A number of N-Boc-protected peptide nucleic acids (PNA) monomers containing 5-aryl-and 5-alkynyl-uracil bases have been synthesized using different palladium-catalyzed cross- coupling reactions. Starting from the base-unprotected 5-iodo-uracil PNA monomer, only the Stille couplings were accomplished successfully, while Suzuki couplings with boronic acids containing the same aryl groups failed. During Sonogashira couplings with terminal ...