The present invention is a process for preparing (3R)-2′-(4-bromo-2-fluorobenzyl)spiro-[pyrrolidine-3,4′(1′H)-pyrrolo[1,2-a]pyrazine]-1′,2,3′,5(2H′)-tetraone, comprising, for example, the following steps: (1) reacting a compound of the formula (III) wherein R is a C1-6 alkyl group, with 2,5-dimethoxytetrahydrofuran to provide a compound of the formula (IV) wherein R is a C1-6 alkyl group; (2) reacting the compound of the formula (IV) wherein R is a C1-6 alkyl group, with trichloroacetyl chloride to provide a compound of the formula wherein R is a C1-6 group; and (3) reacting the compound provided in step (2) with 4-bromo-2-fluorobenzylamine to provide (3R)-2′-(4-bromo-2-fluorobenzyl)spiro-[pyrrolidine-3,4′(1′H)-pyrrolo[1,2-a]pyrazine]-1′,2,3′,5(2H′)-tetraone.