a new and more efficient synthesis of combretastatin a - 3 was completed starting from methyl gallate and isovanillin with aldehyde 5 and phosphonium salt 8 as key intermediates . conversion of combretastatin a - 3 to a series of diphosphate prodrugs containing selected anions was achieved . both the diphosphate sodium and potassium salts displayed aqueous solubility in excess of 220 mg / ml at room temperature and good cancer cell line inhibitory activity .