[O:1]1[C:3]2([CH2:8][CH2:7][O:6][CH2:5][CH2:4]2)[CH2:2]1.[CH3:9][CH:10]([CH3:18])[C:11](=[O:17])[CH2:12][C:13](OC)=[O:14].[O-]CC.[Na+].Cl>C(O)C>[C:11]([CH:12]1[CH2:2][C:3]2([CH2:4][CH2:5][O:6][CH2:7][CH2:8]2)[O:1][C:13]1=[O:14])(=[O:17])[CH:10]([CH3:18])[CH3:9]6.6 g 1,6-Dioxaspiro[2.5]octane and 8.2 ml methyl 4-methyl-3-oxopentanoate are dissolved in 30 ml ethanol, cooled to 0° C. and treated in portions with 3.9 g sodium ethoxide. After completion of the addition the mixture is warmed to room temperature and stirred for 18 hours. Then the mixture is poured in 200 ml ice water, acidified to pH 3 by addition of 1 M hydrochloric acid and extracted for three times with ethylacetate. The combined organic phases are dried with sodium sulphate and the solvents are evaporated in vacuo. The residue is chromatographed on silica gel (petrole ether/ethylacetate 100:0 to 50:50).
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