a suspension of 2.56 g of paraformaldehyde in 8 ml of water and 0.8 ml of concentrated hydrochloric acid is refluxed at 80° C. for 1 hour. One-tenth of the thus-obtained clear solution is added dropwise, after cooling to room temperature, to a solution of 3.8 g (10 mmol) of 2-amino-3-(5-benzyloxyindol-3-yl)-4-methoxybutyric acid isopropyl ester in 500 ml of water and 10 ml of concentrated hydrochloric acid (pH=3). After 1/2 hour of agitation, an estimate of the amount of amino compound still remaining is made by thin-layer chromatography, and a corresponding quantity of formaldehyde solution is added. Thereupon, the mixture is stirred for another hour and then extracted twice by shaking with 50 ml of toluene, respectively. The organic phase is discarded. The aqueous phase is adjusted, after adding 100 ml of toluene, to a pH of 5.3 with 27% strength sodium hydroxide solution. After extraction by shaking, the mixture is additionally extracted by shaking twice with 50 ml of toluene; these 3 organic phases are combined, dried over sodium sulfate, filtered, and concentrated, thus obtaining 3.3 g (85%) of 6-benzyloxy-4-methoxymethyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid isopropyl ester as an oil.
