Synthesis of N-Fused Polycyclic Indoles via Ligand-Free Palladium-Catalyzed Annulation/Acyl Migration Reaction

Visible-light-induced radical cascade cyclization: a catalyst-free synthetic approach to trifluoromethylated heterocycles

A visible-light-promoted research protocol for constructing dihydropyrido[1,2-a]indolone skeletons is herein described proceeding through a cascade cyclization mediated by trifluoromethyl radicals. This method allows the efficient synthesis of various indole derivatives without the need of photocatalysts or transition-metal catalysts. Mechanism experiments indicate that the process involves a radical chain process initiated by the homolysis of Umemoto's reagent. This straightforward method enables a rapid access to heterocycles containing a trifluoromethyl group.

Synthesis of N-Fused Polycyclic Indole Derivatives via Ru(II)-Catalyzed C-H Bond Activation and Intramolecular Hydroarylation

A new synthesis of N-fused tetracyclic indole derivatives and their related polycyclic analogues has been developed based on ruthenium(II)-catalyzed C-H activation and intramolecular hydroarylation. A series of polycyclic indoles with a 3-formyl group have been prepared in good to high yields. Various aliphatic and aromatic amines have been studied to form a transient directing group with the aldehyde for the catalytic process. A significant impact of the structures of the aromatic amines was identified, and 1-naphthylamine was shown to enable the catalytic process. DFT computations were performed to gain further insight into the role of the transient directing groups.

Rh(III)-Catalyzed Successive C-H Activations of 2-Phenyl-3H-indoles and Cyclization Cascades to Construct Highly Fused Indole Heteropolycycles

Rh(III)-catalyzed successive C-H activations of 2-phenyl-3H-indoles and cyclization cascades with diazo compounds were developed to construct highly fused indole heteropolycycles with a broad range of substrates and good yields. In particular, this transformation included two successive C-H activations and unusual [3+3] and [4+2] sequential cyclization cascades, in which the diazo compound played a different role in the two cyclization processes, while simultaneously forming a highly fused polycyclic indole scaffold with a new quaternary carbon center.

Access to thiionized-, selenolized-, and alkylated 5-alkylidene 3-pyrrolin-2-one derivatives via a regioselective oxidative annulation reaction

A metal-free regioselective oxidative annulation reaction of readily available 2,4-pentanediones with primary amines has been described. This protocol provides a divergent strategy for the incorporation of various radical donors into 5-alkylidene 3-pyrrolin-2-one skeletons, producing a variety of thiionized-, selenolized-, and alkylated 5-alkylidene 3-pyrrolin-2-one derivatives. Moreover, the diverse synthetic transformations of the 5-alkylidene 3-pyrrolin-2-one products were also investigated.

Natural Product-Oriented Photo-Induced Denitrogenative Annulations of 1-Alkenylbenzotriazoles

The photo-induced denitrogenative annulations of a variety of 1-alkenylbenzotriazoles were investigated. By judiciously manipulating the structural variations of 1-alkenylbenzotriazoles, two characteristic polycyclic skeletons associated with monoterpene indole alkaloids were constructed through a diverted and controllable manner. The present work not only enriches the photochemistry of 1-alkenylbenzotriazoles, but also offers a unified approach to access skeletally diverse indole alkaloid scaffolds.

Synthesis of 3-substituted 2,3-dihydropyrazino[1,2-a]indol-4(1H)-ones by sequential reactions of 2-indolylmethyl acetates with α-amino acids†

The synthesis of 2,3-dihydropyrazino[1,2-a]indol-4(1H)-ones from the sequential reaction of amino acid methyl esters with readily available indole-2-ylmethyl acetates is described. The reaction proceeds in situ under basic conditions of highly unstable and reactive 2-alkylideneindolenines followed by Michael-type addition of α-amino acid methyl esters/intramolecular cyclization.

The synthesis of 2,3-dihydropyrazino[1,2-a]indol-4(1H)-ones through the in situ generation of 2-methide-2H-indole intermediate I starting from 2-indolylmethyl acetates under basic conditions/nucleophilic Michael addition/cyclization cascade reaction.

Facile synthesis of the spiro-pyridoindolone scaffold via a gold-catalysed intramolecular alkynol cyclisation/hydroindolylation

A simple approach for the synthesis of pyridoindolone scaffolds with a spiroannulated tetrahydrofuran ring is described. The overall process comprises intramolecular sequential gold-catalysed 5-endo-dig alkynol cycloisomerization and subsequent addition of indole C2 to the in situ generated oxocarbenium cation.

Tetrahydroxydiboron and Nickel Chloride Cocatalyzed Rapid Radical Cyclization toward Pyrrolizidine and Indolizidine Alkaloids

A novel tetrahydroxydiboron and nickel chloride cocatalyzed radical cyclization cascade with a broad substrate scope and an ultrashort reaction time was developed. The mechanistic investigation indicated that the reaction might involve a homocleavage of tetrahydroxydiboron and nickel hydride intermediates. This approach enables the simple and efficient synthesis of a series of heteropolycycles.

Transition Metal-Free, Methoxide-Catalyzed Synthesis of Pyridoindolones

A simple transition metal-free strategy for the synthesis of pyrido[1,2-a]indolone derivatives has been devised through sodium methoxide-catalyzed intramolecular cyclization of 2-alkenylated N-pyrimidyl indoles. The reactions involved a Smiles rearrangement/cyclization cascade, which resulted in a new series of N-fused indoles, potentially applicable skeletons in medicinal chemistry. This reaction presents simple eco-friendly reaction conditions, a high atom- and cost-economy, a short reaction time, and a broad range of substrate scope with high reaction efficiency.

A palladium-catalyzed cascade approach for the synthesis of 3,3a,4,6,7,8,9,9a-octahydro-1H-benzo[f]isoindole-1,5(2H)-diones

A synthetic method for the preparation of 3,3a,4,6,7,8,9,9a-octahydro-1 H-benzo[ f]isoindole-1,5(2 H)-diones from 1,6-dienes and vinyl iodides is developed using PdCl2(PPh3)2 as the catalyst. The presented approach exhibits a good functional group tolerance and affords moderate yields of the products. A mechanism is also proposed.

Recent advances in the synthesis of pyrrolo[1,2-a]indoles and their derivatives

The pyrrolo[1,2-a]indole unit is a privileged heterocycle found in numerous natural products and has been shown to exhibit diverse pharmacological properties. Thus, recent years have witnessed immense interest from the synthesis community on the synthesis of this scaffold. In light of the ever-increasing demand for pyrrolo[1,2-a]indoles in drug discovery, this review provides an overview of recent synthesis methods for the preparation of pyrrolo[1,2-a]indoles and their derivatives. The mechanistic pathway and stereo-electronic factors affecting the yield and selectivity of the product are briefly explained. Furthermore, we have attempted to demonstrate the utility of the developed methods in the synthesis of bioactive molecules and natural products, wherever offered.

Total Synthesis of (+)-Alsmaphorazine C and Formal Synthesis of (+)-Strictamine: A Photo-Fries Approach

A bioinspired photo-Fries/imine capture cascade reaction was developed in continuous-flow mode, which facilitated the rapid construction of a series of diversely functionalized 2,7-heterocycle-fused tetrahydrocarbazoles, the ubiquitous core structures embedded in strychnos and akuammiline-type monoterpene indole alkaloids. The synthetic utility of this novel method has been preliminarily explored by the first total synthesis of (+)-alsmaphorazine C and formal synthesis of (+)-strictamine in a concise and efficient manner.

Brønsted Acid Catalyzed (4 + 2) Cyclocondensation of 3-Substituted Indoles with Donor–Acceptor Cyclopropanes

Acylcyclopropanes are employed as useful donor–acceptor cyclopropanes that undergo formal (4 + 2) cyclocondensation with N-unprotected 3-substituted indoles in the presence of a Brønsted acid catalyst. The reaction involves the simultaneous alkylation of both the N and C-2 positions of the indole and provides access to the 8,9-dihydropyrido[1,2-a]indole scaffold that is the central core of several biologically relevant indole alkaloids in excellent yields and good selectivities.

Pd-catalyzed arylation/aza-Michael addition cascade to C2-spiroindolines and azabicyclo[3.2.2]nonanones

A palladium-catalyzed arylation/aza-Michael addition cascade reaction of β-substituted cyclic enones and 2-haloanilines has been reported. Using 1 mol% Pd(PPh₃)₄ as a catalyst, C2-spiroindolines are accessed via an intermolecular vinylogous arylation of β-alkyl cyclic enones and 2-haloanilines followed by an intramolecular aza-Michael addition. The functional group tolerance of this transformation is examined by 18 examples in up to 93% yield. In the second part, we developed an α'-arylation/aza-Michael addition cascade strategy to construct azabicyclo[3.2.2]nonanones catalyzed by Pd(MeCN)₂Cl₂·PPh₃. This study provides a quick route to complex and useful spiro- and bridged-heterocycles from readily available starting materials in good yields with high regioselectivity.

Synthesis of polycyclic indoles via organocatalytic bicyclization of α-alkynylnaphthalen-2-ols with nitrones

A new organocatalytic bicyclization of α-alkynylnaphthalen-2-ols with nitrones was first reported, leading to the convergent synthesis of polycyclic indoles with substantial substitution diversity in generally good yields through scission of the N-O bond of nitrones via [3,3]-sigmatropic rearrangement. This transformation showcases the use of a quinine catalyst in a complicated cascade system that has been shown to effectively construct polycyclic heterocycles via alkyne difunctionalization.

Formal oxo- and aza-[3 + 2] reactions of α-enaminones and quinones: a double divergent process and the roles of chiral phosphoric acid and molecular sieves

A double divergent process has been developed for the reaction of α-enaminones with quinones through facile manipulation of catalyst and additive, leading to structurally completely different products. The two divergent processes, which involve formal aza- and oxo-[3 + 2] cycloaddition reactions, are mediated by chiral phosphoric acid and molecular sieves, respectively. While inclusion of phosphoric acid in the reaction switched the reaction pathway to favor the efficient formation of a wide range of N-substituted indoles, addition of 4 Å molecular sieves to the reaction switched the reaction pathway again, leading to enantioselective synthesis of 2,3-dihydrobenzofurans in excellent yields and enantioselectivities under mild conditions. Studies in this work suggest that the chiral phosphoric acid acts to lower the transition state energy and promote the formation of amide intermediate for the formal aza-[3 + 2] cycloaddition and the molecular sieves serve to facilitate proton transfer for oxo-[3 + 2] cycloaddition. The reactivity of α-enaminones is also disclosed in this work.

Fe-Catalyzed Sequential C(sp3)-H/N-H Annulation of 2-Methylindoles with Ethyl Trifluoropyruvate at Room Temperature: Construction of Pyrrolo[1,2-α]indoles

An efficient and benign iron-catalyzed room-temperature method was developed for direct sequential C(sp³)-H/N-H annulation to construct pyrroloindole scaffolds. This strategy features cheap and readily available raw materials and mild room-temperature reaction conditions and provides a green and practical method for the one-pot rapid synthesis of a wide range of diversely functionalized pyrrolo[1,2-α]indoles.

Visible-light-induced intramolecular radical cascade of α-bromo-N-benzyl-alkylamides: a new strategy to synthesize tetracyclic N-fused indolo[2,1-a]isoquinolin-6(5H)-ones

Polycyclic indole scaffolds are ubiquitous in pharmaceuticals and natural products and in materials science. Here, we present a visible-light-initiated intramolecular aryl migration/desulfonylation/cyclization cascade reaction for the synthesis of tetracyclic indolo[2,1-a]isoquinolin-6(5H)-ones. This protocol not only exhibited a wide substrate scope but also provided a mild route to access a variety of tetracyclic N-fused indoles.

Construction of Bisindolines via Oxidative Coupling Cyclization

Herein, we report a general approach for the efficient construction of three-dimensional bisindolines via oxidative coupling cyclization in an intermolecular manner. This reaction is featured by its operational simplicity, metal-free conditions, lack of protecting group, and high selectivity. Notably, a wide range of anilines are suitable in this intermolecular cyclization, furnishing corresponding bisindolines in up to 98% yield.

Tandem Ullmann-Goldberg Cross-Coupling/Cyclopalladation-Reductive Elimination Reactions and Related Sequences Leading to Polyfunctionalized Benzofurans, Indoles, and Phthalanes

On exposure to a combination of Cu[I]- and Pd[0]-based catalysts, compounds such as 1 and 7 engage in tandem Ullmann-Goldberg cross-coupling and cyclopalladation-reductive elimination reactions to give benzofurans such as 8. Related reactions involving hetero-Michael additions of o-halogenated phenols or anilines to propiolates and the Pd[0]-catalyzed cyclization of the resulting conjugates provide, in a one-pot process, alternately functionalized benzofurans, indoles, or phthalanes.

Synthesis of chromeno[4,3-b]quinolines and spirobenzofuran-3,3′-quinolines through silver-mediated Appel reaction/C–Br bond cleavage/double selective rearrangement sequence

A reduction and sequential silver-mediated Appel reaction/C–Br cleavage/double rearrangement cascade strategy was developed for the selective synthesis of chromeno[4,3-b]quinolines and spirobenzofuran-3,3′-quinolines.