... four possible stereoisomers with the following configurations: (2S,3R), (2R,3S), (2S,3S) and (2R,3R). However, the name ... L-threonine is used for one single enantiomer, (2S, ... -3-hydroxybutanoic acid. The second stereoisomer (2S,3S), which is rarely present in nature, is ... L-allo-threonine. The two stereoisomers (2R,3S)- and (2R,3R)-2-amino-3-hydroxybutanoic ...