The Synthesis of Pamidronic Derivatives in Different Solvents: An Optimization and a Mechanistic Study

The synthesis of pamidronic acid and sodium pamidronate dihydrate from β-alanine and P-reagents (phosphorus trichloride and phosphorous acid) was investigated at 75°C in different solvents, and the preparation was optimized. In sulfolane, the use of 2 equiv of phosphorus trichloride and phosphorous acid was found the optimum to lead to pamidronic acid in a yield of 63%. In methanesulfonic acid, 3.2 equiv of phosphorus trichloride was necessary without any phosphorous acid to give pamidronate dihydrate in the best yield (57%) after hydrolysis and pH adjustment. In the first case, the P-nucleophile may be (HO)2P–O–PCl–O–P(OH)2 or (HO)2P–O–PCl2, whereas in the second case, the P-reactant is probable Cl2P–O–S(O)2Me. It can be said that the mechanism proposed for the formation of pamidronic acid is highly influenced by the solvent used, as it determines the necessary P-reagent(s). Our results promote the “on purpose” planning of the synthesis of dronates.