You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Please write the molecular formula of the molecule tert-butyl 4-[7-but-2-ynyl-2-chloro-1-(isocyanomethyl)-6-oxopurin-8-yl]piperazine-1-carboxylate .
user
C20H24ClN7O3
assistant