A synthetic process for producing bortezomib comprising converting racemic boronic esters, such as the pinacol α-aminoboronic ester, into mixtures of diastereomers [6] by reaction with a suitably protected L-phenylalanine derivative (see Scheme 3), such as N—BOC-L-phenylalanine. The protecting group of the L-phenylalanine moiety is then removed, such as by reacting the diastereomers [6] with an acid, such as hydrochloric acid, to form a mixture of amine salt diastereomers [7] which is then subjected to conditions under which the desired diastereomer (R,S)-[7] is selectively isolated, such as by crystallization, chromatography or stereoselective hydrolysis. The separated desired diastereomer (R,S)-[7] is then converted into bortezomib or bortezomib anhydride.