Sodium hydride → 5-(Bicyclo[3.2.1]oct-2-ylidene)pentanoic acid

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A mixture of 3.92 g (93 mmol) of sodium hydride (57% oil dispersion) and 30 ml of dry dimethylsulfoxide was heated at 70°-80° under nitrogen until gas evolution ceased (2.5 hr). The mixture was cooled, and 19.7 g (44.3 mmol) of 4-(carboxybutyl)triphenylphosphonium bromide (Aldrich No. 15,794-5) was added, followed by 20 ml of dimethylsulfoxide. The resulting deep red mixture was stirred for 45 min at room temperature under nitrogen before a solution of 5.0 g (40.2 mmol) of bicyclo[3.2.1]octan-2-one (Aldrich No. 11,903-2) in 20 ml of dimethylsulfoxide was added dropwise over 1.75 hr. The resulting mixture was stirred for 16 hr at room temperature before it was quenched by the addition of 125 ml of water. The resulting solution was extracted with three 250 ml portions of ether/ethyl acetate-1:1 (V/V) to remove neutral by products. The remaining aqueous phase was acidified by the addition of 15 ml of concentrated hydrochloric acid and extracted with three 250 ml portions of ether-hexane-1:1 (V/V). These combined acidic ether-hexane extracts were dried (MgSO4), and evaporated in vacuo to yield 6.0 g of crude 5-(bicyclo[3.2.1]oct-2-ylidene)-pentanoic acid as an oil. This oil was purified by chromatography on silica gel 60 (elution with CHCl3 containing 0.5 % formic acid) to yield 4.1 g (49%) of pure material as a colorless oil.

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