An amino or thiol linker building block for the synthesis of amino or thiol functionalized amino acids and generally of the following structure: Formula XII 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 inline 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.