p-Toluenesulfonic acid → 6-Fluoro-2,3-dihydro-2',5'-dioxo-spiro[4H-1-benzopyran-4,4'-imidazolidine]-2-carboxylic acid 3,6,9,12-tetraoxatridecyl ester

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A mixture consisting of 6-fluoro-2,3-dihydro-2',5'-dioxo-spiro[4H-1-benzopyran-4,4'-imidazolidine]-2-carboxylic acid (2.8 g, 0.01 mol) (obtained through the process as described in Example 1), 3,6,9,12-tetraoxatridecanol (2.1 g, 0.01 mol) and p-toluenesulfonic acid (1.9 g, 0.01 mol) in 80 ml of toluene was heated at reflux temperature for 7.0 hours under a Dean-Stark trap. The reaction mixture was evaporated in vacuo to give a semi-solid residue which was partitioned between 50 ml of chloroform and 50 ml of water. The organic layer was dried over anhydrous sodium sulfate, filtered and evaporated in vacuo to give a pale yellow oil which was chromatographed on silica gel, eluting with ethyl acetate to give 3.9 g (82.5%) of the desired refined compound as a colorless oil.

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