Question

Suggest a method for the synthesis of the natural L enantiomer of alanine from the readily available L enantiomer of lactic acid.

Short answer

L-Lactic acid D-alanine

Step-by-step solution

Step-1. Enantiomers of alanine:

Alanine exists in two enantiomeric forms, that is, L- and D-alanine. D-alanine is the D-enantiomer of alanine. It is a metabolite found in E. coli strain. L-enantiomer is a pyruvate family amino acid and has (S)-configuration whereas D-alanine has (R)-configuration.

Step-2. Synthesis of D-alanine from L-Lactic acid:

Firstly, L-Lactic acid is made to react with tosyl chloride and pyridine, in which pyridine acts as a base and abstracts proton from hydroxyl group of lactic acid and tosyl group gets attached to the oxygen and good leaving group in the form of tosylate is obtained. On treatment with excess ammonia in next step, tosylate leaves and amide ion acts as a nucleophile and attacks at carbon to which tosylate is attached. After intramolecular proton exchange with acid, we get our required product, that is, D-alanine.

L-Lactic acid D-alanine