The present invention relates to a process for preparing (2R,3S)-1-halo-2-hydroxy-3-(protected)amino-4-substituted butane comprising contacting a mutagenized Rhodococcus erythropolis having ATCC deposit no. PTA-6648 with (3S)-1-halo-2-oxo-3-(protected)amino-4-substituted butane substrate. The present invention also relates to a process comprising mixing (2R,3S)-1-halo-2-hydroxy-3-(protected)amino-4-substituted butane with at least one base in the presence of at least one solvent to produce a reaction mixture containing (2R,3S)-1,2-epoxy-3-(protected)amino-4-substituted butane. The present invention further relates to a process comprising crystallizing the (2R,3S)-1,2-epoxy-3-(protected)amino-4-substituted butane out of the reaction mixture by concurrently adding water and the reaction mixture together. The present invention is also directed to a mutagenized Rhodococcus erythropolis having ATCC deposit no. PTA-6648.