Divergent NHC-catalyzed oxidative transformations of 3-bromoenal: Selective synthesis of 2H-pyran-2-ones and chiral dihydropyranones

摘要

A selective synthesis of either 2H-pyran-2-ones (4) or chiral dihydropyranones (6) from the same substrates of 3-bromoenals and 1,3-dicarbonyl compounds upon oxidative N-heterocyclic carbene catalysis is presented. It is shown that the oxidative transformation of 3-bromoenals under NHC catalyst can be well controlled to proceed through two pathways, i.e., elimination of reducible β-bromide or by an external oxidant 3, allowing the selective generation two sorts of unsaturated acyl azoliums, respectively.