Carbohydrates as chiral templates: Stereoselective Strecker synthesis of D‐α‐amino nitriles and acids using O‐pivaloylated D‐galactosylamine as the auxiliary

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Abstract
The asymmetric synthesis of N‐galactosyl D‐α‐amino nitriles is accomplished by application of 2,3,4,6‐tetra‐O‐pivaloyl‐β‐D‐galactopyranosylamine (4) as the stereodifferentiating template in Strecker reactions. Aldimines 6 derived from 4 and aromatic or aliphatic aldehydes react with trimethylsilyl cyanide in 2‐propanol or tetrahydrofuran in the presence of Lewis acids, e.g. ZnCl2 or SnCl4′ to give the N‐galactosyl α‐amino nitriles 5 in high yield and with diastereomeric ratios between 6:1 and 13:1 favoring the D diastereomers. Pure D‐amino nitrile diastereomers D‐5 are obtained in high yields by recrystallization from n‐heptane. Acid‐catalyzed hydrolysis of compounds D‐5 delivers the free D‐amino acids. A mechanistic interpretation of the stereoselection observed is given in terms of stereoelectronic, steric and complexing effects exhibited by the carbohydrate.