Sulfuric Acid → Ethyl {4-[2-(allyloxy)-4-methylphenyl]-2-methyl-5,6,7,8-tetrahydro[1]benzothieno[2,3-b]pyridin-3-yl}(tert-butoxy)acetate

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To a stirred solution of ethyl {4-[2-(allyloxy)-4-methylphenyl]-2-methyl-5,6,7,8-tetrahydro[1]benzothieno[2,3-b]pyridin-3-yl}(hydroxy)acetate (Step 7, 720 mg, 1.59 mmol) in dichloromethane (2.5 mL) was added tert-butyl acetate (2.5 mL) followed by concentrated sulphuric acid (244 μL, 4.78 mmol) and the reaction was stirred at room temperature for 2 hours. The reaction mixture was quenched by addition of a 1 M aqueous sodium hydroxide solution until the solution was at pH 5. The volatile solvents were removed in vacuo, and the remaining aqueous layer was extracted with ethyl acetate (30 mL). The organic layer was washed with brine (10 mL), dried over MgSO4 and concentrated in vacuo. The residue was purified using ISCO Companion with a Redisep silica gel 12 g cartridge and a gradient of heptane and ethyl acetate (0% to 40%). Product containing fractions were concentrated in vacuo to give a mixture of diastereoisomers of the title compound as a colourless oil, 370 mg, in a 45% yield. Material is shown to be a mixture of diastereomers, in an approximately 80:20 ratio.

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