Hydroxyamine hydrochloride → 4-Pyridinecarboxaldehyde, 2,5-dichloro-, oxime

A solution of lithium di-isopropylamide was prepared by the addition of 100 ml of 1.5 molar (M) butyllithium (in a hexane solution) to 14.2 g of di-isopropylamine in 100 ml of THF at less than -30° C. After stirring for 20 minutes the solution was cooled to below -70° C. and a solution of 20.8 g of 2,5-dichloropyridine in 56 ml of THF was added at such a rate that the temperature was maintained at below -70° C. After the addition was complete, the reaction temperature was stirred at below -70° C. for 30 minutes. A solution of 13.2 g of DMF in 30 ml THF was added at the same temperature and stirred for an additional 30 minutes. The reaction mixture was allowed to warm to room temperature then poured into a mixture containing 250 g of ice and 45 ml of concentrated HCl and stirred for 15 minutes. The acidic solution was neutralized to about pH 6-7 with 3 normal (N) NaOH. The organic layer was separated and the aqueous layer extracted with ethyl ether. The organic solutions were combined, washed with saturated NaCl solution, dried and evaporated to dryness to give about 22 g of 2,5-dichloro-4-pyridinecarboxaldehyde. This aldehyde was dissolved in 100 ml of 2-propanol, mixed with 13.8 g of hydroxylamine hydrochloride and 20 drops of concentrated HCl and heated on a steambath for 1 hour. The mixture was then poured onto 200 g of ice, stirred well and filtered leaving a solid residue. The solid was vacuum dried to give 22.8 g (85%) of the desired 2,5-dichloro-4-pyridinecarboxaldehyde oxime ##STR6##