The production of pure diastereomers and the quality control of chiral drug substances have become key issues for the pharmaceutical industry and regulatory agencies due to differences in their biological properties in vivo. This study discusses the unusual case study for analytical method development and preparative purification of diastereomers derived from the conjugation of l‐ and d‐valine ester with the HIV‐1 protease inhibitor atazanavir prodrug. While the l‐valine ester conjugate diastereomers could be successfully separated using acetonitrile and 10 mM ammonium acetate on SunFire‐C18 OBD column, the corresponding d‐valine ester conjugate diastereomers co‐eluted under the same method conditions. This observation led to a series of method development and optimization work that resulted in an improved chromatographic condition, using acetonitrile:2‐propanol (70:30) v/v combination as an organic modifier with 0.1% HCOOH on SunFire‐C18 column. The same method was further scaled up for isolation of individual diastereomers on preparative high‐performance liquid chromatography.

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