[4 + 2] Benzannulation of 3-Alkenylpyrroles/Thiophenes with Propargylic Alcohols: Access to Substituted Indoles, Benzothiophenes, and Aza[5]helicenes

Synthesis of functionalized indoles via cascade benzannulation strategies: a decade's overview

Indoles are one of the most prominent aromatic heterocycles in the organic chemistry field. Due to their widespread presence in various natural products, alkaloids, drugs, approved medicines, etc. the synthesis and functionalization of indoles are of great interest. This review emphasizes recent developments and techniques in the domino cascade cyclization process in the last decade starting from the various building blocks. In particular, this review depicts several intriguing benzannulation methods of creating a benzene ring on a pre-existing pyrrole nucleus in an inter/intramolecular fashion under metal-catalyzed/metal-free approaches. Different subsections focus on gradual timely developments in this complementary area and a detailed analysis of the mechanisms and reactivity patterns. As a complementary method, this review gives a significant incentive to various annulation strategies and also gives an overview of the remaining challenges and upcoming possibilities.

Stitching Ynones with Nitromethanes: Domino Synthesis of Functionally Enriched Benzofurans and Benzothiophenes

A convenient one-pot benzannulation of regioisomeric 2- or 3-substituted furan and thiophene ynones with a range of nitromethanes has been discovered to directly access densely and diversely functionalized benzofurans and benzothiophenes. In this protocol, the nitro group in nitromethanes functions as recursive carbanion activator to setup tandem Michael addition-6π-electrocyclization, and its eventual sacrificial elimination facilitates aromatization and overall benzannulation. This benzannulation was also explored with furan/thiophene based o-halo ynones wherein a Michael addition-S_NAr process operates and nitromethanes leave their imprint to deliver nitro substituted benzo-furans and -thiophenes.

Rhodium-Catalyzed Enantioselective Synthesis, Structures, and Properties of Single and Double Azahelicene-Like Molecules

The enantioselective synthesis of aza[6] and [7]helicene-like molecules have been achieved by the cationic rhodium(I)/axially chiral biaryl bisphosphine complex-catalyzed intramolecular [2+2+2] cycloaddition of cyanodiynes. This protocol was successfully applied to the diastereo- and enantioselective synthesis of an S-shaped double aza[6]helicene-like molecule with a high ee value of 89 %. Although no epimerization and racemization were observed in the double carbo[6]helicene-like molecule at 80 °C, epimerization and racemization of the double aza[6]helicene-like molecule proceeded at 80 °C. This double aza[6]helicene-like molecule showed good fluorescent quantum yields and chiroptical responses under both neutral and acidic conditions.

Domino Reaction of 2,4-Diyn-1-ols with 1,3-Dicarbonyl Compounds: Direct Access to Aryl/Heteroaryl-Fused Benzofurans and Indoles

A domino propargylation/furanylation (intramolecular exo-dig-cyclization)/benzannulation reaction of 2,4-diyn-1-ols with 1,3-dicarbonyl compounds has been developed for the first time. This provides a novel and effective method for the preparation of aryl/heteroaryl-fused benzofurans from easily accessible starting materials in a single step. The methodology was extended to pyrrolyl-benzannulation to obtain aryl/heteroaryl-fused indoles. Further, application of this approach in the synthesis of eustifoline D and dictyodendrin structural frameworks has been demonstrated.

One-Pot Arylative Benzannulation of 2-Carbonyl-3-propargyl Indoles with Boronic Acids Leading to Arylated Carbazoles

Arylative annulation of 2-carbonyl-3-propargyl indoles with boronic acids under sequential palladium/triflic acid catalysis is described. The present strategy to provide di- and triaryl carbazoles in one pot involves benzannulation through difunctionalization of alkynes. The strategy showed a good substrate scope with respect to boronic acids as well as 2-carbonyl-3-propargyl indoles to afford the corresponding carbazoles in decent yields.

Recent Progress in the Synthesis of Benzo[b]thiophene

:

Due to such a wide range of applicability, the synthesis of benzo[b]thiophene derivatives has attracted intensive research. Numerous mild and efficient approaches for the synthesis of benzo[b]thiophenes have been developed over the years. Different catalysts and substrates have been applied for benzo[b]thiophene synthesis. This review will focus on the studies in the construction of benzo[b]thiophene skeleton, which date back from 2012.

Multicomponent benzannulation of allylic P-ylides with isocyanates or aldehydes for construction of anilines and biaryls

The reactivity of allylic phosphorus ylides generated in situ from alkoxycarbonylmethylenephosphoranes and propiolates is investigated toward isocyanates and aromatic aldehydes, which leads to one-pot multicomponent benzannulations for efficient synthesis of polysubstituted anilines and biaryls, respectively. The mechanism may involve a tandem [2+2] cycloaddition/fragmentation/Wittig/cyclization/elimination/aromatization sequence.

Synthesis of 1,6-Dihydropyridine-3-carbonitrile Derivatives via Lewis Acid-Catalyzed Annulation of Propargylic Alcohols with (E)-3-Amino-3-phenylacrylonitriles

A novel Lewis acid-catalyzed, highly efficient, practical, and atom-economical protocol for the synthesis of functionalized 1,2-dihydropyridine-3-carbonitrile derivatives in the presence of Bi(OTf)₃ (10 mol %) in tetrahydrofuran (2.0 mL) at 80 °C for 8 h in air is described, starting from readily accessed propargylic alcohols and (E)-3-amino-3-phenylacrylonitriles. This cycloaddition protocol, which is scalable and proceeds under mild conditions, is amenable to the gram-scale construction of valuable 1,2-dihydropyridine-3-carbonitriles. Furthermore, the good functional group compatibility and broad scope of this strategy were demonstrated by a broad range of propargylic alcohols and (E)-3-amino-3-phenylacrylonitriles, with yields ranging from 34 to 96%.

Highly selective Diels-Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole

A highly regio-, chemo- and stereoselective divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles is herein described, starting from 2-formylpyrrole and employing Diels-Alder and Heck arylation reactions. 3-(N-Benzyl-2-pyrrolyl)acrylates and 4-(pyrrol-2-yl)butenones underwent a highly endo-Diels-Alder cycloaddition with maleimides to furnish octahydropyrrolo[3,4-e]indoles, which served as precursors in the regioselective synthesis of aza-polycyclic skeletons via an intramolecular Heck arylation reaction. Through the latter reaction, the 3-(N-benzyl-2-pyrrolyl)acrylates give rise to 3-(pyrrolo[2,1-a]isoindol-3-yl)acrylates. A further oxidative aromatization of the polycyclic intermediates provides the corresponding polycyclic pyrrolo-isoindoles and isoindolo-pyrrolo-indoles. A theoretical study on the stereoselective Diels-Alder reactions, carried out by calculating the endo/exo transition states, revealed the assistance of non-covalent interactions in governing the endo stereocontrol.

One-Pot Assembly of 3-Hydroxycarbazoles via Uninterrupted Propargylation/Hydroxylative Benzannulation Reactions

A novel strategy for the synthesis of 3-hydroxycarbazoles involving the consecutive propargylation/palladium-catalyzed hydroxylative benzannulation of indole-2-carbonyls with propargylic alcohols has been exploited. This one-pot procedure leads to a wide range of substituted 3-hydroxycarbazoles in high yield with a broad substrate scope. The method was further extended to access furano-carbazole derivatives from dialkynols via tandem annulations.

Metal-free propargylation/aza-annulation approach to substituted β-carbolines and evaluation of their photophysical properties

An efficient acid-catalyzed propargylation/aza-annulation sequence was developed under metal-free reaction conditions, thus leading to a one-pot synthesis of a variety of substituted β-carbolines starting from propargylic alcohols and indole 2-carbonyls. This versatile strategy was further extended to the synthesis of 5-azaindoles and 5-azabenzothiazoles. Optical properties suggested that manipulation of electron donor and acceptor moieties on β-carbolines has an impact on their ground and excited state electronic behavior. This leads to blue or green emission and should facilitate the development of organic light emitting diodes (OLEDs). Electrochemical and stability studies revealed that 4a-6 shows ease of redox activity and photostability during illumination.

Zn(OTf)2-Catalyzed Formal [3 + 3] Cascade Annulation of Propargylic Alcohols with 2-Aminochromones: Accessing the Chromeno[2,3-b]pyridines

A Zn(OTf)₂-catalyzed formal [3 + 3] cascade annulation strategy for the synthesis of functionalized chromeno[2,3-b]pyridines has been developed using propargylic alcohols and 2-aminochromones as the substrates. The protocol provides a convenient and atom-economical method of accessing a broad range of chromeno[2,3-b]pyridine derivatives in excellent yields with good functional-group tolerance. The method is also effective on the gram scale, which highlights the inherent practicality of this synthetic transformation.

Amine-catalyzed and functional group-controlled chemo- and regioselective synthesis of multi-functionalized CF3-benzene via a metal-free process

A novel strategy for the synthesis of CF₃-containing multi-substituted benzenes with high chemo- and regioselectivities under metal-free and air-tolerant conditions was established.

Sodium halides as the source of electrophilic halogens in green synthesis of 3-halo- and 3,n-dihalobenzo[b]thiophenes

A convenient methodology for the synthesis of mono- and di-halogenated benzo[b]thiophenes is described herein, which utilizes copper(II) sulfate pentahydrate and various sodium halides in the presence of substituted 2-alkynylthioanisoles. The proposed method is facile, uses ethanol as a green solvent, and results in uniquely substituted benzo[b]thiophene structures with isolated yields up to 96%. The most useful component of this methodology is the selective introduction of bromine atoms at every available position (2-7) around the benzo[b]thiophene ring, while keeping position 3 occupied by a specific halogen atom such as Cl, Br or I. Aromatic halogens are useful reactive handles; therefore, the selective introduction of halogens at specific positions would be valuable in the targeted synthesis of bioactive molecules and complex organic materials via metal-catalyzed cross coupling reactions. This work is a novel approach towards the synthesis of dihalo substituted benzo[b]thiophene core structures, which provides a superior alternative to the current methods discussed herein.

One-Pot Consecutive Sulfonamidation/ipso-Cyclization Strategy for the Construction of Azaspirocyclohexadienones

Harnessing of Morita-Baylis-Hillman (MBH) carbonates of acetylenic aldehydes as handy synthons has allowed a facile synthesis of azaspirocyclohexadienones by sequential DABCO-promoted sulfonamidation/ICl-mediated ipso-iodocyclization reactions. A variety of MBH-carbonates having aryl or heteroaryl groups on the alkyne functionality fruitfully participated in the one-pot ipso-annulation reaction to provide the corresponding 3-iodo spirocyclohexadienones. The sulphonamide functionality was further utilized to construct the tricyclic fused-sultam framework.

Microwave-assisted synthesis of novel hetero[5]helicene-like molecules and coumarin derivatives

A concise and efficient approach to design and synthesize hetero[5]helicene-like molecules and coumarin derivatives is reported.