Asymmetric Michael addition of nitrobenzyl pyridines to enals via iminium catalysis

The asymmetric Michael addition of nitrobenzyl pyridines to α,β-unsaturated aldehydes is described. This unprecedented transformation highlights the possibility of extending the nucleophile scope of iminium catalysis to include diaryl compounds in which the reactive methylene centre is not activated by classical electron-withdrawing groups. Indeed, combining the electronic effects of a p-nitro- or o-nitro-substituted aromatic and of a pyridine system fulfils the requirements for nucleophile activation. The synthetic utility of the method has been demonstrated via rapid access to enantioenriched tetrahydro-1-benzazepines.