Siloxy Esters as Traceless Activators of Carboxylic Acids: Boron‐Catalyzed Chemoselective Asymmetric Aldol Reaction**

Abstract The catalytic asymmetric aldol reaction is among the most useful reactions in organic synthesis. Despite the existence of many prominent reports, however, the late‐stage, chemoselective, catalytic, asymmetric aldol reaction of multifunctional substrates is still difficult to achieve. Herein, we identified that in situ pre‐conversion of carboxylic acids to siloxy esters facilitated the boron‐catalyzed direct aldol reaction, leading to the development of carboxylic acid‐selective, catalytic, asymmetric aldol reaction applicable to multifunctional substrates. Combining experimental and computational studies rationalized the reaction mechanism and led to the proposal of Si/B enediolates as the active species. The silyl ester formation facilitated both enolization and catalyst turnover by acidifying the α‐proton of substrates and attenuating poisonous Lewis bases to the boron catalyst.