A building block used for derivatization of carboxylic acids or activated esters with methytetrazine moiety. The stability of Methyltetrazine Amine is substantially improved compared to hydrogen substituted tetrazine-tmine. Superior stability of methyltetrazine-amine allows this reagent to be used in wider range of chemical transformations. Long-term storage of methyltetrazine-amine, especially in aqueous buffer, is also greatly improved compared to Tetrazine Amine. This reagent is supplied as HCl salt for improved stability and handling.