To a stirred suspension of crude 2-tert.-butoxycarbonyl-1,2,3,4-tetrahydro-pyrido[4,3-b]indol (33.5 mmol) and caesium carbonate (25.1 g, 77.1 mmol) in dry acetone is added ethyl bromoacetate (5.6 ml, 50.3 mmol). The reaction mixture is stirred at rt overnight and then filtered over a small plug of Celite. The filtrate is concentrated under reduced pressure and the residue is purified by silica gel column chromatography (hexane/EtOAc 5:1) to afford pure sub-title compound as yellow oil (8.45 g) in 70% yield (over two steps). tR (LC-2) 2.46 min; ESI-MS (positive ion): m/z 381.54 [M+Na]+ (calcd 358.43 for C20H26N2O4).
