A novel synthetic method of peptide nucleic acid (PNA) oligomers using Boc/Cbz-protected PNA trimer blocks in the solution phase

In order to use peptide nucleic acid (PNA) oligomers for therapeutic purposes, a high purity level is essential and, in particular, large-scale production should be easy. However, in the traditional solid-phase synthesis method, the generation of n - 1 or n - 2 deleted oligomers cannot be completely excluded, and purification of crude oligomers is difficult because the target and impurities overlap. Also, for these reasons, scale-up is also limited. Here, we report the efficient synthesis of PNA oligomers in the solid phase using fully protected PNA trimers instead of monomers. Fully protected PNA trimer blocks were successfully synthesized via two different routes in the solution phase. When PNA oligomers were synthesized in the solid phase using trimer blocks as monomers, a higher purity level of PNA could be synthesized compared to when oligomers were synthesized using monomers. When using this method, n - 1 or n - 2 truncated products cannot be systematically produced. Furthermore, the trimer blocks coupling method is successful even for sequences containing purine-rich and/or gamma PNAs, which are difficult to synthesize via the conventional solid-phase peptide synthesis approach. Finally, two hexameric and one dodecameric PNA oligomers were completely synthesized in the solution phase, with the former showing more than 90% crude purity and the latter showing 70%. We propose that this method may be able to solve several problems related to solid-phase synthesis at once.