Tandem Nazarov cyclization-michael addition sequence catalyzed by an Ir(III) complex

Aptamer recognition-promoted specific intercalation of iridium complexes in G-quadruplex DNA for label-free and enzyme-free phosphorescence analysis of kanamycin

In consideration of relevance of antibiotic with food security, it is extremely desirable to propose sensitive and credible methods for antibiotic screening. Nevertheless, most of known approaches are developed based on fluorescence technique, which suffered from the interferences of background fluorescence and autoluminescence, and tedious labeling procedures, ascribing to the deficiency of high-performance and multifunctional dyes. Herein, we developed a novel iridium (III) complex (Ir-QAU)-based aptamer-promoted phosphorescence sensor for label-free, enzyme-free and highly sensitive detection of target antibiotic (kanamycin, Kan) based on target-switched hybridizing chain reaction (HCR). Ir-QAU was elaborately devised to present a signal-on response to G-quadruplex (G4) DNA against other DNAs due to its specific intercalation in G4 DNA and subsequent restriction of intra-molecular rotation. The recognition of H1 by Kan promoted the formation of Kan@H1 complexes, which hybridized with H2 and H3 via toehold-mediated hybridization reaction, subsequently switching HCR to produce large numbers of G4 DNA. Compared to Kan absence, abundant Ir-QAU was locked in G4 DNA to yield a significantly increased luminescence, which switches the luminescence analysis process of Kan with a limit of detection down to 0.38 pM. Furthermore, the Ir-QAU-based sensor was triumphantly applied to detect Kan in milk sample. We anticipate this work will disclose a new way to development of high-efficiency and practical luminescence sensor, and show a great potential for antibiotic-related food security.

Oxa-Nazarov Cyclization-Michael Addition Sequence for the Rapid Construction of Dihydrofuranones

The Nazarov cyclization has been established as a powerful tool in constructing cyclopentenone skeletons. In sharp contrast, oxa-Nazarov cyclization that affords dihydrofuranones, a new type of product, has been less explored. In this work, we report the I2-catalyzed oxa-Nazarov cyclization-Michael addition cascade between vinyl α-diketones and enones. The protocol allows access to a range of functionalized dihydrofuranones with good to high yields, and diverse further transformations on the products have been achieved. Furthermore, the mechanistic studies reveal that the 1,2-hydride shift occurs simultaneously during the dihydrofuranone formation.

Gold self-relay catalysis for accessing functionalized cyclopentenones bearing an all-carbon quaternary stereocenter

A new gold(i) self-relay catalysis consisting of a 3,3-rearrangement, Nazarov cyclization and Michael addition cascade of 1,3-enyne acetates with aurones and their derived azadienes is reported, producing functionalized cyclopentenones.

Asymmetric Nazarov Cyclizations of Unactivated Dienones by Hydrogen-Bond-Donor/Lewis Acid Co-Catalyzed, Enantioselective Proton-Transfer

We report an enantioselective Nazarov cyclization catalyzed by chiral hydrogen-bond-donors in concert with silyl Lewis acids. The developed transformation provides access to tri-substituted cyclopentenones in high levels of enantioselectivity (up to 95% e.e.) from a variety of simple unactivated dienones. Kinetic and mechanistic studies are consistent with a reversible 4π-electrocyclization C-C bond-forming step followed by rate- and enantio-determining proton-transfer as the mode of catalysis.

Novel d-Annulated Pentacyclic Steroids: Regioselective Synthesis and Biological Evaluation in Breast Cancer Cells

The acid-catalyzed cyclization of benzylidenes based on 16-dehydropregnenolone acetate (16-DPA) was studied. It was found that these compounds readily undergo regioselective interrupted Nazarov cyclization with trapping chloride ion and an efficient method of the synthesis of d-annulated pentacyclic steroids based on this reaction was proposed. The structures of the synthesized pentacyclic steroids were determined by NMR and X-ray diffraction. It was found that the reaction affords a single diastereomer, but the latter can crystallize as two conformers depending on the structure. Antiproliferative activity of synthesized compounds was evaluated against two breast cancer cell lines: MCF-7 and MDA-MB-231. All tested compounds showed relatively high antiproliferative activity. The synthetic potential of the protocol developed was illustrated by the gram-scale experiment.

Enantiodivergent synthesis of 1,2-bis(diphenylphosphino)ethanes via asymmetric [3 + 2]-cycloaddition

The present work provides efficient access to chiral 1,2-bis(diphenylphosphino)ethanes (DPPEs) directly from diphosphine oxide alkenes and azomethine ylides via asymmetric [3 + 2] cycloaddition. Both enantiomers of the products can be obtained in good to excellent diastereo- and enantioselectivities (up to >20 : 1 dr and 99% ee).

Nickel/Copper Dual Catalysis for Sequential Nazarov Cyclization/Decarboxylative Aldol Reaction

A nickel/copper cocatalyzed sequential Nazarov cyclization/decarboxylative aldol reaction is reported. Various β-hydroxycyclopentenones containing three contiguous stereocenters were prepared in good yields with high diastereoselectivities. A gram-scale reaction and further derivatizations were easily achieved to highlight the synthetic utility of this transformation. Preliminary attempts for sequential asymmetric transformation were also conducted.

One-Pot Generation of Bicyclo[3.1.0]hexanols and Cyclohexanones by Double Interrupted Nazarov Reactions

Organoaluminum-mediated double interrupted Nazarov cyclization to access bicyclo[3.1.0]hexanols via nucleophilic methyl attack followed by Simmons-Smith-type electrophilic cyclopropanation is reported. These alcohols can undergo ring opening to afford cyclohexanones or cyclohexenones, broadening the range of scaffolds available via interrupted Nazarov reaction beyond the usual cyclopentanoid products. Throughout the sequence, a total of four new C-C bonds are formed, along with four new stereogenic centers.

Nickel-catalyzed enantioselective sequential Nazarov cyclization/decarboxylation

The nickel-catalyzed enantioselective sequential Nazarov cyclization/decarboxylation has been developed.

Assigning regioisomeric or diastereoisomeric relations of problematic trisubstituted double-bonds through heteronuclear 2D selective J-resolved NMR spectroscopy

Although one of the first 2D NMR methods, but so far neglected, selectiveJ-resolved NMR spectroscopy offers a unique opportunity to help organic chemists in structure elucidation, avoiding natural and non-natural product misassignments.

Efficient catalytic enantioselective Nazarov cyclizations of divinyl ketoesters

An efficient catalytic enantioselective Nazarov cyclization of divinyl ketoesters is developed using a BOX/Cu(ii) catalyst, affording various optically active cyclopent-2-enone esters.

Experimental and computational study of the catalytic asymmetric 4π-electrocyclization of N-heterocycles

The first asymmetric metal-catalyzed Nazarov cyclization of N-heterocycles has been developed. The use of a chiral catalyst allows the enantioselective electrocyclization of N-heterocycles under mild conditions and the corresponding products are obtained in good yields with excellent enantio- and diastereoselectivity. The reaction mechanism and the absolute configuration of the obtained products are explained by means of computational studies.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

Efficient Nazarov cyclization/Wagner-Meerwein rearrangement terminated by a Cu(II)-promoted oxidation: synthesis of 4-alkylidene cyclopentenones

The discovery and elucidation of a new Nazarov cyclization/Wagner-Meerwein rearrangement/oxidation sequence is described that constitutes an efficient strategy for the synthesis of 4-alkylidene cyclopentenones. DFT computations and EPR experiments were conducted to gain further mechanistic insight into the reaction pathways.

Experimental and theoretical studies on the Nazarov cyclization/Wagner-Meerwein rearrangement sequence

Highly functionalized cyclopentenones can be generated by a chemoselective copper(II)-mediated Nazarov/Wagner-Meerwein rearrangement sequence of divinyl ketones. A detailed investigation of this sequence is described including a study of substrate scope and limitations. After the initial 4π electrocyclization, this reaction proceeds via two different sequential [1,2]-shifts, with selectivity that depends upon either migratory ability or the steric bulkiness of the substituents at C1 and C5. This methodology allows the creation of vicinal stereogenic centers, including adjacent quaternary centers. This sequence can also be achieved by using a catalytic amount of copper(II) in combination with NaBAr(4)(f), a weak Lewis acid. During the study of the scope of the reaction, a partial or complete E/Z isomerization of the enone moiety was observed in some cases prior to the cyclization, which resulted in a mixture of diastereomeric products. Use of a Cu(II)-bisoxazoline complex prevented the isomerization, allowing high diastereoselectivity to be obtained in all substrate types. In addition, the reaction sequence was studied by DFT computations at the UB3LYP/6-31G(d,p) level, which are consistent with the proposed sequences observed, including E/Z isomerizations and chemoselective Wagner-Meerwein shifts.

Asymmetric Diels-Alder and Ficini reactions with alkylidene β-ketoesters catalyzed by chiral ruthenium PNNP complexes: mechanistic insight

Hydride abstraction from the β-position of the enolato ligand of the previously reported complex [Ru(3a-H)(PNNP)]PF(6) (5a; 3a-H is the enolate of 2-tert-butoxycarbonylcyclopentanone) with (Ph(3)C)PF(6) gives the dicationic complex [Ru(6a)(PNNP)](2+) (7a) as a single diastereoisomer, which contains the unsaturated β-ketoester 2-tert-butoxycarbonyl-2-cyclopenten-1-one (6a) as a chelating ligand. The methyl analogue 2-methoxycarbonylcyclopentanone (3b) gives [Ru(3b-H)(PNNP)]PF(6) as a mixture of noninterconverting diastereoisomers (ester group of 3b trans to P, 5b; or to N, 5c), which were separated by column chromatography. Hydride abstraction from 5b (or 5c) yields diastereomerically pure [Ru(6b)(PNNP)](2+) (7b or 7c). Complexes 7b and 7c do not interconvert at room temperature in CD(2)Cl(2) and form opposite enantiomers of the Diels-Alder adduct upon reaction with Dane's diene (1 equiv). X-ray studies of 7a, 5b, and 5c give insight into the origin of enantioselection and the sense of asymmetric induction in the previously reported asymmetric Diels-Alder and Ficini cycloaddition reactions with 2,3-disubstituted butadienes and ynamides, respectively. Stoichiometric reactions (substrate coordination, cycloaddition, and product displacement) between [Ru(OEt(2))(2)(PNNP)](2+) (2), 6b (or 6a), and Dane's diene (15, to give estrone derivatives) or N-benzyl-N-(cyclohexylethynyl)-4-methylbenzenesulfonamide (17, to give cyclobutenamides) suggest that product displacement from the catalyst is turnover limiting.

Conjugate addition-initiated Nazarov cyclization

A reaction sequence involving the 1,6-conjugate addition of a nucleophile to a dienyl diketone followed by Nazarov cyclization is described. Several nucleophiles are identified as competent initiators for the sequence. A different reaction outcome is observed when catalytic amounts of nucleophile are employed, involving elimination of the nucleophile after the electrocyclization.

Understanding the fate of the oxyallyl cation following Nazarov electrocyclization: sequential Wagner-Meerwein migrations and the synthesis of spirocyclic cyclopentenones

A general reaction sequence is described that involves Nazarov cyclization followed by two sequential Wagner-Meerwein migrations, to afford spirocyclic compounds from divinyl ketones in the presence of 1 equiv of copper(II) complexes. A detailed investigation of this sequence is described including a study of substrate scope and limitations. It was found that after 4π electrocyclization, two different pathways are available to the oxyallyl cation intermediate: elimination of a proton can give the usual Nazarov cycloadduct, or ring contraction can give an alternative tertiary carbocation. After ring contraction, either [1,2]-hydride or carbon migration can occur, depending upon the substitution pattern of the substrate, to furnish spirocyclic products. The rearrangement pathway is favored over the elimination pathway when catalyst loading is high and the copper(II) counterion is noncoordinating. Several ligands were found to be effective for the reaction. Thus, the reaction sequence can be controlled by judicious choice of reaction conditions to allow selective generation of richly functionalized spirocycles. The three steps of the sequence are stereospecific: electrocyclization followed by two [1,2]-suprafacial Wagner-Meerwein shifts, the ring contraction and then a hydride, alkenyl, or aryl shift. The method allows stereospecific installation of adjacent stereocenters or adjacent quaternary centers arrayed around a cyclopentenone ring.

Divergent reaction pathways of a cationic intermediate: rearrangement and cyclization of 2-substituted furyl and benzofuryl enones catalyzed by iridium(III)

In contrast to 2-substituted pyrrole enones, furyl and benzofuryl enones do not undergo the Nazarov electrocyclization. Instead, these furyl and benzofuryl enones exhibit unusual rearrangement sequences in the presence of catalytic amounts of [IrBr(CO)(DIM)((R)-(+)-BINAP)](SbF(6))(2) (1; DIM = diethylisopropylidene malonate) and AgSbF(6) (1:1). A 1,2-H shift followed by intramolecular Friedel-Crafts alkylation leads to synthetically valuable cyclohexanones with furanylic quaternary centers. The electrophilicity of 1 is essential for this rearrangement.

A highly reactive dicationic iridium(III) catalyst for the polarized Nazarov cyclization reaction

Pushing the Nazarov Envelope

A new electrophilic complex [IrBr(CO)(diethylisopropylidene malonate)((R )-(+)-BINAP)](SbF6)2 (2 ) exhibits unusual activity in the catalysis of polarized Nazarov cyclization. Aryl vinyl ketones that show poor reactivity with well-known catalysts such as [Ir(CH3)(CO)(1,2-diiodobenzene)(dppe)](B(Arf)4−)2 (1 ), Sc(OTf)3 + LiClO4 and Cu(ClO4)2, can be cyclized with 2 + AgSbF6 (1:1) under mild conditions with concurrent AgBr precipitation.