Methanesulfonic Acid → Ethyl 7-oxoazepane-4-carboxylate

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In a 250 ml three necked flask fitted with reflux condenser, magnetic stirrer and dropping funnel under inert atmosphere, 5.6 ml (5.98 g, 34 mmoles, 1 eq) of ethyl 4-oxocyclohexanecarboxylate 27 were dissolved in 50 ml of CHCl3. 6.65 g (102 mmole, 3 eq) of sodium azide were added, followed by 22.1 ml (32.8 g, 341 mmoles, 10 eq) of methanesulfonic acid dissolved in 20 ml of over 30 min. The mixture was heated to reflux for 1 hour and cooled down to 10° C. 200 nl of saturated sodium bicarbonate solution were added, the mixture decanted and the aqueous phase extracted with 3×150 ml of CHCl3, the combined organic phases were dried over magnesium sulfate, filtered and concentrated to dryness. The residue was purified by PrepLC (800 g SiO2, CH2Cl2/MeOH, 97/3) to give 5.1 g of crude compound. It was crystallised from 40 ml of diethylether to give 4.41 g of pure ester 28, 70% yield.

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