Brønsted Acid-Catalyzed Formal [2 + 2 + 1] Annulation for the Modular Synthesis of Tetrahydroindoles and Tetrahydrocyclopenta[ b]pyrroles

Recent advances in metal-free catalysts for the synthesis of N-heterocyclic frameworks focusing on 5- and 6-membered rings: a review

The tremendous potential of transition metal-free multi-component reactions (MCR) in the synthesis of N-heterocyclic frameworks is examined in this review, offering a complete overview of this subject matter. The discussion on the mechanistic rationale of the reaction routes and intermediates provides profound insights into the underlying changes, encouraging deeper investigation into various molecular frameworks. This review serves as a doorway to study the practicality of exploiting these reactions for the efficient and uncomplicated synthesis of specific nitrogen heterocycles. Specifically, we reveal the potential of transition metal-free catalysts in this field. Because of their extensive scope and diversity, these reactions enable the synthesis of various heterocycles that contain nitrogen, which include 5-membered (carbazole, pyrimidines, and pyrroles) and 6-membered rings (piperidine, pyridine, quinoline, diazinane, pyrazine, quinoxaline, and 1,2,3-triazine). In addition, the compatibility of transition metal-free catalysts with various functional groups and substrates not only increases the synthetic value of these compounds but also broadens their relevance in the domains of medical chemistry, materials science, and other relevant areas of study. To motivate future study and development in this field, the successful examples described in this review highlight the potential of transition metal-free catalysts as powerful instruments for the quick and efficient synthesis of nitrogen heterocycles. In general, this review provides a thorough and insightful examination of transition metal-free catalysts, highlighting the relevance of these compounds in contemporary organic synthesis and their potential to revolutionize the field of nitrogen heterocycle synthesis.

Synthesis of 2,3-Disubstituted Pyrroles by Lewis Acid Promoted Cyclization of N-Sulfonyl Vinylogous Carbamates and Amides

A three-step sequence to construct pyrroles from three components including 2,2-dimethoxyethylamine, aryl/alkyl sulfonyl chlorides and alkynes bearing electron-withdrawing groups is presented. The pyrroles are proposed to arise via a 5-exo-trig cyclization proceeding through both oxocarbenium and N-sulfonyliminium ions. This modular route allows for the variability at the N-sulfonyl group, the C2 and C3 substituents for rational vectoring of the pyrrole nucleus for downstream processes.

Synthesis of Pyrrole via Formal Cycloaddition of Allyl Ketone and Amine under Metal-Free Conditions

A new metal-free synthesis of pyrrole from allyl ketone and amine has been established. The reaction proceeds via an thiolative activation of the C-C double bond with dimethyl(methylthio)sulfonium trifluoromethanesulfonate, followed by a nucleophilic ring-opening addition of primary amine to the generated episulfonium intermediate, and then an internal condensation and aromatization. This mild procedure provides a novel strategy to the construction of substituted pyrroles through a formal [4 + 1] cycloaddition reaction.

Direct synthesis of pentasubstituted pyrroles and hexasubstituted pyrrolines from propargyl sulfonylamides and allenamides

Multisubstituted pyrroles are important fragments that appear in many bioactive small molecule scaffolds. Efficient synthesis of multisubstituted pyrroles with different substituents from easily accessible starting materials is challenging. Herein, we describe a metal-free method for the preparation of pentasubstituted pyrroles and hexasubstituted pyrrolines with different substituents and a free amino group by a base-promoted cascade addition-cyclization of propargylamides or allenamides with trimethylsilyl cyanide. This method would complement previous methods and support expansion of the toolbox for the synthesis of valuable, but previously inaccessible, highly substituted pyrroles and pyrrolines. Mechanistic studies to elucidate the reaction pathway have been conducted.

Recent Advancements in Pyrrole Synthesis

This review article features selected examples on the synthesis of functionalized pyrroles that were reported between 2014 and 2019. Pyrrole is an important nitrogen-containing aromatic heterocycle that can be found in numerous compounds of biological and material significance. Given its vast importance, pyrrole continues to be an attractive target for the development of new synthetic reactions. The contents of this article are organized by the starting materials, which can be broadly classified into four different types: substrates bearing π-systems, substrates bearing carbonyl and other polar groups, and substrates bearing heterocyclic motifs. Brief discussions on plausible reaction mechanisms for most transformations are also presented.

Synthesis of functionalized tetrahydropyrans via cascade cycloaddition involving silyloxyallyl cation intermediates

An expedient synthesis of highly substituted tetrahydrobenzofuran via an unsymmetrical silyloxyallyl cation is reported.

Intermolecular scandium triflate-promoted nitrene-transfer [5 + 1] cycloadditions of vinylcyclopropanes

Sc(OTf)3-promoted [5 + 1] cycloaddition of vinylcyclopropanes with PhINTs is reported, enabling the regioselective preparation of a range of 1,2,3,6-tetrahydropyridine scaffolds under mild conditions. This represents the second example of a [5 + 1] nitrene-transfer cycloaddition and exhibits complementary substrate scope to the antecedent work, expanding the range of N-heterocycles accessible via this strategy.