You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Provide the molecular formula for the IUPAC named substance (4aR,7aR)-6,6-dioxo-4-phenyl-1-propyl-1,2,4a,5,7,7a-hexahydrothieno[3,4-b]pyrazin-1-ium-3-one .
user
C15H21N2O3S+
assistant