Abstract: An amino or thiol linker building block for the synthesis of amino or thiol functionalized amino acids and generally of the following structure: is provided. Such building block may be introduced in the 5? end position of an amino acid under standard coupling conditions. Such building block allows in-line coupling control, “trityl-on” purification, and solid support functionalization/derivatization. Such building block is a stable, solid compound and can therefore be easily handled. With the building block of the present invention, deprotection may be carried out under standard detritylation conditions.