Recent progress in the improvement of the coupling efficiency of "difficult sequences" in peptide synthesis.

Even though chemistry is now in place that potentially allows high coupling efficiencies to be attained, successful coupling is usually a challenge when so-called "difficult sequences" is encountered in peptide synthesis. Some factors that affect the coupling efficiency have been discussed and related methods to overcome those obstacles have been introduced in present review. All suggestions proposed here are valuable and also feasible to improve the coupling completeness in both liquid-phase or solid-phase