Zopolrestat can be obtained by several different ways: 1) The reaction of 2-(4-oxo-3,4-dihydrophthalazin-1-yl)acetic acid ethyl ester (I) with 2-chloroacetonitrile by means of potassium tert-butoxide in DMF gives 2-[3-(cyanomethyl)-4-oxo-3,4-dihydrophthalazin-1-yl]acetic acid ethyl ester (II), which is cyclized with 2-amino-4-(trifluoromethyl)thiophenol (III) in refluxing ethanol yielding zopolrestat ethyl ester (IV). Finally, this compound is hydrolyzed with KOH in methanol/water/THF. 2) Compound (IV) can also be obtained by cyclization of (II) with 4-chloro-3-nitrobenzotrifluoride (V) in hot DMF saturated with H2S. 3) Compound (II) can also be obtained as follows: The reaction of phthalazine (I) with aqueous formaldehyde gives 2-[3-(hydroxymethyl)-4-oxo-3,4-dihydrophthalazin-1-yl]acetic acid ethyl ester (VI), which is treated with PBr3 in ethyl ether yielding the bromomethyl derivative (VII). Finally, this compound is treated with potassium cyanide and KI in acetone/water.
5) Esterified intermediate (IV) can also be obtained as follows: The reaction of 2-bromo-5-(trifluoromethyl)acetanilide (VIII) with phosphorus pentasulfide in refluxing benzene gives the corresponding thioamide (IX), which is cyclized by means of NaH in hot N-methylpyrrolidone yielding 2-methyl-5-(trifluoromethyl)benzothiazole (X). The bromination of (X) with N-bromosuccinimide (NBS) and benzoyl peroxide in refluxing CCl4 affords the corresponding bromomethyl derivative (XI), which is then condensed with phthalazinone (I) by means of NaH in DMF.