(2S)-3-phenyl-2-(pyrazine-2-carbonylamino)propanoic acid; N-Pyrazinylcarbonyl-L-phenylalanine; Bortezomib Acid Impurity; (pyrazine-2-carbonyl)-L-phenylalanine; N-(2-pyrazinylcarbonyl)-L-phenylalanine; (S)-3-Phenyl-2-(pyrazine-2-carboxamido)propanoic acid; Bortezomib Acid Analog (USP)