N-benzylisatin (see references)
NaOH (SDFCL)

H₂O₂ (Qualigen)

N-benzylisatin (1.5g, 6.3 mmol) was dissolved in  NaOH solution (0.756g was dissolved in 15cm³ DM water). Clear orange color solution was observed. The reaction mass was cooled to 15 to 20 °C. H₂O₂ (4.6 cm³, 50% w/w) was added dropwise (exothermic reaction). The reaction mass was heated to 50 to 60 °C and was stirred at that temperature for 2-3 h. The reaction was monitored by TLC(25% EA/ Hexane, 0.6 Rf of N-benzylisatin). After cooling to RT, the mixture was acidified with HCl (2N) up to pH 2~3 to give an off-white solid material which was filtered via Buchner funnel, washed with water (10 cm³) and the material dried under vacuum for 3-4 h. Yield: 1.1g (78%)

1H NMR (CDCl₃, 400MHz): δ 12.0 (broad, 1H), δ7.90 (d, 1H), δ 7.56 (dd, 1H), δ 7.4 (dd, 2H), δ 7.31 (d, 2H), δ 7.25 (m, 1H), δ 6.81 (d, 1H), δ 6.65 (m, 1H), δ 4.3 (s, 2H).

(M+H): 228.20

Nard C. Sumpter Chem. Rev., 1944, 34(3), pp 393-434,