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General procedure: A mixture of compound 5 (1 eq) and HMTA (4 eq) in HOAc was heated at 120° C. under nitrogen for 1.5 h. The reaction mixture turned to dark-yellow in the period. LCMS indicated that the reaction was completed. After cooled to RT, the solvent was removed under reduced pressure, the product was purified by Prep-HPLC to give compound 6. 2-(8-Formyl-7-hydroxy-6-methoxy-4-methyl-2-oxo-2H-chromen-3-yl)-N-(2-methoxyethyl)acetamide was obtained by the above procedure from amine A1.18% yield