[CH2:1]([S:3]([C:5]1[O:6][C:7]2[C:12]([C:13](=[O:17])[C:14]=1[CH2:15][OH:16])=[CH:11][CH:10]=[CH:9][CH:8]=2)=[O:4])[CH3:2].[O:18]1[CH:23]=[CH:22][CH2:21][CH2:20][CH2:19]1.C1(C)C(S(O)(=O)=O)=CC=CC=1.O>ClCCl.C(OCC)(=O)C>[CH2:1]([S:3]([C:5]1[O:6][C:7]2[C:12]([C:13](=[O:17])[C:14]=1[CH2:15][O:16][CH:19]1[CH2:20][CH2:21][CH2:22][CH2:23][O:18]1)=[CH:11][CH:10]=[CH:9][CH:8]=2)=[O:4])[CH3:2]To 2-ethylsulfinyl-3-hydroxymethyl chromen-4-one (5 g, 19.8 mmol) dissolved in dichloromethane (20 mL) in a 25 mL round bottom flask is added dihydropyran (29.7 mmol) and toluenesulfonic acid (0.99 mol). The reaction mixture is stirred for 3 hours after which it is poured into a separatory funnel and water is added. Ethyl acetate is then added and the layers are separated. The organic layer is washed with water (3×10) and brine and dried over sodium sulfate and filtered. The filtrate is concentrated in vacuo and the residue obtained is purified by silica gel flash chromatography (ethyl ether: hexanes) to yield the above product.
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