2. The
(1R)-nopadiene is prepared from commercially available (Aldrich Chemical Company, Inc.) (1R)-(−)-nopol according to the following procedure.
2 A
1000-mL, three-necked flask is equipped with a mechanical stirrer,
internal thermometer and nitrogen inlet. The flask is charged with
125 g (0.752 mol) of (1R)-(−)-nopol and
500 mL of pyridine. Stirring is begun and the solution is cooled to −10°C in an
ice–salt bath under
nitrogen.
p-Toluenesulfonyl chloride (175 g, 0.918 mol) is added in one portion under an inert atmosphere via Gooch tubing (the checkers used a
powder funnel for the addition). The temperature rises to 40°C for 15–20 min, but returns to 5°C, where it is maintained for 2 hr. Twenty 1-mL portions of water are next introduced at such a rate that the temperature does not exceed 5°C. The reaction mixture is poured into
1 L of ether and extracted with ice-cold
5 M sulfuric acid until the aqueous layer remains acidic, then with saturated
CuSO4 solution until the aqueous layer remains blue. The ethereal phase is washed with two 200-mL portions each of water and
5% sodium bicarbonate solution prior to drying over
magnesium sulfate and solvent evaporation. A solid residue is obtained. If this material is dark, it may be dissolved in
hexane and filtered through a pad of Celite to remove the black impurity. The tosylate is recrystallized by dissolving it in
500 mL of hot hexane and cooling to −78°C. Six such recrystallizations give material with mp
51.0–51.8°C and
[α]D25 −25.6° (C
2H
5OH, c 0.03). The yield is
62–72%. A
2000-mL, three-necked flask is equipped with a mechanical stirrer, an internal thermometer, and a nitrogen inlet. The flask is charged with
200 g (0.624 mol) of (1R)-nopyl tosylate and 1000 mL of dimethyl sulfoxide that has been freshly distilled from
calcium hydride at 40 mm. The stirring solution is cooled briefly in a
cold-water bath and
69.0 g (0.615 mol) of freshly sublimed potassium tert-butoxide is added rapidly while
nitrogen is flowing above the solution (the checkers used
potassium tert-butoxide from a freshly
opened bottle). (The base must be the limiting reagent to offset isomerization of the product diene.) The temperature rises to approximately 45°C, and a brown color develops. As the reaction proceeds, the color dissipates to a light yellow. After the initial exotherm subsides, the mixture is heated at 75°C for 10 hr, cooled to room temperature, and diluted with
800 mL of hexane. The lower layer, mostly
dimethyl sulfoxide, is diluted with 1 L of water and extracted with two
100-mL portions of hexane. The combined
hexane layers are washed with water (5 × 200 mL), dried over
magnesium sulfate, and rotary-evaporated at 40 mm and 25°C to leave a yellow oil. Distillation through a
5-in. Vigreux column gives
69.4–74.0 g (
75–80%) of
(1R)-nopadiene as a clear, colorless oil, bp
78–79°C/25 mm.
12. The product exhibits
[α]D24 −21.9° (C
2H
5OH, c 1.8) and the following
1H NMR spectrum at 300 MHz in CDCl
3 solution δ: 0.72 (s, 3 H), 1.24 (m, 1 H), 1.33 (s, 3 H), 1.60 (s, 1 H), 2.11 (m, 1 H), 2.60 (m, 1 H), 2.70 (m, 2 H), 2.99 (s, 2 H), 5.77 (s, 1 H), 5.99 (s, 1 H).