Silica gel (5 g) and MnO2 (4.60 g, 52.9 mmol) were ground to a homogenous consistency and the mixture was added to a solution of 1-(1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-6,8-dimethoxy-3,4-dihydro-isoquinoline-4-carboxylic acid ethyl ester (Note 1) (1.39 g, 3.53 mmol) in 180 mL of benzene. The reaction flask, equipped with a Dean-Stark trap and a condenser, was then heated to 100° C., with azeotropic removal of water (18 ml). After 4 hours, the reaction was cooled to room temperature and filtered through a plug of celite, rinsing with 3% MeOH in DCM. The filtrate was concentrated to give 1.30 g (94%) of 1-(1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-6,8-dimethoxy-isoquinoline-4-carboxylic acid ethyl ester as a white solid. H1-NMR (CDCl3): δ, 8.85 (s, 1H), 8.07 (d, 111, J=2.1 Hz), 7.92 (m, 2H), 7.75 (m, 2H), 6.62 (d, 1H, J=2.4 Hz), 5.62 (s, 2H), 4.36 (q, 2H, J=7.4), 4.03 and 3.98 (s, 3H each), 1.36 (t, 3H, J=6.9 Hz).
