A Catalytic Deprotection of S,S‐, S,O‐ and O,O‐Acetals Using Bi(NO3)3⋅5 H2O under Air

S,S‐Acetals are smoothly deprotected with air in the presence of a catalytic amount of Bi(NO₃)₃⋅5 H₂O (1–50 mol %) under ambient conditions to regenerate the original carbonyl compounds in good to excellent yield. This mild, simple, and environmentally benign system is successfully applied to the deprotection of S,O‐ and O,O‐acetals and is compatible with various functional groups. From the mechanistic study of the reaction, the catalytic cycle is considered to be composed of the following four steps: (1) the nitrososulfonium ion of the S,S‐acetal is formed by attack of nitrosonium ion (NO ⁺) generated from Bi(NO₃)₃⋅5 H₂O through the equilibrium with NO₂, (2) the nitrososulfonium ion is hydrolyzed to afford the hemithioacetal and thionitrite, (3) the hemithioacetal collapses to the original carbonyl compound and thiol, which is oxidized by NO ⁺ to give disulfide and NO via thionitrite, and (4) the NO captures molecular oxygen from air to regenerate NO₂.