Lewis Acid Catalyzed [4 + 3] Cycloaddition of Propargylic Alcohols with Azides

Lewis Acid Catalyzed [4 + 2] Annulation of Propargylic Alcohols with 2-Vinylanilines

An elegant Lewis acid catalyzed, protection-free, and straightforward synthetic strategy for the assembly of a series of sophisticated polycyclic quinoline skeletons employing propargylic alcohols and 2-vinylanilines as the substrates in the presence of Yb(OTf)3 (10 mol %) and AgOTf (10 mol %) in tetrahydrofuran has been described. This annulation protocol, which proceeds through a sequential Meyer-Schuster rearrangement/nucleophilic substitution/deprotonation sequence, provides a versatile, practical, and atom-economical approach for accessing quinoline derivatives in moderate-to-good yields.

Synthesis of Blue Emissive Quaternary 9,9-Disubstituted N-Methyl-7-azaindole-Appended (Phenylethynyl)-fluorene Derivatives

A highly functionalized 9,9-disubstituted (phenylethynyl)-fluorene-appended N-methyl-7-azaindole derivatives has been synthesized from various fluorene propargylic alcohols and substituted-7-azaindoles using BF₃OEt₂ as a catalyst. The scope of the reaction was demonstrated by selecting a range of fluorene propargylic alcohols and substituting 7-azaindoles. A plausible reaction mechanism for forming title compounds via propargylic carbocation is postulated. The synthetic transformation of the products has been demonstrated by the Suzuki coupling and Click reaction. The Suzuki coupled compounds 5a-5e were evaluated for photophysical properties such as absorption, solvatochromism, emission, and Stokes shift and found blue emissive in nature.

Radical annulation of a designed diene system: access to nitro-benzo[b]azepines

Herein, we describe a novel O₂N˙-triggered ordered addition 7-endo cyclization reaction with excellent chemo- and regioselectivity. With such a strategy, structurally diverse nitro-benzo[b]azepines were prepared with 28 examples. Large-scale operation and handy N-Ts and N-Cbz deprotection reveal the promising utility of this methodology. Mechanistic studies suggest that the reaction proceeds through a radical pathway.

BF3·OEt2 catalyzed synthesis of functionalized 9-fluorene-9-ylidene appended quinazolin-4-ones

An efficient method for the construction of the quinazolinone skeleton from the reaction of 9-(phenylethynyl)-9H-fluoren-9-ols with substituted 2-aminobenzamides catalyzed by boron trifluoride is achieved.

Synthesis of indolo- and benzothieno[3,2-c]quinolines via POCl3 mediated tandem cyclization of o-alkynylisocyanobenzenes derived from o-alkynyl-N-phenylformamides

A synthesis of indolo[3,2-c]quinolines and benzothieno[3,2-c]quinolines has been developed employing o-alkynyl-N-phenylformamide derivatives as the substrates. The reaction proceeded via a tandem process involving POCl3‒assisted intramolecular cyclization of the firstly formed...

Photoredox/nickel dual-catalyzed regioselective alkylation of propargylic carbonates for trisubstituted allenes

Herein, a highly regioselective alkylation of propargylic carbonates for trisubstituted allenes with alkyl 1,4-dihydropyridine derivatives (1,4-DHPs) is developed via a photoredox/nickel dual-catalyzed process, which represents the first direct approach to access alkylated allene products without alkyl organometallic reagents. This method features a broad substrate scope and mild conditions. A hypothetical mechanism with an alkyl radical and an allenyl Ni(III) species is proposed. Benzylation products were also obtained to be the complement building blocks for the potential synthesis of pharmaceuticals.

Synthesis of tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols

A facile and efficient route to tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols via acid-catalyzed dehydrative annulation reactions is described. This reaction proceeds through a cascade sequence of Friedel–Crafts-type alkylation followed by intramolecular “Click” reaction, involving the formation of multiple chemical bonds in a single operation with excellent atom-economy and broad functional group tolerance.

The synthesis of tetrahydro-β-carbolines from 2-indolylmethyl azides and propargylic alcohols via acid-catalyzed dehydrative [4 + 2]-annulation reaction is described.

Recent Advances in Transition-Metal-Free (4+3)-Annulations

(4+3)-Annulations are incredibly versatile reactions which combine a 4-atom synthon and a 3-atom synthon to form both 7-membered carbocycles as well as heterocycles. We have previously reviewed transition-metal-catalyzed (4+3)-annulations. In this review, we will cover examples involving bases, NHCs, phosphines, Lewis and Brønsted acids as well as some rare examples of boronic acid catalysis and photocatalysis. In analogy to our previous review, we exclude annulations involving cyclic dienes like furan, pyrrole, cyclohexadiene or cyclopentadiene, as Chiu, Harmata, Fernándes and others have recently published reviews encompassing such substrates. We will however discuss the recent additions (2010–2020) to the literature on (4+3)-annulations involving other types of 4-atom-synthons.

1 Introduction

2 Bases

3 Annulations Using N-Heterocyclic Carbenes

3.1 N-Heterocyclic Carbenes (NHCs)

3.2 N-Heterocyclic Carbenes and Base Dual-Activation

4 Phosphines

5 Acids

5.1 Lewis Acids

5.2 Brønsted Acids

6 Boronic Acid Catalysis and Photocatalysis

7 Conclusion

Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

Herein, we report a unified approach to azepines by dearomative photochemical rearrangement of aromatic N-ylides. Deprotonation of quaternary aromatic salts with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or N,N,N',N'-tetramethylquanidine (TMG) under visible light irradiation provides mono- and polycyclic azepines in yields up to 98%. This ring-expansion presents a new mode of access to functionalized azepines from N-heteroarenes using two straightforward steps and simple starting materials.

Rapid construction of cyclopenta[b]naphthalene frameworks from propargylic alcohol tethered methylenecyclopropanes

We have developed a new synthetic methodology for the rapid construction of cyclopenta[b]naphthalene frameworks from the reaction of propargylic alcohol tethered methylenecyclopropanes with mesyl chloride in the presence of triethylamine through cascade cyclization. The reaction can be performed under mild conditions without the use of transition metals, affording the target products in moderate to good yields, and this cyclization reaction process can be scaled up to a gram-scale synthesis.

Recent Advances in Transition-Metal-Catalyzed (4+3)-Cycloadditions

A (4+3)-cycloaddition combines a four-atom synthon and three-atom synthon to form seven-membered rings. In the past decade, many improvements have been made to this class of cycloaddition, including excellent diastereo- and enantioselectivities, both intra- and intermolecularly. Through the strategic use of transition-metal catalysts, acids, bases, and organocatalysts, it is possible to perform the cycloaddition on a variety of substrates, generating novel seven-membered rings. With these advances, (4+3)-cycloaddition has also been applied to the synthesis of biologically relevant compounds and natural products. We exclude the cycloadditions of cyclic dienes such as furan, pyrrole, cyclohexadiene or cyclopentadiene as Chiu, Harmata, Mascareñas­ and others have recently published thorough reviews on that topic. We will however discuss the recent additions (2009–2020) to the literature for the (4+3)-cycloadditions involving other types of four-atom synthons.

1 Introduction

2 Rhodium

2.1 Cyclopropanation/Cope Rearrangement

2.2 C–H activation

3 Gold, Silver

4 Copper

5 Palladium, Platinum, Iridium

6 Dual-Activation

7 Conclusion

Silver Trifluoromethanesulfonate-Catalyzed Annulation of Propargylic Alcohols with 3-Methyleneisoindolin-1-one

A silver-catalyzed formal [3 + 3] annulation of 3-methyleneisoindolin-1-one with alkynol for the synthesis of 1,5-dihydroindolizin-3(2H)-one derivatives is disclosed. The protocol allows practical synthesis of N-heterocyclic scaffolds with a broad scope of functional groups and could be efficiently scaled up to gram scale, which incarnates a potential application of this methodology. In addition, a range of chlorine anion substitution of alkenes can be constructed by adjusting the structure of the alkynol substrates with the use of TMSCl.

Photoredox/palladium co-catalyzed propargylic benzylation with internal propargylic carbonates

The developed photo/palladium dual catalytic system provided a novel route to internal propargylic benzylation products. A radical coupling mechanism between the propargylic radical and benzyl radical was proposed.

Lewis or Brønsted acid-catalysed reaction of propargylic alcohol-tethered alkylidenecyclopropanes with indoles and pyrroles for the preparation of polycyclic compounds tethered with indole or pyrrole motif

We developed a facile synthetic method to access the cyclopenta[b]naphthalene skeleton from Lewis or Brønsted acid-catalysed propargylic alcohol-tethered alkylidenecyclopropanes with indole and pyrrole derivatives.

Metal-free aza-Claisen type ring expansion of vinyl aziridines: an expeditious synthesis of seven membered N-heterocycles

A metal-free approach for the synthesis of 7-membered aza-heterocycles has been developed by the intermolecular [5 + 2] cycloaddition of non-activated vinylaziridines and alkynes. This method has a broad substrate scope under mild reaction conditions to afford structurally diverse 7-membered N-heterocycles in high yield up to 92%.

TMSBr-Promoted Cascade Cyclization of ortho-Propynol Phenyl Azides for the Synthesis of 4-Bromo Quinolines and Its Applications

Difficult-to-access 4-bromo quinolines are constructed directly from easily prepared ortho-propynol phenyl azides using TMSBr as acid-promoter. The cascade transformation performs smoothly to generate desired products in moderate to excellent yields with good functional groups compatibility. Notably, TMSBr not only acted as an acid-promoter to initiate the reaction, and also as a nucleophile. In addition, 4-bromo quinolines as key intermediates could further undergo the coupling reactions or nucleophilic reactions to provide a variety of functionalized compounds with molecular diversity at C4 position of quinolines.

Gold-Catalyzed Hydroamination of Propargylic Alcohols: Controlling Divergent Catalytic Reaction Pathways To Access 1,3-Amino Alcohols, 3-Hydroxyketones, or 3-Aminoketones

A versatile approach to the valorization of propargylic alcohols is reported, enabling controlled access to three different products from the same starting materials. First, a general method for the hydroamination of propargylic alcohols with anilines is described using gold catalysis to give 3-hydroxyimines with complete regioselectivity. These 3-hydroxyimines can be reduced to give 1,3-amino alcohols with high syn selectivity. Alternatively, by using a catalytic quantity of aniline, 3-hydroxyketones can be obtained in high yield directly from propargylic alcohols. Further manipulation of the reaction conditions enables the selective formation of 3-aminoketones via a rearrangement/hydroamination pathway. The utility of the new chemistry was exemplified by the one-pot synthesis of a selection of N-arylpyrrolidines and N-arylpiperidines. A mechanism for the hydroamination has been proposed on the basis of experimental studies and density functional theory calculations.

Gold catalysed transformation of 2-arylindoles to terphenyl amines via 3-dienyl indoles and Brønsted acid promoted formation of 2-carboxyindoles to 3-indenylindoles via 3-allenylindoles

Gold-catalysed intramolecular reaction of unprotected/protected 3-dienyl indoles leads to the formation of functionalised terphenylamines with high efficiency; this reaction can also be accomplished in one pot by starting with 2-arylindole and propargylic alcohols. This reaction occurs via 3-dienyl indoles and proceeds likely by gold-catalysed cascade cyclisation, aromatisation and fragmentation of the indole heterocyclic moiety. We have also found that PTSA can work well giving comparable yields, but at least one mole equivalent of PTSA is required. On the other hand, 3-indenylindoles have been synthesised from indole-2-carboxylates (2-carboxyindoles) and propargylic alcohols through Brønsted acid mediation. Many of these indenylindoles exhibit isomerism (probably diastereomers) in the solution state (1H NMR). Two examples of indolo-indenyl acetates have been prepared by reacting the above terphenylanilines with PhI(OAc)2. Key products are characterised by X-ray crystallography.

Ring-opening formal hetero-[5+2] cycloaddition of 1-tosyl-2,3-dihydro-1H-pyrroles with terminal alkynes: entry to 1-tosyl-2,3-dihydro 2,3-dihydro-1H-azepines

Lewis acid-catalyzed formal hetero-[5+2] cycloaddition of 2,3-dihydro-1H-pyrroles with alkynes through C(sp²)–N bond cleavage toward 2,3-dihydro-1H-azepines is described.

Electrochemical Oxidation with Lewis-Acid Catalysis Leads to Trifluoromethylative Difunctionalization of Alkenes Using CF3SO2Na

The directly external oxidant-free oxytrifluoromethylation and aminotrifluoromethylation of alkenes have been developed through the constant current electrolysis synergizing with a Lewis-acid catalysis protocol. By using sodium trifluoromethanesulfinate as the trifluoromethyl source, the method enabled difunctionalization of C-C double bonds of a wide range of styrene derivatives.

An efficient approach to 4-chloro quinolines via TMSCl-mediated cascade cyclization of ortho-propynol phenyl azides

A novel and efficient strategy for the synthesis of 4-chloro quinolines via the TMSCl-mediated cascade cyclization of easily prepared ortho-propynol phenyl azides is developed.

A BF3·Et2O catalyzed atom-economical approach to highly substituted indole-3-carbinols from nitrosobenzenes and propargylic alcohols

The BF₃·Et₂O catalyzed tandem reaction of nitrosobenzenes and propargylic alcohols for the formation of highly substituted indole-3-carbinols.

Lewis Acid Catalyzed Dehydrogenative Coupling of Tertiary Propargylic Alcohols with Quinoline N-Oxides

An unprecedented Lewis acid catalyzed, high-efficiency synthesis of valuable 2-(quinolin-2-yl)prop-2-en-1-ones via dehydrogenative coupling of propargylic alkynols with quinoline N-oxides is described. This protocol, which tolerates a broad range of functional groups, provides a straightforward pathway to the products 2-(quinolin-2-yl)prop-2-en-1-one scaffolds in satisfactory yields. The conversion could be scaled up to gram scale efficiently, which underlines a latent application of this methodology.

Catalyst free synthesis of α-fluoro-β-hydroxy ketones/α-fluoro-ynols via electrophilic fluorination of tertiary propargyl alcohols using Selectfluor™ (F-TEDA-BF4)

A facile access to α-fluoro-β-hydroxyketones via electrophilic fluorination.

Cascade reaction of alkynols with 1-(2-aminophenyl)prop-2-ynols to form a fused 5,5,6-tricyclic system: formation of four bonds in a single reaction

A novel cascade reaction of internal alkynols with 1-(2-aminophenyl)prop-2-ynols has been developed to form a new N,O-containing fused 5,5,6-tricyclic skeleton.