Reaction of linear conjugated enynones, 1,5-diarylpent-2-en-4-yn-1-ones [Ar¹C≡CCH=CHC (=O)Ar² ], with 3-oxo-3-phenylpropanenitrile (NCCH₂COPh) in the presence of sodium methoxide MeONa as a base in MeOH at room temperature for 4–26 h affords polyfunctional δ-diketones as a product of regioselective Michael addition to the double carbon–carbon bond of starting enynones. The δ-diketones have been formed as mixtures of two diastereomers in a ratio of 2.5:1 in good general yields of 53–98%. A synthetic potential of the obtained δ-diketones has been demonstrated by heterocyclization with hydrazine into substututed 5,6-dihydro-4H-1,2-diazepine.