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In this study, the key intermediate N 1 , N 3 -disubstituted 1,3,5-triazone of ensitrelvir fumaric acid, approved in Japan for the treatment of SARS-CoV-2 infection under the emergency regulatory approval system, was produced from S -ethylisothiourea hydrobromide and aminomethyl triazole with CDI by four-step telescoped strategy including CDI-activated, condensation, CDI-cyclization, and N 1 -alkylation. The strategy with simple conditions and operations had a total yield of 53% on a gram scale. The strategy for synthesizing the key N 1 , N 3 -disubstituted 1,3,5-triazone intermediate of ensitrelvir might provide a new avenue for further research and development of ensitrelvir analogs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)