You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Can you give the molecular formula of the molecule [[(3aS,4S,7R,7aR)-7-methyl-2-(2-methyl-5-nitrophenyl)-1,3-dioxo-3a,7a-dihydro-4,7-epoxyisoindol-4-yl]-acetyloxymethyl] acetate ?
user
C21H20N2O9
assistant