Two isomers can be formed when introducing a substituent at the imidazole nitrogen of histidine, the π- and the τ-isomer. Bachem explicitly designates the position of the substituent, if it is unambiguous, e.g. Fmoc-His(1-Trt)-OH (= Fmoc-His(τ-Trt)-OH, the bulkiness of the Trt moiety prevents the formation of the π-isomer), Boc-His(3-Bom)-OH (= Boc-His(π-Bom)-OH); H-His(Bzl)-OH on the other hand is obtained as a mixture of  π- and τ-isomer (Bachem: 3- and 1-isomer).

We recommend searching the appropriate subfamily of the Histidine family instead of entering the derivative.