Acetaldehyde Silyl Enol Ethers in Enantioselective Mukaiyama Aldol Reactions: Enzyme-Like Organocatalysis in Action

Confinement-Controlled, Either syn- or anti-Selective Catalytic Asymmetric Mukaiyama Aldolizations of Propionaldehyde Enolsilanes

Protected aldols (i.e., true aldols derived from aldehydes) with either syn- or anti- stereochemistry are versatile intermediates in many oligopropionate syntheses. Traditional stereoselective approaches to such aldols typically require several nonstrategic operations. Here we report two highly enantioselective and diastereoselective catalytic Mukaiyama aldol reactions of the TBS- or TES- enolsilanes of propionaldehyde with aromatic aldehydes. Our reactions directly deliver valuable silyl protected propionaldehyde aldols in a catalyst controlled manner, either as syn- or anti- isomer. We have identified a privileged IDPi catalyst motif that is tailored for controlling these aldolizations with exceptional selectivities. We demonstrate how a single atom modification in the inner core of the IDPi catalyst, replacing a CF₃-group with a CF₂H-group, leads to a dramatic switch in enantiofacial differentiation of the aldehyde. The origin of this remarkable effect was attributed to tightening of the catalytic cavity via unconventional C–H hydrogen bonding of the CF₂H group.

pKa Modulation of Pyrrolidine-Based Catalytic Polymers Used for the Preparation of Glycosyl Hydrazides at Physiological pH and Temperature

Inspired by the ability of enzymes to use the surrounding hydrophobic and/or polarizable groups to modulate the pK_a of a given amino acid, we designed a series of soluble polymers able to decrease the basicity of pyrrolidine (from 11.2 to 8.6 pK_a units), which clearly increases its aminocatalytic activity at physiological pH in C═N bond formation reactions via ion iminium activation. Other parameters such as charge density, hydrophobic/hydrophilic balance, and aggregation state have been studied as important factors in the catalytic activity of the polymers for a given substrate. To demonstrate the utility of our approach, an optimal pyrrolidine-based catalytic polymer has been used for the formation of C-N bonds between hydrazides and free sugars as the model system for the preparation of glycoconjugates.