Synthesis and fluorescence of xanthone amino acids

Fmoc- and Boc-protected α-amino acids bearing xanthone as side chain are easily accessible by palladium-catalyzed cross-coupling of a xanthone triflate with appropriate amino acid residues. The xanthone triflate was obtained in three steps taking advantage of a high-yield and mild copper-catalyzed diarylether formation. The new xanthone amino acid (Xan-aa) shows typical xanthone fluorescence properties suitable for studying folding processes of polypeptides by triplet–triplet energy transfer (TTET).