A
3-l., three-necked, round-bottomed flask is fitted with a nitrogen-inlet tube, a pressure-equalizing addition funnel, and an air-driven stirring apparatus. After the system has been thoroughly purged with
nitrogen, it is charged with a solution of
100 g. (0.615 mole) of 1,2,3,4,5,6-hexamethylbicyclo[2.2.0]hexa-2,5-diene (Hexamethyldewarbenzene (Note 1)) in
200 ml. of toluene. The reaction mixture is cooled to 0° with a
bath of ice water and stirred rapidly while a solution of
130 g. (0.640 mole) of 85% m-chloroperbenzoic acid (Note
2) in
1.5 l. of chloroform is added dropwise over 3–4 hours. Throughout the addition and for 4 hours following its completion the reaction mixture is kept at 0°, after which it is stirred at room temperature for 36 hours. The white precipitate that forms during the reaction (mainly
m-chlorobenzoic acid) is removed by filtration through a
650-ml., sintered-glass funnel (porosity 10–15 μm), and the filter cake is washed with two
100-ml. portions of chloroform. Combination of the filtrates gives a chloroform–toluene solution, which is condensed on a
rotary evaporator with the
water bath at 35–40°. When all of the
chloroform has been removed, the residual
toluene solution is diluted with
500 ml. of pentane and refiltered. The filtrate is washed with four
200-ml. portions of 10% aqueous sodium hydroxide and two 250-ml. portions of water, dried over
anhydrous magnesium sulfate, and evaporated to dryness on a rotary evaporator with the
water bath at 50°. Traces of
toluene are removed under high vacuum (0.02 mm.) at 25°, and the residue is distilled (Note
3). Collection of material boiling between
72° and 95° (4 mm.) gives about
50 g. of crude product, which is further purified by dissolving it in
50 ml. of pentane and cooling the
pentane solution at −78° for several hours. The resulting white crystals are collected by filtration and dried, giving
31.3–37.5 g. (
29–34%) of pure
5-acetyl-1,2,3,4,5-pentamethylcyclopentadiene, m.p.
54–56° (Note
4) and (Note
5).