Head-to-tail cyclization for the synthesis of naturally occurring cyclic peptides on organophosphorus small-molecular supports

Abstract

To achieve head-to-tail cyclic peptides via the liquid-phase on-support cyclization and synergistic self-cleavage strategy, 4,4′-bis(diphenylphosphinyloxyl) diphenyl ketoxime (BDKO) and 4-diphenyl phospholoxy benzyl alcohol (DPBA) were designed and prepared as small-molecular supports of greener peptide synthesis. In the process of the BDKO (or DPBA) support assisted Boc strategy for liquid-phase peptide chain extension, the last amino acid at the N-terminus of the peptide chain ends with a Fmoc protected amino acid. With the removal of the Fmoc group, head-to-tail cyclization and self-cleavage of the peptide chain synchronously occurred on the BDKO (or DPBA) support to release the target cyclic peptide and the original support itself. The target cyclopeptides can be easily separated and purified, and the support can be recycled for reuse directly. The synthesis of naturally occurring cyclic peptides such as mortiamides A–E, gramicidin S and cyclo(Phe-Pro) on the BDKO (or DPBA) support in a resin-free manner was then successfully demonstrated.