*3.9. General Procedure for the α-Hydroxylation of β-Keto Ester 9*

Into a flame-dried Schlenk flask under argon atmosphere at 0 °C, a mixture of β-keto ester **9** (0.1 mmol, 24.4 mg, ω = 95%) and *N*-(4-bromobenzylidene)-4-methylbenzenesulfonamide (**12**) (1 equivalent, 33.8 mg) was added. Catalyst **III**, **IV**, **VII**, **VIII**, or **IX** (5 mol%) was dissolved in anhydrous methyl *tert*-butyl ether (MTBE, 5 mL) and slowly added via syringe into the reaction mixture. After addition of H2O2 (1 equivalent, 35% in water, 8.6 μL), the reaction mixture was stirred for 20 h at room temperature. After 24 h at 25 ◦C, the reaction mixture was filtrated trough a plug of anhydrous Na2SO4 and washed with dichloromethane.
