Sodium borohydride → (7R,8S,9R)-2,3,8-Trimethyl-7,8-dihydroxy-9-phenyl-7,8,9,10-tetrahydroimidazo[1,2-h][1,7]naphthyridine

8 g of (8S,9R)-2,3,8-trimethyl-8-hydroxy-9-phenyl-7,8,9,10-tetrahydroimidazo[1,2-h][1,7]naphthyridin-7-one are suspended in 160 ml of anhydrous methanol, and the suspension is treated with 2 g of sodium borohydride using a spatula and stirred at room temperature for 24 h, a solution being formed. It is then concentrated to dryness on a vacuum rotary evaporator, the residue is partitioned between water and dichloromethane (50 ml each), and the aqueous phase is adjusted to pH 8 using dilute hydrochloric acid and extracted twice with 200 ml of dichloromethane each time. The organic phases are combined, washed with a little water and dried over anhydrous sodium sulfate, the solvent is stripped off in vacuo and the residue obtained is washed with stirring in acetone. 5.3 g of the title compound are obtained as colorless crystals of melting point 180° C.