Dimethyl (3-oxo-1,3-dihydroisobenzofuran-1-yl)phosphonate is a reagent used in the synthesis of 4-[3-(4-Cyclopropanecarbonylpiperazine-1-carbonyl)-4-fluorobenzyl]-2H-phthalazin-1-one, an inhibitor of both PARP-1 AND PARP-2 that shows standalone activity against BRCA1-deficient breast cancer cell lines.