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Solution-Phase Cyclization

Overview

Solution-phase cyclization is a low-concentration amide bond formation used to close the macrocyclic ring of head-to-tail cyclic peptides. The key to successful cyclization is high dilution to minimize intermolecular (oligomerization) reactions and favor the intramolecular macrolactamization.

Procedure

  1. Dissolve the linear peptide precursor to a concentration of <3 mM in a 1:1 mixture of ACN:THF.

    Note

    The low concentration is critical. Higher concentrations will lead to significant dimer and oligomer formation. Calculate the required volume of solvent based on the amount of linear precursor.

  2. Add 10 eq DIPEA to the solution.

  3. Add 2 eq COMU coupling reagent.

  4. Stir overnight at room temperature.

    Tip

    Monitor the reaction by LCMS. For most sequences, cyclization is complete within 12-16 hours. Difficult sequences may require longer reaction times or alternative coupling reagents.

  5. Concentrate the reaction mixture by rotary evaporation.

  6. Purify by reverse-phase HPLC or silica gel chromatography as appropriate.

Capping Agent Side Reactions

Be cautious with the use of capping agents (e.g., acetic anhydride) during the linear peptide synthesis if the peptide is destined for solution-phase cyclization. Residual capping agent or capped deletion sequences can complicate purification. Ensure thorough washing after any capping steps.