Probing the Reactivity of Cyclic N,O-Acetals versus Cyclic O,O-Acetals with NaBH4 and CH3MgI

An operationally straightforward, project-like laboratory experiment has been developed in which students directly compare the reactivity of two heterocycles, a cyclic O,O-acetal (standard CO protecting group) and a cyclic N,O-acetal (oxazolidine), toward sodium borohydride and methylmagnesium iodide. Students synthesize a substrate containing both structural features, and then experimentally establish the chemo- and regioselective ring-opening of the N,O-acetal by identifying their 2-amino-1-alkanol products through spectral analysis. This experiment demonstrates that nonaromatic N- and O-heterocycles beyond those usually encountered by students in introductory organic textbooks, such as O,O-acetals and epoxides, are also reactive to nucleophiles, and it emphasizes the great effect on chemical reactivity of a seemingly minor structural alteration: exchanging O for N in a heterocycle.