[CH:1]1([C@@H:6]([C:10]2[CH:15]=[CH:14][C:13]([CH3:16])=[CH:12][CH:11]=2)[C:7]([OH:9])=[O:8])[CH2:5][CH2:4][CH2:3][CH2:2]1.S(=O)(=O)(O)O.[CH3:22][C:23]([CH3:25])=[CH2:24]>ClCCl>[CH:1]1([C@@H:6]([C:10]2[CH:15]=[CH:14][C:13]([CH3:16])=[CH:12][CH:11]=2)[C:7]([O:9][C:23]([CH3:25])([CH3:24])[CH3:22])=[O:8])[CH2:5][CH2:4][CH2:3][CH2:2]1160 g (732.9 mmol) of (2S)-cyclopentyl-(4-methylphenyl)acetic acid were initially charged in 480 ml of dichloromethane, and 2.1 ml of sulfuric acid and 172 ml (1722.4 mmol) of condensed 2-methylprop-1-ene were added at 10° C. The mixture was stirred at RT overnight. If required, the addition of sulfuric acid and 2-methylprop-1-ene was repeated until all the starting material had been consumed. After the reaction had ended, 21 g of potassium carbonate were added and the mixture was stirred for another 2-3 h (evolution of gas). The mixture was then diluted with 700 ml of water and extracted with ethyl acetate. The organic phase was dried over sodium sulfate, filtered and concentrated on a rotary evaporator. The crude product was purified by chromatography on silica gel (mobile phase cyclohexane/ethyl acetate 4:1). This gave 172 g (85% of theory) of the title compound.
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