Sodium Hydroxide → (3R,4S)-3,4-dimethyl-1-oxo-1,2,3,4-tetrahydro-pyrazino[1,2-a]indole-7-carboxylic acid

To a solution of ethyl (3R,4S)-3,4-dimethyl-1-oxo-1,2,3,4-tetrahydropyrazino[1,2-a]indole-7-carboxylate (851 mg, 3.0 mmol) in ethanol (20 mL) is added 1M NaOH solution (7.0 mL, 7.0 mmol). The reaction mixture is heated at 70° C. for 2 h. The solvent is removed and the residue is acidified using 1M HCl solution until the pH is 5. The resulting white solid is filtered and dried to afford the title compound (697 mg, 91%).