Oxonium salt leaving groups

This is an Sn2 process, since inversion is found at R. Another good leaving group is NTS2 ditosylamines react quite well with acetate ion in dipolar aprotic solvents RNTs2 4- OAc — ROAc. Ordinary primary amines have been converted to acetates and benzoates by the Katritzky pyrylium-pyridinium method (p. 447). Quaternary ammonium salts can be cleaved by heating with AcO in an aprotic solvent. Oxonium ions can also be used as substrates RsO -f R COO —> R COOR R2O. 

Contrary to the oxonium intermediate 115 (Section III,C,2), the intermediate 211 having the alkoxy leaving group readily opens its heterocyclic ring under these conditions. Diketones 205 (R3 is not H) thus formed are then cyclized into a-naphthols 204 (or /3-naphthols of type 209). If anhydro-bases 212 were formed, they would have to take part in the analogous conversions described previously. Obviously, the same mechanism may be applied to the scheme of formation of a-naphthols 204 from 2-benzopyrylium salts under the action of dimethylamine hydrochloride in ethanol (77KGS996) (cf. Section III,C,4,a,iv). 

Ionization after an initial reaction that converts one functional group into a leaving group, as in protonation of an alcohol to give an oxonium ion or conversion of a primary amine to a diazonium salt, both of which ionize to the corresponding carbocation ... 

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