Boron trifluoride etherate → 2-Benzhydryl-2H-pyran-4(3H)-one

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A solution of boron trifluoride diethyl etherate (7.8 g, 55 mmol) in dry ether (50 ml) was added to a stirred mixture of E-1-methoxy-3-trimethylsilyloxybuta-1,3-diene (8.3 g, 48 mmol), diphenylacetaldehyde 1 (11.4 g, 58 mmol) and dry ether (300 ml) cooled to −78° C. After one hour, the mixture was allowed to reach 0° C. for three hours. The deep red reaction mixture was quenched with saturated aqueous NaHCO3, and the mixture was allowed to come to room temperature. The organic phase was separated and the aqueous phase was extracted with ether (3×70 ml). The combined organic phases were washed with brine, and dried over anhydrous Na2SO4. Evaporation of solvent under reduced pressure and purification of the crude product by chromatography (hexane/ethyl acetate 8:2) gave 2-diphenylmethyl-2,3-dihydro-4H-pyran-4-one 2 (10.2 g, 80.2%, yield) as a yellow solid.

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