Organic Syntheses, CV 6, 774
Submitted by U. Hengartner
1 and Vera Chu.
Checked by William B. Farnham and William A. Sheppard.
1. Procedure
A
3-l., three-necked, round-bottomed flask fitted with a
dropping funnel (Note
1), a
mechanical stirrer, and a distillation head with a
thermometer and
efficient Liebig condenser is charged with
1.4 l. of xylene (Note
2). The
xylene is stirred and heated to boiling with a heating mantle, while 179 g. of a solution containing
43 g. (0.80 mole) of sodium methoxide in
methyl alcohol (Note
3) is added over 20 minutes. During this period 450 ml. of solvent is distilled.
After addition is complete,
300 ml. of xylene is added and the distillation continued until the vapor temperature rises again to 138°. During that time an additional 250 ml. of distillate is collected, leaving a white suspension to which
18 ml. of dimethylsulfoxide is added. A solution of
100 g. (0.633 mole) of ethyl 4-oxohexanoate (Note
4) in
200 ml. of xylene is then added (Note
1) to the vigorously stirred
sodium methoxide suspension over 25 minutes, while 900 ml. of distillate is collected continuously, maintaining the vapor temperature at 134–137°. The orange-colored mixture is stirred and heated for an additional 5 minutes, then cooled to room temperature. Addition of 165 ml. of water with vigorous stirring over a 5-minute period (Note
5) gives two clear phases, which are cooled in an
ice bath and acidified by adding
82 ml. (0.98 mole) of 12 N hydrochloric acid with vigorous stirring. After the mixture is stirred at 0° for another 1.5 hours, the crystalline product is collected by suction filtration and carefully washed successively with
100-ml. and 50-ml. portions of ice-cooled diethyl ether (Note
6).
The crude product is dissolved in 1 l. of boiling water, and the solution is filtered quickly through a preheated
fritted-disk funnel (Note
7). The filtrate is concentrated on a hot plate at atmospheric pressure to a volume of 550–600 ml. and allowed to stand at 0° overnight. The crystals are collected by filtration and dried at 85°, yielding
50.0–50.6 g. (
70–71%) of
2-methyl-1,3-cyclopentanedione, m.p.
210–211° (Note
8).
2. Notes
1. A
500-ml. dropping funnel, provided with a
calcium sulfate drying tube, was used.
2.
Reagent grade xylene, b.p. 138–141°, from Fisher Scientific Company, was used.
3. The
sodium methoxide solution was prepared as follows:
203 g. of methyl alcohol, available from Fisher Scientific Company, is placed in a
500-ml., two-necked flask under an inert atmosphere. The flask is equipped with a
magnetic stirring bar and a
reflux condenser provided with a calcium sulfate drying tube. Freshly cut, clean
sodium (23 g., 1.0 mole) is added in small pieces at such a rate that reflux is maintained. The mixture is stirred until all the
sodium has reacted.
5. The temperature of the mixture is kept at 25–35°.
6. The
ether washings contain 11 g. of a brown viscous oil containing various condensation products.
7. A small amount of insoluble, tarry material is removed by this filtration. Preheating the funnel is necessary, since the product crystallizes easily on cooling. Dark-colored impurities in crude
2-methyl-1,3-cyclopentanedione can be removed by recrystallization from
methanol.
8.
2-Methyl-1,3-cyclopentanedione, [
Org. Synth., Coll. Vol. 5, 747 (1973)] m.p.
210–212°, exists in the enol form in solution. It has the following spectral properties: UV (0.1
N HCl) nm. max. (ε): 252 (19,000);
1H NMR (dimethyl sulfoxide-
d6) δ (multiplicity, number of protons, assignment): 1.51 (s, 3H, C
H3), 2.39 (s, 4H, 2C
H2).
3. Discussion
This preparation is referenced from:
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved