Acetic Anhydride → Ethyl 5-acetylamino-4,5,6,7-tetrahydro-3-methylthio-4-oxobenzo[c]thiophene-1-carboxylate

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A mixture of ethyl 5-azido-4,5,6,7-tetrahydro-3-methylthio-4-oxobenzo[c]thiophene-1-carboxylate (3.62 g), palladium-carbon (5%, 50% wet, 4.0 g), acetic acid (70 ml) and acetic anhydride (35 ml) was subjected to a catalytic reduction reaction at room temperature under 1 atm. The insoluble substances were filtered off; the filtrate was concentrated under reduced pressure; the residue was dissolved in ethyl acetate-tetrahydrofuran (3:1, v/v). This solution was washed with water, a saturated aqueous solution of sodium hydrogen carbonate and water in that order, and dried (MgSO4), after which the solvent was distilled off to yield ethyl 5-acetylamino-4,5,6,7-tetrahydro-3-methylthio-4-oxobenzo[c]thiophene-1-carboxylate (2.81 g, 73.8%), which was then recrystallized from chloroform-ethanol. Colorless needles. Melting point 215-216° C.

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