Step 1: Understand the question.
This question asks for the correct IUPAC name of Buspirone, a non-benzodiazepine anxiolytic. The challenge lies in two structural details: the length of the alkyl chain linking the piperazine and the spiro ring, and the correct numbering of the dione positions.
Step 2: Buspirone's core structure.
Buspirone has: (a) an azaspiro[4.5]decane-7,9-dione (piperidine-2,4-dione fused as a spiro system), (b) an 8-nitrogen carrying a propyl chain (3 carbons, not 4), (c) that propyl chain ending in a piperazine ring, (d) the piperazine also carrying a pyrimidin-2-yl group.
Step 3: Chain length is the critical differentiator.
Options 1 and 2 both use "propyl" (3-carbon chain) while options 3 and 4 use "butyl" (4-carbon chain). Buspirone has a propyl spacer between the azaspiro nitrogen and the piperazine. So options 3 and 4 are immediately wrong.
Step 4: Distinguish options 1 and 2.
Options 1 and 2 differ only in spacing of the bracket notation. Both state "propyl" and "decane-7,9-dione." Option 1 writes "azaspiro[4.5]decane" without internal spaces; option 2 adds a space before the bracket. According to IUPAC nomenclature, the fused ring descriptor is written without internal spaces. Option 1 is therefore the correctly formatted name.
Step 5: Confirm.
The accepted IUPAC name for Buspirone uses a propyl chain and no extra spaces inside the azaspiro descriptor, matching option 1.
Answer: Option (1) — 8-[4-(4-pyrimidin-2-yl piperazin-1-yl)propyl]-8-azaspiro[4.5]decane-7,9-dione