You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named [1-[7-but-2-ynyl-1-methyl-2,6-dioxo-3-(oxolan-2-ylmethyl)purin-8-yl]piperazin-2-yl] 2,2,2-trifluoroacetate by IUPAC nomenclature, provide its molecular formula.
user
C21H25F3N6O5
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