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Figure 1 Reaction formula of N-methyl-O-tert-butyl-L-threonine

L-threonine was placed in a round-bottom flask, anhydrous ethanol and a rotor were added to the round-bottom flask, and the round-bottom flask was placed in a silicone oil heating furnace and heated to 80°C. When the liquid became uniform, potassium hydroxide was added to the round-bottom flask. After reacting for 1 hour, the liquid changed from turbid to transparent. Tert-butyllithium was added, and the liquid became transparent and pale yellow. The reaction was continued for 2 hours; the liquid color remained unchanged. The reaction progress was monitored by thin-layer chromatography. The reaction was stopped after L-threonine had completely reacted. After cooling to room temperature, the mixture was filtered to obtain a pale yellow powder. Anhydrous dioxane and a rotor were added to a round-bottom flask. O-tert-butyl-L-threonine was suspended in the dioxane solution and subjected to acylation with fluorenemethyloxycarbonyl azide. The reaction progress was monitored by thin-layer chromatography. The reaction was stopped after O-tert-butyl-L-threonine had completely reacted, yielding crude N-Fmoc-N-methyl-O-tert-butyl-L-threonine. This crude product was extracted with ethyl acetate at pH 9-10 and then purified by recrystallization.