Selective and Multiple Functionalization of Pyridines and Alkaloids via Mg- and Zn-Organometallic Intermediates

Regioselective magnesiations of functionalized arenes and heteroarenes using TMP2Mg in hydrocarbons

We report the preparation of a new hydrocarbon-soluble magnesium amide TMP₂Mg (TMP = 2,2,6,6-tetramethylpiperidyl). This base showed excellent properties for the regioselective magnesiation of various arenes and heteroarenes bearing ethyl esters and carbamates under very mild reaction conditions. Subsequent trapping with aryl iodides (Negishi cross-coupling) gave access to a range of highly functionalized valuable building blocks.

Regioselective Magnesiations and Zincations of Aromatics and Heterocycles Triggered by Lewis Acids

Mixed TMP‐bases (TMP=2,2,6,6‐tetramethylpiperidyl), such as TMPMgCl ⋅ LiCl, TMP₂Mg ⋅ 2LiCl, TMPZnCl ⋅ LiCl and TMP₂Zn ⋅ 2LiCl, are outstanding reagents for the metalation of functionalized aromatics and heterocycles. In the presence of Lewis acids, such as BF₃ ⋅ OEt₂ or MgCl₂, the metalation scope of such bases was dramatically increased, and regioselectivity switches were achieved in the presence or absence of these Lewis acids. Furthermore, highly reactive lithium bases, such as TMPLi or Cy₂NLi, are also compatible with various Lewis acids, such as MgCl₂ ⋅ 2LiCl, ZnCl₂ ⋅ 2LiCl or CuCN ⋅ 2LiCl. Performing such metalations in continuous flow using commercial setups permitted practical and convenient reaction conditions.

Enantioselective Total Synthesis of (+)-Garsubellin A

Garsubellin A is a meroterpene capable of enhancing the enzyme choline acetyltransferase whose decreased level is believed to play a central role in the symptoms of Alzheimer's disease. Due to the potentially useful biological activity together with the novel bridged and fused cyclic molecular architecture, garsubellin A has garnered substantial synthetic interest, but its absolute stereostructure has been undetermined. We report here the first enantioselective total synthesis of (+)‐garsubellin A. Our synthesis relies on stereoselective fashioning of a cyclohexanone framework and double conjugate addition of 1,2‐ethanedithiol that promotes aldol cyclization to build the bicyclic [3.3.1] skeleton. The twelve‐step, protecting group‐free synthetic route has enabled the syntheses of both the natural (−)‐garsubellin A and its unnatural (+)‐antipode for biological evaluations.

Image-Based Morphological Profiling Identifies a Lysosomotropic, Iron-Sequestering Autophagy Inhibitor

Chemical proteomics is widely applied in small-molecule target identification. However, in general it does not identify non-protein small-molecule targets, and thus, alternative methods for target identification are in high demand. We report the discovery of the autophagy inhibitor autoquin and the identification of its molecular mode of action using image-based morphological profiling in the cell painting assay. A compound-induced fingerprint representing changes in 579 cellular parameters revealed that autoquin accumulates in lysosomes and inhibits their fusion with autophagosomes. In addition, autoquin sequesters Fe²⁺ in lysosomes, resulting in an increase of lysosomal reactive oxygen species and ultimately cell death. Such a mechanism of action would have been challenging to unravel by current methods. This work demonstrates the potential of the cell painting assay to deconvolute modes of action of small molecules, warranting wider application in chemical biology.

Cinchona Alkaloids-Derivatives and Applications

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

Regioselective Metalation and Functionalization of the Pyrazolo[1,5- a]pyridine Scaffold Using Mg- and Zn-TMP Bases

A regioselective functionalization of the pyrazolo[1,5- a]pyridine scaffold using Mg- and Zn-TMP bases (TMP = 2,2,6,6-tetramethylpiperidyl) in the presence or absence of BF₃·OEt₂ is described. Also, various functionalized pyrazolo[1,5- a]pyridines bearing an ester function (and an NHBoc or ethyl group) are magnesiated and functionalized, leading to polysubstituted heterocycles. Additionally, a sulfoxide directed ortho-metalation, followed by the transition-metal-free amination of a pyrazolo[1,5- a]pyridine sulfoxide, using a magnesium amide, is reported.

Base-Controlled Regioselective Functionalization of Chloro-Substituted Quinolines

We prepared a number of di- and trifunctionalized quinolines by selective metalation of chloro-substituted quinolines with metal amides followed by reaction with different electrophiles. Metalation of the C-3 position of the quinolinic ring with lithium diisopropylamide at -70 °C is easy to achieve, whereas reaction with lithium-magnesium and lithium-zinc amides affords C-2 or C-8 functionalized derivatives in a regioselective fashion. These complementary methods could be rationalized by DFT calculations and are convenient strategies toward the synthesis of bioactive quinoline derivatives such as chloroquine analogues.

Zn-, Mg-, and Li-TMP Bases for the Successive Regioselective Metalations of the 1,5-Naphthyridine Scaffold (TMP=2,2,6,6-Tetramethylpiperidyl)

A set of successive regioselective metalations and functionalizations of the 1,5-naphthyridine scaffold are described. A combination of Zn-, Mg-, and Li-TMP (TMP=2,2,6,6-tetramethylpiperidyl) bases and the presence or absence of a Lewis acid (BF₃ ⋅OEt₂ ) allows the introduction of up to three substituents to the 1,5-naphthyridine core. Also, a novel "halogen dance" reaction was discovered upon metalation of an 8-iodo-2,4-trifunctionalized 1,5-naphthyridine allowing a fourth regioselective functionalization. Additionally, reactions leading to key 1,5-naphthyridines for the preparation of OLED materials and a potential antibacterial agent were performed.

C-H Functionalization of Azines

Azines, which are six-membered aromatic compounds containing one or more nitrogen atoms, serve as ubiquitous structural cores of aromatic species with important applications in biological and materials sciences. Among a variety of synthetic approaches toward azines, C-H functionalization represents the most rapid and atom-economical transformation, and it is advantageous for the late-stage functionalization of azine-containing functional molecules. Since azines have several C-H bonds with different reactivities, the development of new reactions that allow for the functionalization of azines in a regioselective fashion has comprised a central issue. This review describes recent advances in the C-H functionalization of azines categorized as follows: (1) S_NAr reactions, (2) radical reactions, (3) deprotonation/functionalization, and (4) metal-catalyzed reactions.

Selective functionalization of complex heterocycles via an automated strong base screening platform

An automated high throughput screening approach to optimizing strong base-mediated metalations.

Iron-Catalyzed Acylation of Polyfunctionalized Aryl- and Benzylzinc Halides with Acid Chlorides

FeCl2 (5 mol %) catalyzes a smooth and convenient acylation of functionalized arylzinc halides at 50 °C (2-4 h) and benzylic zinc chlorides at 25 °C (0.5-4 h) with a variety of acid chlorides leading to polyfunctionalized diaryl and aryl heteroaryl ketones.

Synthetic and reactivity studies of hetero-tri-anionic sodium zincates

The blue, red and green spheres represent three different anions present within a series of novel hetero-trianionic sodium zincates. The syntheses and structures of the complexes are reported as well as their reactivities with important organic molecules.

The medicinal chemist's toolbox for late stage functionalization of drug-like molecules

The advent of modern C–H functionalization chemistries has enabled medicinal chemists to consider a synthetic strategy, late stage functionalization (LSF), which utilizes the C–H bonds of drug leads as points of diversification for generating new analogs.

Unified Protocol for Cobalt-Catalyzed Oxidative Assembly of Two Aryl Metal Reagents Using Oxygen as an Oxidant

The first cobalt-catalyzed oxidative cross-coupling reaction of two aryl metal reagents is described. An equivalent amount of two aryl Grignard or lithium reagents, after mediation by an equivalent amount of simple ClTi(OEt)3, was facilely assembled under the catalysis of 1 mol % of CoCl2/10 mol % of DMPU using oxygen. The cross-couplings between various aryl metal reagents, especially between two structurally similar aryl Grignard reagents, proceeded smoothly and selectively and, thus, provided a highly general and efficient method for the construction of biaryl compounds.

Iron catalyzed oxidative assembly of N-heteroaryl and aryl metal reagents using oxygen as an oxidant

An equivalent amount of N-heteroaryl and aryl Grignard or lithium reagents, after mediation by an equivalent of titanate, was facilely coupled to furnish N-heteroaryl-aryl compounds under the catalysis of FeCl3/TMEDA at ambient temperature using oxygen as an oxidant. Most of the common N-heteroaryls were all good candidates, and thus provided a general, green and pratical protocol for the flexible construction of various N-heteroaryl-aryl structures.

TMP-magnesium and TMP-zinc bases for the regioselective metalation of the cinnoline scaffold

A regioselective functionalization of cinnolines in positions 3 and 8 using metalations has been developed. This involves either the use of a frustrated Lewis pair consisting of BF3·Et2O and TMP2Mg·2LiCl or the in situ generated base TMP2Zn·2MgCl2·2LiCl. Successive metalations allow the preparation of 3,8-disubstituted cinnolines. Various functionalizations by acylation, allylation, and cross-coupling reactions with aryl halides or alkenyl iodides were carried out successfully.

Deproto-metallation using mixed lithium–zinc and lithium–copper bases and computed CH acidity of 2-substituted quinolines

The metallated quinolines were functionalized by iodolysis or aroylation.

Brønsted acid catalyzed intramolecular benzylic cyclizations of alkylpyridines

Aldehyde and ketone electrophiles incorporated into the side chains of 2- and 4-alkylpyridines participate in intramolecular aldol-like condensations with pyridine benzylic carbons in the presence of Brønsted acid catalysts.

Synthesis and characterization of novel linear and cross-linked polyurethane urea elastomers with 2,3-diaminopyridine in the main chain

Novel polyurethane urea containing pyridine moieties in their main chains have been prepared by a two-step solution polymerization procedure. These polyurethane urea elastomers were synthesized by chain-extending isocyanate end-capped prepolymers with 2,3-diaminoyridine and different cross-linkers. The isocyanate-terminated prepolymers were obtained from poly(tetramethylene oxide) glycol of molecular weight 1400 (Terathane 1400) and 1,6-hexamethylene diisocyanate. Including pyridine derivatives into the structural compositions leads to a change in properties that were investigated using Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mechanical measurements and contact angle and fluorescence analysis. The results show that intermolecular hydrogen bonds have formed between pyridine groups and urethane or urea groups, affecting all the obtained properties.