In a
2-l. three-necked flask equipped with a
thermometer, a
mechanical stirrer, and a
dropping funnel (Note
1) is placed a mixture of
45.1 g. (0.45 mole) of a dihydroxycyclopentene mixture (Note
2), 200 ml. of water, and
300 ml. of methylene chloride. After this mixture has been cooled to −5° to 0° by means of an external
cooling bath (Note
3), the addition of a solution of
100 g. (1.0 mole) of chromium trioxide and
160 ml. of concentrated sulfuric acid in 450 ml. of water is begun. The solution of the oxidant is added dropwise, and with stirring, at such a rate that the temperature of the reaction mixture remains between −5° and 0°. After the addition is complete (Note
4), the mixture is stirred at −5° to 0° for 1 hour and then
200 ml. of chloroform is added. The resulting mixture is stirred for 10 minutes, the organic layer is separated, and then the aqueous layer is extracted with two
200-ml. portions of a mixture (1:1 by volume) of chloroform and methylene chloride. The combined organic extracts are washed with 100 ml. of water, dried over anhydrous
magnesium sulfate, and concentrated under reduced pressure at room temperature. The yield of
2-cyclopentene-1,4-dione (Note
5), which crystallizes as yellow plates melting at
30.0–32°, is
17–22 g. (
39–50% based on the
dihydroxycyclopentene mixture) (Note
6).