Plecanatide Synthesis via Fragment Segmentation

The patent divides the 16-residue plecanatide into three smaller fragments (6 residues, 7 residues, and 3 residues) that are synthesized separately using solid phase peptide synthesis. These fragments are then coupled in solution phase to form the complete peptide. This segmentation approach prevents the cumulative coupling efficiency problems that occur in linear synthesis of long chains, as each fragment is synthesized with high efficiency and the final assembly involves only three coupling steps rather than fifteen sequential couplings.

By synthesizing smaller fragments separately and coupling them in solution phase, the patent reduces the number of purification cycles needed. Each fragment can be purified once after synthesis, and the final fragment coupling requires minimal purification since the fragments have distinct properties. This approach eliminates the need for multiple purification cycles that would be required if all deletion impurities from linear synthesis needed to be removed, thereby improving isolated yield.

The patent strategically designs the fragment boundaries and protecting group schemes to create local differences in properties among the fragments. The use of different protecting groups (Fmoc, Boc, Alloc) on different fragments and the specific choice of fragment lengths (6, 7, 3 residues) creates sufficient property differences that result in well-resolved HPLC peaks during purification. This local quality differentiation ensures that even though the fragments are coupled in a modular fashion, their purification is straightforward due to their distinct chromatographic behavior.