On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects

Molecular Dynamics Investigations of Dienolate [4 + 2] Reactions

We report quantum mechanics calculations and quasiclassical trajectory simulations of [4 + 2] reactions using three common dienolate substrates: siloxy dienes, Li dienolates, and conjugated Pd enolates. Asynchronous transition structures and unequal bond formation were invariably found, with average time gaps of developing bonds ranging from 26.5 to >251.0 fs. The results display a spectrum of dynamically concerted and stepwise [4 + 2] reactions, offering insights into the origin of the stereochemical outcomes of such reactions.

Investigation of thia-Diels-Alder Reactions by Ultrafast Transient Absorption Spectroscopy and DFT Calculations

The thia-Diels-Alder reaction represents a versatile synthetic method for the preparation of six-membered sulfur-containing compounds. However, the mechanism of the thia-Diels-Alder reactions remains unclear. In this work, time-resolved spectroscopic experiments and DFT calculations demonstrate that phenacyl sulfide undergoes Norrish II cleavage to produce thioaldehyde, and ortho-hydroxy benzhydryl alcohol occurs in a dehydration reaction to generate o-QMs using diphenylphosphate as the catalyst. Then, the thia-Diels-Alder reaction takes place between thioaldehyde and o-QMs by an asynchronous concerted mechanism. The illustration of the thia-Diels-Alder reaction mechanism not only provides important support for organic synthesis and drug design but also enhances fundamental insights into reaction pathways and catalytic processes in the field of chemical synthesis.

A concise update on the synthetic transformation of aurones via asymmetric cycloaddition, annulation, and Michael/Mannich reactions

This review provides a comprehensive overview of the significance of aurone cores in organic chemistry, highlighting their crucial role as synthetic intermediates. With their innate electrophilic reactivity and convenient accessibility, aurone cores play a vital role in catalysing the development of novel methodologies and facilitating the creation of intricate compounds. The objective of this review is to present a current and insightful compilation that summarizes the progress in aurone synthetic transformations, focusing on diverse cycloaddition ([3 + 2], [4 + 2], [4 + 3], [10 + 2]) and annulation reactions.

A Regioselective [3 + 2] Cycloaddition of Alkynols and Ketones To Access Diverse 1,3-Dioxolane Scaffolds

This study presents a stepwise exoselective [3 + 2] cycloaddition reaction of alkynols with ketones, leading to the synthesis of 4-methylene-1,3-dioxolane derivatives. Remarkably, without any Thorpe-Ingold induced effect, the cyclization reaction was demonstrated with complete regio- and chemoselectivity, which was solely promoted by cesium carbonate. A wide range of unactivated ketones are viable under these mild reaction conditions, and both primary and tertiary alkynols are compatible with these cyclization reactions. We have prepared a diverse array of highly dense exomethylene 1,3-dioxolane rings demonstrating a remarkable tolerance for various functional groups.

Understanding the Molecular Mechanism of Thermal and LA-Catalysed Diels-Alder Reactions between Cyclopentadiene and Isopropyl 3-Nitroprop-2-Enate

The molecular mechanism of the Diels-Alder reaction with the participation of cyclopentadiene and isopropyl 3-nitroprop-2-enate was examined based on wb97xd/6-311+G(d) (PCM) quantum chemical calculations. It was found that the type of mechanism for the conversion of addends depends significantly on the reaction conditions. In less-polar environments, a one-step polar mechanism is realised. In more polar solvents, the formation of "extended"-type zwitterionic intermediates is possible. In contrast, in the presence of an LA-type catalyst, the one-step mechanisms are replaced by respective stepwise mechanisms with zwitterionic or heterocyclic intermediates.

Fully Selective Synthesis of Spirocyclic-1,2-oxazine N-Oxides via Non-Catalysed Hetero Diels-Alder Reactions with the Participation of Cyanofunctionalysed Conjugated Nitroalkenes

Hetero Diels-Alder (HDA) reactions with the participation of E-2-aryl-1-cyano-1-nitroethenes and methylenecyclopentane were evaluated on the basis of experimental as well as quantumchemical data. It was found that contrary to most known HDA reactions, title processes are realised under non-catalytic conditions and with full regiocontrol. The DFT study shows, without any doubt, the polar but single-step reaction mechanism. Deeper exploration using Bonding Evolution Theory (BET) techniques gives a clear image of the sequences of electron density reorganisation along the reaction coordinate. The first C4-C5 bond is created in phase VII by merging two monosynaptic basins, while the second O1-C6 bond is created in the last phase by a donation of the nonbonding electron density of O1 to C6. Based on the research, we can conclude that the analysed reaction proceeds according to a two-stage one-step mechanism.

Synthesis of New Polyheterocyclic Pyrrolo[3,4-b]pyridin-5-ones via an Ugi-Zhu/Cascade/Click Strategy

A diversity-oriented synthesis (DOS) of two new polyheterocyclic compounds was performed via an Ugi-Zhu/cascade (N-acylation/aza Diels-Alder cycloaddition/decarboxylation/dehydration)/click strategy, both step-by-step to optimize all involved experimental stages, and in one pot manner to evaluate the scope and sustainability of this polyheterocyclic-focused synthetic strategy. In both ways, the yields were excellent, considering the high number of bonds formed with release of only one carbon dioxide and two molecules of water. The Ugi-Zhu reaction was carried out using the 4-formylbenzonitrile as orthogonal reagent, where the formyl group was first transformed into the pyrrolo[3,4-b]pyridin-5-one core, and then the remaining nitrile group was further converted into two different nitrogen-containing polyheterocycles, both via click-type cycloadditions. The first one used sodium azide to obtain the corresponding 5-substituted-1H-tetrazolyl-pyrrolo[3,4-b]pyridin-5-one, and the second one with dicyandiamide to synthesize the 2,4-diamino-1,3,5-triazine-pyrrolo[3,4-b]pyridin-5-one. Both synthesized compounds may be used for further in vitro and in silico studies because they contain more than two heterocyclic moieties of high interest in medicinal chemistry, as well as in optics due to their high π-conjugation.

Full Regio- and Stereoselective Protocol for the Synthesis of New Nicotinoids via Cycloaddition Processes with the Participation of Trans-Substituted Nitroethenes: Comprehensive Experimental and MEDT Study

[3 + 2] Cycloaddition reactions with the participation of Z-C-(3-pyridyl)-N-methylnitrone and series of E-2-R-nitroethenes were both experimentally and theoretically explored in the framework of Molecular Electron Density Theory. It was found that all considered processes are realized under mild conditions and in full regio- and stereocontrol. The ELF analysis additionally showed that the studied reaction proceeds by a two-stage, one-step mechanism.

Unveiling the Chemistry and Synthetic Potential of Catalytic Cycloaddition Reaction of Allenes: A Review

Allenes with two carbon-carbon double bonds belong to a unique class of unsaturated hydrocarbons. The central carbon atom of allene is sp hybridized and forms two σ-bonds and two π-bonds with two terminal sp² hybridized carbon atoms. The chemistry of allenes has been well documented over the last decades. They are more reactive than alkenes due to higher strain and exhibit significant axial chirality, thus playing a vital role in asymmetric synthesis. Over a variety of organic transformations, allenes specifically undergo classical metal catalyzed cycloaddition reactions to obtain chemo-, regio- and stereoselective cycloadducts. This review briefly describes different types of annulations including [2+2], [2+2+1], [3+2], [2+2+2], [4+2], [5+2], [6+2] cycloadditions using titanium, cobalt, rhodium, nickel, palladium, platinum, gold and phosphine catalyzed reactions along with a mechanistic study of some highlighted protocols. The synthetic applications of these reactions towards the synthesis of natural products such as aristeromycin, ent-[3]-ladderanol, waihoensene(-)-vindoline and (+)-4-epi-vindoline have also been described.

Multiselective Diels-Alder Reaction of α-Arylacroleins Catalyzed by Boron Tribromide-Assisted Chiral Phosphoric Acids

A multiselective Diels-Alder (DA) reaction of α-arylacroleins with cyclopentadiene using BBr₃-assisted chiral BINOL-derived phosphoric acid catalysts has been developed. This unusual exo- and enantioselective DA reaction can be multicontrolled by the chiral cavity of the in situ-formed acid-base cooperative catalysts, in particular, suppressing the competitive hetero Diels-Alder (HDA) reaction effectively.

Asymmetric [4 + 2] cycloaddition synthesis of 4H-chromene derivatives facilitated by group-assisted-purification (GAP) chemistry

In this work, we present a strategy for the preparation of functionalized 4H-chromene derivatives via a Cs₂CO₃-catalyzed [4 + 2] cycloaddition of enantiopure chiral salicyl N-phosphonyl imines with allenoates. Fifteen examples were achieved in excellent yields and diastereoselectivity. The products were purified simply by washing the crude mixture with hexanes following the Group-Assisted Purification (GAP) chemistry/technology to bypass traditional separation methods. The absolute configuration was unambiguously determined by X-ray structure analysis.

A new asymmetric method for the synthesis of highly functionalized 4H-chromenes was developed via Group-Assisted Purification (GAP) chemistry and shown in good to high yield and excellent diastereoselectivity.

On the Question of Zwitterionic Intermediates in the [3 + 2] Cycloaddition Reactions between C-arylnitrones and Perfluoro 2-Methylpent-2-ene

The molecular mechanism of the [3 + 2] cycloaddition reaction between C-arylnitrones and perfluoro 2-methylpent-2-ene was explored on the basis of DFT calculations. It was found that despite the polar nature of the intermolecular interactions, as well as the presence of fluorine atoms near the reaction centers, all reactions considered cycloaddition proceed via a one-step mechanism. All attempts for the localization of zwitterionic intermediates on the reaction paths were not successful. Similar results were obtained regardless of the level of theory applied.

The Participation of 3,3,3-Trichloro-1-nitroprop-1-ene in the [3 + 2] Cycloaddition Reaction with Selected Nitrile N-Oxides in the Light of the Experimental and MEDT Quantum Chemical Study

The regioselective zw-type [3 + 2] cycloaddition (32CA) reactions of a series of aryl-substituted nitrile N-oxides (NOs) with trichloronitropropene (TNP) have been both experimentally and theoretically studied within the Molecular Electron Density Theory (MEDT). Zwitterionic NOs behave as moderate nucleophiles while TNP acts as a very strong electrophile in these polar 32CA reactions of forward electron density flux, which present moderate activation Gibbs free energies of 22.8-25.6 kcal·mol-1 and an exergonic character of 28.4 kcal·mol-1 that makes them irreversible and kinetically controlled. The most favorable reaction is that involving the most nucleophilic MeO-substituted NO. Despite Parr functions correctly predicting the experimental regioselectivity with the most favorable O-CCCl3 interaction, these reactions follow a two-stage one-step mechanism in which formation of the O-C(CCl3) bond takes place once the C-C(NO2) bond is already formed. The present MEDT concludes that the reactivity differences in the series of NOs come from their different nucleophilic activation and polar character of the reactions, rather than any mechanistic feature.