Leveraging Pyrazolium Ylide Reactivity to Access Indolizine and 1,2-Dihydropyrimidine Derivatives

N-Heterocyclic ylides are important synthetic precursors to rapidly build molecular complexity. Pyrazolium ylides have largely been unexplored, and we demonstrate their diverse utility in this report. We show that these readily accessible building blocks can be used to construct different heterocyclic skeletons by varying the coupling partner. Indolizines can be formed via an N-deletion type mechanism when reacting pyrazolium salts with electron deficient alkynes. 1,2-Dihydropyrimidines can be formed via a rearrangement mechanism when reacting pyrazolium ylides with isocyanates. These reactions enable access to valuable heteroarenes without the need for transition metal catalysis, high temperatures, or strong bases.

Photocatalytic Boryl Radicals Triggered Sequential B─N/C─N Bond Formation to Assemble Boron-Handled Pyrazoles

Vinyldiazo compounds are one of the most important synthons in the construction of a cyclic ring. Most photochemical transformations of vinyldiazo compounds are mainly focusing on utilization of their C═C bond site, while reactions taking place at terminal nitrogen atom are largely unexplored. Herein, a photocatalytic cascade radical cyclization of LBRs with vinyldiazo reagents through sequential B─N/C─N bond formation is described. The reaction starts with the addition of LBRs (Lewis base-boryl radicals) at diazo site, followed by intramolecular radical cyclization to access a wide range of important boron-handled pyrazoles in good to excellent yields. Control experiments, together with detailed mechanism studies well explain the observed reactivity. Further studies demonstrate the utility of this approach for applications in pharmaceutical and agrochemical research.

Domino Aldol-SNAr-Dehydration Sequence for [3+3] Annulations to Prepare Quinolin-2(1H)-ones and 1,8-Naphthyridin-2(1H)-ones

A domino aldol-SNAr-dehydration [3+3] annulation strategy has been utilized to fuse six-membered cyclic amides onto aromatic substrates. 2-Arylacetamides have been reacted with 2-fluorobenzaldehyde derivatives activated toward SNAr reaction by an electron-withdrawing substituent (NO2, CN, CF3, CO2Me) at C5 to prepare 3,6-disubstituted quinolin-2(1H)-ones. Additionally, 3-substituted 1,8-naphthyridin-2(1H)-ones have been similarly derived from 2-fluoronicotinaldehyde. Fifteen examples are reported, and two possible mechanistic scenarios are presented and discussed.

Towards DNA-Encoded Micellar Chemistry: DNA-Micelle Association and Environment Sensitivity of Catalysis

The development of DNA-compatible reaction methodologies is a central theme to advance DNA-encoded screening library technology. Recently, we were able to show that sulfonic acid-functionalized block copolymer micelles facilitated Brønsted acid-promoted reactions such as the Povarov reaction on DNA-coupled starting materials with minimal DNA degradation. Here, the impact of polymer composition on micelle shape, and reaction conversion was investigated. A dozen sulfonic acid-functionalized block copolymers of different molar mass and composition were prepared by RAFT polymerization and were tested in the Povarov reaction, removal of the Boc protective group, and the Biginelli reaction. The results showed trends in the polymer structure-micellar catalytic activity relationship. For instance, micelles composed of block copolymers with shorter acrylate ester chains formed smaller particles and tended to provide faster reaction kinetics. Moreover, fluorescence quenching experiments as well as circular dichroism spectroscopy showed that DNA-oligomer-conjugates, although highly water-soluble, accumulated very effectively in the micellar compartments, which is a prerequisite for carrying out a DNA-encoded reaction in the presence of polymer micelles.

Fused 1,5-Naphthyridines: Synthetic Tools and Applications

Heterocyclic nitrogen compounds, including fused 1,5-naphthyridines, have versatile applications in the fields of synthetic organic chemistry and play an important role in the field of medicinal chemistry, as many of them have a wide range of biological activities. In this review, a wide range of synthetic protocols for the construction of this scaffold are presented. For example, Friedländer, Skraup, Semmlere-Wolff, and hetero-Diels-Alder, among others, are well known classical synthetic protocols used for the construction of the main 1,5-naphthyridine scaffold. These syntheses are classified according to the nature of the cycle fused to the 1,5-naphthyridine ring: carbocycles, nitrogen heterocycles, oxygen heterocycles, and sulphur heterocycles. In addition, taking into account the aforementioned versatility of these heterocycles, their reactivity is presented as well as their use as a ligand for metal complexes formation. Finally, those fused 1,5-naphthyridines that present biological activity and optical applications, among others, are indicated.

Synthetic Strategies, Reactivity and Applications of 1,5-Naphthyridines

This review covers the synthesis and reactivity of 1,5-naphthyridine derivatives published in the last 18 years. These heterocycles present a significant importance in the field of medicinal chemistry because many of them exhibit a great variety of biological activities. First, the published strategies related to the synthesis of 1,5-naphthyridines are presented followed by the reactivity of these compounds with electrophilic or nucleophilic reagents, in oxidations, reductions, cross-coupling reactions, modification of side chains or formation of metal complexes. Finally, some properties and applications of these heterocycles studied during this period are examined.

Chiral piperidines from acyclic amines via enantioselective, radical-mediated δ C-H cyanation

Piperidines are the most prevalent heterocycle found in medicines. Yet, while they are often chiral, there remain no robust methods for their asymmetric syntheses. To solve this challenge, we have interrupted the century-old Hofmann-Löffler-Freytag (HLF) reaction to afford this privileged heterocycle. The catalytic, regio- and enantio- selective δ C-H cyanation of acyclic amines described herein, incorporates a carbonyl equivalent selectively at the δ position. This δ C-H cyanation is enabled by a chiral Cu catalyst, which both initiates and terminates intramolecular hydrogen atom transfer (HAT) by an N-centered radical relay mechanism. The broad scope and utility of this highly enantioselective method for δ C-C formation is presented, as well as conversion of the resulting enantioenriched δ amino nitriles to a family of chiral piperidines. Experiments probing the chemo-, regio-, and enantio- selectivity of this HAT mechanism are also included to enable extension to other stereoselective δ C-H functionalizations.

Selenium-Catalyzed Carbonylative Synthesis of 3,4-Dihydroquinazolin-2(1H)-one Derivatives with TFBen as the CO Source

An efficient and general carbonylative procedure for the synthesis of 3,4-dihydroquinazolin-2(1H)-one from 1-(halomethyl)-2-nitrobenzenes and aryl/alkyl amines have been explored. In this approach, to avoid of using toxic CO gas, a solid and stable CO precursor, TFBen (benzene-1,3,5-triyl triformate), was utilized. With elemental selenium as the catalyst, a variety of aryl/alkyl amines has been tolerated well to afford the corresponding 3,4-dihydroquinazolin-2(1H)-one products in moderate to excellent yields under mild reaction condition.

Palladium-catalyzed coupling reactions on functionalized 2-trifluoromethyl-4-chromenone scaffolds. Synthesis of highly functionalized trifluoromethyl-heterocycles

The chromenone core is a ubiquitous group in biologically-active natural products and has been extensively used in organic synthesis. Fluorine derived compounds, including those with a trifluoromethyl group (-CF₃), have shown enhanced biological activities in numerous pharmaceuticals compared with their non-fluorinated analogs. We have found that 2-trifluoromethyl chromenones can be readily functionalized in the 8- and 7-positions, providing chromenones cores of high structural complexity which are excellent precursors for numerous trifluoromethyl-heterocycles.

Multicomponent Reactions

Multicomponent Reactions appear to be ideal for any form of synthesis, because of their numerous advantages in terms of sustainability and selectivity in building up complex molecular architectures, with high molecular diversity. This Special Issue collects seven contributions which expand our knowledge about Multicomponent Reactions, providing a good overview about innovative reactivities and applications.

Quinazolin-4(3H)-ones and 5,6-Dihydropyrimidin-4(3H)-ones from β-Aminoamides and Orthoesters

Quinazolin-4(3H)-ones have been prepared in one step from 2-aminobenzamides and orthoesters in the presence of acetic acid. Simple 2-aminobenzamides were easily converted to the heterocycles by refluxing in absolute ethanol with 1.5 equivalents of the orthoester and 2 equivalents of acetic acid for 12–24 h. Ring-substituted and hindered 2-aminobenzamides as well as cases incorporating an additional basic nitrogen required pressure tube conditions with 3 equivalents each of the orthoester and acetic acid in ethanol at 110 °C for 12–72 h. The reaction was tolerant towards functionality on the benzamide and a range of structures was accessible. Workup involved removal of the solvent under vacuum and either recrystallization from ethanol or trituration with ether-pentane. Several 5,6-dihydropyrimidin-4(3H)-ones were also prepared from 3-amino-2,2-dimethylpropionamide. All products were characterized by melting point, FT-IR, ¹H-NMR, ¹³C-NMR, and HRMS.

Synthesis of 2H-Chromenones from Salicylaldehydes and Arylacetonitriles

An efficient and convenient protocol for the synthesis of 2H-chromenones has been developed. In the presence of ^tBuOK in DMF, good to excellent yields of various chromenones were obtained from the corresponding salicylaldehydes and arylacetonitriles. No protection of inert gas atmosphere is required here.

Ruthenium-Catalyzed Amination of Secondary Alcohols using Borrowing Hydrogen Methodology

A new ruthenium complex catalyzes the amination of primary and secondary alcohols and the regioselective mono- and sequential diamination of diols via the borrowing hydrogen pathway. Several variations on new intra- and intermolecular cyclizations of aminoalcohols, diols and diamines lead to heterocyclic ring systems.

Metal-free, ionic liquid-mediated synthesis of functionalized quinolines

An expedient and metal-free synthetic protocol for construction of substituted quinolines has been developed from anilines and phenylacetaldehydes using imidazolium cation-based ionic liquids as the reaction medium. Mechanistic analysis indicated that the reaction occurs through C-C and C-N bond formation to produce isolable 2,3-disubstituted quinoline intermediates, which undergo C-C bond cleavage to produce 3-substituted quinolines. The reaction proceeds smoothly with a range of functionalities in good to excellent yields. Advantages of this protocol include metal-free, environmentally friendly, recyclable reaction media, higher yields and shorter reaction times, and thus is promising for the efficient combinatorial synthesis of structurally diverse 2,3-disubstituted and 3-substituted quinolines.

Syntheses of Benzo[b]furan-6-carbonitrile and 6-Cyanobenzo[b]furan-2-boronic Acid Pinacol Ester

6-Cyanobenzo[b]furan-2-boronic acid pinacol ester (10) is a potentially useful 2-point scaffold for the construction of specific compounds or compound libraries with benzofuran cores. Using a per-iodination/de-iodination strategy coupled with a Sonogashira alkynylation and Cu-catalyzed heteroannulation, we have developed a procedure that allows the preparation of benzo[b]furan-6-carbonitrile (9) and 6-cyanobenzo[b]furan-2-boronic acid pinacol ester (10) in gram quantities.