Methyl iodide → 3-(4-Methoxy-benzyl)-1-methyl-2,4-dioxo-1,3,8-triaza-spiro[4.5]decane-8-carboxylic acid tert-butyl ester

390 mg (1 mmol) of 3-(4-methoxybenzyl)-2,4-dioxo-1,3,8-triazaspiro[4.5]decane-8-carboxylic acid tert-butyl ester were solubilized in 5 mL of DMF under nitrogen before adding 61 mg (1.5 mmol) of sodium hydride in mineral oil 60%. The reaction mixture was agitated for 5 minutes before the addition of 70 μL (1.1 mmol) of iodomethane. After 2 hours of agitation at room temperature, the reaction mixture was quenched with 20 mL of water and then extracted with diethyl ether (2×20 mL). The organic layers were dried over sodium sulfate, filtrated and evaporated in vacuo to give quantitatively (404 mg) 3-(4-methoxy-benzyl)-1-methyl-2,4-dioxo-1,3,8-triaza-spiro[4.5]decane-8-carboxylic acid tert-butyl ester as a yellowish oil.