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General procedure for the synthesis of tert-butyl 1-oxa-5-azaspiro[2.3]hexane-5-carboxylate from 1-Boc-3-methyleneazetidine: To a solution of 1,1-dimethylethyl 3-chloromethyl-1,3-methylvinyl-1-carboxylate (2.96 g, 17.5 mmol) in chloroform (180 mL) was added 3-chloroperoxybenzoic acid (77%. 13.9 g, 62.0 mmol). The reaction mixture was stirred at room temperature for 2 days. Upon completion of the reaction, the reaction was quenched with a 1:1 mixture of 10% sodium thiosulfate and saturated sodium bicarbonate solution (150 mL). The organic phase was separated, dried with anhydrous sodium sulfate, filtered and concentrated to give an oily residue. This residue was purified by fast chromatography (eluent: 15-50% ethyl acetate/hexane) to afford 1,1-dimethylethyl-1-oxa-5-azaspiro[2.3]hexane-5-carboxylate (1.65 g, 51% yield). The product was analyzed by GC-MS with molecular formula C9H15NO3 and molecular ion peak m/z of 185.

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