You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Convert the IUPAC name of a molecule 1-[3-[5-(2-phenylpyrimidin-5-yl)-1H-imidazol-2-yl]azetidin-1-yl]butan-1-one into molecular formula.
user
C20H21N5O
assistant