Phosphoramidate compound and preparation method and crystal thereof

A method for preparing a composition enriched in a crystalline form of compound 61501b wherein the purity of the compound 61501b is not less than 90%, and

• the X-ray powder diffraction pattern of the crystalline form of compound 61501b has peaks at the following 2θ angles:
• 7.2°±0.3°, 9.6°±0.3° and 23.1°±0.3°,
• comprising:
• reacting a compound (61501h) with a compound (61501g) under alkaline conditions, and then reacting with a compound (61501f) to obtain a compound 61502:
• and
• obtaining the composition enriched in the crystalline form of compound 61501b by isomer separation of the compound 61502, the isomer separation comprising:
• dissolving the compound 61502 in an organic solvent and stirring for dissolving;
• and adding an anti-solvent dropwise to the above solution system, and crystallizing, thereby giving the compound 61501b: