Synthesis of optically pure γPNA monomers: a comparative study

We report a systematic study examining two synthetic routes, reductive amination and Mitsunobu coupling, for preparation of chiral γ-peptide nucleic acid (γPNA) monomers and oligomers. We found that the reductive amination route is prone to epimerization, even under mild experimental conditions. The extent of epimerization could be minimized by utilizing a bulky protecting group such as PhFl; however, it is difficult to remove in the subsequent oligomer synthesis stage. On the other hand, we found that the Mitsunobu route produced optically superior products using standard carbamate protecting groups.