[F-].[K+].[CH3:3][O:4][CH:5]([O:12][CH3:13])/[CH:6]=[CH:7]\[C:8]([O:10][CH3:11])=[O:9].[C:14]([O:20][C:21]([CH3:24])([CH3:23])[CH3:22])(=[O:19])[CH2:15][C:16]([CH3:18])=[O:17]>C(O)(C)(C)C>[C:21]([O:20][C:14]([CH:15]([C:16](=[O:17])[CH3:18])[CH:6]([CH:5]([O:12][CH3:13])[O:4][CH3:3])[CH2:7][C:8]([O:10][CH3:11])=[O:9])=[O:19])([CH3:24])([CH3:23])[CH3:22]Into a 500-ml reactor are placed 40 g of potassium fluoride, 40 ml of dry tert-butanol, 123 g of methyl cis-4,4-dimethoxy-2-butenate and 36 g of tert-butyl acetoacetate. The mixture is heated at 100° C. with stirring on an oil bath for two days. On completion of the reaction, the tert-butanol is distilled off from the mixture. The residue is dissolved in ethyl acetate, and the solution washed with an aqueous solution of common salt and then dried. The solvent is removed from the product. The resulting residue is purified by a silica gel column and distilled at reduced pressure, giving methyl 4-tert-butoxycarbonyl-3-dimethoxymethyl-5-oxohexanoate (compound (5), R1 =t-Bu, R2 =R3 =CH3) in a yield of 95.4%, b.p. 72°-76° C./0.014 mm Hg.
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