Peptide−Heterocycle Hybrid Molecules: Solid-Phase Synthesis of a 400-Member Library of N-Terminal 2-Iminohydantoin Peptides

General Installation of (4H)-Imidazolone cis-Amide Bioisosteres Along the Peptide Backbone

Imidazolones represent an important class of heterocycles present in a wide range of pharmaceuticals, metabolites, and bioactive natural products and serve as the active chromophore in green fluorescent protein. Recently, imidazolones have received attention for their ability to act as a nonaromatic amide bond bioisotere which improves pharmacological properties. Herein, we present a tandem amidine installation and cyclization with an adjacent ester to yield (4H)-imidazolone products. Using amino acid building blocks, we can access the first examples of α-chiral imidazolones that have been previously inaccessible. Additionally, our method is amenable to on-resin installation which can be seamlessly integrated into existing solid-phase peptide synthesis protocols. Finally, we show that peptide imidazolones are potent cis-amide bond surrogates that preorganize linear peptides for head-to-tail macrocyclization. This work represents the first general approach to the backbone and side-chain insertion of imidazolone bioisosteres at various positions in linear and cyclic peptides.

Heterocycles as a Peptidomimetic Scaffold: Solid-Phase Synthesis Strategies

Peptidomimetics are a privileged class of pharmacophores that exhibit improved physicochemical and biological properties. Solid-phase synthesis is a powerful tool for gaining rapid access to libraries of molecules from small molecules to biopolymers and also is widely used for the synthesis of peptidomimetics. Small molecules including heterocycles serve as a core for hundreds of drugs, including peptidomimetic molecules. This review covers solid-phase synthesis strategies for peptidomimetics molecules based on heterocycles.

Solid-Phase Synthesis of Pyrrole Derivatives through a Multicomponent Reaction Involving Lys-Containing Peptides

The synthesis of pyrroles has received considerable attention because of their biological and pharmaceutical activities. Herein we describe a solid-phase multicomponent reaction that utilizes Lys as a N donor, β-nitrostyrenes, 1,3-dicarbonyl compounds, and FeCl₃ as an easily accessible catalyst under microwave irradiation to afford the subsequent pyrrole derivatives in high conversions. The strategy combines three of the most powerful tools in modern synthetic chemistry: the solid-phase mode, microwave activation, and a multicomponent reaction. The excellent results in terms of rapidity, versatility, and purity obtained herein support once again that this combined strategy is efficient for gaining chemical diversity.

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.