Preparation method of PGF-2alpha derivative

The invention discloses a preparation method of a PGF-2alpha derivative, which comprises the following steps: methylating a compound as shown in a formula 2 by using a lithium methide reagent at the temperature of 70 DEG C below zero to 50 DEG C below zero to obtain a compound as shown in a formula 3; performing reduction, deprotection, Wittig reaction and macrocyclic lactonization reaction on the compound in the formula 3 to synthesize a compound 6a, and performing chromatographic separation on 6a to obtain a compound in a formula 6b; carrying out hydrolysis and ring opening on the compound in the formula 6b to prepare a 15-methylprostaglandin F2alpha compound, and further salifying to obtain 15-methylprostaglandin F2alpha tranexamic acid amine salt, or carrying out ester exchange reaction with methanol to obtain 15-methylprostaglandin F2alpha methyl ester. The synthesis route is moderate in length, the 15R-epimer and the 15S-, 5, 6-trans isomer can be effectively removed, and the high-purity 15-methylprosta