Unexpected Highly Chemoselective Deprotection of the Acetals from Aldehydes and Not Ketones: TESOTf−2,6-Lutidine Combination

Acetal functions are recognized as good protecting groups of carbonyl groups. Although many deprotecting methods of acetals to carbonyl functions have already been developed, there is no methodology which can deprotect acetals in the presence of ketals because the usual acidic or radical reactions occur more easily via the more stable cationic or radical intermediates from the ketals. On the other hand, this new method can proceed in a reverse manner to that described in previous reports. That is, the method can deprotect aliphatic acetals in the presence of ketals. The reaction condition is common for silylation, i.e., the TESOTf-2,6-lutidine combinations. Although the TMSOTf-2,6-lutidine combination can also deprotect acetals, it lacks chemoselectivity in deprotection of the acetals from aldehydes and ketones. The treatment of acetals with TESOTf and 2,6-lutidine in CH2Cl2 followed by a H2O workup gave the corresponding aldehydes. Of course, the compounds, which have both acetal and hydroxyl functions afforded the compounds obtained by the usual silylation of an alcohol and deprotection of an acetal without any problem. However, deprotection of the ketals from ketones was not observed during the conversion reaction of acetals from aldehydes. This chemoselectivity was confirmed in the reactions of the compounds that have the acetal and ketal in the same molecule. In both cases, the acetal functions were deprotected to give aldehydes with intact ketals. Furthermore, under the conditions described here, many functional groups such as methoxy, acetoxy, allyl alcohol, and silyloxy ether are intact. This method is very mild and available for many compounds.