[OH:1][CH:2]([CH3:10])/[CH:3]=[CH:4]/[C:5]([O:7][CH2:8][CH3:9])=[O:6].N1C=CC=CC=1.[CH3:17][O:18][C:19](Cl)=[O:20]>CN(C1C=CN=CC=1)C.C1COCC1>[CH3:17][O:18][C:19]([O:1][CH:2]([CH3:10])/[CH:3]=[CH:4]/[C:5]([O:7][CH2:8][CH3:9])=[O:6])=[O:20]rac-Ethyl (2E)-4-hydroxypent-2-enoate (21) (10.09 g, 70 mmol), DMAP (1.71 g, 14 mmol) and pyridine (17 mL, 210 mmol) are dissolved in THF (400 mL). The solution is cooled to 1° C. Methylchloroformate (16.1 mL, 210 mmol) in THF (50 ml) is then added dropwise over 3 hours under stirring at 1° C. After 2 hours methylchloroformate (199 ml, 2.50 mol) in THF (150 ml) is added dropwise in four portions. After every portion, the reaction mixture is stirred for 5 hours at 1° C. The reaction mixture is quenched with 10% HCl (0.5 l) and extracted with tBuOMe (1.5 and 0.5 l). The organic phases are washed with 10% HCl (0.6 l), saturated NaHCO3 (0.3 l) and brine (2×0.2 l) successively. The combined organic phases are dried with N2SO4 and evaporated under reduced pressure. The residue is purified by flash chromatography (tBuOMe:heptane fraction 15:85 to 25:75) and yields 1 (rac-Ethyl (2E)-4-(methoxycarbonyloxy)pent-2-enoate) (8.64 g, 61%) as a yellowish oil.
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