Linagliptin Dimer Impurity

8-((R)-3-aminopiperidin-1-yl)-1-((4-((2-((8-((R)-3-aminopiperidin-1-yl)-7-(but-2-yn-1-yl)-3-methyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-1-yl)methyl)-4-methyl-1,4-dihydroquinazolin-4-yl)methyl)quinazolin-2-yl)methyl)-7-(but-2-yn-1-yl)-3-methyl-1H-purine-2,6(3H,7H)-dione; 8-[(3R)-3-Amino-1-piperidinyl]-1-[[4-[[2-[[8-[(3R)-3-amino-1-piperidinyl]-7-(2-butyn-1-yl)-2,3,6,7-tetrahydro-3-methyl-2,6-dioxo-1H-purin-1-yl]methyl]-1,4-dihydro-4-methyl-4-quinazolinyl]methyl]-2-quinazolinyl]methyl]-7-(2-butyn-1-yl)-3,7-dihydro-3-methyl-1H-purine-2,6-dione