N#N.[OH:3][CH2:4][C:5]1[O:9][C:8]([CH:10]=[O:11])=[CH:7][CH:6]=1.C1(C)C=CC(S([O-])(=O)=O)=CC=1.[NH+]1C=CC=CC=1.[O:29]1[CH:34]=[CH:33][CH2:32][CH2:31][CH2:30]1>C(Cl)Cl.O>[O:29]1[CH2:34][CH2:33][CH2:32][CH2:31][CH:30]1[O:3][CH2:4][C:5]1[O:9][C:8]([CH:10]=[O:11])=[CH:7][CH:6]=1In a flame dried round-bottomed flask under inert atmosphere (N2), to a mixture of 5-hydroxymethyl-2-furaldehyde (100 g, 0.79 mol) and pyridinium toluene-4-sulfonate (10 g, 0.04 mol) in CH2Cl2 (1 L) was added 3,4-dihydro-2H-pyran (150 mL, 1.62 mol) while keeping the internal temperature below 28° C. (water bath). The reaction mixture was stirred at rt for 5 h. Water (1 L) was added, the layers separated and the org. layer washed with water (500 mL) and evaporated to dryness to give crude 5-(tetrahydro-pyran-2-yloxymethyl)-furan-2-carbaldehyde as a yellow oil.
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