An efficient synthesis of 3-alkyl 1, 5, 3-dioxazepanes was developed for subsequent use in double-Mannich reactions with a variety of carbon-based nucleophiles. It was found that addition of methyltrichlorosilane to the dioxazepane led to a long-lasting reactive species that reacted rapidly with acid-sensitive ketones and β-ketoesters to afford azabicyclo [3.3. 1] nonanes in good yield.