Reaction Steps
In a round-bottom flask under argon, 2,2,6-trimethyl-1,4-cyclohexanedione (15.40 g, 101.19 mmol) was dissolved in 2,3-butanediol (90 ml) and abs. toluene (90 ml), and trimethyl orthoformate (33.21 ml, 303.56 mmol) and p-toluenesulfonic acid (1.22 g, 7.08 mmol) were added. The resulting reaction mixture was stirred at 50° C. for 7 h. After cooling to room temperature, water and toluene were added and the aqueous phase was extracted repeatedly with toluene. The combined organic phases were dried over magnesium sulfate, filtered and concentrated under reduced pressure. By column chromatography purification of the resulting crude product (ethyl acetate/heptane gradient), 2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-one (20.01 g, 88% of theory) was obtained. In a round-bottom flask under argon, 2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-one (10.00 g, 44.58 mmol) was then dissolved in abs. tetrahydrofuran (50 ml) and added dropwise to a solution of a lithium acetylide/ethylenediamine complex (6.28 g, 57.96 mmol, 85% pure) in abs. tetrahydrofuran (70 ml). On completion of addition, the reaction mixture was stirred at room temperature for 4 h, then water was added and the mixture was concentrated under reduced pressure. The remaining residue was admixed with water and dichloromethane, and the aqueous phase was extracted repeatedly with dichloromethane. The combined organic phases were dried over magnesium sulfate, filtered and concentrated under reduced pressure. By column chromatography purification of the crude product obtained (ethyl acetate/heptane gradient), 8-ethynyl-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (10.02 g, 85% of theory) was isolated as a colorless solid. Subsequently, copper(I) iodide (46 mg, 0.24 mmol) and bis(triphenylphosphine)palladium(II) chloride (126 mg, 0.18 mmol) were initially charged under argon in a baked-out round-bottom flask, and abs. toluene (8 ml) and ethyl (2Z)-3-iodohex-2-enoate (321 mg, 1.19 mmol) were added. Stirring at room temperature for 10 min was followed by the dropwise addition of a solution of 8-ethynyl-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-ol (300 mg, 1.19 mmol) in abs. toluene (2 ml) and of diisopropylamine (0.34 ml, 2.39 mmol). The resulting reaction mixture was stirred at room temperature for 3 h and then water was added. The aqueous phase was extracted repeatedly with dichloromethane. The combined organic phases were dried over magnesium sulfate, filtered and concentrated under reduced pressure. By final column chromatography purification of the crude product obtained (using an ethyl acetate/heptane gradient), ethyl (2Z)-3-[(8-hydroxy-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-yl)ethynyl]hex-2-enoate (440 mg, 94% of theory) was isolated in the form of a colorless oil. 1H-NMR (400 MHz, CDCl3 δ, ppm) 5.97 (s, 1H), 5.49 (s, 1H), 4.18 (m, 3H), 3.59 (m, 1H), 2.22 (m, 2H), 2.02 (m, 1H), 1.92 (m, 4H), 1.58 (m, 3H), 1.24 (t, 3H), 1.20-1.12 (m, 12H), 0.91 (t, 3H).
