Diastereoselective Formation of Cyanohydrins from α-Alkoxy Aldehydes

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