p-Nitrophenyl Ethylthioester in Enantioselective Organocatalytic Michael Additions: Different Behaviour of β-Aryl and β-Alkyl Enals

Although several nucleophiles derived from arylacetic acids have been described in catalysis via iminium activation, none have presented good enantioselectivity and reactivity with both beta-alkyl and beta-aryl enals. This study on the Michael addition of p-nitrophenylacetic thioesters suggests that this behavior is due to the different reactivity and distinct tendency to reversibility exhibited by these types of enal. Independent optimization of the conditions for beta-alkyl and beta-aryl enals provide good yields and enantioselectivities with both substrates, affording Michael adducts that are versatile building blocks for the preparation of an interesting variety of arylacetic acid derivatives.