Three stereoisomers (cis-L, trans-D and cis-D) of N-Fmoc-glycyl-4-(thymin-1-yl)proline and their pentafluorophenyl esters have been prepared for use in the synthesis of novel peptide nucleic acids. In addition, N-Fmoc-glycyl-4-(N6-benzoyladenin- 9-yl)proline, N-Fmoc-glycyl-4-(N4-benzoylcytosin -1-yl)proline and N-Fmoc-glycyl-4-(N2-isobutyrylguan in-9-yl)proline and their pentafluorophenyl esters of the cis-D series have been synthesized.