Under nitrogen, 6 g (43.5 millimoles) of finely pulverized potassium carbonate is stirred for 10 minutes at 95° C. in 25 ml of absolute dimethylformamide. Then, while the mixture is hot, 10 g (29.6 mmol) of 5-benzyloxy-3-(1-isopropylamino-2-methoxyethyl)indole is added and the mixture is stirred approximately 10 minutes at 95° C. until the compound has been dissolved. Thereupon, likewise at 95° C., a solution of 34.5 mmol of glycine anisaldehyde isopropyl ester in 25 ml of dimethylformamide is added dropwise in a time period of 30 minutes. The solution is agitated until the starting indole can no longer be detected in a thin-layer chromatogram. After cooling, the product is filtered off by suction from potassium carbonate and rinsed with toluene. After adding 100 ml of toluene, 200 ml of 1N hydrochloric acid is added and the mixture stirred for 3 hours at room temperature. The toluene phase is separated, and the aqueous acidic phase is extracted by shaking with 100 ml of toluene. The organic phase is discarded. The acidic phase is cooled to 5° C., combined with 100 ml of toluene, and adjusted to pH 10-12 with 4N sodium hydroxide solution. After extraction by shaking, the mixture is again extracted by shaking with 100 ml of toluene, and the combined organic phase is washed with 50 ml of water, dried, filtered, and concentrated, thus obtaining 70% 2-amino-3-(5-benzyloxyindol-3-yl)-4-methoxybutyric acid isopropyl ester as an oil.
