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