Method E: 6-Hydroxy-1,2,3,4-tetrahydroquinoline (25 g, 172.9 mmol) was dissolved in a solvent mixture of dioxane (200 mL) and 1N NaOH aqueous solution (200 mL), followed by addition of di-tert-butyl dicarbonate (56.6 g, 259.4 mmol). The reaction mixture was stirred at room temperature overnight. Upon completion of the reaction, the organic and aqueous layers were separated and the aqueous layer was extracted with ethyl acetate (2 x 200 mL). The organic layers were combined, washed with saturated saline (1 × 200 mL) and dried over anhydrous sodium sulfate. The organic layer was concentrated under reduced pressure to give the crude product. The target product tert-butyl 6-hydroxy-3,4-dihydro-2H-quinoline-1-carboxylate (33.0 g, 76.7% yield) was purified by fast column chromatography (Biotage system, eluent: 10% ethyl acetate/hexane).
