Bortezomib Impurity C

N,N’,N”-((2S,2’S,2”S)-2,2′,2”-(((1R,1’R,1”R)-(1,3,5,2,4,6-trioxatriborinane-2,4,6-triyl)tris(pentane-1,1-diyl))tris(azanediyl))tris(3-phenylpropanoyl))tris(pyrazine-2-carboxamide)