Hydrazone Resins for Cyclization and Ligation

Our novel hydrazone resins provide a reliable and convenient method for the synthesis of peptide hydrazides, which are useful intermediates applied in certain ligation techniques. The hydrazone linker is completely stable in the course of standard Fmoc SPPS. Linker cleavage occurs with 95% TFA (e.g. TFA/H2O/TIS, 95:2.5:2.5), which directly affords the desired peptide as a hydrazide. The hydrazone linker tolerates treatment with 5% TFA/DCM, thus permitting selective removal of Mtt and similar acid-labile protecting groups for on-resin side-chain functionalization.

One application of peptide hydrazides is their conversion to C-terminal azides with isoamyl nitrite under acidic condi- tions, and subsequent head-to-tail cyclization forming a native peptide bond. In case the target peptide carries an N-ter- minal cysteine, the conversion to the azide can be carried out using sodium nitrite at pH 3. Addition of a thiol at neutral pH leads to the desired head-to-tail cyclization.

Further applications of peptide hydrazides include the conjugation to biomacromolecules or carrier molecules though hydrazone ligation, i.e. the reaction of hydrazides with aldehyde or keto groups. Peptide hydrazides can also be conver ted to peptide thioesters, which are intermediates for application in native chemical ligation (NCL), or used directly as thioester surrogates in NCL.

One example for an application of this method is the synthesis of the peptide cyclo-[RGDfK(TPP)]. This cyclic RGD peptide was prepared by synthesizing the linear peptide on hydrazone resin, followed by selective Mtt removal on the single Lysine in the sequence, and on-resin functionalization of the liberated ́amino group with a triphenylphosphonium group. Cleavage from the resin afforded the TPP-functionalized linear peptide which was cyclized utilizing isoamyl nitrite under acidic conditions.