Metal-free reductive desulfurization of C-sp3-substituted thiols using phosphite catalysis.

Phosphines and phosphites are critical tools for non-metal desulfurative methodologies due to the strength of the P[double bond, length as m-dash]S bond. An overarching premise in these methods has been that stoichiometric (or excess) P(iii) reagent is required for reactivity. Despite decades of research, a desulfurative process that is catalytic in phosphine/phosphite has not been reported. Here, we report the successful merging of two thermal radical processes: the desulfurization of unactivated and activated alkyl thiols and the reduction of P(v) = S to P(iii) by reaction with a silyl radical species. We employ catalytic trimethyl phosphite, catalytic azo-bis(cyclohexyl)nitrile, and two equivalents of tris(trimethylsilyl)silane as the stoichiometric reductant and sulfur atom scavenger. This method is tolerant of common organic functional groups and affords products in good to excellent yields.

Phosphine-Dependent Photoinitiation of Alkyl Thiols under Near-UV Light Facilitates User-Friendly Peptide Desulfurization.

Peptides are steadily gaining importance as pharmaceutical targets, and efficient, green methods for their preparation are critically needed. A key deficiency in the synthetic toolbox is the lack of an industrially viable peptide desulfurization method. Without this tool, the powerful native chemical ligation reaction typically used to assemble polypeptides and proteins remains out of reach for industrial preparation of drug targets. Current desulfurization methods require very large excesses of phosphine reagents and thiol additives or low-abundance metal catalysts. Here, we report a phosphine-only photodesulfurization (POP) using near-UV light that is clean, high-yielding, and requires as little as 1.2 equiv phosphine. The user-friendly reaction gives complete control to the chemist, allowing solvent and reagent selection based on starting material and phosphine solubility. It can be conducted in a range of solvents, including water or buffers, on protected or unprotected peptides, in low or high dilution and on gram scale. Oxidation-prone amino acids, π-bonds, aromatic rings, thio-aminal linkages, thioesters, and glycans are all stable to the POP reaction. We highlight the utility of this approach for desulfurization of industrially relevant targets including cyclic peptides and glucagon-like peptide 1 (GLP-1(7-36)). The method is also compatible with NCL buffer, and we highlight the robustness of the approach through the one-pot disulfide reduction/multidesulfurization of linaclotide, aprotinin, and wheat protein.

Recent advances in the synthesis of C-terminally modified peptides.

C-Terminally modified peptides are important for the development and delivery of peptide-based pharmaceuticals because they impact peptide activity, stability, hydrophobicity, and membrane permeability. Additionally, the vulnerability of C-terminal esters to cleavage by endogenous esterases makes them excellent pro-drugs. Methods for post-SPPS C-terminal functionalization potentially enable access to libraries of modified peptides, facilitating tailoring of their solubility, potency, toxicity, and uptake pathway. Apparently minor structural changes can significantly impact the binding, folding, and pharmacokinetics of the peptide. This review summarizes developments in chemical methods for C-terminal modification of peptides published since the last review on this topic in 2003.

Raising the Bar On-Bead: Efficient On-Resin Synthesis of α-Conotoxin LvIA.

α4/7-Conotoxin LvIA is an isoform-selective inhibitor of the α3ß2 nicotinic acetylcholine receptor. An efficient strategy for the synthesis of this toxin is critical to advancing its utility as a probe for receptor function and as a potential pharmaceutical lead target. On-resin methods for peptide synthesis offer potential synthetic advantages; however, strategies for on-resin formation of multiple disulfides have historically been low-yielding. Here, we harness the reactivity of the Allocam protecting group and employ 3-amino acid spacer strategy to synthesize α4/7-conotoxin LvIA via three different on-resin strategies, each of which results in an isolated yield higher than prior fully on-resin approaches.

Facile triflic acid-catalyzed α-1,2-cis-thio glycosylations: scope and application to the synthesis of S-linked oligosaccharides, glycolipids, sublancin glycopeptides, and TN/TF antigens.

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts. We here report a facile approach to access α-1,2-cis-S-linked glycosides using triflic acid as a catalyst to promote the glycosylation of a series of thiols with d-glucosamine, galactosamine, glucose, and galactose electrophiles. This method is broadly applicable for the stereoselective synthesis of S-linked glycopeptides, oligosaccharides and glycolipids in high yield and excellent α-selectivity. Many of the synthetic limitations associated with the preparation of these S-linked products are overcome by this catalytic method.

Bringing Macrolactamization Full Circle: Self-Cleaving Head-to-Tail Macrocyclization of Unprotected Peptides via Mild N-Acyl Urea Activation.

We establish herein conditions for the cyclization of unprotected N-acyl urea-linked peptides to form macrocyclic peptides mediated by N-terminal cysteine. We report a detailed investigation of the parameters of the reaction, including variation of the reaction conditions, the C-terminal residue, and the macrocycle size. C-Terminal epimerization was not observed. The synthesis of macrocyclic targets ranging from tetrapeptides to the disulfide-linked 14-mer, sunflower trypsin inhibitor 1 are demonstrated. For most substrates, hydrolysis and head-to-tail dimer formation are avoided.

Synthetic and Computational Study of Tin-Free Reductive Tandem Cyclizations of Neutral Aminyl Radicals.

5- exo, 5- exo Cyclizations of conformationally unbiased propargylic aminyl radicals proceed with excellent yield, chemoselectivity, and diastereoselectivity under tin-free reductive cyclization conditions, regardless of the electronic environments and intermediate radical stabilization resulting from various olefin substituents. These conditions avoid the need for slow addition of initiator and reductant. By contrast, analogous 6- exo, 5- exo cyclizations require substituents capable of intermediate radical stabilization to avoid premature reduction products. These experimental results are corroborated by computations that further establish the reactivity of these aminyl radicals upon exposure to tin-free cyclization conditions.

Epimerization-free access to C-terminal cysteine peptide acids, carboxamides, secondary amides, and esters via complimentary strategies.

C-Terminal cysteine peptide acids are difficult to access without epimerization of the cysteine α-stereocenter. Diversification of the C-terminus after solid-phase peptide synthesis poses an even greater challenge because of the proclivity of the cysteine α-stereocenter to undergo deprotonation upon activation of the C-terminal carboxylic acid. We present herein two general strategies to access C-terminal cysteine peptide derivatives without detectable epimerization, diketopiperazine formation, or piperidinylalanine side products.

Tin-Free Access to the ABC Core of the Calyciphylline A Alkaloids and Unexpected Formation of a D-Ring-Contracted Tetracyclic Core.

A tin-free strategy for the successful cyclization of a variety of internal alkyne-containing N-chloroamine precursors to the ABC core via cyclization of a neutral aminyl radical is established. Deuterium labeling experiments confirm that the solvent is the primary source of the final H atom in the cyclization cascade. These conditions enabled a streamlined route to a ß-ketoester intermediate poised for intramolecular Knoevenagel condensation to construct the seven-membered D-ring of calyciphylline A alkaloids. However, exposure to CsF in t-BuOH at elevated temperatures led to an unexpected decarboxylation to form a D-ring-contracted tetracyclic core.

Sequence Diversification by Divergent C-Terminal Elongation of Peptides.

Sequence diversification at the C terminus is traditionally limited by significant epimerization of the C-terminal residue during its activation toward nucleophilic attack, thus mandating repetition of the peptide synthesis for each targeted variation. Here, we accomplish divergent C-terminal elongation of a single peptide substrate with concomitant resin cleavage via displacement of an N-acyl urea moiety. Sterically hindered amino acids such as Ile and Pro are well-tolerated in this approach, which proceeds reasonable conversion and no detectable epimerization of the starting peptide's C-terminal amino acid.

Total Chemical Synthesis of Human Thyroid-Stimulating Hormone (hTSH) ß-Subunit: Application of Arginine-tagged Acetamidomethyl (AcmR) Protecting Groups.

The ß-subunit of human thyroid stimulating hormone (hTSH) has been synthesized as a single glycoform bearing a chitobiose disaccharide at the native glycosylation site. Key to the successful completion of this synthesis was the introduction of an arginine-tagged acetamidomethyl group, which served to greatly facilitate handling of a glycopeptide fragment with poor aqueous solubility. This general solution to the challenge of working with intractable peptides is expected to find wide use in protein synthesis.

Exploiting the MeDbz Linker To Generate Protected or Unprotected C-Terminally Modified Peptides.

C-terminally modified peptides are important targets for pharmaceutical and biochemical applications. Known methods for C-terminal diversification are limited mainly in terms of the scope of accessible modifications or by epimerization of the C-terminal amino acid. In this work, we present a broadly applicable approach that enables access to a variety of C-terminally functionalized peptides in either protected or unprotected form. This chemistry proceeds without epimerization of C-terminal Ala and tolerates nucleophiles of varying nucleophilicity. Finally, unprotected peptides bearing nucleophilic side chain groups can be selectively functionalized by strong nucleophiles, whereas macrocyclization is observed for weaker nucleophiles. The potential utility of this method is demonstrated through the divergent synthesis of the conotoxin conopressin G and GLP-1(7-36) and analogs.

Direct palladium-mediated on-resin disulfide formation from Allocam protected peptides.

The synthesis of disulfide-containing polypeptides represents a long-standing challenge in peptide chemistry, and broadly applicable methods for the construction of disulfides are in constant demand. Few strategies exist for on-resin formation of disulfides directly from their protected counterparts. We present herein a novel strategy for the on-resin construction of disulfides directly from Allocam-protected cysteines. Our palladium-mediated approach is mild and uses readily available reagents, requiring no special equipment. No reduced peptide intermediates or S-allylated products are observed, and no residual palladium can be detected in the final products. The utility of this method is demonstrated through the synthesis of the C-carboxy analog of oxytocin.

Copper-Catalyzed Asymmetric Propargylation of Cyclic Aldimines.

The copper-catalyzed asymmetric propargylation of cyclic aldimines is reported. The influence of the imine trimer to inhibit the reaction was identified, and equilibrium constants between the monomer and trimer were determined for general classes of imines. Asymmetric propargylation of a diverse series of N-alkyl and N-aryl aldimines was achieved with good to high asymmetric induction. The utility was demonstrated by a titanium catalyzed hydroamination and reduction to generate the chiral indolizidines (-)-crispine A and (-)-harmicine.

Real-Time, Selective Detection of Copper(II) Using Ionophore-Grafted Carbon-Fiber Microelectrodes.

Rapid, selective detection of metals in complex samples remains an elusive goal that could provide critical analytical information for biological and environmental sciences and industrial waste management. Fast-scan cyclic voltammetry (FSCV) using carbon-fiber microelectrodes (CFMs) is an emerging technique for metal analysis with broad potential applicability because of its rapid response to changes in analyte concentration and minimal disturbance to the analysis medium. In this communication, we report the first effective application of covalently modified CFMs to achieve highly selective, subsecond Cu(II) measurements using FSCV. A two-part strategy is employed for maximum selectivity: Cu(II) binding is augmented by a covalently grafted ionophore, while binding of other metals is prevented by chemical blocking of nonselective surface adsorption sites. The resulting electrodes selectively detect Cu(II) in a complex medium comprising several interfering metals. Overall, this strategy represents a transformative innovation in real-time electrochemical detection of metal analytes.

Enantioselective Synthesis of Caprolactam and Enone Precursors to the Heterocyclic DEFG Ring System of Zoanthenol.

The enantioselective synthesis of both caprolactam and enone synthons for the DEFG ring system of zoanthenol are described. The evolution of this synthetic approach proceeds first through a synthesis using the chiral pool as a starting point. Challenges in protecting group strategy led to the modification of this approach beginning with (±)-glycidol. Ultimately, an efficient approach was developed by employing an asymmetric hetero-Diels-Alder reaction. The caprolactam building block can be converted by an interesting selective Grignard addition to the corresponding enone synthon. Addition of a model alkyne provides support for the late-stage addition of a hindered alkyne into the caprolactam building block.

Toward the ABCD Core of the Calyciphylline A-Type Daphniphyllum Alkaloids: Solvent non-Innocence in Neutral Aminyl Radical Cyclizations.

The Daphniphyllum alkaloids remain an attractive target in the synthetic community because of their unique framework and promising biological activities. We have shown that the ABC core of the calyciphylline A-type alkaloids can be rapidly accessed via the tandem cyclization of a neutral aminyl radical with a polarized cyclic olefin. Deuterium labeling experiments and reactions omitting a tin hydride reagent suggest that the solvent is the major source of the terminating hydrogen atom in the cyclization cascade. Incorporation of an internal alkyne in the radical pathway was tolerated in the reaction, and it provided the necessary atoms to enable completion of the D ring of the calyciphylline A-type alkaloids.

Rapid access to the heterocyclic core of the calyciphylline A and daphnicyclidin A-type Daphniphyllum alkaloids via tandem cyclization of a neutral aminyl radical.

A streamlined approach to the tertiary amine-containing core of the calyciphylline A and daphnicyclidin A-type Daphniphyllum alkaloids is presented. A known carvone derivative is converted into the core structure in only four synthetic operations, and it is well poised for further elaboration. The key enabling methodology is a radical cyclization cascade beginning with addition of a secondary, neutral aminyl radical to the ß-position of an enone, followed by trapping with a pendant alkyne.