[CH2:1]([O:4][CH:5]1[CH2:10][CH2:9][CH2:8][CH2:7][O:6]1)[C:2]#[CH:3].C([Mg]Br)C.[O:15]1[CH2:20][CH2:19][C:18](=[O:21])[CH2:17][CH2:16]1>C1COCC1>[OH:21][C:18]1([C:3]#[C:2][CH2:1][O:4][CH:5]2[CH2:10][CH2:9][CH2:8][CH2:7][O:6]2)[CH2:19][CH2:20][O:15][CH2:16][CH2:17]1Tetrahydro-2-(2-propynyloxy)-2H-pyran (21 g, 150 mmol) was converted to the corresponding magnesium anion by deprotonation with ethyl magnesium bromide (75 mL of a 2M solution, 150 mmol) according to the method described in Org. Synth., 60: 81, 7 (1981). The resulting anion was cooled to -20° C. and tetrahydro-4H-pyran-4-one (14.8 g, 148 mmol) in dry THF (30 mL) was added dropwise and the resulting solution stirred for three hours. The reaction was quenched by addition of crushed ice and saturated aqueous NH4Cl. The resulting two-layered mixture was extracted with ether (3×150 mL). The combined organic layers were dried over MgSO4, filtered, and concentrated in vacuo. Purification by chromatography on silica gel (200 g, 20% ethyl acetate: hexanes) provided 4-hydroxy-4-[3-(tetrahydropyran-2-yloxy)prop-1-ynyl]tetrahydropyran(31.4 g, 88%).
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