1,3,5-Triazinanes, as a kind of versatile building block, are applied in the synthesis of chromeno [2,3-d]pyrimidin-5-one derivatives via two different reaction modes, which perfectly exhibits the powerful function of 1,3,5-triazinane as a three-atom synthon along with the structure variation of another substrate. The two annulation reactions proceed under mild conditions and bear broad substrate scope and high yield.

A microwave-promoted multicomponent reaction of 3-formylchromones, amines, and paraformaldehyde was achieved under catalyst-free and solvent-free conditions, delivering 5H-chromeno­[2,3-d]­pyrimidin-5-one derivatives in good to excellent yields via an unexpected annulation pathway, which further expanded the synthetic application of paraformaldehyde as a C1 building block.