Expedient synthesis of threo-beta-hydroxy-alpha-amino acid derivatives: phenylalanine, tyrosine, histidine, and tryptophan

Total Synthesis of Dragocins A-C through Electrochemical Cyclization

The first total synthesis of dragocins A-C, remarkable natural products containing an unusual C4' oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C-H oxidation on a late-stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9-membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.

Continuous-Flow Chemistry and Photochemistry for Manufacturing of Active Pharmaceutical Ingredients

An active pharmaceutical ingredient (API) is any substance in a pharmaceutical product that is biologically active. That means the specific molecular entity is capable of achieving a defined biological effect on the target. These ingredients need to meet very strict limits; chemical and optical purity are considered to be the most important ones. A continuous-flow synthetic methodology which utilizes a continuously flowing stream of reactive fluids can be easily combined with photochemistry, which works with the chemical effects of light. These methods can be useful tools to meet these strict limits. Both of these methods are unique and powerful tools for the preparation of natural products or active pharmaceutical ingredients and their precursors with high structural complexity under mild conditions. This review shows some main directions in the field of active pharmaceutical ingredients' preparation using continuous-flow chemistry and photochemistry with numerous examples of industry and laboratory-scale applications.

Atroposelective Total Synthesis of Darobactin A

A concise, modular synthesis of the novel antibiotic darobactin A is disclosed. The synthesis successfully forges the hallmark strained macrocyclic ring systems in a sequential fashion. Key transformations include two atroposelective Larock-based macrocyclizations, one of which proceeds with exquisite regioselectivity despite bearing an unprotected alkyne. The synthesis is designed with medicinal chemistry considerations in mind, appending key portions of the molecule at a late stage. Requisite unnatural amino acid building blocks are easily prepared in an enantiopure form using C-H activation and decarboxylative cross-coupling tactics.

Total Synthesis of Ecumicin

The first total synthesis of the potent anti-mycobacterial cyclic depsipeptide natural product ecumicin is described. Synthesis was achieved via a solid-phase strategy, incorporating the synthetic non-proteinogenic amino acids N-methyl-4-methoxy-l-tryptophan and threo-β-hydroxy-l-phenylalanine into the growing linear peptide chain. The synthesis employed key on-resin esterification and dimethylation steps as well as a final macrolactamization between the unusual N-methyl-4-methoxy-l-tryptophan unit and a bulky N-methyl-l-valine residue. The synthetic natural product possessed potent antimycobacterial activity against the virulent H37Rv strain of Mycobacterium tuberculosis (MIC₉₀ = 312 nM).

Use of Bromine and Bromo-Organic Compounds in Organic Synthesis

Bromination is one of the most important transformations in organic synthesis and can be carried out using bromine and many other bromo compounds. Use of molecular bromine in organic synthesis is well-known. However, due to the hazardous nature of bromine, enormous growth has been witnessed in the past several decades for the development of solid bromine carriers. This review outlines the use of bromine and different bromo-organic compounds in organic synthesis. The applications of bromine, a total of 107 bromo-organic compounds, 11 other brominating agents, and a few natural bromine sources were incorporated. The scope of these reagents for various organic transformations such as bromination, cohalogenation, oxidation, cyclization, ring-opening reactions, substitution, rearrangement, hydrolysis, catalysis, etc. has been described briefly to highlight important aspects of the bromo-organic compounds in organic synthesis.

Practical synthesis of anti-β-hydroxy-α-amino acids by Pd(II) -catalyzed sequential C(sp(3) )-H functionalization

An improved and practical procedure for the stereoselective synthesis of anti-β-hydroxy-α-amino acids (anti-βhAAs), by palladium-catalyzed sequential C(sp(3) )-H functionalization directed by 8-aminoquinoline auxiliary, is described. followed by a previously established monoarylation and/or alkylation of the β-methyl C(sp(3) )-H of alanine derivative, β-acetoxylation of both alkylic and benzylic methylene C(sp(3) )-H bonds affords various anti-β-hydroxy-α-amino acid derivatives. As an example, the synthesis of β-mercapto-α-amino acids, which are highly important to the extension of native chemical ligation chemistry beyond cysteine, is described. The synthetic potential of this protocol is further demonstrated by the synthesis of diverse β-branched α-amino acids. The observed diastereoselectivities are strongly influenced by electronic effects of aromatic AAs and steric effects of the linear side-chain AAs, which could be explained by the competition of intramolecular C-OAc bond reductive elimination from Pd(IV) intermediates vs. intermolecular attack by an external nucleophile (AcO(-) ) in an SN 2-type process.

Synthesis of fully protected, reverse N-prenylated (2S,3R)-3-hydroxytryptophan, a unique building block of the cyclomarins

The rather exotic amino acid of the cyclomarins, is obtained in enantio- and diastereomerically pure and fully protected form.

Studies Toward the Total Synthesis and Stereochemical Assignment of Microspinosamide

Efforts toward the total synthesis and stereochemical assignment of the cyclic depsipeptide natural product microspinosamide are described. A single diastereoisomer was targeted corresponding to the predicted structure of the natural product incorporating a (2S, 3R)-β-hydroxy-p-bromophenylalanine residue. Assembly was achieved through the initial synthesis of a cyclic depsipeptide and a linear peptide thioester fragment by solid-phase peptide synthesis, followed by fusion of the two fragments through a native chemical ligation–oxidation protocol. Extensive spectroscopic analysis showed structural differences to the isolated natural product, suggesting that a diastereoisomer of microspinosamide had been synthesised. This work lays the foundation for the future synthesis of the correct diastereoisomer.

Synthetic Amino Acids for Applications in Peptide Ligation–Desulfurization Chemistry

Native chemical ligation is a powerful tool for the convergent assembly of homogeneous peptide and protein targets from unprotected peptide fragments. The method involves the chemoselective coupling of a peptide thioester with a peptide bearing an N-terminal cysteine (Cys) residue and is mediated by the nucleophilic Cys thiol functionality. A widely adopted extension of the technique for the disconnection of protein targets at alanine (Ala) ligation junctions has been the application of post-ligation desulfurization protocols for the mild removal of the Cys thiol moiety. Recently, attention has turned to the construction of synthetic amino acid building blocks bearing suitably positioned β-, γ-, or δ-thiol ligation auxiliaries with a view to expanding the scope of the ligation–desulfurization manifold. To date, several thiol-derived amino acids have been prepared, greatly increasing the generality and flexibility of chemoselective ligation technologies for the chemical synthesis of diverse protein targets. This review will highlight the current synthetic approaches to these important amino acid building blocks.

Realizing serine/threonine ligation: scope and limitations and mechanistic implication thereof

Serine/Threonine ligation (STL) has emerged as an alternative tool for protein chemical synthesis, bioconjugations as well as macrocyclization of peptides of various sizes. Owning to the high abundance of Ser/Thr residues in natural peptides and proteins, STL is expected to find a wide range of applications in chemical biology research. Herein, we have fully investigated the compatibility of the STL strategy for X-Ser/Thr ligation sites, where X is any of the 20 naturally occurring amino acids. Our studies have shown that 17 amino acids are suitable for ligation, while Asp, Glu, and Lys are not compatible. Among the working 17 C-terminal amino acids, the retarded reaction resulted from the bulky β-branched amino acid (Thr, Val, and Ile) is not seen under the current ligation condition. We have also investigated the chemoselectivity involving the amino group of the internal lysine which may compete with the N-terminal Ser/Thr for reaction with the C-terminal salicylaldehyde (SAL) ester aldehyde group. The result suggested that the free internal amino group does not adversely slow down the ligation rate.

Concise Total Synthesis of (+)-Bionectins A and C

The concise and efficient total synthesis of (+)-bionectins A and C is described. Our approach to these natural products features a new and scalable method for erythro-β-hydroxytryptophan amino acid synthesis, an intramolecular Friedel-Crafts reaction of a silyl-tethered indole, and a new mercaptan reagent for epipolythiodiketopiperazine (ETP) synthesis that can be unravelled under very mild conditions. In evaluating the impact of C12-hydroxylation, we have identified a unique need for an intramolecular variant of our Friedel-Crafts indolylation chemistry. Several key discoveries including the first example of permanganate-mediated stereoinvertive hydroxylation of the α-stereocenters of diketopiperazines as well as the first example of a direct triketopiperazine synthesis from a parent cyclo-dipeptide are discussed. Finally, the synthesis of (+)-bionectin A and its unambiguous structural assignment through X-ray analysis provides motivation for the reevaluation of its original characterization data and assignment.

Conformationally constrained histidines in the design of peptidomimetics: strategies for the χ-space control

A successful design of peptidomimetics must come to terms with χ-space control. The incorporation of χ-space constrained amino acids into bioactive peptides renders the χ¹ and χ² torsional angles of pharmacophore amino acids critical for activity and selectivity as with other relevant structural features of the template. This review describes histidine analogues characterized by replacement of native α and/or β-hydrogen atoms with alkyl substituents as well as analogues with α, β-didehydro unsaturation or C^α-C^β cyclopropane insertion (ACC derivatives). Attention is also dedicated to the relevant field of β-aminoacid chemistry by describing the synthesis of β²- and β³-models (β-hHis). Structural modifications leading to cyclic imino derivatives such as spinacine, aza-histidine and analogues with shortening or elongation of the native side chain (nor-histidine and homo-histidine, respectively) are also described. Examples of the use of the described analogues to replace native histidine in bioactive peptides are also given.

Novel Cinchona alkaloid derived ammonium salts as catalysts for the asymmetric synthesis of beta-hydroxy alpha-amino acids via aldol reactions

Using the cinchonidine-derived phase-transfer catalyst de veloped by Park and Jew as a lead structure, we have prepared novel chiral ammonium salts and investigated their efficacy for the preparation of beta-hydroxy alpha-amino acids via asymmetric aldol reactions. The modifications were performed at C3 of the cinchonidine nucleus and include dimers as well as catalysts possessing electron-deficient alkyne and alkene moieties. Some of the new catalysts yielded improvements relative to the Park-Jew catalyst in the aldol reaction.