A Hydroxy-Substituted Trialkyloxonium Ion and Its Oxonium Alkoxide Zwitterion

Trialkyl oxonium salts are among the most powerful electrophiles known. They are immediately quenched by nucleophiles like alcohols and halide ions, and are therefore used in unreactive solvents and have non-nucleophilic counterions. Any proposal to incorporate a nucleophilic hydroxy substituent into a trialkyl oxonium species would thus be ill-considered, and pairing an R₃O⁺ cation with an alkoxide counterion would be an abstract notion, at best. Here, we describe how these combinations are made possible within the context of the robust environment of the heterotriquinane ring system. A stable, C₃ symmetric oxatriquinanetriol can be simply accessed by reaction of the tris-epoxide of 1,4,7-cyclononatriene with aqueous acid. The three OH groups of this hemispherical cation point into the endo space of the bowl and participate in strong intramolecular hydrogen bonding. A close balance in the dynamics of proton exchange between these hydroxy functions and amine bases is predicted by modeling. In practice, treatment of oxatriquinanetriol with the non-nucleophilic base 2-tert-butyl-1,1,3,3-tetramethylguanidine (Barton's base) leads to a zwitterionic oxonium alkoxide.