From CO2 to 4H-Quinolizin-4-ones: A One-Pot Multicomponent Approach via Ag2O/Cs2CO3 Orthogonal Tandem Catalysis

Rh(III)-Catalyzed C-H Activation/Annulation for the Construction of Quinolizinones and Indolizines

A catalytic-condition-controlled synthesis strategy was reported to build quinolizinone and indolizine derivatives from the easily available enamide and triazole substrates with high regioselectivity and good functional group tolerance. More especially, this transformation has successfully fulfilled a C-H bond activation of terminal olefin from enamides followed by a [3 + 3] and a [2 + 3] cyclization cascade under different catalytic conditions, respectively, to provide two kinds of potentially biologically active heterocyclic scaffolds with a ring-junction nitrogen atom. Mechanistically, the methoxyamine formyl group serves as either a traceless directing group (DG) or an oxidizing DG via the C-N and C-C cleavage in this protocol.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Diastereoselective and E/Z-Selective Synthesis of Functionalized Quinolizine Scaffolds via the Dearomative Annulation of 2-Pyridylacetates with Nitroenynes

An organocatalytic Michael/aza-Michael cascade reaction was developed to build the functionalized quinolizine scaffolds in 60-82% yields, excellent diastereoselectivities, and E/Z selectivities. This protocol involves the [3 + 3] annulations of 2-pyridylacetates with nitroenynes through the dearomative strategy in the presence of an organic base under mild conditions. The versatile late-stage derivatizations further demonstrated the synthetic utility of this methodology.

Ag2O versus Cu2O in the Catalytic Isomerization of Coordinated Diaminocarbenes to Formamidines: A Theoretical Study

DFT theoretical calculations for the Ag₂O-induced isomerization process of diaminocarbenes to formamidines, coordinated to Mn(I), have been carried out. The reaction mechanism found involves metalation of an N-H residue of the carbene ligand by the catalyst Ag₂O and the formation of a key transition state showing a μ-η²:η² coordination of the formamidinyl ligand between manganese and silver, which allows a translocation process of Mn(I) and silver(I) ions between the carbene carbon atom and the nitrogen atom, before the formation of the formamidine ligand is completed. Calculations carried out using Cu₂O as a catalyst instead of Ag₂O show a similar reaction mechanism that is thermodynamically possible, but highly unfavorable kinetically and very unlikely to be observed, which fully agrees with experimental results.

DBU-catalyzed dearomative annulation of 2-pyridylacetates with α,β-unsaturated pyrazolamides for the synthesis of multisubstituted 2,3-dihydro-4H-quinolizin-4-ones

DBU-catalyzed dearomative [3 + 3] annulation of 2-pyridylacetates and α,β-unsaturated pyrazolamides for the synthesis of multisubstituted 2,3-dihydro-4H-quinolizin-4-ones was developed.

One-Pot Synthesis of 3-Substituted 4H-Quinolizin-4-ones via Alkyne Substrate Control Strategy

Three-substituted 4H-quinolizin-4-ones were obtained via a facile method with good selectivity and high efficiency. On the basis of alkyne substrate control, the mild and cost-efficient reaction has a broad substrate scope (20 examples, up to 93% yield) and is also easy to scale up. Active sites on the products allow for further modifications. The alkyne substrate control strategy could be further extended to achieve more complex three-substituted 4H-quinolizin-4-one skeletons.

Carbon dioxide based methodologies for the synthesis of fine chemicals

Rapid environmental changes triggered by the increase in the concentration of heat-absorbing gases such as CO2 in the atmosphere have become a major cause of concern. One of the ways to counter this growing threat will be to efficiently convert atmospheric CO2 into value-added products via the development of efficient transition-metal-catalyzed processes. Conversion of CO2 into bulk products such as CH3OH and methane as well as its incorporation into commercial polyurethane synthesis has been achieved and reviewed extensively. However, the efficient transformation of CO2 into fine chemicals and value-added chemicals has many fold advantages. Recent years have seen a rapid rise in the number of metal-mediated protocols to achieve this goal of converting CO2 into fine chemicals. These are essential developments given the requirement of several commodities and fine chemicals in various industrial processes and the utilization of atmospheric CO2 will help provide a sustainable solution to the current environmental problems. Accordingly, we present here a comprehensive compilation of catalytic processes, involving CO2 as the C1 source for reacting with substrates such as alkanes, alkenes, alkynes, amines, acid chlorides, alcohols, allyl boronates, alkenyl triflates, and many others to provide easy access to a wide variety of useful molecules. Such a technology would certainly prove to be beneficial in solving the problems associated with the environmental accumulation of CO2.

Enzymatic synthesis of 2-hydroxy-4H-quinolizin-4-one scaffolds by integrating coenzyme a ligases and a type III PKS from Huperzia serrata

2-Hydroxy-4H-quinolizin-4-one scaffolds were enzymatically synthesized by integrating three enzymes including phenylacetate-CoA ligase (PcPCL) from an endophytic fungus Penicillium chrysogenum MT-12, malonyl-CoA synthase (AtMatB) from Arabidopsis thaliana, and a type III polyketide synthase (HsPKS3) from Chinese club moss Huperzia serrata. The findings paved the way to produce these kinds of structurally interesting alkaloids by engineered microorganisms.

One-pot enzymatic synthesis of 2-hydroxy-4H-quinolizin-4-one scaffolds was developed by integrating three enzymes PcPCL, AtMatB, and HsPKS3.

Computational Investigation of Nickel-Mediated B–H Activation and Regioselective Cage B–C(sp2) Coupling of o-Carborane

Density functional theory (DFT) methods including LC-ωPBE, CAM-B3LYP, B3LYP, and B3LYP-D3, combined with double Zeta all-electron DZVP basis set, have been employed to conduct computational investigations on nickel-mediated reaction of o-carboranylzirconacycle, n-hexene, and 2-bromophenyltrimethylsilylacetylene in toluene solution. A multistep mechanism leading to the C,C,B-substituted carborane-fused tricyclics, including (1) sequential insertion of alkene and alkyne into Ni–C bonds; (2) double 1,2-migration of the TMS group; (3) B–H activation assisted by Cs2CO3 additive; and (4) reduction cage B–C (sp2) coupling, was proposed. Among these steps, the B–H activation of o-carborane was located as rate-determining step (RDS). With assistance of Cs2CO3 additive (replaced by K2CO3 in simulation), the RDS free-energy barrier at PCM-LC-ωPBE/DZVP level was calculated to be 23.1–23.9 kcal·mol−1, transferring to a half-life of 3.9–15.1 h at 298 K. The predicted half-life coincides well with 80% experimental yields of C,C,B-substituted carborane-fused tricyclics after 12 h. Kinetic data obtained by employing LC-ωPBE method also reproduced the experimental diastereoselective ratio well. Various B–H activation pathways with and without Cs2CO3 additive were taken into consideration, which illustrates Cs2CO3 as an essential guarantee for smooth occurrence of this reaction at room temperature.

C-H Bond Carboxylation with Carbon Dioxide

Carbon dioxide is a nontoxic, renewable, and abundant C₁ source, whereas C-H bond functionalization represents one of the most important approaches to the construction of carbon-carbon bonds and carbon-heteroatom bonds in an atom- and step-economical manner. Combining the chemical transformation of CO₂ with C-H bond functionalization is of great importance in the synthesis of carboxylic acids and their derivatives. The contents of this Review are organized according to the type of C-H bond involved in carboxylation. The primary types of C-H bonds are as follows: C(sp)-H bonds of terminal alkynes, C(sp² )-H bonds of (hetero)arenes, vinylic C(sp² )-H bonds, the ipso-C(sp² )-H bonds of the diazo group, aldehyde C(sp² )-H bonds, α-C(sp³ )-H bonds of the carbonyl group, γ-C(sp³ )-H bonds of the carbonyl group, C(sp³ )-H bonds adjacent to nitrogen atoms, C(sp³ )-H bonds of o-alkyl phenyl ketones, allylic C(sp³ )-H bonds, C(sp³ )-H bonds of methane, and C(sp³ )-H bonds of halogenated aliphatic hydrocarbons. In addition, multicomponent reactions, tandem reactions, and key theoretical studies related to the carboxylation of C-H bonds are briefly summarized. Transition-metal-free, organocatalytic, electrochemical, and light-driven methods are highlighted.