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Methyl (S)-4-((tert-butoxycarbonyl)amino)-5-hydroxypentanoate (25 g, 0.1 mol, 1.0 eq.) was used as a raw material, which was dissolved in a mixed solvent of toluene (300 mL) and acetic acid (150 mL), and the reaction was heated and refluxed for 5 hours. Upon completion of the reaction, the reaction solution was cooled to room temperature and subsequently concentrated under reduced pressure to remove the solvent. The concentrated residue was placed in an ice bath and the pH was adjusted to 7-8 by slowly adding saturated sodium bicarbonate solution. the aqueous phase was extracted three times with ethyl acetate, and the organic phases were combined, washed with saturated brine and dried over anhydrous sodium sulfate. After filtration, the filtrate was concentrated to give the crude product. The crude product was purified by solvent recrystallization with mixed solvents of ethyl acetate and petroleum ether to afford the target product tert-butyl (S)-(6-oxotetrahydro-2H-pyran-3-yl)carbamate (8.0 g, 37.2% yield) as a white powder. The product was analyzed by GC-MS and the molecular ion peak was 215.

[1] Tetrahedron Letters, 1989, vol. 30, # 29, p. 3803 - 3804
[2] Journal of Organic Chemistry, 1991, vol. 56, # 13, p. 4167 - 4176
[3] Patent: WO2018/67422, 2018, A1. Location in patent: Page/Page column 47; 48
[4] Patent: WO2015/168246, 2015, A1. Location in patent: Page/Page column 64; 75