2-Methyl-2-phenyl- (CAS no.: ), which is also known as 4-Pentenal, 2-methyl-2-phenyl-, could be produced through the following synthetic routes.

A. N-Bromomethylphthalimide. A 1-L, three-necked, round-bottomed flask is fitted with a mechanical stirrer, a 125-mL pressure-equalizing dropping funnel, and a thermometer. The flask is charged with 50.0 g (0.28 mol) of N-hydroxymethylphthalimide and 200 mL of 48% aqueous hydrobromic acid. The flask is immersed in an ice bath, and 75 mL of concentrated sulfuric acid is added with stirring over a period of about 15 min. On completion of the addition, the flask is removed from the ice bath, heated at 60–70°C for 5 hr, and then cooled overnight in a refrigerator. The solid is collected by suction filtration using a 125-mm glass funnel with a coarse frit. The crude product is washed thoroughly with three 100-mL portions of cold water, two 50-mL portions of cold 10% aqueous ammonium hydroxide, and finally with three 100-mL portions of cold water. The crude product thus obtained is completely dried under reduced pressure at room temperature over phosphorus pentoxide to give 57.1–63.8 g (85–95%) of N-bromomethylphthalimide as a light-tan solid, mp 142–147°C. Although the material thus obtained may be used in the next step without further purification, it may also be recrystallized from dry acetone, mp 147–148°C (lit. mp 148°C, 148–149°C).

B. Diethyl phthalimidomethylphosphonate. A 500-mL, one-necked, round-bottomed flask equipped with a magnetic stirring bar and an efficient reflux condenser (ca. 80 cm long) is charged with 51.2 g (0.21 mol) of dry N-bromophthalimide and 43.1 g (0.26 mol) of freshly distilled triethyl phosphite. The mixture is immersed in an oil bath and the temperature of the oil bath gradually increased over about 15 min to 85–100°C, whereupon the solid dissolves and a vigorous, exothermic reaction ensues. When the reaction has subsided, the oil bath is lowered and the condenser is removed. The flask is fitted for simple distillation, and ethyl bromide and other volatile materials are distilled from the reaction mixture over a period of 3–4 hr by continued heating at 115°C (oil bath). The resulting light-yellow oil is cooled to room temperature, whereupon it solidifies. The crude product is collected by suction filtration and washed with three 100-mL portions of cold petroleum ether (bp 60–68°C) to give 50.1–56.3 g (80–90%) of diethyl phthalimidomethylphosphonate as white crystals, mp 60–63°C, which are used in the next step without further purification. Recrystallization of this material from diethyl ether/petroleum ether (bp 60–68°C) affords pure 3, mp 65–67°C (lit., 67°C).

C. Diethyl N-benzylideneaminomethylphosphonate. A 2-L, two-necked, round-bottomed flask is equipped with a mechanical stirrer and a 125-mL pressure-equalizing dropping funnel fitted with a calcium chloride drying tube. The flask is charged with 50.0 g (0.17 mol) of diethyl phthalimidomethylphosphonate dissolved in 750 mL of absolute ethanol. To this solution is then added 6.4 g (0.19 mol) of 95% hydrazine in 50 mL of absolute ethanol, and the resulting mixture is stirred overnight at room temperature. The dropping funnel is replaced with a reflux condenser bearing a calcium chloride drying tube, and the mixture is heated at reflux for 4 hr and then cooled to 0–5°C in an ice bath. The precipitated phthalhydrazide is collected by suction filtration and thoroughly washed with three 125-mL portions of benzene. The excess solvents and hydrazine are completely removed under reduced pressure on a rotary evaporator and then under high vacuum (0.1 mm). The crude diethyl aminomethylphosphonate thus obtained is dissolved in 350 mL of reagent-grade benzene, and the solution is cooled overnight in the refrigerator. Any additional phthalhydrazide that precipitates is removed by suction filtration and washed with two 25-mL portions of benzene. The filtrate and washings are combined in a 1-L, one-necked flask equipped with magnetic stirring bar, and the solution is cooled to 5–10°C, at which time 21.2 g (0.20 mol) of freshly distilled benzaldehyde is added in one portion with stirring. The mixture is stirred for 4 hr at room temperature, the flask is fitted with a Dean–Stark trap and a reflux condenser and heated overnight at reflux with constant removal of water. The solution is cooled to approximately room temperature, and the excess solvents are removed under reduced pressure. The crude product is purified by vacuum distillation, bp 145–149°C (0.05 mm), to give 37.5–40.2 g (84–90%) of pure diethyl N-benzylideneaminomethylphosphonate as a light-yellow oil.

D. 2-Methyl-2-phenyl-4-pentenal. A dry, 100-mL, three-necked, round-bottomed flask with 14/20 joints is fitted with a magnetic stirrer, a reflux condenser, and a rubber septum. The flask is charged with 50 mL of anhydrous tetrahydrofuran and cooled to -78°C in a dry ice–isopropyl alcohol bath, and a solution of butyllithium (12.0 mmol) in hexane is added with stirring. To this stirred solution is added dropwise via syringe a solution of 3.06 g (12.0 mmol) of diethyl N-benzylideneaminomethylphosphonate in 5 mL of anhydrous tetrahydrofuran, and the colored solution is stirred an additional hour at -78°C. A solution containing 1.20 g (10.0 mmol) of freshly distilled acetophenone in 5 mL of anhydrous tetrahydrofuran is added dropwise, and the cooling bath is removed. The solution is stirred for 1 hr at room temperature and then at reflux for 2 hr. After the solution is cooled to room temperature, it is poured into a 250-mL, round-bottomed flask and the solvents are removed under reduced pressure on a rotary evaporator. The yellow residue is partitioned between 50 mL of ether and 50 mL of saturated sodium chloride. The layers are separated and the aqueous phase is extracted with three 25-mL portions of ether. The combined organic layers are washed with 50 mL of saturated sodium chloride and dried over sulfate. Magnesium sulfate is removed by filtration, and the excess solvents are then completely removed under reduced pressure on a rotary evaporator. The resulting yellow solid is dried under reduced pressure and transferred to a 100-mL, two-necked, round-bottomed flask that is fitted with a magnetic stirring bar, a nitrogen inlet, and a rubber septum. The flask is charged with 50 mL of anhydrous tetrahydrofuran and flushed thoroughly with dry nitrogen. The resulting solution of the 2-azadiene is cooled to -78°C, and a solution of butyllithium (12.0 mmol) in hexane is added dropwise via syringe. The deeply colored solution is stirred at -78°C for 1 hr, at which time 1.81 g (15.0 mmol) of freshly distilled allyl bromide is added. The cooling bath is removed, and the solution is stirred for 2 hr at room temperature. The reaction is added to 50 mL of 3 N aqueous hydrochloric acid, and the resulting heterogeneous mixture is stirred vigorously for 18 hr at room temperature. After the addition of 50 mL of saturated sodium chloride, the layers are separated, and the aqueous layer is extracted with three 75-mL portions of ether. The combined organic layers are washed with 75-mL portions of saturated aqueous sodium bicarbonate and saturated sodium chloride, and the washings are back washed with a 50-mL portion of ether. The combined organic layers are dried over magnesium sulfate, and the excess solvents are removed under reduced pressure on a rotary evaporator. Distillation of the resulting yellow oil under reduced pressure gives 1.30–1.45 g (75–83%) of pure 2-methyl-2-phenyl-4-pentenal as a colorless liquid, bp 70–73°C (0.1 mm).