Switchable selectivity in an NHC-catalysed dearomatizing annulation reaction.

The development of general catalytic methods for the regio- and stereoselective construction of chiral N-heterocycles in a diversity-oriented fashion remains a formidable challenge in organic synthesis. N-heterocyclic carbene (NHC) catalysis has been shown to produce a variety of outcomes, but control of the reactivity has rarely been demonstrated. Here we report a switchable catalytic activation of enals with aromatic azomethine imines that provides high selectivity using NHC organocatalysts. The original selectivity corresponds to the acidity of the base used in the reaction. The catalytically generated chiral homoenolate or enol intermediate undergoes enantioselective annulation with electrophiles such as N-iminoquinolinium ylides, N-iminoisoquinolinium ylides and β-N-iminocarboline ylides. The good-to-high overall yields, high regioselectivities and excellent enantioselectivities observed are controlled by the catalyst and reaction conditions.