Sequential three-component reactions: synthesis, regioselectivity and application of functionalized dihydropyridines (DHPs) for the creation of fused naphthyridines

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.

Green synthesis of C5–C6-unsubstituted 1,4-DHP scaffolds using an efficient Ni–chitosan nanocatalyst under ultrasonic conditions

A heterogeneous and magnetically recyclable Ni–chitosan nanocatalyst was synthesized and thoroughly characterized by powder Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDX) spectroscopy, etc. It was effectively utilized in the eco-friendly synthesis of new C5–C6-unsubstituted 1,4-DHPs under ultrasonic irradiation. The important focus of the methodology was to develop an environmentally friendly protocol with a short reaction time and a simple reaction procedure. The other advantages of this protocol are a wide substrate scope, a very good product yield, the use of an eco-friendly solvent and a recyclable nanocatalyst, as well as reaction at room temperature.

Metal-free synthesis of quinoline-2,4-dicarboxylate derivatives using aryl amines and acetylenedicarboxylates through a pseudo three-component reaction

An efficient, useful and one-pot protocol for the synthesis of quinoline-2,4-dicarboxylate scaffolds is accomplished from aryl amines and dimethyl/diethyl acetylenedicarboxylates using 20 mol% molecular iodine as a catalyst in acetonitrile at 80 °C. In addition, the mechanistic explanation for the formation of the desired products is disclosed. The pivotal role of molecular iodine in the formation of the major products, diester quinoline derivatives, and the minor product, triesters, in two cases is described in the mechanism. The notable advantages of this method are non-involvement of a metal catalyst, avoiding of metal contamination in the final product as well as waste generation, use of a low cost and eco-friendly catalyst, ease of handling, high regioselectivity, shorter reaction time, the formation of one C-N and two C-C bonds and a broad substrate scope with good yields.

Metal-free multicomponent reactions: a benign access to monocyclic six-membered N-heterocycles

The advent of multicomponent reactions in the synthesis of heterocycles and their ever burgeoning applications in drug development, materials chemistry, and catalysis, have attracted a great deal of current scientific interest. In particular, the metal-free multicomponent synthesis of six membered N-heterocycles has undergone intensive research over the last two decades offering an environmentally benevolent means contrary to traditional metal catalysed reactions. To the best of our knowledge, there exists no exclusive review on the metal-free multicomponent synthesis of six membered N-heterocyles, and hence the present report highlights the progress on metal-free multicomponent reactions with their advantages and mechanistic insights to access monocyclic six-membered N-heterocycles including pyridine, pyrimidine, pyrazine, triazine and their hydrogenated derivatives. The literature is covered since 2000, and the contents offer not only striking methods for divergent synthesis of six-membered N-heterocycles but also put forward some new insights into the exploration of metal-free multicomponent chemistry.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Pd-Catalyzed One-Pot Stepwise Synthesis of Benzo[b][1,6]naphthyridines from 2-Chloroquinoline-3-carbonitriles Using Sulfur and Amines As Nucleophiles

A palladium-catalyzed one-pot stepwise coupling-annulation reaction of 2-chloroqunoline-3-carbonitriles enabled the direct synthesis of sulfur-substituted benzo[b][1,6]naphthyridines via multiple bond formation. The reaction provided an unusual mode for cyclization as sodium sulfide, a soft nucleophile, preferred to attack on the carbon of the nitrile group rather than on the C-C triple bond. The developed chemistry was extended with the secondary amines as nucleophiles to afford nitrogen-substituted benzo[b][1,6]naphythyridines while primary amines afforded hydroamination products . The hydromination products were transformed to benzo[b][1,6]naphthyridones via a base-mediated cyclization reaction. The  developed protocol features inexpensive and easily synthesizable starting materials, easy operations, and a high efficiency and tolerance to a broad range of substrates.

Synthesis of functionalized dihydro-2-oxypyrroles and tetrahydropyridines using 2,6-pyridinedicarboxylic acid as an efficient and mild organocatalyst

Simple and mild one-pot protocols for the synthesis of diversely functionalized dihydro-2-oxypyrroles and tetrahydropyridines using catalytic amounts of 2,6-PDCA.

Ultrasound mediated, green innovation for the synthesis of polysubstituted 1,4-dihydropyridines

An atom efficient protocol via a one-pot four-component cyclocondensation reaction catalyzed by copper(i) iodide in aqueous medium under ultrasonic irradiation has been developed for the synthesis of novel polysubstituted 1,4-dihydropyridines.

Cubic Perovskite ZnTiO3 Nanopowder as a Recyclable Heterogeneous Catalyst for the Synthesis of 1,6-Naphthyridines in Water

Cubic perovskite ZnTiO₃ nanopowder has been prepared by means of a sustainable sol-gel method. The nanopowder (12-20 nm) has been thoroughly characterized by N₂ sorption analysis, high-resolution (HR) TEM, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), SEM, and FTIR analyses. The environmentally benign synthesis of highly substituted 1,6-naphthyridines catalyzed by ZnTiO₃ nanopowder in aqueous media has been demonstrated. It requires two different catalytic functions, that is, an acid one, which is provided by Ti^IV ions, and a basic one, which is provided by the oxide ion incorporated within the ZnTiO₃ metal oxide framework. 1,6-Naphthyridines are of tremendous biological importance, and this method is simple and environmentally friendly. The greenness of the process was satisfactorily established as water was exploited as the reaction medium and there was very high atom economy.

Efficient synthesis of functionalized spiro[indoline-3,4′-pyridines] and spiro[indene-2,4′-pyridines] via a three-component reaction

Three-component reactions for the synthesis of polysubstituted spiro[indoline-3,4′-pyridines] and spiro[indene-2,4′-pyridines] in good yields and with high diastereoselectivity.