The disclosure relates to a method for preparing morphine-6-glucuronide or one of the deritives thereof comprising reacting a compound of formula (I): wherein R1 is as defined in the disclosure, with a glucuronic acid derivative of formula (II): wherein PG, X, and R4 are as defined in the disclosure, in the presence of an aromatic solvent and trimethylsilyl trifluoromethanesulfonate; (ii) reacting the product obtained in step (i) with a strong basic agent; and then (iii) recovering the product obtained in step (ii).