A. (Z)-2,3-Dibromo-2-propenoic acid ethyl ester.² Ethyl propiolate (9.01 g, 91.8 mmol) and carbon tetrachloride (75 mL) are added in sequence to a 250-mL, round-bottomed flask equipped with a Teflon-coated magnetic stirring bar (Note 1). The flask is fitted with a 30-mL, pressure-equalizing addition funnel to which elemental bromine (5.00 mL, 97.0 mmol) is added by transfer pipette (Note 2). The flask is partially immersed in an oil bath heated to 70°C. After 10 min for temperature equilibration, dropwise addition of bromine is initiated and completed within 10 min. Complete bromine transfer is achieved by rinsing the addition funnel with a 5-mL portion of carbon tetrachloride. The red-brown solution is heated at 70°C for 30 min, allowed to cool to ambient temperature, and carbon tetrachloride and excess bromine are removed by rotary evaporation. The pale yellow concentrate is applied to the top of a short column of flash-grade silica gel (150 g, 7-cm diam × 10 cm) loaded with 5% ethyl acetate-hexanes solution; the product dibromide elutes rapidly with this solvent system (< 1 L total solvent volume, 150-mL fractions, (Note 3)). Fractions containing the dibromide (R_f = 0.66, 5% ethyl acetate-hexanes) are combined and concentrated. The last traces of solvent are removed at 1 mm (0.5 hr) to afford (Z)-2,3-dibromo-2-propenoic acid ethyl ester (21.5–22.0 g, 91–93%) as a colorless liquid (Note 4), (Note 5).

B. (Z)-2-Bromo-5-(trimethylsilyl)-2-penten-4-ynoic acid ethyl ester. An oven-dried, 500-mL, modified Schlenk flask (Kjeldahl shape with 24/40 female top), equipped with a Teflon-coated magnetic stirring bar and rubber septum, is connected at the side-arm to a source of argon and vacuum. The flask is charged with (Z)-2,3-dibromo-2-propenoic acid ethyl ester prepared in part A (21.5 g, 83.4 mmol) and N,N-dimethylformamide (150 mL). With efficient stirring, it is alternately evacuated and flushed with argon at ambient temperature to deoxygenate the solution (Note 6), (Note 7). The flask is cooled in ice and (trimethylsilyl)acetylene (20.0 mL, 142 mmol) and N,N-diisopropylethylamine (24.7 mL, 142 mmol) are added in sequence (Note 8). The resulting pale yellow solution is deoxygenated as above. Cuprous iodide (3.18 g, 16.7 mmol) and tetrakis(triphenylphosphine)palladium (4.82 g, 4.17 mmol) are added, followed by a third deoxygenation cycle (Note 9). The brown reaction mixture is maintained at 0°C while the progress of reaction is monitored by thin layer chromatography (TLC), as follows: An aliquot (ca. 0.1 mL) of the reaction mixture is withdrawn with an argon-flushed hypodermic syringe and is injected into a small test tube containing ethyl ether (0.25 mL) and water (0.5 mL). After the biphasic mixture is swirled briefly, the ether layer is allowed to separate and is analyzed by TLC using 20% ethyl ether-hexanes as eluent (R_f of the starting dibromide = 0.67, of the product = 0.76, visualization with ultraviolet light). When TLC analysis indicates consumption of the starting dibromide, typically 3–6 hr at 0°C, the reaction mixture is poured into a 1-L separatory funnel containing water (300 mL), saturated aqueous ammonium chloride solution (75 mL), and the tetrasodium salt of ethylenediaminetetraacetic acid (4.50 g, (Note 10)). The resulting brown suspension is extracted with four 150-mL portions of 50% ethyl acetate-hexanes. The organic layers are combined and washed with three 100-mL portions of water, then dried over sodium sulfate and concentrated. The brown oil thus obtained is filtered through a short column of flash-grade silica gel (150 g, 7-cm diam × 10 cm), using 20% toluene-hexanes as eluent (< 1.5 L total solvent volume, 150-mL fractions). Fractions containing product (R_f = 0.23, 20% toluene-hexanes) are combined and concentrated. The light brown concentrate is combined with silicone oil (7.0 g) in a 250-mL, 14/20 round-bottomed flask. An insulated 6-in Vigreux column and a short-path distillation head are affixed for vacuum distillation. The system is evacuated (0.9 mm) and the pot is heated to 110°C whereupon bis(trimethylsilyl)butadiyne sublimes from the mixture. A heat gun is used to assist in the transfer of this component; the flow of condenser water is initiated only after this phase of the distillation. A liquid forerun is collected (ca. 0.5 mL, head temperature 94°C, pot temperature ~125°C) prior to collection of the product. (Z)-2-Bromo-5-(trimethylsilyl)-2-penten-4-ynoic acid ethyl ester is collected (bp 105–108°C, 0.9 mm, pot temperature < 125°C) as a pale yellow oil (10.95–12.85 g, 48–56%) and is shown by capillary GC to be 87–94% pure (contaminants include bis(trimethylsilyl)butadiyne, <2% and (E)-2-[(trimethylsilyl)ethynyl]-5-(trimethylsilyl)-2-penten-4-ynoic acid ethyl ester, 6–11%; (Note 11),(Note 12),(Note 13)).