A.
3,4,5-Trimethyl-2,5-heptadien-4-ol. Lithium wire (1/8 in.) is cut up into approximately 1-cm lengths and washed with
hexane (Note
1). A mixture of the cut-up
lithium (21 g, 3.0 mol) in 100 mL of diethyl ether from a freshly opened can is stirred well under
argon in a
2-L, three-necked, round-bottomed flask equipped with a
reflux condenser and a
250-mL addition funnel.
2-Bromo-2-butene (cis-trans mixture) is purified and dried by passing it through a
2 × 15-cm column of basic alumina (Note
2). An addition funnel is charged with
2-bromo-2-butene (200 g, 1.48 mol) and a small amount of the alkene is added dropwise to the flask until reaction begins, as shown by warming of the reaction mixture and formation of bubbles on the surface of the
lithium (Note
3). At this point the mixture is diluted with an additional
900 mL of fresh diethyl ether, and the remainder of the
2-bromo-2-butene is added at a rate sufficient to maintain gentle reflux. Stirring is continued for 1 hr following completion of this addition, after which
ethyl acetate (66 g, 0.75 mol) diluted with an equal volume of fresh
diethyl ether is added dropwise via the addition funnel. The reaction mixture turns from yellow-orange to milky-yellow with this addition. It is then poured into
2 L of saturated aqueous ammonium chloride. The ethereal layer is separated, and the aqueous layer is adjusted to approximately pH 9 with
hydrochloric acid. The aqueous layer is extracted three times with
diethyl ether. The combined ethereal layers are dried over
magnesium sulfate, filtered, and concentrated to 100–200 mL by rotary evaporation.
B.
1,2,3,4,5-Pentamethylcyclopentadiene. A mixture of
13 g (0.068 mol) of p-toluenesulfonic acid monohydrate and
300 mL of diethyl ether is stirred under
argon in a
1-L, three-necked, round-bottomed flask equipped with a reflux condenser and a 250-mL addition funnel. The concentrate from above is added as quickly as possible to the flask from the addition funnel, maintaining a gentle reflux. As the reaction proceeds, a water layer separates. The mixture is stirred for 1 hr after the addition is completed and then washed with saturated aqueous
sodium bicarbonate until the washings remain basic. The ethereal layer is separated, and the combined aqueous layers are extracted three times with
diethyl ether. The combined ethereal layers are dried over
sodium sulfate.
Diethyl ether is removed by rotary evaporation, and the crude product is distilled under reduced pressure (bp
55–60°C, 13 mm): yield
73–75 g (
73–75%) (Note
4).