Alkylation of 3-ethyl-2-methyl-4-oxo-4,5,6,7-tetrahydroindole with bromoesters: benzenesulfonyl as convenient nitrogen protecting group

α-Alkylation of 3-ethyl-2-methyl-4-oxo-1H-4,5,6,7-tetrahydroindole (3) with bromoesters gives 3-ethyl-2-methyl-4-oxo-1H-4,5,6,7-tetrahydro-5-indoleacetate (5a), 3-ethyl-2-methyl-4-oxo-1H-4,5,6,7-tetrahydro-5-indole-2-propionate (5b), and 3-ethyl-2-methyl-4-oxo-1H-4,5,6,7-tetrahydro-5-indole-3-propionate (6)