A new synthetic method for cycloheptanone derivatives was developed on the basis of a novel [5 + 2] cycloaddition reaction using a dicobalt hexacarbonyl propargyl cation species and a silyloxyallene. The method can be applied for constructing functionalized hydroazulene skeletons, and a total synthesis of sesquiterpene furanether B was achieved through the reactions involving transformation of the dicobalt acetylene complex into a maleic anhydride derivative and the crucial oxidative transannular ether ring formation.