h. Heating at 50 ℃ for extra 1-2 h was operated when the reaction was not complete. Upon the completion of the hydrolysis, the solvent was removed in vacuo. The crude was dissolved with water and acidified with HCl (0.5 M in water) until no more precipitate is formed, the resulting precipitate was collected via filtration, washed with water and dried in vacuo for the next step without further purification.