You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
4-ethyl-11-[[(2R)-oxolan-2-yl]methyl]-5-phenyl-2,3,7,8,11-pentazatricyclo[7.4.0.02,6]trideca-1(9),3,5,7,12-pentaen-10-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H21N5O2
assistant