A catalytic deprotection of S,S-, S,O- and O,O-acetals using Bi(NO3)(3)center dot 5 H2O under air

S,S-Acetals are smoothly deprotected with air in the presence of a catalyti c amount of Bi(NO3)(3).5 H2O (1-50 mol %) under ambient conditions to regen erate 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 functio nal groups. From the mechanistic study of the reaction, the catalytic cycle is considered to be composed of the following four steps: (1) the nitrosos ulfonium 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 nitrososulronium ion is hydrolyzed to afford the Hemithioacetal and thionit rite, (3) the Hemithioacetal collapses to the original carbonyl compound an d 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.