This compound can be obtained by two related ways: 1) The cyclization of 2-formyl-4-methyl-1,2,3,4-tetrahydroisoquinolone-3-carboxylic acid (I) with 2-chloro-5-hydroxy-1,4-naphthoquinone (II) by means of sodium acetate in hot acetic anhydride gives 9-hydroxy-14-methyl-5,8,13,14-tetrahydrobenz[5,6]isoindolo[2,1-b] isoquinoline-8,13-dione (III), which is condensed with dibenzyl phosphonate (IV) by means of N-chlorosuccinimide (NClS) and NaH in benzene to afford (14-methyl-8,13-dioxo-5,8,13,14-tetrahydrobenz[5,6]isoindolo[2,1-b] isoquinolin-9-yl)phosphoric acid dibenzyl ester (V). Finally, this compound is partially hydrolyzed with NaI in refluxing acetone, and the resulting sodium salt treated with aqueous HCl. 2) The condensation of 5-hydroxy-1,4-naphthoquinone (VI) with phosphonate (IV) by means of Cl2SO2 followed by a treatment with NaHCO3 gives (1,4-naphthoquinon-5-yl)phosphoric acid dibenzyl ester (VII), which is then cyclized with isoquinoline (I) in hot acetic anhydride to compound (V), already obtained.