To a cold −45° C. solution of dimethyl(2S)-5-methoxypyrrolidine-1,2-dicarboxylate (3.30 g, 15.20 mmol) and bistrimethylsilylacetylene (5.20 g, 30.4 mmol, 2.0 equiv) in methylene chloride (45 mL) was added a solution of tin (IV) chloride (1 M in methylene chloride, 20.0 mL, 20.0 mmol, 1.3 equiv) dropwise via syringe over 15 minutes. To the dark yellow solution was added solid aluminum chloride (2.77 g, 20.8 mmol, 1.4 equiv) in one portion. The resulting mixture was allowed to warm to room temperature and stirred at room temperature for 48 hours. The reaction mixture was carefully poured into aqueous sodium bicarbonate solution (100 mL) with ice cooling. A white precipitate forms and 1 M HCl (ca. 50 mL) was added until the solids dissolved. This mixture was extracted with ethyl acetate (2×200 mL). The combined organic layers were filtered, dried (sodium sulfate), filtered, and concentrated. The residue was chromatographed on a Biotage flash 40 M eluting with 70% hexane/30% ethyl acetate to afford (2S,5S)-5-((trimethylsilyl)ethynyl)pyrrolidine-1,2-dicarboxylate (trans compound Rf of 0.3 in 70% hexane/30% ethyl acetate) and dimethyl(2S,5R)-5-((trimethylsilyl)ethynyl)pyrrolidine-1,2-dicarboxylate (cis compound Rf of 0.2 in 70% hexane/30% ethyl acetate). Data for example ID: MS (DCI/NH3) m/e 284 (M+H)+; The compound exists as a mixture of rotomers. 1H NMR (300 MHz, CDCl3): δ ppm 4.60 (d, 1H), 4.51 (d, 1H), 4.30 (d, 1H), 4.24 (d, 1H), 3.62 (s, 3H), 3.59 (s, 3H), 3.57 (s, 3H), 3.54 (s, 3H), 2.40-2.28 (m, 2H), 2.11-2.04 (m, 2H), 1.90-1.81 (m, 4H), 0.0 (s, 18H). Data for example 1C: MS (DCI/NH3) m/e 284 (M+H)+; 1H NMR (300 MHz, CDCl3): δ ppm 4.55-4.40 (m, 1H), 4.20-4.15 (m, 1H), 3.59 (s, 6H), 2.15-1.89 (m, 4H), 0.00 (s, 9H).
