Carbon chemical diastereotopic effects

A method for stereoselective synthesis of a-amino acids can be used to illustrate a diastereoselective chemical reaction." " The key is the stereoselective reduction of a carbon-nitrogen double bond in which the hydrogen atom addition is highly preferred from one diastereotopic face. The sequence is shown below for the synthesis of D-alanine. The optical purity observed is 96% in this instance, with optical yields of 92-97% reported for other amino acids. The chirality which is present in the reactant molecule directs the course of the addition of hydrogen in the step in which the new chiral center is created. This occurs as the result of a steric effect. It is easier for the reactant to approach the surface of the hydrogenation catalyst from the less bulky side of the molecule. After the reduction, the newly created chiral product is obtained by a reaction which also releases the original chiral reactant in a form in which it can be reconverted to the chiral hydrazine. 

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