1,4,7-trimethyloxatriquinane: S_N2 reaction at tertiary carbon

The synthesis of 1,4,7-trimethyloxatriquinane (1), a 3-fold tertiary alkyl oxonium salt, is described. Compound 1 is inert to solvolysis with alcohols, even at elevated temperatures, but undergoes facile substitution with the strongly nucleophilic azide anion. Since an S_N1 pathway is excluded, the only reasonable mechanistic interpretation for the reaction between 1 and N₃ ^- is S_N2, despite the fact that substitution is occurring at a tertiary carbon center. This finding is supported by computational modeling and a study of the reaction kinetics, and is also consistent with observed solvent and salt effects.