The present invention relates to an improved process for the preparation of a compound of formula (I), wherein PG1 may be independently selected from tert-butyloxycarbonyl (Boc), phthaloyl, 9-fluorenylmethyloxycarbonyl (Fmoc), triphenylmethyl (Trityl), carboxybenzyl (Cbz), trifluoroacetyl, benzyl (Bn), benzylidene, methanesulfonyl (Mesyl), toluene sulfonyl (Tosyl) or acyl; its isolation as solid and use for the preparation of the compound of formula (IV), in particular the compound of formula (IV) i.e. [(1R)-3-methyl-1[[(2S)-1-oxo-3-phenyl-2-[(pyrazinylcarbonyl) amino]propyl]amino]butyl] boronic acid with more than 99.95% chiral purity, as measured by HPLC.