NHC-Catalyzed Benzannulation via 1,6-Addition to Conjugated Dienals

Abstract

Significance: Song, Chi and co-workers report an N-heterocyclic carbene (NHC) catalyzed synthesis of multisubstituted benzenes and 3-ylidenephthalides. Starting from readily accessible α,β-γ,δ-diunsaturated aldehydes and 1,3-diketones or β-keto esters, the target compounds were obtained in moderate to good yields using 10 mol% of precatalyst A in the presence of an excess of oxidant B under basic conditions. Comment: As pointed out by previous reports on the NHC-based activation of conjugated dienals, nucleophilic attacks preferably occur in the β-position of such systems. The authors have successfully circumvented this problem by 'blocking' the corresponding carbon atom with an additional substituent (R 1 ), thus resulting in a preferred 1,6-addition of the nucleophile.