Synthesis of 1,4-Dihydropyridines and Related Heterocycles by Iodine-Mediated Annulation Reactions of N-Cyclopropyl Enamines

Photocatalyzed Selective Hydrocarbonation of Alkenes with Hantzsch Esters toward 4-Alkyl-Hantzsch Esters

Here, we describe a mild photoredox-neutral reaction system that enables the selective hydrocarbonation of alkenes with Hantzsch esters, affording structurally diverse 4-alkyl-Hantzsch esters. This straightforward protocol can be performed under an air atmosphere without the need for any transition metals. The synthetic potential of this method is well exemplified by the late-stage structural modification of a series of pharmaceutically relevant complex molecules.

Synthesis of 2,2-difunctionalized 2H-azirines via I2-mediated annulation of enamines

Various 2,2-difunctionalized 2H-azirines were synthesized via I2-mediated annulation reactions of readily accessible enamines in the presence of nitrogen or non-nitrogen nucleophiles. The features of the present synthesis process also include no use of transition metals, simple operation, mild reaction conditions, a broad substrate scope, and gram-scale synthesis.

Construction of 1,4-Dihydropyridines: The Evolution of C4 Source

The field of cascade cyclization for the construction of 1,4-dihydropyridines (1,4-DHPs) has been continuously expanding during the last decades because of their broad-spectrum biological and synthetic importance. To date, many methods have been developed, mainly including the Hantzsch reaction, Hantzsch-like reaction and newly developed cascade cyclization, in which various synthons have been successively developed as C4 sources of 1,4-DHPs. This review presents the cascade cyclization synthesis strategy for the construction of 1,4-DHPs according to various C4 sources from carbonyl compounds, alkenyl fragments, alcohols, aliphatic amines, glycines and other C4 sources.

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.

I2-mediated coupling of quinazolinone enamines with 2-aminopyridines: a new strategy to access spiroquinazolinones

Herein, we report a novel, rapid and efficient route to the spiroquinazolinone framework via an umpolung strategy mediated by molecular iodine. A library of functionalized spiroquinazolinone iodide salts was synthesized in moderate to good yields under ambient, metal-free and mild conditions. The current methodology opens up a new efficient and concise strategy for the construction of spiroquinazolinones.

Synthesis of Quinazolinone-Fused Tetrahydroisoquinolines and Related Polycyclic Scaffolds by Iodine-Mediated sp3 C-H Amination

An iodine-mediated intramolecular sp³ C-H amination reaction producing quinazolinone-fused polycyclic skeletons from 2-aminobenzamide precursors is reported. This reaction does not use transition metals, has a broad substrate scope, and can be used on a gram scale. Under the optimal reaction conditions, a variety of quinazolinone-fused tetrahydroisoquinolines and derivatives of Rutaecarpine were synthesized from readily accessible compounds. The reaction proceeds well with crude 2-aminobenzamide derivatives, allowing for the synthesis of the products from simple 2-aminobenzoic acids and tetrahydroisoquinolines without purification of the 2-aminobenzamide intermediates. Preliminary biological experiments have identified Cereblon (CRBN) inhibitory activity and relevant anti-myeloma medicinal properties in some of these polycyclic products.

Activation of aminocyclopropanes via radical intermediates

Aminocyclopropanes are versatile building blocks for accessing high value-added nitrogen-containing products. To control ring-opening promoted by ring strain, the Lewis acid activation of donor-acceptor substituted systems is now well established. Over the last decade, alternative approaches have emerged proceeding via the formation of radical intermediates, alleviating the need for double activation of the cyclopropanes. This tutorial review summarizes key concepts and recent progress in ring-opening transformations of aminocyclopropanes via radical intermediates, divided into formal cycloadditions and 1,3-difunctionalizations.

Preparation of Substituted Pyridines via a Coupling of β-Enamine Carbonyls with Rongalite-Application for Synthesis of Terpyridines

A Hantzsch-type strategy for the synthesis of 2,3,5,6-tetrasubstituted pyridines via an oxidative coupling of β-enamine carbonyl compounds with rongalite was developed. This method employs rongalite as a C1 unit for the assembly of a pyridine ring at C-4 position, offering a facile method for the preparation of substituted pyridine derivatives with a broad functional group tolerance. In particular, this method allows us to prepare terpyridine derivatives, which are important ligands or structural fragments for catalysts and 3D metal–organic frameworks.

Iodine-Mediated C═C Double Bond Cleavage toward Pyrido[2,1-b]quinazolinones

A transition-metal-free C═C double bond cleavage reaction employing molecular iodine is described. In the presence of K₂CO₃ as the base, I₂-mediated C═C bond cleavage followed by intramolecular annulation of N-(2-vinylaryl)pyridin-2-amine substrates produces pyrido[2,1-b]quinazolinones and related heterocyclic compounds. This reaction can be completed on a gram scale and has been successfully applied to the synthesis of compounds with important biological properties, including efflux pump inhibitory and antiallergic activities.