methyl (2S)-2-[(2-chloro-5-nitro-pyrimidin-4-yl)-isopropyl-amino]butanoate → (7S)-2-chloro-8-isopropyl-7-ethyl-5,7-dihydropteridin-6-one

Methyl (2S)-2-[(2-chloro-5-nitro-pyrimidin-4-yl)-isopropyl-amino]butanoate 43b (8.74 g, 27.7 mmol) was dissolved in 100 mL of acetic acid followed by the addition of iron powder (6.20 g, 0.11 mol). The reaction mixture was heated to 65° C., stirred for 4 hours and filtered. The filter cake was washed with dichloromethane (100 mL). The filtrate was concentrated under reduced pressure, then added with 100 mL of water and filtered. The filter cake was dried by heating to obtain the crude compound (7S)-2-chloro-8-isopropyl-7-ethyl-5,7-dihydropteridin-6-one 43c (7.17 g) as a gray solid, which was used in the next step without further furification.