You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named methyl (E)-4-[(1R,2S,4R,6S,8aS)-4,6-dihydroxy-5,5,8a-trimethylspiro[3,4,4a,6,7,8-hexahydro-1H-naphthalene-2,2'-oxirane]-1-yl]-2-(oxiran-2-yl)but-2-enoate by IUPAC nomenclature, provide its molecular formula.
user
C21H32O6
assistant