Tributylphosphine → 3,3-difluoro-3,4-dihydro-2H-thieno[3,4-b][1,4]-dioxepine-6,8-dicarboxylic acid diethyl ester

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3,3-Difluoro-3,4-dihydro-2H-thieno[3,4-b][1,4]-dioxepine-6,8-dicarboxylic acid diethyl ester was synthesized by adding dropwise 3.4 mL (17.2 mmol) of azodicarboxylic acid diisopropyl ester (ADC) to a mixture of 1.77 g (6.67 mmol) of 3,4-dihydroxythiophen-2,5-dicarboxylic acid diethyl ester, 748 mg of 2,2-difluoro-1,3-propanediol and 4.3 mL of tributylphosphine in 12 mL of absolute tetrahydrofuran under argon at 20° C. with ice-cooling. After completion of the addition of ADC the resulting mixture was allowed to stand at room temperature for 4 days. The tetrahydrofuran was then distilled off and the residue heated for 12.5 hours at 100° C. after which it was dissolved in ethyl acetate. The ethyl acetate solution was washed three times with saturated sodium bicarbonate solution, then with deionized water, dried with anhydrous magnesium sulphate, concentrated and the residue chromatographically purified on a kieselgel 60 column with heptane/ethyl acetate as eluant. 335 mg (corresponding to a yield of 15%) of the desired product was obtained with a melting point of 121–122° C., as characterized by:

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