Diels-Alder and Friedel-Crafts reactions of [3]cumulene 1 were investigated. Although the Diels-Alder reaction with cyclopentadiene selectively occurred on the C2, 3 double bond to give tetrasubstituted allenyl products, the Friedel-Crafts addition of electron-rich heteroaromatics and subsequent protonation cleanly occurred on the C3, 4 double bond to afford tetrasubstituted conjugated dienes.