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Tham khảo tài liệu 'organocatalysis episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Nucleophilic Carbenes as Organocatalysts 169 Table 2 Reaction of cinnamaldehyde and derivatives with activated ketonesa o o IMesHCI KOffiu THF -5 u-5 1 mmol o Ph- R 2 equiv Entry Ar R 5 Yield l ub 1c Ph cf3 a 84 66 34 2c d Ph cf3 a 84 64 36 3e Ph cf3 a 92 68 32 4c 4- MeO C6H4 cf3 b 92 66 34 5c 4- Me2N C6H4 cf3 c 74 70 30 6f Ph C O Me d 55 58 42 7 Ph CO2Me e 78 50 50 8c 4- MeO C6H4 CO2Me f 94 47 53 9 4- Me2N C6H4 CO2Me g 98 44 56 a General reaction conditions IMes-HCl 0.05 mmol DBU 0.05 mmol THF 2.5 ml cinnamaldehyde derivative 0.5 mmol ketone 1.0 mmol 16 h at rt. Yield given for the isolated mixture of diastereomers b Determined by GC-MS c Reaction conditions IMes-HCl 0.05 mmol KOtBu 0.1 mmol THF 3 ml cinnamaldehyde derivative 1 mmol ketone 2.0mmol 16 h at rt d 30-mmol scale e 10-mmol scale f Run at 60 C 5 Conjugate Umpolung of Crotonaldehyde Derivatives Crotonaldehyde derivates aliphatically substituted a P-unsaturated aldehydes were also successfully used in the NHC-catalyzed lactone formation Scheme 11 . Good yields up to 90 and good stereoselectivities up to 93 7 were obtained in these transformations. In these cases DBU was found to give better results than KOtBu. 170 F Glorius K. Hirano Dimerization complete conversion by 1H NMR Lactam formation IMesHCI 15mol DBU 10 mol 0.1 M fert-BuOH 60 C 14h cis trans 10 examples 51-75 cis trans 1.5 1 to 10 1 Scheme 10. Conjugate umpolung using different imine substrates Sohn et al. 2005 6 Conjugate Umpolung of a-Substituted Cinnamaldehyde Derivatives A particularly challenging class of substrates are a-substituted cinnam-aldehyde derivatives. Under conditions optimized for the previously mentioned reactions using IMes as the catalyst the use of a-methyl cin-namaldehyde and trifluoroacetophenone did not give any of the desired product. This can easily be understood when analyzing the structure of the conjugate enamine of a-methyl cinnamaldehyde in the conjugated planar conformation. This planar arrangement is disfavored .