N-heterocyclic-carbene-catalyzed asymmetric oxidative hetero-Diels-Alder reactions with simple aliphatic aldehydes.

N-heterocyclic carbene (NHC) catalysis has emerged as a powerful tool for synthesis. Among its involvement in several cycloaddition-type reactions for the construction of lactams and lactones, which are abundant in biologically active molecules and broadly used as synthons for complex molecules, NHC catalysis through formal [3+3] or [4+2] strategies is noteworthy. In particular, [4+2] cycloadditions have received more attention than others. In the literature, most NHC-catalyzed [4+2] hetero-Diels–Alder reactions involve an enolate intermediate. To date, NHCcatalyzed enolate formation is achieved from functionalized aldehydes, ketenes, or esters (Scheme 1). Treatment of these