Reaction Steps
Pent-2-ynoic acid (1.50 g, 15.29 mmol) was dissolved in conc. acetic acid (15 ml), finely powdered sodium iodide (6.88 g, 45.87 mmol) was added and the mixture was stirred at a temperature of 110° C. for 3 h. After cooling to room temperature, methyl tert-butyl ether (MTBE) and saturated sodium thiosulfate solution were added. The aqueous phase was extracted repeatedly with MTBE, and 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), it was possible to obtain (2Z)-3-iodopent-2-enoic acid (2.10 g, 61% of theory) in the form of a colorless solid. In a round-bottom flask under argon, (2Z)-3-iodopent-2-enoic acid (500 mg, 2.21 mmol) was then dissolved in abs. dichloromethane, and oxalyl chloride (0.16 ml, 1.88 mmol) was added dropwise. Following the addition of catalytic amounts of N,N-dimethylformamide, the reaction solution was stirred at a temperature of 60° C. for 3 h, and after cooling to room temperature, n-propylamine (78 mg, 1.33 mmol) and triethylamine (0.19 ml, 1.33 mmol) were added dropwise. After addition of water and dichloromethane, the aqueous phase was extracted repeatedly with dichloromethane and 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), it was possible to obtain (2Z)-3-iodo-N-propylpent-2-enamide (230 mg, 74% of theory) in the form of a colorless solid. 1H-NMR (400 MHz, CDCl3 δ, ppm) 6.27 (s, 1H), 5.73 (br. t, 1H, NH), 3.34 (m, 2H), 2.65 (q, 2H), 1.60 (m, 2H), 1.12 (t, 3H), 0.97 (t, 3H). Subsequently, copper(I) iodide (30 mg, 0.16 mmol) and bis(triphenylphosphine)palladium(II) chloride (84 mg, 0.12 mmol) were initially charged under argon in a baked-out round-bottom flask, and abs. toluene (4 ml) and (2Z)-3-iodo-N-propylpent-2-enamide (214 mg, 0.80 mmol) were added. Stirring at room temperature for 10 minutes 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 (200 mg, 0.80 mmol) in abs. toluene (1 ml) and of diisopropylamine (0.22 ml, 1.60 mmol). The resulting reaction mixture was stirred at room temperature for 4 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), (2Z)-3-ethyl-5-(8-hydroxy-2,3,7,9,9-pentamethyl-1,4-dioxaspiro[4.5]dec-6-en-8-yl)-N-propylpent-2-en-4-ynamide (280 mg, 85% of theory) was isolated in the form of a colorless, highly viscous oil. 1H-NMR (400 MHz, CDCl3 δ, ppm) 6.83 (br. t, 1H, NH), 5.99 (s, 1H), 5.42 (s, 1H), 4.23 (m, 1H), 3.59 (m, 1H), 3.29 (m, 2H), 2.29 (q, 2H), 2.21 (br. s, 1H, OH), 2.02 (d, 1H), 1.92 (s, 3H), 1.85 (d, 1H), 1.57 (m, 2H), 1.23 (m, 3H), 1.19-1.12 (m, 12H), 0.92 (t, 3H).
