Linagliptin Impurity L Catalogue L562377 CAS 666816-91-7 M.W 416.47 M.F C20H28N6O4 Specifications FAQs (R)-tert-butyl (1-(7-(but-2-yn-1-yl)-3-methyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-8-yl)piperidin-3-yl)carbamate