Zr(OBu^t)₄ as an effective promoter for the Meerwein-Ponndorf-Verley alkynylation and cyanation of aldehydes: Development of new asymmetric cyanohydrin synthesis

Zr(OBu^t)₄ can serve as an effective promoter for the Meerwein-Ponndorf-Verley alkynylation of aldehydes and also facilitate MPV type cyanide transfer to aldehyde carbonyls with commercially available acetone cyanohydrin under mild conditions. Based on this finding, a new procedure for asymmetric cyanohydrin synthesis has been developed employing (4R,5R)-2,2-dimethyl-α,α,α′,α′-tetraph enyl-1,3-dioxolane-4,5-dimethanol (TADDOL, 6) as a chiral ligand. For instance, sequential treatment of CH₂Cl₂ solution of 6 (1 equiv.) with Zr(OBu^t)₄ (1 equiv.) and acetone cyanohydrin (2 equiv.) at room temperature for 1 h, and subsequent reaction with 3-phenylpropanal at -40°C for 7.5 h resulted in formation of the corresponding cyanohydrin 5g [R=Ph(CH₂)₂] in 63% isolated yield with 85% ee. The scope and limitations of this method have been clarified with various aldehydes as substrates.