You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Name the molecular formula for the molecule established as 1,3-dihydroxy-2-[(3R,4S,5S,6R,7R)-4,5,6-trihydroxy-7-(hydroxymethyl)oxocan-3-yl]xanthen-9-one using the IUPAC nomenclature.
user
C21H22O9
assistant