Exploring the Vinylogous Reactivity of Cyclohexenylidene Malononitriles: Switchable Regioselectivity in the Organocatalytic Asymmetric Addition to Enals Giving Highly Enantioenriched Carbabicyclic Structures

Recent Advances in Organic Small-Molecule Fluorescent Probes Based on Dicyanoisophorone Derivatives

Fluorescent probe technology holds great promise in the fields of environmental monitoring and clinical diagnosis due to its inherent advantages, including easy operation, reliable detection signals, fast analysis speed, and in situ imaging capabilities. In recent years, a wide range of fluorescent probes based on diverse fluorophores have been developed for the analysis and detection of various analytes, yielding significant achievement. Among these fluorophores, the dicyanoisophorone-based fluorophores have garnered significant attention. Dicyanoisoporone exhibits minimal fluorescence, yet possesses a robust electron-withdrawing capability, rendering it suitable for constructing of D-π-A structured fluorophores. Leveraging the intramolecular charge transfer (ICT) effect, such fluorophores exhibit near-infrared (NIR) fluorescence emission with a large Stokes shift, thereby offering remarkable advantages in the design and development of NIR fluorescence probes. This review article primarily focus on small-molecule dicyanoisoporone-based probes from the past two years, elucidating their design strategies, detection performances, and applications. Additionally, we summarize current challenges while predicting future directions to provide valuable references for developing novel and advanced fluorescence probes based on dicyanoisoporone derivatives.

Colorimetric and fluorometric bimodal amine chemosensor based on deprotonation-induced intramolecular charge transfer: Application to food spoilage detection

Amine derivatives are signature organic compounds generated from rotten protein food. Thus, sensitive detection of the presence of amines in protein foods can be a critical technique for monitoring their quality. In this study, we develop an organic chemosensor probe, 4-(2-(3-(dicyanomethylene)- 5,5-dimethylcyclohex-1-en-1-yl)vinyl)-N,N-diethylbenzenaminium chloride (DEAH), to effectively detect amines through discernible bimodal (colorimetric and fluorometric) changes. By exploiting the amine-triggered intramolecular charge transfer behavior, DEAH exhibits rapid color changes (<1 s) with an excellent detection limit (36.9 nM) and also fluorescence turn-on in response to amine gas. Moreover, it possesses detection capabilities in versatile conditions, including solutions, solids, and coated films, suggesting its practical applicability. In particular, DEAH shows dramatic color change from yellow to violet with exceptional color difference (△Eab) over 98, repeatable usability, and excellent selectivity to amines. Based on these compelling advantages, we successfully demonstrate real-time monitoring of amine gas generated from spoiled protein foods using DEAH-coated films.

The Role of Aromatic Alcohol Additives on Asymmetric Organocatalysis Reactions: Insights from Theory

The presence of an aromatic additive has been seen to enhance, often significantly, the enantioselectivity and yield in asymmetric organocatalysis. Considering their success across a dizzying range of organocatalysts and organic transformations, it would seem unlikely that a common principle exists for their functioning. However, the current investigations with DFT suggest a general principle: the phenolic additive sandwiches itself, through hydrogen bonding and π⋅⋅⋅π stacking, between the organocatalyst coordinated electrophile and nucleophile. This is seen for a wide range of experimentally reported systems. That such complex formation leads to enhanced stereoselectivity is then demonstrated for two cases: the cinchona alkaloid complex (BzCPD), catalysing thiocyanation (2-naphthol additive employed), as well as for L-pipecolicacid catalysing the asymmetric nitroaldol reaction with a range of nitro-substituted phenol additives. These findings, indicating that dual catalysis takes place when phenolic additives are employed, are likely to have a significant impact on the field of asymmetric organocatalysis.

Diastereoselective access to [4,4]-carbospirocycles: governance of thermodynamic enolates with an organocatalyst in vinylogous cascade annulation

A vinylogy concept driven annulation strategy is developed to access [4,4]-carbospirocycles from alkylidene malononitriles and cyclopentene-1,3-diones. The reaction is catalyzed by an inexpensive organocatalyst and products with three stereocenters were obtained as a single diastereomer in high yields. The spiro-selectivity originates from the reaction of the thermodynamic enolate intermediate which is fundamentally intriguing.

Observation of macrophage autophagy in the healing of diabetic ulcers via a lysosome-targeting polarity-specific two-photon probe

As a disease with high incidence, mutilation, and fatality rates, diabetic ulcers (DUs) have become a difficult and complicated disease of widely concern in recent years due to the unclear healing mechanism. The main reason for the delayed healing in DU patients is the unduly long chronic inflammation window, and the polarization state of macrophages plays a key role in this process. Since autophagy is believed to be closely related to the polarization trend of macrophages, recent studies have shown that autophagy is closely related to the healing of DU. To this end, a lysosome-targeting polarity-sensitive probe, XZTU-VIS, was developed to monitor the changes in lysosomal polarity, thereby assessing the autophagy of macrophages in mice suffering from DU. The experimental results showed that under two-photon fluorescence microscopy, the green channel fluorescence signal of XZTU-VIS decreased significantly during autophagy. In the meantime, DU models established using BV-2 cells and mice showed a process that could cause inflammation and the release of ROS, thereby inducing autophagy.

A polarity-dependent two-photon fluorescent probe for evaluation of autophagy in the process of diabetic mouse skin ulcer-induced inflammation was constructed.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.

Extended Enolates: Versatile Intermediates for Asymmetric C-H Functionalization via Noncovalent Catalysis

Catalyst-controlled functionalization of unmodified carbonyl compounds is a relevant operation in organic synthesis, especially when high levels of site- and stereoselectivity can be attained. This objective is now within reach for some subsets of enolizable substrates using various types of activation mechanisms. Recent contributions to this area include enantioselective transformations that proceed via transiently generated noncovalent di(tri)enolate-catalyst coordination species. While relatively easier to form than simple enolate congeners, di(tri)enolates are ambifunctional in nature and so control of the reaction regioselectivity becomes an issue. This Minireview discusses in some detail this and other problems, and how noncovalent activation approaches based on metallic and metal free catalysts have been developed to advance the field.

Rapid Evaluation of the Mechanism of Buchwald-Hartwig Amination and Aldol Reactions Using Intramolecular 13C Kinetic Isotope Effects

A practical approach is introduced for the rapid determination of ¹³C kinetic isotope effects that utilizes a "designed" reactant with two identical reaction sites. The mechanism of the Buchwald-Hartwig amination of tert-butylbromobenzene with primary and secondary amines is investigated under synthetically relevant catalytic conditions using traditional intermolecular ¹³C NMR methodology at natural abundance. Switching to 1,4-dibromobenzene, a symmetric bromoarene as the designed reactant, the same experimental ¹³C KIEs are determined using an intramolecular KIE approach. This rapid methodology for KIE determination requires substantially less material and time compared to traditional approaches. Details of the Buchwald-Hartwig amination mechanism are investigated under varying synthetic conditions, namely a variety of halides and bases. The enantioselectivity-determining step of the l-proline catalyzed aldol reaction is also evaluated using this approach. We expect this mechanistic methodology to gain traction among synthetic chemists as a practical technique to rapidly obtain high-resolution information regarding the transition structure of synthetically relevant reactions under catalytic conditions.

Chemodivergent reactions

An important strategy for the efficient generation of diversity in molecular structures is the utilization of common starting materials in chemodivergent transformations. The most studied solutions for switching the chemoselectivity rely on the catalyst, ligand, additive, solvent, temperature, time, pressure, pH and even small modifications in the substrate. In this review article several processes have been selected such as inter- and intramolecular cyclizations, including carba-, oxa-, thia- and oxazacyclizations promoted mainly by Brønsted or Lewis acids, transition metals and organocatalysts, as well as radical reactions. Catalyst-controlled intra- and intermolecular cyclizations are mainly described to give five- and six-membered rings. Cycloaddition reactions involving (2+2), (3+2), (3+3), (4+1), (4+2), (5+2), (6+2) and (7+2) processes are useful reactions for the synthesis of cyclic systems using organocatalysts, metal catalysts and Lewis acid-controlled processes. Addition reactions mainly of carba- and heteronucleophiles to unsaturated conjugated substrates can give different adducts via metal catalyst-, Lewis acid- and solvent-dependent processes. Carbonylation reactions of amines and phenols are carried out via ligand-controlled transition metal-catalyzed multicomponent processes. Ring-opening reactions starting mainly from cyclopropanols, cyclopropenols and epoxides or aziridines are applied to the synthesis of acyclic versus cyclic products under catalyst-control mainly by Lewis acids. Chemodivergent reduction reactions are performed using dissolving metals, sodium borohydride or hydrogen transfer conditions under solvent control. Oxidation reactions include molecular oxygen under solvent control or using different dioxiranes, as well as chemodivergent palladium catalyzed cross-coupling reactions using boronic acids are applied to aromatic and allenic compounds. Other chemodivergent reactions such as alkylations and allylations under transition metal catalysis, dimerization of acetylenes, bromination of benzylic substrates, and A³-couplings are performed via catalyst- or reaction condition-dependent processes.

Lewis acid-catalyzed asymmetric reactions of β,γ-unsaturated 2-acyl imidazoles

The investigation of diverse reactivity of β,γ-unsaturated carbonyl compounds is of great value in asymmetric catalytic synthesis. Numerous enantioselective transformations have been well developed with β,γ-unsaturated carbonyl compounds as nucleophiles, however, few example were realized by utilizing them as not only nucleophiles but also electrophiles under a same catalytic system. Here we report a regioselective catalytic asymmetric tandem isomerization/α-Michael addition of β,γ-unsaturated 2-acyl imidazoles in the presence of chiral N,N′-dioxide metal complexes, delivering a broad range of optically pure 1,5-dicarbonyl compounds with two vicinal tertiary carbon stereocenters in up to >99% ee under mild conditions. Meanwhile, stereodivergent synthesis is disclosed to yield all four stereoisomers of products. Control experiments suggest an isomerization process involved in the reaction and give an insight into the role of NEt₃. In addition, Mannich reaction and sulfur-Michael addition of β,γ-unsaturated 2-acyl imidazoles proceed smoothly as well under the same catalytic system.

The investigation of reactivity of β,γ-unsaturated carbonyl compounds is of great synthetic value, especially in asymmetric transformations. Here, the authors report a catalytic asymmetric tandem isomerization/α-Michael addition of β,γ-unsaturated 2-acyl imidazoles in presence of chiral N,N′-dioxide metal catalysts.

Introducing ortho-methoxyl group as a fluorescence-enhancing and bathochromic-shift bi-functional strategy for typical cysteine sensors

A practical strategy of introducing ortho-methoxyl group was explored to achieve the fluorescence-enhancing and bathochromic-shift bi-functional optimization. It was tested in the Cys sensing ISOPH-X series, thus the successful case, ISOPH-2, was obtained. It realized the optimization in a simple and compatible way. The corresponding strategy was basically established during the confirmation of checkpoints including applicable steadiness (over 24 h), wide pH range (7.0-9.0), rapid response (20 min), good biocompatibility, high sensitivity (LOD = 0.072 nm), high selectivity and biological monitoring of Cys in living cells as well as C. elegans. In this work, the o-methoxyl introduction strategy led to a 15 nm red shift and a near 4-fold fluorescence enhancement. This strategy could be combined with the double bond-introducing approach. Compared with reported strategies, by breaking the dilemma between red shift and strong fluorescent intensity, this strategy might offer beneficial information for exploiting better sensors with more fluorophores and mechanisms for their targets.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

NHC-Mediated Synthesis of Tricyclic Spirocarbocycles via an Intramolecular Stetter Reaction of Cyclic Enal-Enones

A general and efficient method for the synthesis of tricyclic spirocarbocycles is described. Various cyclic enal-enones were reacted with an N-heterocyclic carbene, and an intramolecular Stetter reaction proceeded smoothly to give various tricyclic spiro-1,4-diketones in 31-72% yields. The ring size of the spiro compounds can be easily controlled using different cyclic enals and enones or by altering the length of the carbon tether.

Pot and time economies in the total synthesis of Corey lactone

The Corey lactone is a highly versatile intermediate for the synthesis of a variety of prostaglandin hormones that natively control a multitude of important physiological processes. Starting from commercially available compounds, we herein disclose a time-economical, one-pot enantioselective preparation of the Corey lactone by virtue of a new diphenylprolinol silyl ether-mediated domino Michael/Michael reaction to afford the substituted cyclopentanone core in a formal (3 + 2) cycloadditive fashion. More broadly, this work advances the on-demand, gram-scale synthesis of high-value targets involving chemically orthogonal transformations, whereby distinct reactions of acids, bases, organometalics, reductants and oxidants can be carried out in a single reaction vessel in a sequential fashion.

Lewis Base/Brønsted Acid Co-Catalyzed Asymmetric Thiolation of Alkenes with Acid-Controlled Divergent Regioselectivity

A divergent strategy for the facile preparation of various enantioenriched phenylthio-substituted lactones was developed based on Lewis base/Brønsted acid co-catalyzed thiolation of homoallylic acids. The acid-controlled regiodivergent cyclization (6-endo vs. 5-exo) and acid-mediated stereoselective rearrangement of phenylthio-substituted lactones were explored. Experimental and computational studies were performed to clarify the origins of the regioselectivity and enantioselectivity. The calculation results suggest that C-O and C-S bond formation might occur simultaneously, without formation of a commonly supposed catalyst-coordinated thiiranium ion intermediate and the potential π-π stacking between substrate and SPh as an important factor in the enantio-determining step. Finally, this methodology was applied in the rapid syntheses of the bioactive natural products (+)-ricciocarpin A and (R)-dodecan-4-olide.

Construction of Spirofused Tricyclic Frameworks by NHC-Catalyzed Intramolecular Stetter Reaction of a Benzaldehyde Tether with a Cyclic Enone

Various benzaldehyde tethers with a cyclic enone were prepared from commercially available 2-hydroxybenzaldehydes via a three-step sequence involving triflate formation, Sonogashira cross-coupling, and regioselective hydrogenation. These substrates were then exposed to an N-heterocyclic carbene, whereupon intramolecular Stetter reaction proceeded smoothly to give various spirofused tricyclic 1,4-diketones in 30-87% yields. Furaldehyde and nicotinaldehyde derivatives also participated in the reaction under the Stetter conditions.

Ligand and counteranion enabled regiodivergent C-H bond functionalization of naphthols with α-aryl-α-diazoesters

An unprecedented ligand- and counteranion-controlled and site-selectivity switchable direct C–H bond functionalization of unprotected naphthols with α-aryl-α-diazoesters was developed.

Here, an unprecedented ligand and counteranion-controlled and site-selectivity switchable direct C–H bond functionalization of unprotected naphthols with α-aryl-α-diazoesters was developed. In this transformation, site selectivities are realized by turning on/off the coordination between metal complexes and hydroxy groups. The preliminary mechanism revealed that the interaction between the hydroxy group and gold catalyst plays a key role in switching the site-selectivity of gold-carbene. This protocol potentially provides a novel design for C–H bond functionalization.

A chiral-at-metal asymmetric catalyzed vinylogous Michael addition of ortho-methyl aromatic nitro compounds for isoxazole derivative synthesis

A chiral-at-metal rhodium(iii) complex catalyzed asymmetric vinylogous Michael addition of 5-methyl 4-nitroisoxazoles with α,β-unsaturated 2-acyl imidazoles has been developed, delivering the corresponding adducts with up to 96% yield and 97% ee.

Diastereoselective Rh-catalyzed decarboxylative allylation to form quaternary stereocenters using sulfinimine as the directing group

In this paper, we reported for the first time that the diastereoselective Rh-catalyzed decarboxylative allylation of chiral sulfinimines could be used to form quaternary stereocenters.

α-Alkylation of Chiral Sulfinimines for Constructing Quaternary Chiral Carbons by Introducing Removable Directing Groups

This study developed a facile and efficient synthetic strategy to construct quaternary chiral centers at the α-position of imines and ketones. High regioselectivity and diastereoselectivity were achieved through the synergetic effect of electron-withdrawing directing groups and N-tert-butyl sulfinamide as chiral auxiliaries. Either of them could be removed under the optimized conditions without any epimerization.