Michael reaction of acyclic #beta#-enaminesters derived from #alpha#-alkyl-#beta#-ketoesters and chiral #alpha#-methylbenzylamine: Stereoselective generation of quaternary carbon centres

摘要

Michael reaction of electrophilic olefins with acyclic #beta#-enaminoester substrates derived from #alpha#-substituted #beta#-ketoesters and (R)- or (S)-#alpha#-methylbenzylamine under neutral conditions resulted in the enantioselective formation of quaternary cen-tres. The addition of acrolein and methyl vinyl ketone to the enaminoesers (+)-8, (+)-10 and (+)-5 followed by aldol condensation of the resulting adducts produces the corresponding cyclohexenones (+)-12, (-)-12, (+)-17, and (-)-18, (+)-21 and (-)-22 with good optical purities. Similarly, keto diesers (-)-14 and (-)-19 are obtained from the reaction of ethyl acry-late with enaminoester (+)-10 and of methyl acrylate with (+)-5. Transition states of these reactions have been explored with the help of PM3 semi-empirical calculations using MOPAC 6.00. The energy differeces calculated between the two-competing transition states are in agreement with the observation that the alkylations take place preferentially on the less hindered sideof the #pi#-bond opposite to the phenyl group.