Catalytic Asymmetric Rearrangement of α,α-Disubstituted α-Siloxy Aldehydes to Optically Active Acyloins Using Axially Chiral Organoaluminum Lewis Acids

A catalytic asymmetric rearrangement of α,α-dialkyl-α-siloxy aldehydes has been developed based on the design of a stereochemically defined, axially chiral organoaluminum Lewis acid 3. For instance, treatment of (S,S)-4 (1.1 equiv) with Me3Al in toluene at room temperature for 30 min generated (S,S)-3 (5 mol %), and subsequent reaction with α-siloxy aldehyde 1a (R = CH2Ph) at −20 °C for 12 h resulted in the smooth rearrangement to afford the corresponding α-siloxy ketone 2a (R = CH2Ph) in 96% isolated yield with 87% ee (S). The scope of this unprecedented stereoselective rearrangement has been investigated with representative substrates, in which impressive kinetic resolution of racemic, α,α-disubstituted α-siloxy aldehydes has also been achieved. These results clearly demonstrate the utility of the present approach for the catalytic enantioselective synthesis of various acyloins and tertiary α-hydroxy aldehydes not readily accessible by the previously known asymmetric methodologies.