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1.6g (7.4 mmol) of Boc-L-allylglycine was dissolved in methanol (25 ml), followed by bubbling with ozone gas at -78 C. After bubbling until color of the solution turned blue, oxygen was bubbled until the blue color disappeared. After returning to room temperature, 752 mg (7.4 mmol) of <strong>[2133-34-8]L-azetidine-2-carboxylic acid</strong> and 470 mg (7.4 mmol) of sodium cyanoborohydride were added, followed by stirring for one hour. After vacuum concentration of the reaction mixture, 2.1 g of compound (3-3) (yield 93%) was obtained by short-pass silica gel chromatography with silica gel (after removing the residue of sodium cyanoborohydride and a small amount of byproduct derived from L-allylglycine with a mobile phase: a mixture solution of ethyl acetate: methanol = 5:1 (v/v), methanol solution was passed through).