Abstract: We have recently reported an experimental and computational study on [1,5]-H shifts in acetal-ketenimines and acetal-carbodiimides in which the acetalic H atom migrates to the central electrophilic carbon atom of the heterocumulenic fragment. These rearrangements occur under unusual mild thermal conditions, and were chemically interpreted as hydride shifts. The resulting intermediates undergo 6π electrocyclic ring closure to give, respectively, quinoline and quinazoline rings. As a continuation we considered the replacement of the ketenimine and carbodiimide functions by cumulene fragments. As expected, the thermal treatment of several acetal-allenes led to 1,2-dihydronaphthalenes by a similar [1,5]-H/6π electrocyclic ring closure tandem process. Next, as we will describe in this communication, we essayed the cyclization of acetal and dithioacetal-allenes, by activating the electron-rich cumulene function with a protic acids Surprisingly, these reactions provided new heterocyclic fused indenes as reaction products. A mechanistic rationale for explaining these results will be also disclosed.