2-Monosubstituted 1,3-dioxolanes and dithiolanes act as hydride-releasing fragments, transferring intramolecularly their acetalic H atom to the central carbon of ketenimine functions. The presumed products of these migrations, o-quinomethanimines, undergo in situ 6 pi-electrocyclization. A computational study supports this mechanism and the hydride-shift character of the first step. Carbodiimides were also suitable substrates, although less reactive.