(E)-4-(Dimethylamino)but-2-enoyl Chloride-d6 is an intermediate used in the synthesis of Afatinib-d6 (A355303), which is the d6 labeled analogue of Afatinib (A355300), an aminocrotonylamino-substituted quinazoline derivative used for treating cancer and diseases of the respiratory tract, lungs, gastrointestinal tract, bile duct, and gallbladder.