The first synthesis of octavalene (1a) is reported. The starting material is homobenzvalene (5), to which monobromocarbene is added. The resulting compound 3a takes up bromine across the central bicyclo [1.1. 0] butane bond to form the tribromide 7a which undergoes a cyclopropyl bromide-allyl bromide rearrangement on heating. From the product (1Oa) HBr is eliminated to give a 1, 3-dibromocyclobutane with a 1, 3-butadiene bridge across its 2-and ...