To provide a new process for preparing a compound represented by formula (I) whereby the final product in a pure form can be produced in a large scale in a high yield by applying a one-pot procedure to a process disclosed by WO91/18897; the necessity for time-consuming and high-cost isolation of an intermediate can be avoided; and the necessity for a dangerous reagent (e.g. phosgene) or an environmentally harmful reagent (e.g. tin chloride) can be avoided.

A process for preparing (S)-4-{[3-[2-(dimethylamino)ethyl]-1H- indol-5-yl]methyl}-2-oxazolidinone by one pot synthesis is provided.