Hydrochloric Acid → 1-Cyclopropyl-butane-1,3-dione

Magnesium turnings (3.04 g, 125 mmol), suspended in methanol (145 ml), were heated to reflux under nitrogen for 1 hour, then cooled to room temperature and the β-keto acid from Preparation 2 (25.5 g, 250 mmol) dissolved in methanol (25 ml) was added dropwise, with ice-cooling. The reaction mixture was stirred for 1 hour, at room temperature, and then the solvent was removed under reduced pressure to give the magnesium salt of the acid. Meanwhile, cyclopropane-carboxylic acid (9.91 ml, 125 mmol) was dissolved in dimethylformamide (200 ml). Carbonyldiimidazole (22.4 g, 138 mmol) was then added portionwise, under nitrogen, at 0° C. This reaction mixture was stirred for 1.5 hours, and then the magnesium salt from above was added as a solution in N,N-dimethylformamide (100 ml) at 0° C. The reaction mixture was allowed to stir at room temperature for 92 hours, and then it was poured into 2M aqueous hydrochloric acid (85 ml), followed by dilution with water (170 ml). The mixture was extracted with diethyl ether (6×200 ml), and the combined organic extracts were then washed with brine (3×200 ml), dried over magnesium sulphate and concentrated under reduced pressure. The residual orange oil was purified by flash chromatography on silica gel eluting with pentane:diethyl ether (100:0 then 90:10 then 80:20, by volume) to provide the title compound (7.39 g, 24%) as a yellow oil. 1H NMR (400 MHz, CDCl3): δ=0.83-0.95 (m, 2H), 1.06-1.10 (m, 2H), 1.54-1.63 (m, 1H), 2.00 (s, 3H); LRMS (electrospray): m/z 149 [MNa+].