Pilot Plant PAT Approach for the Diastereoselective Diimide Reduction of Artemisinic Acid

In this study, an attractive route for the diastereoselective synthesis of dihydroartemisinic acid (DHAA) starting from artemisinic acid (AA) is presented. Diimide was used as a reducing agent, which was generated by two different methods: (1) by the reaction of hydrazine monohydrate and hydrogen peroxide and (2) by the reaction of hydrazine monohydrate and oxygen. Both methods were found to be suitable for the diimide reduction of AA showing full conversion and a high diastereoselectivity. Due to advantages in the crystallization step of DHAA, the second option for generation of diimide was chosen for the pilot plant scale-up. The reaction and the crystallization process development as well as the batch production in the pilot plant were monitored and controlled using dispersive Raman spectroscopy as PAT tool. Three DHAA batches in kilogram scale were successfully produced by the reaction of artemisininic acid, hydrazine monohydrate, and a gas mixture of nitrogen and oxygen (containing 5% v/v oxygen) in 2-propanol at 40 degrees C. Excellent yields of >90% (including the crystallization, isolation, and drying step) as well as high diastereoselectivities (>= 97:3) of the products were achieved by the elaborated pilot plant manufacturing processes.