An overcrowded silanone [Tbt(Mes)Si=O; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = mesityl] (8) was generated via intermediary 1,2,4-oxazasiletes by the reaction of a stable silylene-isocyanide complex with nitrile oxides. in a THF solution, silanone 8 underwent intramolecular cyclization with migration of a trimethylsilyl group in Tbt group to the oxygen atom of the Si=O double bond.