Sulfuric acid protonated ethers, cleavage

Regioselective debenzylation can be achieved by treatment with Lewis acids such as ferric chloride and S11CI4 or under acetolysis conditions with acetic anhydride and sulfuric acid, and several examples are depicted in Scheme 2.3.9 Acetolysis results in cleavage of the most acid-sensitive benzyl group. In general, primary benzyl ethers can be selectively acetolysed in the presence of secondary benzyl ethers. The regioselectivity of the reaction can be explained as follows sulfuric acid protonates acetic anhydride followed by the formation of an acetyl ion and acetic acid. The acetyl ion reacts with the sterically most accessible oxygen which is at... 

HPA is very soluble in oxygen-containing polar solvents such as water, alcohol, ether, and ketone. In these polar solvents, HPA works as an efficient acid catalyst for alkene hydration, the Prins reaction, nucleophilic cleavage of alicyclic and cyclic ethers, esterification, hydrolysis, transesterification, and acetalization. The catalytic activity of HPA in these reactions is much higher than that of ordinary protonic acids such as sulfuric acid and p-toluenesulfonic acid, and the activation energies of the HPA-catalyzed reactions are remarkably reduced, owing to stabilization of the cationic reaction intermediates by the heteropoly anion [1,4]. 

Similar to alcohols, aliphatic ethers [54], thiols [55], and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in magic acid carbocations are subsequently formed upon raising the temperature. Promoted sulfides, excluding tertiary alkyl, are resistant to cleavage up to -i-70 °C [56]. 

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