A method for removal of N-BOC protecting groups from substrates on TFA-sensitive resins

Diastereoselective Reductive Etherification Via High-Throughput Experimentation: Access to Pharmaceutically Relevant Alkyl Ethers

This manuscript describes the development of the first diastereoselective intermolecular synthesis of alkyl ethers via reductive etherification of diverse ketones or aldehydes with alcohols. Key to this development was the use of low-temperature high-throughput experimentation (HTE) technologies that enabled rapid reaction optimizations and parallel synthesis. A broad scope of pharmaceutically relevant substrates was surveyed, which formed alkyl ethers effectively. In addition, we demonstrated that the diastereoselectivity of this transformation can be readily modulated by prudent selection of the reductant.

N-Terminal Selective C-H Azidation of Proline-Containing Peptides: a Platform for Late-Stage Diversification

A methodology for the C-H azidation of N-terminal proline-containing peptides was developed employing only commercially available reagents. Peptides bearing a broad range of functionalities and containing up to 6 amino acids were selectively azidated at the carbamate-protected N-terminal residue in presence of the numerous other functional groups present on the molecules. Post-functionalizations of the obtained aminal compounds were achieved: cycloaddition reactions or C-C bond formations via a sequence of imine formation/nucleophilic addition were performed, offering an easy access to diversified peptides.

Solid-Phase Synthesis of Biaryl Cyclic Lipopeptides Derived from Arylomycins

An efficient approach for the solid-phase synthesis of N-methylated tailed biaryl cyclic lipopeptides based on the structure of arylomycins was established. Each of these analogues incorporates an N-terminal linear lipopeptide attached to a biaryl cyclic tripeptide containing a Phe-Tyr, a Tyr-Tyr, or a His-Tyr linkage. This methodology first involved an intramolecular Suzuki-Miyaura arylation of a linear peptidyl resin incorporating the corresponding halogenated amino acid at the N-terminus and a boronotyrosine at the C-terminus. After N-methylation of the resulting biaryl cyclic peptidyl resin, the N-methylated lipopeptidyl tail was then assembled. The biaryl cyclic lipopeptides were purified and characterized.

Monofluoroalkene-Isostere as a 19 F NMR Label for the Peptide Backbone: Synthesis and Evaluation in Membrane-Bound PGLa and (KIGAKI)3

Solid-state ¹⁹ F NMR is a powerful method to study the interactions of biologically active peptides with membranes. So far, in labelled peptides, the ¹⁹ F-reporter group has always been installed on the side chain of an amino acid. Given the fact that monofluoroalkenes are non-hydrolyzable peptide bond mimics, we have synthesized a monofluoroalkene-based dipeptide isostere, Val-Ψ[(Z)-CF=CH]-Gly, and inserted it in the sequence of two well-studied antimicrobial peptides: PGLa and (KIGAKI)₃ are representatives of an α-helix and a β-sheet. The conformations and biological activities of these labeled peptides were studied to assess the suitability of monofluoroalkenes for ¹⁹ F NMR structure analysis.

Total Synthesis and Structural Revision of Cyclotetrapeptide Asperterrestide A

The structural revision of cyclotetrapeptide asperterrestide A has been achieved based on total synthesis and molecular modeling. For these studies, (2 R,3 S)-MePhe(3-OH) and (2 S,3 S)-MePhe(3-OH) suitably protected for peptide synthesis were prepared via a stereoselective reduction of a ketone precursor derived from L- or d-serine, using L-selectride or DIBAL-H. The synthesis of the proposed structure of asperterrestide A (1a) was accomplished by solution-phase synthesis of a linear precursor followed by macrolactamization. The NMR spectra of our synthetic 1a were not identical to those reported for the natural compound. Molecular modeling studies suggested that the correct structure 1b was the one in which the stereochemistry at the α-positions of the Ala and MePhe(3-OH) residues is the opposite to that of the proposed structure. This was confirmed by the total synthesis of 1b and its subsequent structural characterization.

Solid-phase synthesis of biaryl bicyclic peptides containing a 3-aryltyrosine or a 4-arylphenylalanine moiety

A methodology for the solid-phase synthesis of biaryl bicyclic peptides containing a Phe-Phe, a Phe-Tyr or a Tyr-Tyr motif has been devised. This approach comprises two key steps. The first one involves the cyclization of a linear peptidyl resin containing the corresponding halo- and boronoamino acids via a microwave-assisted Suzuki–Miyaura cross coupling. This step is followed by the macrolactamization of the resulting biaryl monocyclic peptidyl resin leading to the formation of the expected biaryl bicyclic peptide. This study provides the first solid-phase synthesis of this type of bicyclic compounds being amenable to prepare a diversity of synthetic or natural biaryl bicyclic peptides.

Modular synthesis of pentaarylcyclopentadienyl Ru-based molecular machines via sequential Pd-catalysed cross couplings

A dissymmetric piano-stool ruthenium(ii) complex as a key building block in the modular synthesis of molecular cogwheel and winch prototypes.

Nα-Amino acid containing privileged structures: design, synthesis and use in solid-phase peptide synthesis

Fmoc-protected Nα-amino acid containing heterocyclic privileged structures, O-(1-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)-l-serine and O-((S)-5-oxo-2,3,5,7a-tetrahydro-1H-pyrrolizin-7-yl)-l-serine, were synthesized on the solid phase from simple commercially available building blocks under mild conditions. The amino acid side-chain is composed of tetramic acid, a natural product derived privileged structure. The key transformation was the formation of cyclic enol ethers via nonclassical Wittig olefinations of the esters. Solid-phase synthesis represents a method of choice, particularly for the synthesis of peptides. This route is compatible with traditional Merrifield solid-phase peptide synthesis (SPPS), as documented on the preparation of the pentapeptide Leu-enkephalin amide H-Tyr-Gly-Gly-Phe-Leu-NH2 with Phe or Tyr replaced by a novel amino acid.

Synthesis of N-Substituted 3-Amino-4-halopyridines: A Sequential Boc-Removal/Reductive Amination Mediated by Brønsted and Lewis Acids

N-Substituted 3-amino-4-halopyridines are valuable synthetic intermediates, as they readily provide access to imidazopyridines and similar heterocyclic systems. The direct synthesis of N-substituted 3-amino-4-halopyridines is problematic, as reductive aminations and base-promoted alkylations are difficult in these systems. A high yielding deprotection/alkylation protocol mediated by trifluoroacetic acid and trimethylsilyl trifluoromethanesulfonate is described, providing access to a wide scope of N-substituted 3-amino-4-halopyridines. This protocol furnishes many reaction products in high purity without chromatography. Similar reductive amination conditions were also established for deactivated anilines.

Structure-activity relationship studies toward the discovery of selective apelin receptor agonists

Apelin is the endogenous ligand for the previously orphaned G protein-coupled receptor APJ. Apelin and its receptor are widely distributed in the brain, heart, and vasculature, and are emerging as an important regulator of body fluid homeostasis and cardiovascular functions. To further progress in the pharmacology and the physiological role of the apelin receptor, the development of small, bioavailable agonists and antagonists of the apelin receptor, is crucial. In this context, E339-3D6 (1) was described as the first nonpeptidic apelin receptor agonist. We show here that 1 is actually a mixture of polymethylated species, and we describe an alternative and versatile solid-phase approach that allows access to highly pure 27, the major component of 1. This approach was also applied to prepare a series of derivatives in order to identify the crucial structural determinants required for the ligand to maintain its affinity for the apelin receptor as well as its capacity to promote apelin receptor signaling and internalization. The study of the structure-activity relationships led to the identification of ligands 19, 21, and 38, which display an increased affinity compared to that of 27. The latter and 19 behave as full agonists with regard to cAMP production and apelin receptor internalization, whereas 21 is a biased agonist toward cAMP production. Interestingly, the three ligands display a much higher stability in mouse plasma (T1/2 > 10 h) than the endogenous apelin-17 peptide 2 (T1/2 < 4 min).

Distance distributions of end-labeled curved bispeptide oligomers by electron spin resonance

We demonstrate the synthesis of a series of spin-labeled curved oligomers to determine their end-to-end lengths and distance distributions using electron spin resonance. We synthesize shape-persistent macromolecules from conformationally restricted, asymmetric monomers that are coupled through pairs of amide bonds to create water-soluble, spiro-ladder oligomers with well-defined three-dimensional structures. We synthesized seven different macromolecules, each containing eight monomers but differing in the sequence to create macromolecules with different curved shapes. The ends of the oligomers were labeled with nitroxide spin probes, and double electron-electron resonance (DEER) electron spin resonance (ESR) experiments were carried out to obtain quantitative information about the shapes and flexibility of the oligomers. The most probable end-to-end distance of the oligomers ranges from 23 to 36 A, a range of length that we previously accessed by assembling rod-like homo-oligomers that contain 4-8 bisamino acid monomers. The relative distances measured for the oligomers confirm that, by varying the sequence of an oligomer, we are able to control its shape. The shapes of the ESR-derived population distributions allow us to compare the degree of shape persistence and flexibility of spiro-ladder oligomers to other well-studied nanoscale molecular structures such as p-phenylethynylenes.

An efficient protocol for the solid-phase synthesis of malondiamides

A novel and straightforward solid-phase synthesis of malondiamides containing a free nitrogen has been developed. These intermediates, which can be directly obtained in good yield and purity, can be further derivatised. This approach can be used for the synthesis of large split-and-mix-libraries.

Peptide-based inhibitors of hepatitis C virus full-length NS3 (protease-helicase/NTPase): model compounds towards small molecule inhibitors

From L-alpha-aminobutyric acid (Abu) a set of electrophilic and non-electrophilic replacements for the P1 cysteine of substrate and product inhibitors of hepatitis C virus full-length NS3 (protease-helicase/NTPase) serine protease have been synthesised and coupled to a model pentapeptide furnishing a set of hexapeptide inhibitors. Promising inhibitory activities with K(i) values of 0.18 microM (11b, P1 electrophilic alpha,beta-unsaturated ketone), 0.46 microM (12e, P1 electrophilic alkyl ketone) and 0.98 microM (10e, P1 non-electrophilic alkenyl alcohol as diastereomeric mixture). The reference hexapeptide product inhibitor had a K(i) value of 1.54 microM (14, P1 Abu-OH). The electrophilic inhibitors exhibit increased potency as compared with the corresponding product inhibitor, and notably also the non-electrophilic P1 alkenyl alcohol 10e. This represents the first example of non-electrophilic inhibitors that are not P1 amides or product inhibitors.

Acid-labile protecting groups for the synthesis of lipidated peptides

Lipidated peptides and their neolipoprotein derivatives are efficient tools for the investigation of biological processes in molecular detail. These compounds are often acid- and base-labile, and their synthesis requires the use of a combination of blocking groups that can be removed under very mild conditions. In this article we demonstrate that the Boc urethane and different trityl-type protecting groups can be cleaved selectively under acidic conditions that are mild enough to be compatible with the demands of lipopeptide synthesis. Thus, the Boc group was cleaved with TMS triflate in the presence of lutidine, and the methyltrityl (Mtt) and the methoxytrityl (Mmt) group were removed with 1% TFA in dichloromethane in the presence of triethylsilane as cation scavenger. Removal of the phenylfluorenyl group was achieved with up to 3% TFA in dichloromethane in the presence of triethylsilane at 0 degrees C. These protecting-group techniques were successfully applied in the synthesis of differently lipidated H-Ras peptides.