We report a direct, simple, and straightforward approach for the construction of a mixed monosilyl acetal as a new and synthetically valuable functional group by mixing an aldehyde, sodium tert-butoxide and trimethylsilyl azide. We also demonstrate catalyst-dependent chemoselective reaction between mixed monosilyl acetals and silyl ketene acetals through Mukaiyama aldol reactions to give different structures of O-protected β-hydroxy esters in excellent yields with high chemoselectivities. This study provided the existence of an oxonium ion intermediate and of its kinetically controlled reaction with the pre-equilibrated silyl enol ether obtained from (E)- and (Z)-isomerization.