Preparation of optically active allothreonine via optical resolution by replacing crystallization

An attempt was made to use a simple procedure to obtain D- and L-allothreonine-(D- and L-aThr), which are non-proteinogenic alpha-amino acids and are useful as chiral reagents in asymmetric syntheses. DL-aThr that exists as a conglomerate was optically resolved by replacing crystallization with L-alanine (L-Ala) as an optically active co-solute. D-aThr was preferentially crystallized from an aqueous solution of DL-aThr in the presence of L-Ala, as was L-aThr in the presence of D-Ala. Furthermore, a diasteroisomeric mixture of D-aThr and L-threonine (L-Thr) and one of L-aThr and D-Thr were prepared, respectively, by epimerization of L- and D-Thr using salicylaldehyde as the catalyst in acetic acid. Based on the result of the replacing crystallization, D- and L-aThr were separated from aqueous solutions of the diastereoisomeric mixtures in the presence of L- and D-Ala. The partially resolved D- and L-aThr were recrystallized from water to yield the corresponding enantiomers in optically pure forms.