C-Glycosyl Amino Acids through Hydroboration-Cross-Coupling ofexo-Glycals and Their Application in Automated Solid-Phase Synthesis

Asymmetric Synthesis of β,β-Disubstituted Alanines via a Sequential C(sp2)-C(sp3) Cross-Coupling-Hydrogenation Strategy

We report the development of a sequential C(sp2)-C(sp3) Suzuki cross-coupling-asymmetric hydrogenation strategy which allows access to a diverse array of valuable β,β-disubstituted alanine derivatives. This synthesis exhibits broad functional group tolerance, and permits efficient access to β-aryl-β-alkyl, and the more rarely reported β,β-dialkyl Ala derivatives with high yield and excellent enantioselectivity. This transformation has been exhibited on decagram quantity, and can be used to generate Fmoc amino acid derivatives which are useful for SPPS.

Photocatalytic Functionalization of Dehydroalanine-Derived Peptides in Batch and Flow

Unnatural amino acids, and their synthesis by the late-stage functionalization (LSF) of peptides, play a crucial role in areas such as drug design and discovery. Historically, the LSF of biomolecules has predominantly utilized traditional synthetic methodologies that exploit nucleophilic residues, such as cysteine, lysine or tyrosine. Herein, we present a photocatalytic hydroarylation process targeting the electrophilic residue dehydroalanine (Dha). This residue possesses an α,β-unsaturated moiety and can be combined with various arylthianthrenium salts, both in batch and flow reactors. Notably, the flow setup proved instrumental for efficient scale-up, paving the way for the synthesis of unnatural amino acids and peptides in substantial quantities. Our photocatalytic approach, being inherently mild, permits the diversification of peptides even when they contain sensitive functional groups. The readily available arylthianthrenium salts facilitate the seamless integration of Dha-containing peptides with a wide range of arenes, drug blueprints, and natural products, culminating in the creation of unconventional phenylalanine derivatives. The synergistic effect of the high functional group tolerance and the modular characteristic of the aryl electrophile enables efficient peptide conjugation and ligation in both batch and flow conditions.

Nucleophilic Addition of Amides to Haloalkynes: Synthesis of (Z)-β-Halovinyl Amides as Dual Precursors of Alkylidene Carbenes and Allyl Halides

Toward a regioselective method for the synthesis of β-halovinyl amides, we developed a transition-metal-free nucleophilic addition reaction of amides to haloalkynes. The regioselective nucleophilic addition was achieved under solvent-free conditions using phosphonates to protonate the intermediate alkylidene carbenoids, thus suppressing their decomposition. Furthermore, we demonstrate that β-halovinyl amides can serve as dual precursors of allyl halides and alkylidene carbenes to obtain functionalized indoles and pyrrolidones.

Phosphatidylserine Synthase PTDSS1 Shapes the Tumor Lipidome to Maintain Tumor-Promoting Inflammation

Significance:

This study shows that inhibiting the production of ether-phosphatidylserine by targeting phosphatidylserine synthase PTDSS1 limits tumor-associated macrophage expansion and breast tumor growth.

Visible Light Induced Synthesis of (Z)-β-iodoenamides from N-Vinyl Amides Mediated by Ion Pair Charge Transfer State

A facile and novel approach for high chemo- and stereoselective synthesis of (Z)-β-iodoenamides from N-vinyl amides has been developed based on a visible-light-induced ion pair charge transfer state derived from...

Convergent Chemoenzymatic Strategy to Deliver a Diversity of Shigella flexneri Serotype-Specific O-Antigen Segments from a Unique Lightly Protected Tetrasaccharide Core

Progress in glycoscience is strongly dependent on the availability of broadly diverse tailor-made, well-defined, and often complex oligosaccharides. Herein, going beyond natural resources and aiming to circumvent chemical boundaries in glycochemistry, we tackle the development of an in vitro chemoenzymatic strategy holding great potential to answer the need for molecular diversity characterizing microbial cell-surface carbohydrates. The concept is exemplified in the context of Shigella flexneri, a major cause of diarrhoeal disease. Aiming at a broad serotype coverage S. flexneri glycoconjugate vaccine, a non-natural lightly protected tetrasaccharide was designed for compatibility with (i) serotype-specific glucosylations and O-acetylations defining S. flexneri O-antigens, (ii) recognition by suitable α-transglucosylases, and (iii) programmed oligomerization following enzymatic α-d-glucosylation. The tetrasaccharide core was chemically synthesized from two crystalline monosaccharide precursors. Six α-transglucosylases found in the glycoside hydrolase family 70 were shown to transfer glucosyl residues on the non-natural acceptor. The successful proof of concept is achieved for a pentasaccharide featuring the glucosylation pattern from the S. flexneri type IV O-antigen. It demonstrates the potential of appropriately planned chemoenzymatic pathways involving non-natural acceptors and low-cost donor/transglucosylase systems to achieve the demanding regioselective α-d-glucosylation of large substrates, paving the way to microbial oligosaccharides of vaccinal interest.

Development of a Flexible and Robust Synthesis of Tetrahydrofuro[3,4-b]furan Nucleoside Analogues

In the context of a PRMT5 inhibitor program, we describe our efforts to develop a flexible and robust strategy to access tetrahydrofuro[3,4-b]furan nucleoside analogues. Ultimately, it was found that a Wolfe type carboetherification from an alkenol derived from d-glucofuranose diacetonide was capable of furnishing the B-ring and installing the desired heteroaryl group in a single step. Using this approach, key intermediate 1.3-A was delivered on a gram scale in a 62% yield and 9.1:1 dr in favor of the desired S-isomer. After deprotection of 1.3-A, a late-stage glycosylation was performed under Mitsunobu conditions to install the pyrrolopyrimidine base. This provided serviceable yields of nucleoside analogues in the range of 31-48% yield. Compound 1.1-C was profiled in biochemical and cellular assays and was demonstrated to be a potent and cellularly active PRMT5 inhibitor, with a PRMT5-MEP50 biochemical IC₅₀ of 0.8 nM, a MCF-7 target engagement EC₅₀ of 3 nM, and a Z138 cell proliferation EC₅₀ of 15 nM. This work sets the stage for the development of new inhibitors of PRMT5 and novel nucleoside chemical matter for alternate drug discovery programs.

Straightforward synthesis of protected 2-hydroxyglycals by chlorination-dehydrochlorination of carbohydrate hemiacetals

A straightforward and scalable method for the synthesis of protected 2-hydroxyglycals is described. The approach is based on the chlorination of carbohydrate-derived hemiacetals, followed by an elimination reaction to establish the glycal moiety. 1,2-dehydrochlorination reactions were studied on a range of glycosyl chlorides to provide suitable reaction conditions for this transformation. Benzyl ether, isopropylidene and benzylidene protecting groups, as well as interglycosidic linkage, were found to be compatible with this protocol. The described method is operationally simple and allows for the quick preparation of 2-hydroxyglycals with other than ester protecting groups, providing a feasible alternative to existing methods.

Recent Advances in Metal-Catalyzed Alkyl–Boron (C(sp3)–C(sp2)) Suzuki-Miyaura Cross-Couplings

Boron chemistry has evolved to become one of the most diverse and applied fields in organic synthesis and catalysis. Various valuable reactions such as hydroborylations and Suzuki–Miyaura cross-couplings (SMCs) are now considered as indispensable methods in the synthetic toolbox of researchers in academia and industry. The development of novel sterically- and electronically-demanding C(sp3)–Boron reagents and their subsequent metal-catalyzed cross-couplings attracts strong attention and serves in turn to expedite the wheel of innovative applications of otherwise challenging organic adducts in different fields. This review describes the significant progress in the utilization of classical and novel C(sp3)–B reagents (9-BBN and 9-MeO-9-BBN, trifluoroboronates, alkylboranes, alkylboronic acids, MIDA, etc.) as coupling partners in challenging metal-catalyzed C(sp3)–C(sp2) cross-coupling reactions, such as B-alkyl SMCs after 2001.

Solid-surface activated recombinant Rhizopous oryzae lipase expressed in Pichia pastoris and chemically modified variants as efficient catalysts in the synthesis of hydroxy monodeprotected glycals

A highly active, specific and regioselective heterogeneous lipase biocatalyst was developed for the monodeprotection of peracetylated glycals.

4R- and 4S-iodophenyl hydroxyproline, 4R-pentynoyl hydroxyproline, and S-propargyl-4-thiolphenylalanine: conformationally biased and tunable amino acids for bioorthogonal reactions

Bioorthogonal reactions allow the introduction of new functionalities into peptides, proteins, and other biological molecules. The most readily accessible amino acids for bioorthogonal reactions have modest conformational preferences or bases for molecular interactions. Herein we describe the synthesis of 4 novel amino acids containing functional groups for bioorthogonal reactions. (2S,4R)- and (2S,4S)-iodophenyl ethers of hydroxyproline are capable of modification via rapid, specific Suzuki and Sonogashira reactions in water. The synthesis of these amino acids, as Boc-, Fmoc- and free amino acids, was achieved through succinct sequences. These amino acids exhibit well-defined conformational preferences, with the 4S-iodophenyl hydroxyproline crystallographically exhibiting β-turn (ϕ, ψ∼-80°, 0°) or relatively extended (ϕ, ψ∼-80°, +170°) conformations, while the 4R-diastereomer prefers a more compact conformation (ϕ∼-60°). The aryloxyproline diastereomers present the aryl groups in a highly divergent manner, suggesting their stereospecific use in molecular design, medicinal chemistry, and catalysis. Thus, the 4R- and 4S-iodophenyl hydroxyprolines can be differentially applied in distinct structural contexts. The pentynoate ester of 4R-hydroxyproline introduces an alkyne functional group within an amino acid that prefers compact conformations. The propargyl thioether of 4-thiolphenylalanine was synthesized via copper-mediated cross-coupling reaction of thioacetic acid with protected 4-iodophenylalanine, followed by thiolysis and alkylation. This amino acid combines an alkyne functional group with an aromatic amino acid and the ability to tune aromatic and side chain properties via sulfur oxidation. These amino acids provide novel loci for peptide functionalization, with greater control of conformation possible than with other amino acids containing these functional groups.

The road to the synthesis of "difficult peptides"

The last decade has witnessed a renaissance of peptides as drugs. This progress, together with advances in the structural behavior of peptides, has attracted the interest of the pharmaceutical industry in these molecules as potential APIs. In the past, major peptide-based drugs were inspired by sequences extracted from natural structures of low molecular weight. In contrast, nowadays, the peptides being studied by academic and industrial groups comprise more sophisticated sequences. For instance, they consist of long amino acid chains and show a high tendency to form aggregates. Some researchers have claimed that preparing medium-sized proteins is now feasible with chemical ligation techniques, in contrast to medium-sized peptide syntheses. The complexity associated with the synthesis of certain peptides is exemplified by the so-called "difficult peptides", a concept introduced in the 80's. This refers to sequences that show inter- or intra-molecular β-sheet interactions significant enough to form aggregates during peptide synthesis. These structural associations are stabilized and mediated by non-covalent hydrogen bonds that arise on the backbone of the peptide and-depending on the sequence-are favored. The tendency of peptide chains to aggregate is translated into a list of common behavioral features attributed to "difficult peptides" which hinder their synthesis. In this regard, this manuscript summarizes the strategies used to overcome the inherent difficulties associated with the synthesis of known "difficult peptides". Here we evaluate several external factors, as well as methods to incorporate chemical modifications into sequences, in order to describe the strategies that are effective for the synthesis of "difficult peptides". These approaches have been classified and ordered to provide an extensive guide for achieving the synthesis of peptides with the aforementioned features.

The use of tin(IV) chloride to selectively cleave benzyl esters over benzyl ethers and benzyl amines

Benzyl esters are cleaved upon reaction with SnCl4, resulting in isolation of the corresponding carboxylic acid. Importantly, benzyl ethers, amines, and amides do not undergo debenzylation under these conditions, nor do a variety of other common protecting groups for alcohols, thereby rendering SnCl4 selective amongst Lewis acids. The scope, tolerance, and limitations of the strategy are demonstrated through the analysis of several multifunctional substrates, including those bearing Cbz groups.