Boron trifluoride etherate → 2,3,4,5-Tetrahydro-1-benzoxepin-4-one

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To a solution of 4-chromanone (1 eq, 16.8 mmol, 2.5 g) in anhydrous diethyl ether (70 ml) at 0° C., boron trifluoride diethyl etherate (1.1 eq, 18.6 mmol, 2.4 ml) was added added dropwise followed by the dropwise addition of 2M TMS diazomethane in ether (1.1 eq, 18.6 mmol, 9.2 ml), and stirred at 0° C. under N2 for 1 hr. Saturated NaHCO3 aq was carefully added (100 ml) and the reaction extracted into diethyl ether, the organic layer was then washed with water before drying over MgSO4, filtering and absorbing onto silica. The crude material was then purified on the ISCO companion system running a gradient from 0% ethyl acetate/petrol to 20% ethyl acetate/petrol. This gave 2,3-dihydro-5H-benzo[b]oxepin-4-one as a yellow oil (1.15 g, yield 42%).

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