Solid-phase synthesis of peptide–heterocycle hybrids containing a tripeptide-derived 6,6-fused bicyclic subunit

Discovery of novel β-turn mimetic-based peptides as novel quorum sensing inhibitors of gram-negative bacteria

To date, a very limited number of peptides reported as quorum sensing inhibitors. Herein, we report the synthesis and evaluation of a series of β-turn mimetic-based peptides as potent quorum sensing inhibitors and antibiofilm formation. In this series, peptides P1, P4, and P5 showed very promising anti-quorum sensing activity on lasB-gfp reporter strain of Pseudomonas aeruginosa without affecting bacterial growth. Under our condition, these compounds also showed good anti-violacein production of Chromobacterium violaceum. In terms of antibiofilm formation, except P5, two β-turn mimetic-based peptides P1 and P4 showed maximum inhibition of 80% total biomass of Pseudomonas aeruginosa. This report provides the first β-turn mimetic-based scaffold for future drug development.

Polymer-supported stereoselective synthesis of tetrahydro-2H-oxazolo[3,2-a]pyrazin-5(3H)-ones from N-(2-oxo-ethyl)-derivatized dipeptides via eastbound iminiums

Polymer-supported N-(2-oxo-ethyl)-derivatized Ser/Thr/Cys-containing dipeptides were synthesized and subjected to acid-mediated tandem N-acylium ion cyclization-nucleophilic addition to yield tetrahydro-2H-oxazolo[3,2-a]pyrazin-5(3H)-ones. The reaction conditions and building-block combinations for stereoselective synthesis of the newly formed asymmetric carbon were developed. The synthesis was fully compatible with solid-phase peptide synthesis, and the products serve as conformationally constrained peptidomimetics. The traceless synthesis of bicycles is also reported as part of this work.

Regioselective incorporation of backbone constraints compatible with traditional solid-phase peptide synthesis

A protected aldehyde was attached via a two-carbon spacer to a peptide backbone amide nitrogen during a traditional Merrifield solid-phase synthesis. Acid-mediated unmasking of the aldehyde triggered the regioselective formation of cyclic N-acyliminiums between the aldehyde and the neighboring peptide amide nitrogen. In the absence of an internal nucleophile, the cyclic iminiums formed dihydropyrazinones, a six-membered peptide backbone constraint between two peptide amides. In the presence of an internal nucleophile, tetrahydropyrazinopyrimidinediones or tetrahydroimidazopyrazinediones were formed via tandem N-acyliminium ion cyclization-nucleophilic addition. The outcome of this nucleophilic addition was dependent on the substituent on the nitrogen nucleophile.

N-acyliminium intermediates in solid-phase synthesis

N-Acyliminium ions are powerful intermediates in synthetic organic chemistry. Examples of their use are numerous in solution-phase synthesis, but there are unmerited few reports on these highly reactive electrophiles in solid-phase synthesis. The present review covers the literature to date and illustrates the methods used to generate N-acyliminium intermediates on solid support and their further elaboration to a range of pharmacologically interesting peptidomimetics, heterocycles, and other small molecules.