Vildagliptin Dimer Impurity

(2S,2'S)-1,1'-[[(3-Hydroxytricyclo[3.3.1.13,7]dec-1-yl)imino]bis(1-oxo-2,1-ethanediyl)]bis-2-pyrrolidinecarbonitrile; 2-Pyrrolidinecarbonitrile, 1,1'-[[(3-hydroxytricyclo[3.3.1.13,7]dec-1-yl)imino]bis(1-oxo-2,1-ethanediyl)]bis-, (2S,2'S); (2S)-1-[[[2[(2S)-2-Cyanopyrrolidin-1-yl]-2-oxoethyl][(3-hydroxytricyclo[3.3.1.1(3,7)]dec-1-yl)-amino]]acetyl]-pyrrolidine-2-carbonitrile; (2S,2'S)-1,1'-(2,2'-(((1r,3R,5R,7S)-3-hydroxyadamantan-1-yl)azanediyl)bis(acetyl))bis(pyrrolidine-2-carbonitrile);  Vildagliptin Impurity D