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Synthesis of 4-acylcoumarins by NHC-catalyzed nucleophilic substitution
Tetrahedron Letters 59 (2018) 4276–4278
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Synthesis of 4-acylcoumarins by NHC-catalyzed nucleophilic substitution Yumiko Suzuki ⇑, Asuka Ando, Mizuki Nakagawa Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 Japan
a r t i c l e
i n f o
Article history: Received 18 September 2018 Revised 8 October 2018 Accepted 22 October 2018 Available online 23 October 2018 Keywords: N-Heterocyclic carbene Coumarin Quinolinone Acylation Organocatalysis
a b s t r a c t Coumarins are bioactive; consequently their syntheses are of importance to medicinal chemists. We describe the one-step organocatalytic syntheses of 4-acylcoumarins from 4-chlorocoumarin. The leaving group at the 4-position of the coumarin was replaced by aroyl groups that originate from aromatic aldehydes by NHC-catalyzed umpolung. 4-Acylthiocoumarins and 2-acylquinolin-2-ones were also prepared using this method. These are the first examples of nucleophilic substitutions at the b-carbons of enones to afford c-ketoenones. Ó 2018 Elsevier Ltd. All rights reserved.
N-Heterocyclic carbenes (NHCs) are unique organocatalysts that mediate various types of reactions [1], including umpolung [2–7], transesterification [8], oxidative-esterification [9], homoenoate [10], and Diels–Alder reactions [11]. The benzoin condensation [2], Stetter reaction [3], and nucleophilic aroylation [4–7] are representative examples of the umpolung reaction. In 1985, Higashino and co-workers reported the first example of an NHCcatalyzed nucleophilic aroylation [4]. Subsequently, aroylation reactions of various N-heteroarenes [5], imidoyl chlorides [6], and benzene derivatives [7] have been reported. The authors envisaged that the substrate scope of this NHC-catalyzed aroylation reaction could be expanded to include enones bearing leaving groups at their b-carbons, such as 4-chlorocoumarins and 4-chloroquinolin-2-ones. These types of bicyclic conjugated enones have received significant attention in the areas of natural products and medicinal chemistry. Coumarin derivatives with anti-HIV [12], antitumor [13], anticoagulation [14], and anti-inflammatory [15] activities have been reported on numerous occasions. In addition, quinolinone derivatives that exhibit anticancer [16], antithyroid [17], and antibacterial [18] properties have also been reported. Therefore, these enone-bearing heterocycles are attractive as components of chemical libraries, and the syntheses of a wide variety of quinolinone derivatives are of interest.
⇑ Corresponding author. E-mail address: [email protected] (Y. Suzuki). https://doi.org/10.1016/j.tetlet.2018.10.044 0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
To the best of our knowledge, only two methods have been reported for the introduction of aroyl groups at the 4-position of the coumarin core; one requires five steps starting from 4-hydroxycoumarin [19], while the other involves the direct palladium-catalyzed aroylation of 4-coumarinylzinc bromide [20]. Furthermore, there are no reports that describe the introduction of aroyl groups onto 2-quinolones. Reactions of readily available coumarins 1–5 bearing leaving groups at the 4-position with 4-methoxybenzaldehyde (6a) were first examined under standard NHC-catalyzed aroylation conditions [7], in which 1,3-dimethylimizolium iodide was used as the catalyst precursor with sodium hydride in DMF at room temperature (Table 1; For reaction mechanism, see Supplementary data 1). The desired ketone 7a was obtained in satisfactory yields in reactions involving either chloride 1 or bromide 2 (entries 1 and 2). Unfortunately, sulfonyl groups were poor leaving groups (entries 3 and 4), while the methyl ether 5 produced a complex mixture in which 7a was not detected (entry 5). The reaction of 1 with 6a was screened under the same conditions but with various NHC precursors and bases (Table 2). NHCs generated from imidazolium salts B–E bearing a variety of substituents were as effective as A, with the exception of the dichloride D, in which the effect of electron induction presumably lowers the nucleophilicity of the carbene, leading to slightly decreased yield (entries 1–4). The use of triazolium salt F resulted in a further decreased yield (entry 5), while a significantly lower product yield was observed when benzimidazolium salt G was used (entry 6), and the NHC derived from thiazolium salt H was
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Y. Suzuki et al. / Tetrahedron Letters 59 (2018) 4276–4278 Table 1 Leaving-group Scope.a
Table 3 Reaction Optimization: Solvent and Temperature.a
Entry
L
Coumarin
Time (h)
Yield (%)
Entry
Solvent
Temperature (°C)
Time (h)
Yield (%)
1 2 3 4 5
Cl Br OMs OTs OMe
1 2 3 4 5
4 5 7 7 7
67 66 9 –b –c
1 2 3 4 5 6 7 8 9 10 11 12
DMF DMF DMSO DMSO CH3CN CH3CN CH3CN THF THF THF 1,4-Dioxane Toluene
-20 50 r.t. 80 r.t. 50 reflux r.t. 50 reflux r.t. 50
6.5 3 6.5 3 6.5 6 3 6 6 5 6 5.5
63 49 63 57 33 65 34 54 77 63 –b –b
a Reaction conditions: 1–5 (1 mmol), 6a (1.15 mmol), A (10 mol%), NaH (1.6 mmol), and DMF (10 mL). b No reaction. c Complex mixture of products.
Table 2 Reaction Optimization: Catalyst and Base.a
a Reaction conditions: 1 (1 mmol), 6a (1.15 mmol), A (10 mol%), base (1.6 mmol), and solvent (10 mL). b No reaction.
Entry
NHC Precursor
Base
Time (h)
Yield (%)
1 2 3 4 5 6 7 8 9 10
B C D E F G H H A A
NaH NaH NaH NaH NaH NaH NaH Et3N t BuOK DBN
3 4 7 5 7 7.5 7 6 6.5 6.5
67 66 54 63 41 6 –b –b 7 8
a Reaction conditions: 1 (1 mmol), 6a (1.15 mmol), NHC precursor (10 mol%), base (1.6 mmol), and DMF (10 mL). b No reaction.
inert (entries 7 and 8). The use of weaker bases such as tBuOK and DBN in the presence of A also afforded very low yields (entries 9 and 10). Hence, the use of A with sodium hydride was determined to be the optimal combination. Solvents other than DMF were also examined, and reactions were also performed at a variety of temperatures (Table 3). The product yield was barely lower than that observed at room
temperature when the reaction was carried out at 20 °C in DMF, (entry 1), while the higher temperature (50 °C) did not positively affect the yield of the product (entry 2). The decreased yield at the higher temperature could be because of the NaH-absorbed water that is added to the reaction mixture: the undesired substitution reaction with hydroxide to afford 4-hydroxycoumarin occurred. The benzoin condensation product was not detected. The same trend was also observed in other polar solvents, namely DMSO, acetonitrile, and THF, with product yields comparable or higher than those obtained in DMF (entries 3–10). The reaction at 50 °C in THF afforded the product most efficiently (entry 9), while reactions performed at temperatures higher than 50 °C did not afford improved yields of the product (entries 4, 7, and 10). Non-polar solvents, namely 1,4-dioxane and toluene, were unsuitable for this reaction (entries 11 and 12). The aldehyde scope was next explored (Table 4). Reactions with benzaldehydes bearing a variety of substituents, as well as 2-naphthaldehyde and heteroaryl aldehydes afforded the corresponding products in good-to-moderate yields, except for a few examples. The reactivities of the furan moieties in 3-furancarboxaldehyde and the product 7l are believed to be factors responsible for the lower yield. However, the reasons for the low yields observed in the reactions of 4-bromobenzaldehyde and nicotinaldehyde, which gave ketones 7f and 7m, respectively, remain uncertain. Substrates other than 4-chlorocoumarin were also reacted with 4-methoxybenzaldehyde (Table 5). The desired product 8 was obtained in 71% yield from 4-chlorothiocoumarin, while 4-chloroquinolinones bearing methyl and benzyl groups at their 1-positions afforded products 9 and 10 in yields of 66% and 79%, respectively. In conclusion, a straightforward organocatalytic synthesis of 4-aroylcoumarins has been developed. A structurally related thiocoumarin and quinolinone derivatives bearing aroyl groups at their 4-positions are also accessible using this method. NHCs generated from imidazolium salts are the most effective in this transformation compared to those formed from triazolium or benzthiazolium salts, while the thiazolium-salt-based NHC failed to catalyze this
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Table 4 Aldehyde Scope.a
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.tetlet.2018.10.044. References
b
The reaction was carried out in THF (10 mL) at 50 °C instead of DMF at r.t. a Standard conditions: 1 (1 mmol), 6a (1.15 mmol), A (10 mol%), NaH (1.6 mmol), and DMF (10 mL) at r.t.
Table 5 Further Substrate Scope.a
a Reaction conditions: 1 (1 mmol), 6a (1.15 mmol), A (10 mol%), NaH (1.6 mmol), and DMF (10 mL) at r.t.
reaction. To the best of our knowledge, these reactions represent the first examples of nucleophilic substitutions at the b-carbons of enones to furnish the corresponding aroyl-substituted systems. The method facilitates rapid access to this type of c-ketoenone. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Advanced Molecular Transformations by Organocatalysts” from The Ministry of Education, Culture, Sports, Science and Technology, Japan (Grant number: 26105754).
[1] (a) D. Enders, T. Balensiefer, ACC Chem. Res. 37 (2004) 534–541; (b) N-Heterocyclic Carbenes in Synthesis; S.P. Nolan (Ed.), WILEY-VCH GmbH and KGaA, Weinheim, 2006.; (c) N. Marion, S. Die-Gonzale, S.P. Nolan, Angew. Chem. Int. Ed. 46 (2007) 2988–3000; (d) D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 107 (2007) 5606–5655; (e) M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014) 485– 496; (f) D.M. Flanigan, F.R. Michailidis, N.A. White, T. Rovis, Chem. Rev. 115 (2015) 9307–9387. [2] (a) T. Ukai, S. Tanaka, S. Dokawa, J. Pharm. Soc. Jpn. 63 (1943) 269–300; (b) R. Breslow, J. Am. Chem. Soc. 80 (1958) 3719–3726. [3] (a) H. Stetter, Angew. Chem. Int. Ed. 15 (1976) 639–647; (b) H. Stetter, H. Kuhlmann, Org. React. 40 (1991) 407–496. [4] T. Higashino, M. Takemoto, A. Miyashita, E. Hayashi, Chem. Pharm. Bull. 33 (1985) 1395–1399. [5] (a) A. Miyashita, Y. Suzuki, K. Iwamoto, E. Oishi, T. Higashino, Chem. Pharm. Bull. 38 (1990) 1147–1152; (b) A. Miyashita, Y. Suzuki, K. Iwamoto, E. Oishi, T. Higashino, Heterocycles 49 (1998) 405–413. [6] (a) A. Miyashita, H. Matsuda, T. Higasmno, Chem. Pharm. Bull. 40 (1992) 2627–2631; (b) Y. Suzuki, A. Bakar, Y. Tanoi, N. Nomura, M. Sato, Tetrahedron 67 (2011) 4710–4715. [7] (a) Y. Suzuki, T. Toyota, F. Imada, M. Sato, A. Miyashita, Chem. Commun. (2003) 1314–1315; (b) Y. Suzuki, T. Toyota, A. Miyashita, M. Sato, Chem. Pharm. Bull. 54 (2006) 1653–1658. [8] (a) G.A. Grasa, R.M. Kissling, S.P. Nolan, Org. Lett. 4 (2002) 3583–3586; (b) G.W. Nyce, J.A. Lamboy, E.F. Connor, R.M. Waymouth, J.L. Hedrick, Org. Lett. 4 (2002) 3587–3590; (c) E.F. Connor, G.W. Nyce, M. Myers, A. Möck, J.L. Hedrick, J. Am. Chem. Soc. 124 (2002) 914–915. [9] (a) K.Y.K. Chow, J.W. Bode, J. Am. Chem. Soc. 126 (2004) 8126–8127; (b) N.T. Reynolds, J.R. de Alaniz, T. Rovis, J. Am. Chem. Soc. 126 (2004) 9518– 9519; (c) S. De Sarkar, S. Grimme, A. Studer, J. Am. Chem. Soc. 132 (2010) 1190– 1191. [10] (a) S.S. Sohn, E.L. Rosen, J.W. Bode, J. Am. Chem. Soc. 126 (2004) 14370– 14371; (b) C. Burstein, F. Glorius, Angew. Chem. Int. Ed. 43 (2004) 6205–6208; (c) M. He, J.W. Bode, Org. Lett. 7 (2005) 3131–3134; (d) A. Chan, K.A. Scheidt, J. Am. Chem. Soc. 130 (2008) 2740–2741. [11] (a) M. He, J.R. Struble, J.W. Bode, J. Am. Chem. Soc. 128 (2006) 8418–8420; (b) M. He, G.J. Uc, J.W.J. Bode, Am. Chem. Soc. 128 (2006) 15088–15089. [12] (a) R. Dayam, R. Gundla, L.Q. Al-Mawsawi, N. Neamati, Med. Res. Rev. 28 (2008) 118–154; (b) E.B.B. Ong, N. Watanabe, A. Saito, Y. Futamura, K.H. Abd El Galil, A. Koito, N. Najimudin, H. Osada, J. Biol. Chem. 286 (2011) 14049–14056. [13] (a) S. Emami, S. Dadashpour, Eur. J. Med. Chem. 102 (2015) 611–630; (b) X.Y. Huang, Z.J. Shan, H.L. Zhai, L. Su, X.Y. Zhang, Chem. Biol. Drug Des. 78 (2011) 651–658. [14] G. Cravotto, G.M. Nano, G. Palmisano, S. Tagliapietra, Tetrahedron: Asymmetry 12 (2001) 707–709. [15] (a) T. Nakamura, N. Kodama, M. Oda, S. Tsuchiya, Y. Arai, T. Kumamoto, T. Ishikawa, K. Ueno, S. Yano, J. Nat. Med. 63 (2009) 15–20; (b) F. Gosselin, R.A. Britton, I.W. Davies, S.J. Dolman, D. Gauvreau, R.S. Hoerrner, G. Hughes, J. Janey, S. Lau, C. Molinaro, C. Nadeau, P.D. O’Shea, M. Palucki, R. Sidler, J. Org. Chem. 75 (2010) 4154–4160. [16] (a) G.C. Muscia, M. Bollini, A.M. Bruno, S.E. Asis, J. Chil. Chem. Soc. 51 (2006) 859–863; (b) J. Vukanovic, A. Passaniti, T. Hirata, R.J. Traystman, B. Hartley-Asp, J.T. Isaacs, Cancer Res. 53 (1993) 1833–1837. [17] (a) S.G. Taran, O.L. Kodolova, O.V. Gorokhova, V.N. Kravchenko, Chem. Heterocycl. Compd. 33 (1997) 959–963; (b) S.G. Taran, O.V. Gorokhova, N.A. Marusenko, A.V. Turov, Chem. Heterocycl. Compd. 33 (1997) 600–604. [18] (a) M. Azad, M.A. Munawar, H.L. Siddiqui, J. Applied Sci. 7 (2007) 2485–2489; (b) E. Melliou, P. Magiatis, S. Mitaku, A.-L. Skaltsounis, E. Chinou, I. Chinou, J. Nat. Prod. 68 (2005) 78–82; (c) Y.L. Chen, C.H. Chung, I.L. Chen, P.H. Chen, H.Y. Heng, Bioorg. Med. Chem. 10 (2002) 2705–2712. [19] T. Kay, S.E.J. Glue, Tetrahedron Lett. 27 (1986) 113–116. [20] R.D. Rieke, S.H. Kim, Tetrahedron Lett. (2011) 52: 3094–3096. F.R. Lyle, U.S. Patent 5 973 257, 1985, Chem. Abstr. (1985), 65, 2870.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Synthesis of 4-acylcoumarins by NHC-catalyzed nucleophilic substitution Yumiko Suzuki ⇑, Asuka Ando, Mizuki Nakagawa Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554 Japan
a r t i c l e
i n f o
Article history: Received 18 September 2018 Revised 8 October 2018 Accepted 22 October 2018 Available online 23 October 2018 Keywords: N-Heterocyclic carbene Coumarin Quinolinone Acylation Organocatalysis
a b s t r a c t Coumarins are bioactive; consequently their syntheses are of importance to medicinal chemists. We describe the one-step organocatalytic syntheses of 4-acylcoumarins from 4-chlorocoumarin. The leaving group at the 4-position of the coumarin was replaced by aroyl groups that originate from aromatic aldehydes by NHC-catalyzed umpolung. 4-Acylthiocoumarins and 2-acylquinolin-2-ones were also prepared using this method. These are the first examples of nucleophilic substitutions at the b-carbons of enones to afford c-ketoenones. Ó 2018 Elsevier Ltd. All rights reserved.
N-Heterocyclic carbenes (NHCs) are unique organocatalysts that mediate various types of reactions [1], including umpolung [2–7], transesterification [8], oxidative-esterification [9], homoenoate [10], and Diels–Alder reactions [11]. The benzoin condensation [2], Stetter reaction [3], and nucleophilic aroylation [4–7] are representative examples of the umpolung reaction. In 1985, Higashino and co-workers reported the first example of an NHCcatalyzed nucleophilic aroylation [4]. Subsequently, aroylation reactions of various N-heteroarenes [5], imidoyl chlorides [6], and benzene derivatives [7] have been reported. The authors envisaged that the substrate scope of this NHC-catalyzed aroylation reaction could be expanded to include enones bearing leaving groups at their b-carbons, such as 4-chlorocoumarins and 4-chloroquinolin-2-ones. These types of bicyclic conjugated enones have received significant attention in the areas of natural products and medicinal chemistry. Coumarin derivatives with anti-HIV [12], antitumor [13], anticoagulation [14], and anti-inflammatory [15] activities have been reported on numerous occasions. In addition, quinolinone derivatives that exhibit anticancer [16], antithyroid [17], and antibacterial [18] properties have also been reported. Therefore, these enone-bearing heterocycles are attractive as components of chemical libraries, and the syntheses of a wide variety of quinolinone derivatives are of interest.
⇑ Corresponding author. E-mail address: [email protected] (Y. Suzuki). https://doi.org/10.1016/j.tetlet.2018.10.044 0040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
To the best of our knowledge, only two methods have been reported for the introduction of aroyl groups at the 4-position of the coumarin core; one requires five steps starting from 4-hydroxycoumarin [19], while the other involves the direct palladium-catalyzed aroylation of 4-coumarinylzinc bromide [20]. Furthermore, there are no reports that describe the introduction of aroyl groups onto 2-quinolones. Reactions of readily available coumarins 1–5 bearing leaving groups at the 4-position with 4-methoxybenzaldehyde (6a) were first examined under standard NHC-catalyzed aroylation conditions [7], in which 1,3-dimethylimizolium iodide was used as the catalyst precursor with sodium hydride in DMF at room temperature (Table 1; For reaction mechanism, see Supplementary data 1). The desired ketone 7a was obtained in satisfactory yields in reactions involving either chloride 1 or bromide 2 (entries 1 and 2). Unfortunately, sulfonyl groups were poor leaving groups (entries 3 and 4), while the methyl ether 5 produced a complex mixture in which 7a was not detected (entry 5). The reaction of 1 with 6a was screened under the same conditions but with various NHC precursors and bases (Table 2). NHCs generated from imidazolium salts B–E bearing a variety of substituents were as effective as A, with the exception of the dichloride D, in which the effect of electron induction presumably lowers the nucleophilicity of the carbene, leading to slightly decreased yield (entries 1–4). The use of triazolium salt F resulted in a further decreased yield (entry 5), while a significantly lower product yield was observed when benzimidazolium salt G was used (entry 6), and the NHC derived from thiazolium salt H was
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Y. Suzuki et al. / Tetrahedron Letters 59 (2018) 4276–4278 Table 1 Leaving-group Scope.a
Table 3 Reaction Optimization: Solvent and Temperature.a
Entry
L
Coumarin
Time (h)
Yield (%)
Entry
Solvent
Temperature (°C)
Time (h)
Yield (%)
1 2 3 4 5
Cl Br OMs OTs OMe
1 2 3 4 5
4 5 7 7 7
67 66 9 –b –c
1 2 3 4 5 6 7 8 9 10 11 12
DMF DMF DMSO DMSO CH3CN CH3CN CH3CN THF THF THF 1,4-Dioxane Toluene
-20 50 r.t. 80 r.t. 50 reflux r.t. 50 reflux r.t. 50
6.5 3 6.5 3 6.5 6 3 6 6 5 6 5.5
63 49 63 57 33 65 34 54 77 63 –b –b
a Reaction conditions: 1–5 (1 mmol), 6a (1.15 mmol), A (10 mol%), NaH (1.6 mmol), and DMF (10 mL). b No reaction. c Complex mixture of products.
Table 2 Reaction Optimization: Catalyst and Base.a
a Reaction conditions: 1 (1 mmol), 6a (1.15 mmol), A (10 mol%), base (1.6 mmol), and solvent (10 mL). b No reaction.
Entry
NHC Precursor
Base
Time (h)
Yield (%)
1 2 3 4 5 6 7 8 9 10
B C D E F G H H A A
NaH NaH NaH NaH NaH NaH NaH Et3N t BuOK DBN
3 4 7 5 7 7.5 7 6 6.5 6.5
67 66 54 63 41 6 –b –b 7 8
a Reaction conditions: 1 (1 mmol), 6a (1.15 mmol), NHC precursor (10 mol%), base (1.6 mmol), and DMF (10 mL). b No reaction.
inert (entries 7 and 8). The use of weaker bases such as tBuOK and DBN in the presence of A also afforded very low yields (entries 9 and 10). Hence, the use of A with sodium hydride was determined to be the optimal combination. Solvents other than DMF were also examined, and reactions were also performed at a variety of temperatures (Table 3). The product yield was barely lower than that observed at room
temperature when the reaction was carried out at 20 °C in DMF, (entry 1), while the higher temperature (50 °C) did not positively affect the yield of the product (entry 2). The decreased yield at the higher temperature could be because of the NaH-absorbed water that is added to the reaction mixture: the undesired substitution reaction with hydroxide to afford 4-hydroxycoumarin occurred. The benzoin condensation product was not detected. The same trend was also observed in other polar solvents, namely DMSO, acetonitrile, and THF, with product yields comparable or higher than those obtained in DMF (entries 3–10). The reaction at 50 °C in THF afforded the product most efficiently (entry 9), while reactions performed at temperatures higher than 50 °C did not afford improved yields of the product (entries 4, 7, and 10). Non-polar solvents, namely 1,4-dioxane and toluene, were unsuitable for this reaction (entries 11 and 12). The aldehyde scope was next explored (Table 4). Reactions with benzaldehydes bearing a variety of substituents, as well as 2-naphthaldehyde and heteroaryl aldehydes afforded the corresponding products in good-to-moderate yields, except for a few examples. The reactivities of the furan moieties in 3-furancarboxaldehyde and the product 7l are believed to be factors responsible for the lower yield. However, the reasons for the low yields observed in the reactions of 4-bromobenzaldehyde and nicotinaldehyde, which gave ketones 7f and 7m, respectively, remain uncertain. Substrates other than 4-chlorocoumarin were also reacted with 4-methoxybenzaldehyde (Table 5). The desired product 8 was obtained in 71% yield from 4-chlorothiocoumarin, while 4-chloroquinolinones bearing methyl and benzyl groups at their 1-positions afforded products 9 and 10 in yields of 66% and 79%, respectively. In conclusion, a straightforward organocatalytic synthesis of 4-aroylcoumarins has been developed. A structurally related thiocoumarin and quinolinone derivatives bearing aroyl groups at their 4-positions are also accessible using this method. NHCs generated from imidazolium salts are the most effective in this transformation compared to those formed from triazolium or benzthiazolium salts, while the thiazolium-salt-based NHC failed to catalyze this
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Y. Suzuki et al. / Tetrahedron Letters 59 (2018) 4276–4278
Table 4 Aldehyde Scope.a
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.tetlet.2018.10.044. References
b
The reaction was carried out in THF (10 mL) at 50 °C instead of DMF at r.t. a Standard conditions: 1 (1 mmol), 6a (1.15 mmol), A (10 mol%), NaH (1.6 mmol), and DMF (10 mL) at r.t.
Table 5 Further Substrate Scope.a
a Reaction conditions: 1 (1 mmol), 6a (1.15 mmol), A (10 mol%), NaH (1.6 mmol), and DMF (10 mL) at r.t.
reaction. To the best of our knowledge, these reactions represent the first examples of nucleophilic substitutions at the b-carbons of enones to furnish the corresponding aroyl-substituted systems. The method facilitates rapid access to this type of c-ketoenone. Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Advanced Molecular Transformations by Organocatalysts” from The Ministry of Education, Culture, Sports, Science and Technology, Japan (Grant number: 26105754).
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