8-tert-butyl 3-ethyl 1-oxa-2,8-diazaspiro[4.5]dec-2-ene-3,8-dicarboxylate

95080-93-6

159635-49-1

479636-65-2

Compound 7-a (1.0 g, 5.1 mmol) and sodium bicarbonate (2.1 g, 25 mmol) were dissolved in ethyl acetate (10 mL). After the mixture was cooled to 0 °C, ethyl (2Z)-2-chloro-2-(hydroxyimino)acetate (1.2 g, 7.6 mmol, prepared with reference to the literature method, Tetrahedron Letters, 2011, 52(43), 5656-5658) was slowly added. The reaction mixture was stirred at room temperature for 48 hours. After completion of the reaction, it was diluted by adding distilled water (100 mL) and extracted with ethyl acetate. The organic layers were combined, washed sequentially with distilled water and saturated saline, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (eluent: ethyl acetate/hexane, 5:95→3:7) to afford the target product 1-oxa-2,8-diazaspiro[4.5]dec-2-ene-3,8-dicarboxylic acid-8-tert-butyl ester-3-ethyl ester 7-b (1.4 g, 93% yield) as a yellow solid.