To an appropriate reactor equipped with mechanical stirrer was charged acetic acid (12 L), tert-butyl 4-(3-(6-(3,5-dimethoxyphenyl)-2-(methylthio)-7-oxopyrido[2,3-d]pyrimidin-8(7H)-yl)propyl)piperazine-1-carboxylate (2000 g) and triethylamine (639 g, 2.3 eq.). Internal temperature was adjusted to approximately 20° C. and N-chlorosuccinimide (1651 g, 4.5 eq.) was added at 20-30° C. Reaction was stirred for 2 hours. Ethyl acetate (30 L) was added. 5% aqueous NaCl solution (20 L) was added. The organic layer was separated and the aqueous layer was extracted with EtOAc. The combined organic layers were washed with 30% aqueous potassium carbonate solution (14 L). The organic layer was concentrated to ˜12 L and used for next step directly.
