The amino acid N(im)-trityl-D-histidine was dissolved in dichloromethane, followed by the addition of triethylamine and trimethylsilyl chloride (3.5 equivalents). After 2 hours of gentle reflux, the mixture was ready to be used for in situ coupling to the anhydride of the carboxylic acid.
Thus, isovaleric acid was separately dissolved in THF and maintained at -20 ℃ using an ice-salt bath. N-methyl morpholine (1.2 equivalents) was added, followed by the addition of ethyl chloroformate (1.0 equivalent). After 15-20 minutes at -20 ℃, the bis-TMS amino acid reaction mixture was added directly to the anhydride solution and left stirring at room temperature overnight. After 8 hours, the reaction mixture was concentrated in vacuo.
Finally, H-His(Trt)-OH was obtained after purification.
