Trifluoroacetic acid → Ethyl cis-3,4-dimethyl-1-oxo-1,2,3,4-tetrahydropyrazino[1,2-a]indole-7-carboxylate

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Reaction Steps

To a solution of the crude diethyl 1-{3-[(tert-butoxycarbonyl)amino]butan-2-yl}-1H-indole-2,6-dicarboxylate from the preceding reaction in CH2Cl2 (10 mL) is added TFA (10 mL). The mixture is stirred for 1 h at room temperature. The solvent is removed and the residue is dried in vacuo for 1 h. To a solution of the residue in ethanol (100 mL) is added K2CO3 (6.2 g, 44.8 mmol). The reaction mixture is refluxed for 1 h. After cooling to room temperature, the solvent is removed under vacuum and the residue is partitioned between EtOAc and water. The organic layer is separated, washed with brine, dried (MgSO4) and concentrated to afford the crude compound which is purified by flash column chromatography using EtOAc in heptane to afford ethyl trans-3,4-dimethyl-1-oxo-1,2,3,4-tetrahydropyrazino[1,2-α]indole-7-carboxylate (870 mg, 20% for 2 steps) and ethyl cis-3,4-dimethyl-1-oxo-1,2,3,4-tetrahydropyrazino[1,2-a]indole-7-carboxylate (1.9 g, 45% for 2 steps).

Provenance & Evidence