Selective Modification of Ribosomally Synthesized and Post‐Translationally Modified Peptides (RiPPs) through Diels–Alder Cycloadditions on Dehydroalanine Residues

Abstract

We report the late‐stage chemical modification of ribosomally synthesized and post‐translationally modified peptides (RIPPs) by Diels–Alder cycloadditions to naturally occurring dehydroalanines. The tail region of the thiopeptide thiostrepton could be modified selectively and efficiently under microwave heating and transition‐metal‐free conditions. The Diels–Alder adducts were isolated and the different site‐ and endo/exo isomers were identified by 1D/2D 1H NMR. Via efficient modification of the thiopeptide nosiheptide and the lanthipeptide nisin Z the generality of the method was established. Minimum inhibitory concentration (MIC) assays of the purified thiostrepton Diels–Alder products against thiostrepton‐susceptible strains displayed high activities comparable to that of native thiostrepton. These Diels–Alder products were also subjected successfully to inverse‐electron‐demand Diels–Alder reactions with a variety of functionalized tetrazines, demonstrating the utility of this method for labeling of RiPPs.

Cobalt(III)‐Catalyzed C−H Amidation of Dehydroalanine for the Site‐Selective Structural Diversification of Thiostrepton

“…Site‐specific modification in the tail region of thiostrepton should also likely be beneficial for maintaining antibacterial activity. X‐ray structural determination of thiostrepton bound to the 50S ribosomal subunit have established that the tail region is solvent exposed, with modifications to this region generally well tolerated –. Analogs 2 a – l and 4 a – g were consequently evaluated for activity against a broad spectrum of Gram‐positive bacteria (Table ).…”

“…Moreover, by introducing an aldehyde group with C−H amidation, oxime ligation could be employed to install an even greater range of functionality. Many of the analogs maintained potent antibacterial activity with only a moderate reduction relative to thiostrepton perhaps because Dha1 is located in a solvent exposed region of thiostrepton with modifications to this region generally well‐tolerated –. Most importantly, many analogs showed significantly greater solubility in pH 7.2 aqueous solution with up to 28‐fold improvement to 83 μg mL −1 , surpassing the aqueous solubility of all previously reported thiostrepton derivatives prepared by semisynthetic or biosynthetic methods …”

“…Importantly, many of the synthesized thiostrepton derivatives maintained potent antibacterial activity with only moderate reductions relative to 1 . However, hydrophobic functionality has generally been introduced, including in recent work involving cycloaddition to the Dha residues, and aqueous solubility has not been reported for any of these derivatives of 1 . Moreover, physicochemical properties of derivatives of 1 have not been characterized, except for studies by Honek and co‐workers on several thiol Michael adducts showing qualitative increases in overall polarity by RP‐HPLC and improvements in theoretical logP …”

Advances in Asymmetric Amino Acid Synthesis Enabled by Radical Chemistry

“…Dehydroalanine (ΔAla) are naturally occurring non‐coded amino acid, found primarily in peptides. In this context, ΔAla appendage is a useful synthetic tool for simple modification of peptides or proteins by different reactions such as Michael addition, Diels‐Alder cycloaddition and radical coupling under mild conditions . Dehydroalanine derivatives are also widely applied as electrophilic partners in the range of ionic reactions for the asymmetric synthesis of unusual AAs .…”

Cobalt(III)‐Catalyzed C−H Amidation of Dehydroalanine for the Site‐Selective Structural Diversification of Thiostrepton

“…[5] Previously, several studies have been carried out on additions to the Dha residues in 1 (Figure 1, middle). [6,7] Va riable Dha site selectivity was observed depending on the transformation and reaction conditions,with notable site and diastereoselectivity achieved for Rh-catalyzed arylboronic acid additions. [6b] Importantly,many of the synthesized thiostrepton derivatives maintained potent antibacterial activity with only moderate reductions relative to 1.H owever,h ydrophobic functionality has generally been introduced, including in recent work involving cycloaddition to the Dha residues, [7a] and aqueous solubility has not been reported for any of these derivatives of 1.M oreover,p hysicochemical properties of derivatives of 1 have not been characterized, except for studies by Honek and co-workers on several thiol Michael adducts showing qualitative increases in overall polarity by RP-HPLC and improvements in theoretical logP.…”