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, alpha,alpha-dialkyl-alpha-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 alpha-siloxy aldehyde 1a (R = CH2Ph) at -20 degrees C for 12 h resulted in the smooth rearrangement to afford the corresponding alpha-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, alpha,alpha-disubstituted alpha-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 hydroxy aldehydes not readily accessible by the previously known asymmetric methodologies.