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(π)-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(π)-alkylation and provides synthetic methodologies that permit the facile synthesis of histidine N(π)-modified peptides. These agents represent new and potentially important tools for biological studies.