Method for preparing clarithromycin impurity

The invention discloses a method for preparing clarithromycin impurity, relates to the field of medicinal chemistry and in particular relates to a method for preparing clarithromycin impurity P which is expressed as a formula (II). The method comprises the following steps: protecting oxime hydroxyl of erythromycin oxime by using 1,1-Diisopropoxy cyclohexane, selectively protecting hydroxyl of deoxy amino sugar ring by using trimethylsilane, carrying out methylation, deprotection and purification to obtain the high-purity clarithromycin impurity P and providing a reference substance for researching the quality of clarithromycin API. The method for preparing the clarithromycin impurity is high in methylation selectivity and simple and conveninet in after-treatment; the purity of the purified clarithromycin impurity P reaches over 99%.