Influence of Sulfur on Acid-Mediated Enamide Formation

Enzymatic Formation of an Aminovinyl Cysteine Residue in Ribosomal Peptide Natural Products

The carboxyl-terminal (C-terminal) S-[(Z)-2-aminovinyl]-cysteine (AviCys) analogs have been identified in four families of ribosomally synthesized and post-translationally modified peptides (RiPPs): lanthipeptides, linaridins, thioamitides, and lipolanthines. Within identified biosynthetic pathways, a highly reactive enethiol intermediate, formed through an oxidative decarboxylation catalyzed by a LanD-like flavoprotein, can undergo two types of cyclization: a Michael addition with a dehydroamino acid or a coupling reaction initiated by a radical species. The collaborative actions of LanD-like proteins with diverse enzymes involved in dehydration, dethiolation or cyclization lead to the construction of structurally distinct peptide natural products with analogous C-terminal macrocyclic moieties. This concept summarizes existing knowledge regarding biosynthetic pathways of AviCys analogs to emphasize the diversity of biosynthetic mechanisms that paves the way for future genome mining explorations into diverse peptide natural products.

Aryl-, Akynyl-, and Alkenylbenziodoxoles: Synthesis and Synthetic Applications

Hypervalent iodine reagents are in high current demand due to their exceptional reactivity in oxidative transformations, as well as in diverse umpolung functionalization reactions. Cyclic hypervalent iodine compounds, known under the general name of benziodoxoles, possess improved thermal stability and synthetic versatility in comparison with their acyclic analogs. Aryl-, alkenyl-, and alkynylbenziodoxoles have recently received wide synthetic applications as efficient reagents for direct arylation, alkenylation, and alkynylation under mild reaction conditions, including transition metal-free conditions as well as photoredox and transition metal catalysis. Using these reagents, a plethora of valuable, hard-to-reach, and structurally diverse complex products can be synthesized by convenient procedures. The review covers the main aspects of the chemistry of benziodoxole-based aryl-, alkynyl-, and alkenyl- transfer reagents, including preparation and synthetic applications.

Photocatalyzed Oxidative Decarboxylation Forming Aminovinylcysteine Containing Peptides

The formation of (2S,3S)-S-[(Z)-aminovinyl]-3-methyl-D-cysteine (AviMeCys) substructures was developed based on the photocatalyzed-oxidative decarboxylation of lanthionine-bearing peptides. The decarboxylative selenoetherification of the N-hydroxyphthalimide ester, generated in situ, proceeded under mild conditions at −40 °C in the presence of 1 mol% of eosin Y-Na2 as a photocatalyst and the Hantzsch ester. The following β-elimination of the corresponding N,Se-acetal was operated in a one-pot operation, led to AviMeCys substructures found in natural products in moderate to good yields. The sulfide-bridged motif, and also the carbamate-type protecting groups, such as Cbz, Teoc, Boc and Fmoc groups, were tolerant under the reaction conditions.

Aminovinyl Cysteine Containing Peptides: A Unique Motif That Imparts Key Biological Activity

Natural products that contain distinctive chemical functionality can serve as useful starting points to develop Nature's compounds into viable therapeutics. Peptide natural products, an under-represented class of medicines, such as ribosomally synthesized and post-translationally modified peptides (RiPPs), often contain noncanonical amino acids and structural motifs that give rise to potent biological activity. However, these motifs can be difficult to obtain synthetically, thereby limiting the transition of RiPPs to the clinic. Aminovinyl cysteine containing peptides, which display potent antimicrobial or anticancer activity, possess an intricate C-terminal ring that is critical for bioactivity. To date, successful methods for the total chemical synthesis of such peptides are yet to be realized, although several advancements have been achieved. In this perspective, we review this burgeoning class of aminovinyl cysteine peptides and critically evaluate the chemical strategies to install the distinct aminovinyl cysteine motif.

Synthesis of cis-β-Amidevinyl Benziodoxolones from the Ethynyl Benziodoxolone-Chloroform Complex and Sulfonamides

The synthesis of cis-β-amidevinyl benziodoxolones from the ethynyl benziodoxolone-chloroform complex and sulfonamides is reported. Evidence indicates that highly reactive unsubstituted ethynyl benziodoxolone undergoes Michael addition of sulfonamides, including sterically demanding acyclic amino acid derivatives. The synthesis of selectively deuterated cis-β-amidevinyl benziodoxolones and investigation of ethynyl-λ³-iodane reactivity are also described.

Stereoselective synthesis of alkyl-, aryl-, vinyl- and alkynyl-substituted Z-enamides and enol ethers

Enamides and enol ethers are valuable building blocks in synthetic chemistry, yet their stereoselective synthesis can be challenging. Herein, we report a new stereoselective synthesis of vinyl, aryl, alkynyl, alkyl and thio-substituted Z-enamides and enol ethers based on the use of vinylbenziodoxolone (VBX) reagents. The stable VBX reagents were synthesized by stereoselective addition of N- or O-nucleophiles on the corresponding alkynyl reagents in the presence of a catalytic amount of cesium carbonate. The VBX reagents were used in palladium-catalyzed cross-couplings at room temperature to access Z-enamides and enol ethers.