Peptide-heterocycle hybrid molecules: solid-phase-supported synthesis of substituted N-terminal 5-aminotetrazole peptides via electrocyclization of peptidic imidoylazides.

A method for the synthesis of polypeptides modified with a tetrazole ring at the N-terminus is described. Reaction of the N-terminal amino group of solid-supported peptides with arylisothiocyanates generates thiourea intermediates, which upon treatment with Mukaiyama's reagent (2-chloro-1-methylpyridinium iodide) generate electrophilic carbodiimide functionality. Trapping by the azide anion and electrocyclization of the intermediate imidoylazide generates an aryl-substituted 5-aminotetrazole at the N-terminus of the peptide. To prevent competitive cyclization of a neighboring amide N-H into the carbodiimide, there should not be a free N-H at the [X-1] position relative to the activated carbodiimide. Protection of the N-H group at this position or incorporation of a secondary amino acid is thus required for optimal tetrazole formation. Cleavage from the resin releases the hybrid molecules incorporating a 5-aminotetrazole ring conjugated onto a peptidic fragment.