You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (2S,4R,4aR,7R,7aS)-7-hydroxy-2-phenyl-4-[(2R,4R)-2-phenyl-1,3-dioxolan-4-yl]-4,4a,7,7a-tetrahydrofuro[3,2-d][1,3]dioxin-6-one by IUPAC nomenclature, provide its molecular formula.
user
C21H20O7
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