Organocatalytic aza-Michael/retro-aza-Michael reaction: Pronounced chirality amplification in aza-Michael reaction and racemization via retro-aza-Michael reaction

Direct Organocatalytic Reductive Alkylation of Syncarpic Acid: Scope and Applications

Biologically important 4-alkylsyncarpic acids, which resemble the core structure of many natural products, were synthesized in one-pot through the organocatalytic three-component reductive alkylation with excellent yields and C-selectivity. Synthetic applications of 4-alkylsyncarpic acids were demonstrated by converting into the functionally rich molecules through different reactions like Michael, retro-Michael, reduction, and oxidation reactions. In a continuation, formal total synthesis of (±)-triumphalone, (±)-isotriumphalone, and monomeric phloroglucinol derivatives was reported in a few steps starting from 4-alkylsyncarpic acids in overall very good yields. Further showcasing the importance of C-alkylated products, 4-benzylsyncarpic acid and its Michael adduct with methyl vinyl ketone were synthesized in a gram scale without compromising rate/yields.

Recent advances in organocatalytic asymmetric aza-Michael reactions of amines and amides

Nitrogen-containing scaffolds are ubiquitous in nature and constitute an important class of building blocks in organic synthesis. The asymmetric aza-Michael reaction (aza-MR) alone or in tandem with other organic reaction(s) is an important synthetic tool to form new C-N bond(s) leading to developing new libraries of diverse types of bioactive nitrogen compounds. The synthesis and application of a variety of organocatalysts for accomplishing highly useful organic syntheses without causing environmental pollution in compliance with 'Green Chemistry" has been a landmark development in the recent past. Application of many of these organocatalysts has been extended to asymmetric aza-MR during the last two decades. The present article overviews the literature published during the last 10 years concerning the asymmetric aza-MR of amines and amides catalysed by organocatalysts. Both types of the organocatalysts, i.e., those acting through non-covalent interactions and those working through covalent bond formation have been applied for the asymmetric aza-MR. Thus, the review includes the examples wherein cinchona alkaloids, squaramides, chiral amines, phase-transfer catalysts and chiral bifunctional thioureas have been used, which activate the substrates through hydrogen bond formation. Most of these reactions are accompanied by high yields and enantiomeric excesses. On the other hand, N-heterocyclic carbenes and chiral pyrrolidine derivatives acting through covalent bond formation such as the iminium ions with the substrates have also been included. Wherever possible, a comparison has been made between the efficacies of various organocatalysts in asymmetric aza-MR.

Highlight on the Mechanism of Linear Polyamidoamine Degradation in Water

This paper aims at elucidating the degradation mechanism of linear polyamidoamines (PAAs) in water. PAAs are synthesized by the aza-Michael polyaddition of prim-monoamines or bis-sec-amines with bisacrylamides. Many PAAs are water-soluble and have potential for biotechnological applications and as flame-retardants. PAAs have long been known to degrade in water at pH ≥ 7, but their degradation mechanism has never been explored in detail. Filling this gap was necessary to assess the suitability of PAAs for the above applications. To this aim, a small library of nine PAAs was expressly synthesized and their degradation mechanism in aqueous solution studied by 1H-NMR in different conditions of pH and temperature. The main degradation mechanism was in all cases the retro-aza-Michael reaction triggered by dilution but, in some cases, hints were detected of concurrent hydrolytic degradation. Most PAAs were stable at pH 4.0; all degraded at pH 7.0 and 9.0. Initially, the degradation rate was faster at pH 9.0 than at pH 7.0, but the percent degradation after 97 days was mostly lower. In most cases, at pH 7.0 the degradation followed first order kinetics. The degradation rates mainly depended on the basicity of the amine monomers. More basic amines acted as better leaving groups.

Indium(III) Triflate-Catalyzed Reactions of Aza-Michael Adducts of Chalcones with Aromatic Amines: Retro-Michael Addition versus Quinoline Formation

The indium(III) triflate-catalyzed reaction of aza-Michael adducts of chalcones with aromatic amines has been investigated. The Michael adducts derived from substituted anilines and chalcones underwent retro-Michael addition to give the original starting materials, whereas the adducts derived from 1-naphthylamines and chalcones afforded quinolines. A six-membered cyclic transition state has been proposed to explain the retro-Michael addition, while a Povarov mechanism has been put forward to explain the quinoline formation.

Diastereoselective auxiliary- and catalyst-controlled intramolecular aza-Michael reaction for the elaboration of enantioenriched 3-substituted isoindolinones. Application to the synthesis of a new pazinaclone analogue

A new asymmetric organocatalyzed intramolecular aza-Michael reaction by means of both a chiral auxiliary and a catalyst for stereocontrol is reported for the synthesis of optically active isoindolinones. A selected cinchoninium salt was used as phase-transfer catalyst in combination with a chiral nucleophile, a Michael acceptor and a base to provide 3-substituted isoindolinones in good yields and diastereomeric excesses. This methodology was applied to the asymmetric synthesis of a new pazinaclone analogue which is of interest in the field of benzodiazepine-receptor agonists.

Regioselective addition of DDQ on a quinoid ring: an entry into chiral zwitterionic bridging ligands

The Michael-type addition of DDQ on a zwitterionic ligand is described together with the chiral separation and full characterization of the two enantiomers of one derivative. These adducts offer a novel entry into chiral zwitterionic quinoidal ligands.

Preparation and characterization of silicone rubber cured via catalyst-free aza-Michael reaction

A novel silicone rubber of high strength and stable dimension was cured via catalyst-free aza-Michael reaction.

Switchable stereocontrolled divergent synthesis induced by aza-Michael addition of deactivated primary amines under acid catalysis

Switchable tandem intramolecular aza-Michael/Michael and double aza-Michael reactions allow the oriented synthesis of highly functionalised cyclic skeletons. Conjugate addition of deactivated anilines triggers chemo- and stereo-divergent ring-closure reaction pathways with a striking selectivity depending on reaction conditions.

Deviant mass shift of hydrated product ions from sodiated beta-anilinodidrochalcones using an ion-trap mass spectrometer

The fragmentation reactions of sodiated beta-anilinodidrochalcones have been investigated by electrospray ionization multi-stage mass spectrometry (ESI-MS(n)). The fragment ion of sodiated N-benzylidenebenzenamine (P1) easily undergoes ion-molecule reactions with the residual ESI solvent molecules (H2O and CH3OH) in the vacuum system, as verified by MS3 and accurate MS analysis. The formed hydrated ions appear as an unusual leading peak in the profile spectrum, which results in a deviant decreasing mass shift of almost 1 Da. Density functional theory calculations indicate that P1 easily associates with H2O without any energy barrier. Thus, the hydrated P1 exists partially as a loose system of P1 and H2O, which provides a reasonable explanation for the decreasing mass shift of the solvated P1. The above results are important in obtaining structural information from MS(n) spectra and preventing erroneous data interpretation for the analogous adducts.