Phosphoric Acid → 2,3,4-Trimethylhex-2-enoic acid

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Using the procedure of example 1 and the general procedure of H. Vieregge, H. M. Schmidt, J. Renema, H. J. T. Bos and J. F. Arends (Recl. Trav. Chim. Pays-Bas 1966, 85(9-10), 929-951], ethyl 2,3,4-trimethylhex-2-enoate was prepared by the addition of 3-methylpentan-2-one to 1-ethoxypropyne in 46 percent yield. 5.83 g (31.7 mmol) of this ethyl 2,3,4-trimethylhex-2-enoate were dissolved in 100 ml of ethanol/water (1:1) and then 2.93 g (44.4 mmol) of 86 percent strength potassium hydroxide were added. After the mixture had been refluxed for 3 hours, a further 2.93 g (44.4 mmol) of 86 percent strength potassium hydroxide were added, and the reaction mixture was then refluxed for a further 3 hours. After cooling, 300 ml of water were added and the mixture was acidified with concentrated phosphoric acid. The aqueous phase was extracted with 400 ml of ether, and the organic phases were dried and evaporated to dryness on a rotary evaporator. After flash chromatography (pentane/ether, 4:1) over silica gel, 3.77 g (76%) of 2,3,4-trimethylhex-2-enoic acid were isolated, which were taken up in 50 ml of dry tetrahydrofuran. At −10° C., 15.1 ml (24.2 mmol) of 1.6 M butyllithium solution in tetrahydrofuran were added dropwise under nitrogen. After the mixture had been stirred for 15 minutes at this temperature, 30.0 ml (24.2 mmol) of propenyllithium solution in ether were added dropwise over the course of 10 minutes. The cooling was removed and the reaction mixture was heated to 30° C. It was stirred for 1 hour at this temperature, then cooled to 0° C., and 4 ml of acetone, followed by 50 ml of saturated ammonium chloride solution, were added dropwise. After the reaction mixture had been heated to room temperature, it was diluted with 200 ml of water, and the product was extracted by washing three times with 400 ml of ether. The organic extracts were combined, dried and evaporated on a rotary evaporator, and the crude product was subjected to flash chromatography over silica gel with 4 l of pentane/ether (49:1) and 2 l of pentane/ether (19:1). As a result, 0.67 g (15%) of (2E,5E)-5,6,7-trimethylnona-2,5-dien-4-one (4) was isolated as a colorless liquid having an intensive and characteristic odor.

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