Conjugate addition of allylsilanes to α,β-unsaturated acylsilanes

α,β-Unsaturated acylsilanes serve as highly reactive carboxylic acid equivalents in conjugate allylation reactions with allylsilane derivatives.     

A biogenetically inspired heterodimerization approach to the synthesis of the core structure of the alkaloid fissoldhimine

A biogenetically inspired heterodimerization reaction of N-substituted 2-pyrroline equivalents leads to the tricyclic core of the alkaloid fissoldhimine. Thus, pyrrolidin-2-ol derivatives, in which the nitrogen atom is substituted either with urea or thiourea functionality, serve as equivalents of the corresponding N-substituted 2-pyrrolines. Reaction of these compounds under Lewis acidic (e.g., lanthanide triflate) or Brønsted acid conditions leads to a diastereomeric form of the tricyclic core of fissoldhimine. The reaction can be envisaged to occur either via an asynchronous intermolecular inverse electron demand hetero-Diels-Alder reaction, or through a tandem Mannich/ring-closure reaction.     

An efficient and simple β-acylvinyl anion equivalent for cyclohexenones

Dilithio species available from treatment of Δ^3,4bromocyclohexenone ketals with two equivalents of n-butyllithium serve as one-pot equivalents of β-vinyl-carbanions of cyclohexenones. These former ketals are available in good yield from the corresponding β-bromocyclohexenone by direct ketalization under controlled conditions.     

Versatile asymmetric synthesis of the kavalactones: first synthesis of (+)-kavain

[reaction: see text] Three asymmetric pathways to the kavalactones have been developed. The first method is chiral auxiliary-based and utilizes aldol reactions of N-acetyl thiazolidinethiones followed by a malonate displacement/decarboxylation reaction. The second approach uses the asymmetric catalytic Mukaiyama additions of dienolate nucleophile equivalents developed by Carreira and Sato. Finally, tin-substituted intermediates, prepared by either of these routes, can serve as advanced general precursors of kavalactone derivatives via Pd(0)-catalyzed Stille couplings with aryl halides.     

Tandem base-promoted ring opening/brook rearrangement/allylic alkylation of O-silyl cyanohydrins of beta-silyl-alpha,beta-epoxyaldehydes

[reaction: see text]. Metalated O-silyl cyanohydrins of beta-silyl-alpha,beta-epoxyaldehyde have been found to serve as functionalized homoenolate equivalents by a tandem sequence involving a base-promoted ring opening of the epoxide, Brook rearrangement, and alkylation of the resulting allylic anion.     

Glycosyl sulfonium ions as storable intermediates for glycosylations

Glycosyl sulfonium ions, which serve as persistent glycosyl cation equivalents, were prepared by the addition of diorganosulfides to an electrochemically generated glycosyl triflate. Low-temperature and variable-temperature NMR studies were performed to reveal the structure, stability, and reactivity of glycosyl sulfonium ions. The glycosyl sulfonium ions could be used as storable intermediates for reactions with various glycosyl acceptors including thioglycosides to give the corresponding disaccharides.     

α-Chlor-nitrone V: Substitutionsreaktionen an Olefin- und Benzolderivaten. Eine Methode zur Darstellung von β,γ-un-gesättigten und β- arylsubstituierten Aldehyden. Stereospezifische Bildung von tetra-alkylsubstituierten Olefindoppelbindungen. Über synthetische Methoden, 9. (vorläufige) Mitteilung

The Ag⁺-induced α-chloro-aldonitrone/olefin reaction in polar solvents can proceed by substitution, thereby providing a method for the preparation of β, γ-unsaturated aldehydes. Positional as well as configurational retention of the olefinic double bond are mechanistically significant and preparatively useful characteristics of this process. Substitution also occurs with great ease at nucleophilic aromatic nuclei; this offers a simple preparative route to certain β-aryl-aldehydes. The results illustrate a general aspect of the chemistry of α-chloro-aldonitrones: the N-alkenyl-N-alkyl-nitrosonium-ions derived from them can serve as preparative equivalents of the elusive corresponding α-acyl-carbonium-ions.     

Novel ambiphilic dichlorocarbenoid equivalent in alkene cyclopropanation and carbonyl olefination

The Ti-Mg-dichloromethylene complexes derived from the oxidative addition of CCl4 to the Mg-TiCl4 bimetallic species serve as a novel class of ambiphilic dichlorocarbenoid equivalents. Not only is Ti-Mg-dichlorocarbenoid highly selective but also it seems highly reactive in both alkene cyclopropanations and carbonyl dichloromethylenations.     

Unusual ambiphilic carbenoid equivalent in amide cyclopropanation

[reaction: see text] The titanium-methylene complexes derived from the TiCl(4)-Mg-CH(2)Cl(2) system serve as a novel class of ambiphilic carbenoid equivalents, which not only efficiently effect cyclopropanations of a variety amides but also exhibit high chemoselectivity.     

Synthesis of 5-methyl-1,3,2-benzodithiazoles

The novel helerocycles 5-methyl-1,3,2-benzodithiazoles (7) were prepared in 30–50% yields from toluene-3,4-dithiol (6) and appropriate primary amines in the presence of 2 equivalents of triethylamine under high-dilution conditions. These compounds, which exhibit one reversible oxidation potential around +0.90 V vs. SCE, serve as a model study in the quest of synthesizing a new donor, the “boron-nitrogen” analog of tetrathiafulvalene (BNTTF).     

27 Years of Catalytic Carbonylative Coupling Reactions in Hungary (1994–2021)

Palladium-catalyzed carbonylation reactions, in the presence of nucleophiles, serve as very potent tools for the conversion of aryl and alkenyl halides or halide equivalents to carboxylic acid derivatives or to other carbonyl compounds. There are a vast number of applications for the synthesis of simple building blocks as well as for the functionalization of biologically important skeletons. This review covers the history of carbonylative coupling reactions in Hungary between the years 1994 and 2021.     

Tandem base-promoted ring-opening/Brook rearrangement/allylic alkylation of O-silyl cyanohydrins of beta-silyl-alpha,beta-epoxyaldehyde: scope and mechanism

Metalated O-silyl cyanohydrins of beta-silyl-alpha,beta-epoxyaldehyde have been found to serve as functionalized homoenolate equivalents by a tandem sequence involving base-promoted ring opening of the epoxide, Brook rearrangement, and alkylation of the resulting allylic anion. On the basis of mechanistic studies involving competitive experiments using the diastereomeric cyanohydrins, we propose a reaction pathway involving a silicate intermediate 36 formed by a concerted process via an anti-opening of the epoxide followed by the formation of an O-Si bond.     

Reversed-polarity synthesis of diaryl ketones via palladium-catalyzed cross-coupling of acylsilanes

Acylsilanes serve as acyl anion equivalents in a palladium-catalyzed cross-coupling reaction with aryl bromides to give unsymmetrical diaryl ketones. Water plays a unique and crucial activating role in these reactions. High-throughput experimentation techniques provided successful reaction conditions initially involving phosphites as ligands. Ultimately, 1,3,5,7-tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane was identified as giving a longer-lived catalyst with higher turnover numbers. Its use, in conjunction with a palladacycle precatalyst, led to optimal reaction rates and yields. Scope and limitations of this novel method are presented along with initial mechanistic insight.     

Bispyrrolylbenzene Anion Receptor: From Supramolecular Switch to Molecular Logic Gate

We have designed anion receptor 4 based on a conformationally labile bispyrrolylbenzene framework, the conformation of which can be changed by appropriate anionic stimuli. In the absence of fluoride anion, the pyrrole moieties rotate freely at room temperature. However, when the concentration of fluoride anion exceeds 2 equivalents, the rotation of the pyrrole units slows down and the conformation of the receptor changes to anti‐anti. DFT calculations have shown that this change is due to binding of a third fluoride anion through C?H interaction. Anion receptor 4 can also serve as a molecular logic gate. Anionic inputs such as fluoride and dihydrogenphosphate allow the realization of INHIBIT and NAND logic gate functions with absorption and fluorescence as readouts, respectively.     

Organocatalytic umpolung: N-heterocyclic carbenes and beyond

The umpolung strategy encompasses all the methods that make organic molecules react in an inverse manner compared to their innate polarity-driven reactivity. This concept entered the field of organocatalysis when it was recognized that N-heterocyclic carbenes (NHCs) can provide catalytic access to acyl anion equivalents. Since then, tremendous efforts have followed to develop a broad variety of NHC-catalyzed reactions. In addition to this, more recent research developments have shown that other families of organocatalysts are also able to mediate transformations in which inversion of polarity is involved. This tutorial review aims at offering a didactic overview of organocatalytic umpolung and should serve as an inspiration for further progress in this field.     

Combinatorial synthesis of an oligosaccharide library by using beta-bromoglycoside-mediated iterative glycosylation of selenoglycosides: rapid expansion of molecular diversity with simple building blocks

A new method for constructing an oligosaccharide library composed of structurally defined oligosaccharides is presented based on an iterative glycosylation of selenoglycosides. Treatment of 2-acyl-protected selenoglycosides with bromine selectively generates beta-bromoglycosides, which serve as glycosyl cation equivalents in the oligosaccharide synthesis. Thus, the coupling of the bromoglycosides with another selenoglycoside affords the corresponding glycosylated selenoglycosides, which can be directly used to next glycosylation. The iteration of this sequence allows the synthesis of a variety of oligosaccharides including an elicitor active heptasaccharide. A characteristic feature of the iterative glycosylation is that glycosyl donors and acceptors with the same anomeric reactivity can be selectively coupled by activation of the glycosyl donor prior to coupling with the glycosyl acceptor. Therefore, same selenoglycosides can be used for both the glycosyl donors and the acceptors. This feature has been exemplified by a construction of an oligosaccharide library directed to elicitor-active oligosaccharides. The library composed of stereochemically defined oligoglucosides with considerable structural diversity can be constructed starting from simple selenoglycosides.     

Lewis acid-catalyzed conjugate additions of silyloxyallenes: a selective solution to the intermolecular Rauhut-Currier problem

Silyloxyallenes serve as highly useful alpha-acylvinyl anion equivalents. These latent allenolates undergo conjugate additions to alkylidene malonates in the presence of 10 mol % Sc(OTf) 3. The reaction delivers intermolecular Rauhut-Currier products in excellent yields and regioselectivities for a wide scope of substrates. Notably, the formal cross-coupling of two different alpha,beta-unsaturated carbonyl compounds (a cross Rauhut-Currier reaction) is achieved. Preliminary investigations have demonstrated good levels of enantioselectivity for the addition of a racemic silyloxyallene with a chiral Lewis acid.     

Antioxidant activity and total phenolic content of aqueous and methanolic extracts of Jordanian plants: an ICBG project

As part of an International Cooperative Biodiversity Groups (ICBG) program to study Jordan's biodiversity, the relative levels of antioxidant activity and the total phenolic content of aqueous and methanolic extracts of a total of 95 plant species, all of Jordanian origin and those collected at random, have been measured. The total phenolic content of aqueous and methanolic extracts of the investigated plant species ranged from 4.4 to 78.3 mg and from 2.1 to 52.8 mg gallic acid equivalents g(-1) dry weight, respectively, while the total antioxidant capacity ranged from 20.0 to 916.7 and from 15.1 to 915.6 micromol Trolox equivalents g(-1) dry weight, respectively. Based on this collection, approximately 5% of assayed plants showed high levels of antioxidant activity. There was a significant linear correlation between antioxidant activity and total phenolic content for aqueous and methanolic extracts, suggesting that phenolic compounds were the predominant antioxidant components in the investigated plant species. Interestingly, a few of the collected plants had high-antioxidant activity yet "low" phenolic content includes Ceratonia siliqua and Viscum cruciatum. These plants may serve as sources of antioxidants with new chemotypes.     

Unsaturated aldehydes as alkene equivalents in the Diels-Alder reaction

A one-pot procedure is described for using alpha,beta-unsaturated aldehydes as olefin equivalents in the Diels-Alder reaction. The method combines the normal electron demand cycloaddition with aldehyde dienophiles and the rhodium-catalyzed decarbonylation of aldehydes to afford cyclohexenes with no electron-withdrawing substituents. In this way, the aldehyde group serves as a traceless control element to direct the cycloaddition reaction. The Diels-Alder reactions are performed in a diglyme solution in the presence of a catalytic amount of boron trifluoride etherate. Subsequent quenching of the Lewis acid, addition of 0.3% of [Rh(dppp)2Cl] and heating to reflux achieves the ensuing decarbonylation to afford the product cyclohexenes. Under these conditions, acrolein, crotonaldehyde and cinnamaldehyde have been reacted with a variety of 1,3-dienes to afford cyclohexenes in overall yields between 53 and 88%. In these transformations, the three aldehydes serve as equivalents of ethylene, propylene and styrene, respectively.     

Stepwise Halide‐Triggered Double and Triple Catenation of Self‐Assembled Coordination Cages

A simple self‐assembled [Pd₂ L ₄] coordination cage consisting of four carbazole‐based ligands was found to dimerize into the interpenetrated double cage [3 X@Pd₄ L ₈] upon the addition of 1.5 equivalents of halide anions (X=Cl^?, Br^?). The halide anions serve as templates, as they are sandwiched by four Pd^II cations and occupy the three pockets of the entangled cage structure. The subsequent addition of larger amounts of the same halide triggers another structural conversion, now yielding a triply catenated link structure in which each Pd^II node is trans‐coordinated by two pyridine donors and two halide ligands. This simple system demonstrates how molecular complexity can increase upon a gradual change of the relative concentrations of reaction partners that are able to serve different structural roles.