Diastereoselective formation of cyanohydrins from alpha-alkoxy aldehydes

reaction: see text] The reaction of alpha-alkoxy aldehydes with Et4NAg(CN)2 or Me3SiCN in the presence of MgBr2 x OEt2 in CH2Cl2 at 0 degrees C gives the corresponding syn cyanohydrins in good yield with high diastereoselectivity. Excess MgBr2 x OEt2 (typically 5 equiv) is required for high diastereoselectivity. Et4NAg(CN)2 (but not Me3SiCN) is sufficiently reactive to give cyanohydrins at -78 degrees C, and higher diastereoselectivity is obtained at this temperature.