Synthesis of novel halopyridinylboronic acids and esters. Part 4: Halopyridin-2-yl-boronic acids and esters are stable, crystalline partners for classical Suzuki cross-coupling

2,6-Bis(diazaboryl)pyridine: A Superbasic Sterically Demanding Pyridine Ligand

Herein a convenient one-pot route to a sterically demanding superbasic pyridine is presented. Functionalization of the 2- and 6-positions with the strongly σ-donating boryl-groups shifts the calculated gas phase basicity of the pyridine nitrogen atom to 1012 kJ mol^-1 , which outperforms the "proton sponge" 1,8-bis(dimethylamino)naphthalene (996 kJ mol^-1 ). The diazaboryl groups are oriented orthogonally to the pyridine ring and do not block the N-position, which resembles the geometry of commonly used N-heterocyclic carbenes. This allows the substituted pyridine to be used as a neutral N-donor ligand in coordination chemistry that is demonstrated herein with the Lewis adducts of haloboranes. Contrary to NHCs, which can form extraordinarily stable adducts, the pyridine ligand is intended to act as a weaker-coordinating alternative and could allow for alternative ligand chemistry.

Selective palladium-catalysed arylation of 2,6-dibromopyridine using N-heterocyclic carbene ligands

A selective palladium-catalysed arylation of 2,6-dibromopyridine has been developed by employing N-heterocyclic carbene ligands. Selective mono-arylation was performed in water/acetonitrile solvent at ambient temperature and low catalyst loading.

Palladium-catalyzed oxidative CH/CH cross-coupling of pyridine N-oxides with five-membered heterocycles

Using Ag₂CO₃ as an additive, we developed the Pd-catalyzed intermolecular C–H/C–H cross-coupling of pyridine N-oxides with five-membered heterocycles such as 1-benzyl-1,2,3-triazoles, thiophens and furans.

Using halo (het) arylboronic species to achieve synthesis of foldamers as protein–protein interaction disruptors

Protein–protein interactions (PPIs) play a central role in all biological processes and have been the focus of intense investigations from structural molecular biology to cell biology for the majority of the last two decades and, more recently, are emerging as important targets for pharmaceuticals. A common motif found at the interface of PPIs is the α-helix, and apart from the peptidic structures, numerous nonpeptidic small molecules have been developed to mimic α-helices. The first-generation terphenyl scaffold is able to successfully mimic key helix residues and disrupt relevant interactions, including Bcl-xL-Bak interactions that are implicated in apoptosis mechanism. These scaffolds were designed and evaluated in silico. Analysis revealed that substituents on aromatic scaffolds can efficiently mimic side-chain surfaces. Unfortunately, the literature describes a long and difficult procedure to access these aromatic-based scaffolds. The search for new simpler methodology is the aim of the research of our medicinal chemistry team. On the basis of structural requirements, we developed a program concerning the synthesis of new oligo(het)aryl scaffolds produced by iterative couplings of boronic species (garlanding) in which substituents on rings project functionality in spatial orientations that mimic residues of an α-helix.

Synthesis of the phenylpyridal scaffold as a helical peptide mimetic

Phenylpyridal- and phenyldipyridal-based scaffolds have been designed and synthesized as novel helical peptide mimetics. The synthesis required optimisation and selective alkylation in producing 2,6-functionalized 3-hydroxypyridine derivatives for a convergent scheme. The pyridine analogues were coupled by a series of Suzuki/Stille types cross-coupling reactions. A series of biaryl and ter-aryl substituted heterocycles were produced. The synthetic approach was concise and high yielding allowing large variability at the wanted side-chain attachment points. A number of compounds were synthesised to show the versatility of the strategy.

A general solution for unstable boronic acids: slow-release cross-coupling from air-stable MIDA boronates

Many boronic acids, including 2-heterocyclic, vinyl, and cyclopropyl derivatives, are inherently unstable, which can limit their benchtop storage and/or efficient cross-coupling. We herein report the first general solution to this problem: in situ slow release of unstable boronic acids from the corresponding air-stable MIDA boronates. This remarkably general approach has transformed all three classes of these unstable boronic acids into shelf-stable and highly effective building blocks for cross-coupling with a wide range of aryl and heteroaryl chlorides.

Aryl triolborates: novel reagent for copper-catalyzed N arylation of amines, anilines, and imidazoles

The N arylation of primary and secondary aliphatic amines, anilines, and imidazoles with novel potassium aryl triolborates was carried out in the presence of a reoxidant and a catalytic amount of Cu(OAc)(2) (10 mol %). Aryl triolborates were found to be better reagents than aryl boronic acids or potassium aryl trifluoroborates as the former achieved high yields under mild conditions. Coupling of primary and secondary aliphatic amines to give N-aryl amines in excellent yields was performed under oxygen atmosphere. The reactions of anilines and imidazoles to provide N-aryl anilines and N-aryl imidazoles in good yields proceeded smoothly when trimethylamine N-oxide was used as an oxidant.