Formic Acid → Azithromycin

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In a 0.25 L three-necked round bottom flask equipped with a mechanical stirrer, a condenser and thermometer, 25 g of desmethyl-azithromycin was dissolved in 102 mL of acetone. Formaldehyde (5.3 mL) and formic acid (2.6 mL) were added and the clear solution was heated to reflux (˜58° C.). The mixture was maintained under reflux for 3 hours then cooled to less than 40° C. and 102 mL of water was added. The acetone was separated from the reaction mixture by distillation under low vacuum (˜300 mbar). The distillation was stopped when the temperature of the liquid phase reaches ˜40° C./300 mbar. Activated carbon SXI (1.25 g) was added and the suspension was mixed for 1 hour. Then the activated carbon was separated from the solution by filtration on a Büchner filter containing Hyflow. The filter cake was washed with 102 mL of water. The pH was adjusted with an NaOH solution of 47% to 10-10.5 (˜3.14 g). A massive precipitation of Azithromycin was observed. After stirring the suspension for 1 hour, the crude azithromycin was separated by filtration, washed twice with 25 mL of water and dried at 40° C. in a vacuum oven for>24 hours. After drying, 19.7 g of crude azithromycin was obtained (yield 77% based on weight). No impurities that were derivatives of azithromycin were detected.

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