1. The submitters used a purified grade of bromoform purchased from the Fisher Chemical Company.

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° (C₂H₅OH, 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.

3. These phase-transfer conditions are adapted from experimental procedures described earlier.^3,4

4. The dibromocyclopropane is light-sensitive when hot. Exposure to light during distillation produces colored impurities that cause autocatalytic decomposition of the product when subsequently stored in the cold.

5. Both distillations must be performed with a pot temperature below 150°C in order to avoid thermal decomposition; ¹H NMR indicates the product to be a 4 : 1 mixture of diastereomers. All available evidence denotes that both are transformed efficiently into the cyclopentadiene.

6. The ether was freshly distilled from sodium benzophenone ketyl. The checkers used anhydrous ether from a freshly opened can.

7. The methyllithium was purchased from the Aldrich Chemical Company, Inc. and contains lithium bromide.

8. Solvent evaporation was accomplished at 40 mm and 25°C in order to counter product volatility.

9. The checkers used a 1 × 1-in. plug of alumina. The experience of the submitters has been that the use of silica gel at this point causes some decomposition.

10. The checkers found foaming to be a serious problem in this distillation. The problem is ameliorated by use of a ≥50-mL distillation flask.

11. Purified diene polymerizes within 24 hr if stored neat. Its lifetime can be indefinitely prolonged by storage as a 10% by weight solution in hexane under an argon atmosphere.

12. The product exhibits [α]_D24 −21.9° (C₂H₅OH, c 1.8) and the following ¹H NMR spectrum at 300 MHz in CDCl₃ 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).