Investigation of Unanticipated Alkylation at the N(π) Position of a Histidyl Residue Under Mitsunobu Conditions and Synthesis of Orthogonally Protected Histidine Analogues

We had previously reported that Mitsunobu-based introduction of alkyl substituents onto the imidazole N(pi)-position of a key histidine residue in phosphothreonine-containing peptides can impart high binding affinity against the polo-box domain of polo-like kinase 1. Our current paper investigates the mechanism leading to this N(pi)-alkylation and provides synthetic methodologies that permit the facile synthesis of histidine N(pi)-modified peptides. These agents represent new and potentially important tools for biological studies.