowship. Authors thank Dr. J. S. Yadav and Dr. S. Chandrasekhar for valuable suggestions.
7.
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11. For compounds 4a–4e, 4g, 4i, 4l–4o see: Ref. 7c; for compound 4f see: Ref. 7f; for compound 4h see: Ref. 7h; for compound 4p see: Mustafa, C.; Hayreddin, G. Turk. J. Chem. 2008, 32, 55–61; for compound 4p see: Liujuan, C.; Sanzhong, L.; Jiuyuan, L.; Xin, L.; Jin, P. C. Org. Biomol. Chem. 2010, 8, 2627–2632.
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry journal homepage: www.elsevier.com/locate/tetasy
Direct asymmetric Michael addition of ketones to chalcones catalyzed by a hydroxyphthalimide derived triazole–pyrrolidine Togapur Pavan Kumar ⇑, Mohammad Abdul Sattar, Vanka Uma Maheshwara Sarma Division of Natural Products Chemistry, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 3 October 2013 Accepted 1 November 2013
a b s t r a c t An efficient protocol for the enantioselective Michael additions of ketones to chalcones catalyzed by a hydroxyphthalimide linked triazole–pyrrolidine derivative has been developed. The corresponding products, 1,5-dicarbonyl compounds, were obtained in good yields with high levels of stereoselectivities under mild reaction conditions employing benzoic acid as an additive. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction N
Organocatalysis has become an area of intense investigation over the past few years.1 One of the most significant organocatalytic transformations is the asymmetric Michael addition reaction, which results in the formation of functionalized products with multiple stereogenic centers in a single step. Due to its importance, the asymmetric Michael addition reaction has gained much attention from the scientific community.2 A large number of designed organocatalysts bearing a pyrrolidine moiety were developed and investigated for a wide range of organic transformations and were found to be efficient with various levels of success.3,4 The most commonly used Michael acceptors thus far are nitroolefins5 and sulfones,6 while other Michael acceptor substrates have been less well explored. We noticed that enantioselective organocatalytic Michael addition reactions of ketones to chalcones have remained significantly less explored, probably due to the steric and reactivity barriers of the substrates.7 Therefore, an investigation into new catalytic methods for this relatively underexplored transformation is highly desirable. In a continuation of our interest on organocatalysis,8 we recently developed the hydroxyphthalimide derived triazole–pyrrolidine 1 (Fig. 1) as an efficient catalyst for asymmetric Michael addition reactions of ketones to nitroolefins via an enamine mechanism using water as the reaction medium.8g Having been encouraged by this study and considering the consistency of catalytic mechanisms, wherein the privileged chiral pyrrolidine backbone acts as the catalytically active site and the triazole ring with a phthalimide group as the selectivity director,9 we therefore started our investigations by testing whether catalyst 1 would also catalyze asymmetric Michael addition reactions of ketones to chalcones. Herein we report on the use of a hydroxyph-
⇑ Corresponding author. Tel.: +91 40 27191727; fax: +91 40 27160512. E-mail address: [email protected] (T.P. Kumar). 0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetasy.2013.11.002
O
N
N
O
N H
N O
1
Figure 1. Hydroxyphthalimide linked triazole–pyrrolidine.
thalimide linked triazole–pyrrolidine in asymmetric Michael addition reaction of ketones with activated chalcones. 2. Results and discussion In order to evaluate the efficacy of catalyst 1 for asymmetric Michael additions of ketones to chalcones, we initiated our experiments with a model reaction using cyclohexanone 2a as a donor and chalcone 3a as an acceptor (Scheme 1) and the reaction optimization involved variation of solvent, additive, and template. Firstly, the reaction was examined in several solvents using 20 mol % of the catalyst at room temperature and the results are summarized in Table 1. As evident from the survey, the conjugate addition reaction proceeded well in polar solvents such as DMF, MeOH, i-PrOH, and t-BuOH to afford the product 4a in good yields and stereoselectivities (Table 1, entries 3, 7–9), while in other solvents the reaction is relatively less productive. To our delight the
O
O
O
O
1 (10 mol%)
2a
3a
solvent (0.5 mL) rt
4a
Scheme 1. Michael addition of cyclohexanone to chalcone.
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T. P. Kumar et al. / Tetrahedron: Asymmetry 24 (2013) 1615–1619
Table 1 Screening of solventsa
a b c d
Entry
Solvent
Time (d)
Yieldb (%)
1 2 3 4 5 6 7 8 9 10
CHC13 Toluene DMF CH3CN THF Neat MeOH i PrOH t BuOH H2O
6 6 4 5 5 3 4 3 3 5
58 65 73 70 66 78 73 71 69 45
syn/antic
eed (%)
88:12 85:15 91:9 93:7 91:9 94:6 9:1 92:8 95:5 8:2
63 61 70 65 68 81 72 76 74 67
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol). Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
reaction performed under solvent free conditions has resulted in the product with best yield (78%) and stereoselectivities (94:6 syn/anti and 81% ee) among the conditions tested (Table 1, entry 6). Encouraged by solvent screening survey, we then directed our investigations to test the effect of acid additive in the above transformation. As evident from the literature, the presence of an acid additive enhances the efficiency of catalytic cycle by accelerating enamine formation. In anticipation, we examined the effect of various acid additives such as TFA, HCOOH, PhCOOH, PhCH2COOH, PhOH, pTSA, CSA, and HCl. As illustrated in Table 2, from these experiments we found PhCOOH as the best additive in combination with catalyst 1 (89% yield, 97:3 syn/anti, and 86% ee, Table 2, entry 3) and it was selected as the additive for further investigation.
Table 2 Screening of additivesa Entry
Additive
1 2 3 4 5 6 7 8
TFA HCOOH PhCOOH PhCH2COOH PhOH pTSA HCl CSA
Time (d)
Yieldb (%)
5 3 3 3 3 3 5 3
Trace 75 89 82 78 72 Trace 76
syn/antic
eed (%)
— 93:7 97:3 94:6 91:9 93:7 — 86:14
— 77 86 83 80 74 — 63
a Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol), additive (10 mol %). b Isolated yields. c Determined by the 1H NMR of the crude product. d Determined by chiral HPLC.
loading or reaction temperature resulted in longer reaction time with no substantial effect on the yield and selectivity (Table 3, entries 1 and 3). Having established the optimal reaction conditions for Michael addition of cyclohexanone 2a with chalcone 3a, we then examined the scope and limitation of this reaction with different ketones and chalcones under solvent free reaction conditions using catalyst 1 (20 mol %) in combination with benzoic acid (10 mol %) at room temperature. As shown in Tables 4 and 5, chalcones 3b–k reacted smoothly with cyclohexanone 2a (Table 4, entries 1–10) and other six-membered ring ketones 2b–d (Table 5, entries 1–3) under the established reaction conditions and the corresponding Michael products were obtained in good yields with high levels of diastereoselectivities and enantioselectivities regardless of the nature of substitution pattern in chalcones. However, reactions with cyclopentanone and acetone were less compatible, and the resultant products are obtained in moderate yields and selectivities (Table 5, entries 4 and 5). Overall, there is a good substrate scope for the conjugate addition of ketones to chalcones using hydroxyphthalimide derived triazole–pyrrolidine catalyst, providing access to a variety of 1,5-dicarbonyl compounds in high selectivities. The absolute stereochemical outcome of this transformation can be explained by considering the possible transition state model as shown in Figure 2. The pyrrolidine moiety of the catalyst acts as an activation site leading to the formation of enamine on reaction with ketone and the planar triazole ring as a steric controller, which effectively shields the Si face of the enamine and allows the reaction to occur via Re–Re approach.10 While the phthalimide group might participate in H-bonding interaction with the carbonyl group by the intermediacy of benzoic acid resulting in a cavity like arrangement, thus bringing about a tighter transition state and leading to the formation of desired products with high stereoselectivities. 3. Conclusions In conclusion, we have demonstrated the application of hydroxyphthalimide derived triazole–pyrrolidine as an effective organocatalyst for the asymmetric Michael addition of ketones to chalcones. The process was found to be productive in terms of yield and stereoselectivities, when performed under solvent-free reaction conditions using benzoic acid as an additive. Further investigations to extend the scope of this valuable transformation and development of new organocatalysts are underway in our laboratory. 4. Experimental section 4.1. General
We then studied the effect of temperature and catalyst loading to further optimize the reaction conditions and however none of these variations could further improve the reaction either in terms of yield or selectivity. As shown in Table 3, a decrease in catalyst
All solvents and reagents were purified by standard techniques. Crude products were purified by column chromatography on silica gel of 60–120 mesh. IR spectra were recorded on a Perkin–Elmer
Table 3 Effect of temperature and catalyst loadinga
a b c d
Entry
Catalyst (mol %)
1 2 3 4
20 20 10 30
Temp. (°C)
Time (d)
Yieldb (%)
0 50 RT RT
6 2 5 3
85 71 83 90
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol), PhCOOH (10 mol %), neat. Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
syn/antic
eed (%)
94:6 8:2 91:9 97:3
84 58 81 86
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T. P. Kumar et al. / Tetrahedron: Asymmetry 24 (2013) 1615–1619 Table 4 Substrate scope of chalconesa O Ar1 2a
Entry
O
O Ar2
Ar1
1 (20 mol%) PhCOOH (10 mol%)
2
Ar
neat, rt 4b-k
3b-k
Product
O
Time (d)
Yieldb (%)
syn/antic
eed (%)
3
86
92:8
84
3
89
96:4
91
3
90
93:7
87
3
85
97:3
91
3
87
91:9
85
3.5
89
93:7
90
3
92
95:5
88
3.5
86
92:8
83
3
85
97:3
86
3
87
94:6
85
Cl
1
O
O Ph
4b Me
2
O
O Ph
4c NO2
3
O
O Ph 4d
O
Ph
4
O
4e NO2 O
Ph
O
5 4f O
Ph
Cl O
6 4g O
Ph
OMe O
7 4h O
Ph
Me O
8 4i O
Ph
O Cl
9 4j O
Ph
O
Cl
10 4k a b c d
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol). Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
683 spectrometer. Optical rotations were obtained on a Jasco Dip 360 digital polarimeter. 1H and 13C NMR spectra were recorded in CDCl3 solution on a Varian Gemini 200 and Brucker Avance 300. Chemical shifts were reported in parts per million with respect to internal TMS. Coupling constants (J) are quoted in Hz. Mass spectra were obtained on an Agilent Technologies LC/MSD Trap SL. Chiral HPLC analysis was carried out on chiral pak OD-H, IC, or IA columns using a mixture of isopropanol and hexanes as the eluent.
4.1.1. General procedure for the Michael addition of cyclohexa none to chalcones To a mixture of catalyst 1 (20 mol %), and cyclohexanone (5 mmol) was added PhCOOH (10 mol %) and stirred for 20 min. at room temperature. Then, chalcone (1 mmol) was added to the resulting mixture and stirred for appropriate time (Table 3) at room temperature. After completion of the reaction (monitored by TLC), the mixture was purified by silica-gel column chromatography to afford the desired product. Relative and absolute
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T. P. Kumar et al. / Tetrahedron: Asymmetry 24 (2013) 1615–1619
Table 5 Substrate scope of ketonesa O
Ph 4l-p
Product O
Ph
O
neat, rt
3a
2b-f
Ph
1 (20 mol%) PhCOOH (10 mol%)
Ph
Ph
Entry
O
O
Time (d)
Yieldb (%)
syn/antic
eed (%)
3.
O
1
Ph
4l
4
84
92:8
81
Ph
4m
4
82
91:9
79
4.5
83
93:7
80
7:3
62
O O
Ph
O
2
O
Ph
O
3
Ph
4n
Ph
4o
5
79
4p
5
38
N O
Ph
O
4 O
Ph
O
5 a b c d
4. Ph
—
47
Reaction conditions: cyclohexanone (5 mmol), chalcone (1 mmol). Isolated yields. Determined by the 1H NMR of the crude product. Determined by chiral HPLC.
N
N O
N N
O O
H Ar2
5.
O
N
O O
Ar1
Figure 2. Proposed transition state.
configuration of the products was determined by comparison of 1H NMR, 13C NMR, and specific rotation values with those reported in the literature.11 Acknowledgements
6.
T.P.K. thank DST New Delhi for INSPIRE Faculty Award (IFA12CH-30). M.A.S. thank UGC, New Delhi for the award of research fellowship. Authors thank Dr. J. S. Yadav and Dr. S. Chandrasekhar for valuable suggestions.
7.
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11. For compounds 4a–4e, 4g, 4i, 4l–4o see: Ref. 7c; for compound 4f see: Ref. 7f; for compound 4h see: Ref. 7h; for compound 4p see: Mustafa, C.; Hayreddin, G. Turk. J. Chem. 2008, 32, 55–61; for compound 4p see: Liujuan, C.; Sanzhong, L.; Jiuyuan, L.; Xin, L.; Jin, P. C. Org. Biomol. Chem. 2010, 8, 2627–2632.