A series of substituted 2-(3-oxoprop-1-en-1-yl)guanidines was obtained via the interaction of 1,1,3,3-tetramethylguanidine with fused 4 H-pyrans, containing a carbonyl group in the β-position to the oxygen atom. The proposed reaction mechanism includes a conjugate 1,4-addition with subsequent opening of the dihydropyran ring.