Solid-phase synthesis of cyclic hexapeptides wollamides A, B and desotamide B

Solid-phase synthesis of antibacterial cyclohexapeptides including wollamides A, B and desotamide B has been developed. Briefly, the protected linear hexapeptides were assembled on 2-chlorotrityl chloride resin using standard Fmoc chemistry and diisopropylcarbodiimide/hydroxybenzotriazole coupling reagents, cleaved off-resin with hexafluoroisopropanol/dichloromethane to keep side-chain protecting groups intact, and cyclized in solution. Final global removal of all protecting groups using a cocktail of trifluoroacetic acid/triisopropylsilane/dichloromethane afforded the desired cyclic hexapeptides, which were characterized by 1H, 13C NMR, and HRMS. Subsequent investigation of macrocyclization parameters such as terminal residues, coupling reagents, and cyclization concentration revealed the optimized conditions for the synthesis of this class of cyclic hexapeptides.

A meta-selective-C–H alkenylation of phenol-derivatives employing a traceless organosilicon template

A traceless organosilicon template-directed meta-selective-C–H alkenylation of phenols was realized with good yields and high selectivities.

Exo-selective reductive macrocyclization of ynals

A general protocol for the highly exo-selective macrocyclization of ynals using a nickel/N-heterocyclic carbene catalyst system has been developed. A series of 10- to 21-membered macrocycles bearing an exomethylene substituent was synthesized in good yields with excellent regioselectivity (exo/endo >95:5). Very high levels of long-range diastereocontrol can also be achieved for some classes of macrocycles. Complementary to previously reported endo-selective macrocyclizations, this method provides accesses to exoalkylidene macrocycles from simple ynals in high selectivity.

Stereogenic α-carbons determine the shape and topology of [13]-macrodilactones

The configuration of α-stereogenic centers affects the shape and topology of [13]-macrodilactones. When one α-stereogenic center is substituted, it directs the planar chirality of the macrocycle; when two centers are substituted, both the shape and the topology are influenced.

Oxidative α,ω-diyne coupling as an approach towards novel peptidic macrocycles

The Glaser–Hay diyne coupling proved to be an efficient cyclisation approach towards diyne containing peptidic macrocycles.

Lewis acid catalyzed intramolecular [3 + 2] cross cycloadditions of cobalt-alkynylcyclopropane 1,1-diesters with carbonyls for construction of medium-sized and polycyclic skeletons

A Lewis acid catalyzed intramolecular [3 + 2] cross cycloaddition of cobalt-alkynylcyclopropane 1,1-diesters with carbonyls has been successfully developed. Together with simple and efficient postcycloadditions of the cobalt-alkyne moiety, a general and efficient strategy for construction of structurally complex and diverse medium-sized skeletons and related polycycles was supplied successfully.

A clean and selective radical homocoupling employing carboxylic acids with titania photoredox catalysis

A titania photoredox catalysis protocol was developed for the homocoupling of C-centered radicals derived from carboxylic acids. Intermolecular reactions were generally efficient and selective, furnishing the desired dimers in good yields under mild neutral conditions. Selective cross-coupling with two acids proved unsuccessful. An intramolecular adaptation enabled macrocycles to be prepared, albeit in modest yields.

Stereoselective fluorination alters the geometry of a cyclic peptide: exploration of backbone-fluorinated analogues of unguisin A

New methods for enhancing the efficiency of peptide cyclization, and for fine-tuning the conformations of cyclic peptides, are valuable from a drug development perspective. Herein stereoselective fluorination is investigated as a new strategy for achieving these goals. Four vicinal difluorinated analogues of the natural cyclic heptapeptide unguisin A have been efficiently synthesized. The analogues are found to adopt dramatically different secondary structures, controlled by the fluorine stereochemistry.

Glaser–Eglinton–Hay sp–sp coupling and macrocyclization: construction of a new class of polyether macrocycles having a 1,3-diyne unit

Glaser–Eglinton–Hay-type sp–sp coupling, macrocyclization and assembly of skeletally interesting 1,3-diyne unit-based crown ether/polyether macrocycles are presented.

A diversity-oriented synthesis approach to macrocycles via oxidative ring expansion

Macrocycles are key structural elements in numerous bioactive small molecules and are attractive targets in the diversity-oriented synthesis of natural product-based libraries. However, efficient and systematic access to diverse collections of macrocycles has proven difficult using classical macrocyclization reactions. To address this problem, we have developed a concise, modular approach to the diversity-oriented synthesis of macrolactones and macrolactams involving oxidative cleavage of a bridging double bond in polycyclic enol ethers and enamines. These substrates are assembled in only 4–5 synthetic steps and undergo ring expansion to afford highly functionalized macrocycles bearing handles for further diversification. In contrast to macrocyclization reactions of corresponding seco-acids, the ring expansion reactions are efficient and insensitive to ring size and stereochemistry, overcoming key limitations of conventional approaches to systematic macrocycle synthesis. Cheminformatic analysis indicates that these macrocycles access regions of chemical space that overlap with natural products, distinct from currently-targeted synthetic drugs.

Contemporary strategies for peptide macrocyclization

Peptide macrocycles have found applications that range from drug discovery to nanomaterials. These ring-shaped molecules have shown remarkable capacity for functional fine-tuning. Such capacity is enabled by the possibility of adjusting the peptide conformation using the techniques of chemical synthesis. Cyclic peptides have been difficult, and often impossible, to prepare using traditional synthetic methods. For macrocyclization to occur, the activated peptide must adopt an entropically disfavoured pre-cyclization conformation before forming the desired product. Here, we review recent solutions to some of the major challenges in this important area of contemporary synthesis.

Intramolecular Suzuki-Miyaura reaction for the total synthesis of signal peptidase inhibitors, arylomycins A(2) and B(2)

Development of the total syntheses of arylomycins A(1) and B(2) is detailed. Key features of our approach include 1) formation of 14-membered meta,meta-cyclophane by an intramolecular Suzuki-Miyaura reaction; 2) incorporation of N-Me-4-hydroxyphenylglycine into the cyclization precursor, which avoids the late-stage low-yielding N-methylation step; 3) segment coupling of a fully elaborated peptide side chain to the macrocycle, which makes the synthesis highly convergent. Overall, arylomycin A(2) was obtained in 13 steps from L-Tyr for the longest linear sequence, in 13 % overall yield. Arylomycin B(2) was synthesized in 10 steps from L-3-nitro-Tyr, in 10 % overall yield.

Peptoid macrocycles: making the rounds with peptidomimetic oligomers

Macrocyclic constraints are often employed to rigidify the conformation of flexible oligomeric systems. This approach has recently been used to organize the structure of peptoid oligomers, which are peptidomimetics composed of chemically diverse N-substituted glycine monomer units. In this review, we describe advances in the synthesis and characterization of cyclic peptoids. We evaluate how the installation of covalent constraints between the oligomer termini or side chains has been effective in defining peptoid conformations. We also discuss the potential applications for this promising family of macrocyclic peptidomimetics.