[CH3:1][C:2]1[N:3]=[C:4]2[C:13]3[NH:12][C@H:11]([C:14]4[CH:19]=[CH:18][CH:17]=[CH:16][CH:15]=4)[C@@H:10]([OH:20])[C:9](=[O:21])[C:8]=3[CH:7]=[CH:6][N:5]2[C:22]=1[CH3:23].[BH4-].[Na+]>CO.O>[CH3:1][C:2]1[N:3]=[C:4]2[C:13]3[NH:12][C@H:11]([C:14]4[CH:19]=[CH:18][CH:17]=[CH:16][CH:15]=4)[C@@H:10]([OH:20])[C@H:9]([OH:21])[C:8]=3[CH:7]=[CH:6][N:5]2[C:22]=1[CH3:23]6 g (19.52 mmol) of (8R,9R)-2,3-dimethyl-8-hydroxy-9-phenyl-7,8,9,10-tetra-hydroimidazo-[1,2-h][1,7]naphthyridin-7-one (ee>90%, Daicel Chiralcel HPLC) are suspended in 60 ml of methanol and cooled to −5 to 0° C. in a methanol-ice bath. At this temperature, sodium borohydride (0.81 g, 21.47 mmol) is added by spatula during the course of 0.5 h (evolution of gas). After addition is complete, the mixture is stirred for a further 10 min, and then concentrated in a vacuum rotary evaporator at a bath temperature of 40° C. The oily residue obtained is taken up in distilled water and extracted three times with chloroform. The organic phases are combined and washed with a little water, then dried using anhydrous sodium sulfate and filtered. The filtrate is concentrated on a vacuum rotary evaporator and co-evaporated with acetone; the title compound crystallizes out in the course of this. The precipitate is filtered off, washed with acetone and dried to constant weight at 50° C. in a vacuum drying oven. 5.15 g (85.3%, ee>90%, Daicel Chiralcel HPLC) of the title compound are obtained as a colorless crystallizate of melting point 206-9° C.
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