It is shown that the exclusive 1,4-addition ofN-nitromethylphthalimide4ato unhindered α,β-unsaturated ketones and esters can be effected by non-nucleophilic bases, such as sodium hydride or potassium fluoride, in dimethyl sulfoxide. The phthalimido moiety can be removed from the adducts with hydroxylamine or 1,2-dianilinoethane to give the oximes or diphenylimidazolidines of 1,4-ketoaldehydes. The ethylene ketal5of 3-formylcyclohexanone could easily be recovered from the imidazolidine10, but it was not possible to transform11into 3-formylcyclohexanone under similar conditions. It is further shown that reactions, such as the Knoevenagel condensation, or the Paal–Knorr synthesis of pyrroles, can be achieved directly on the adducts without the need for isolating the unstable 4-formyl carbonyl compounds.N-Nitromethylphthalimide thus serves as a new formyl anion equivalent.