Site-selective synthesis of indanyl-substituted indole derivatives via 1,3-dithiane induced Nazarov cyclization

We report an efficient site-selective synthetic method to C2 and C3 indanyl-substituted indole derivatives via 1,3-dithianyl induced Nazarov-type cyclization. In particular, C2-selective indanyl-substituted indoles were directly obtained by a BF3·Et2O-promoted sequence of intramolecular C3-C2 migration and Nazarov-type cyclization process.

Selective production of methylindan and tetralin with xylose or hemicellulose

Indan and tetralin are widely used as fuel additives and the intermediates in the manufacture of thermal-stable jet fuel, many chemicals, medicines, and shockproof agents for rubber industry. Herein, we disclose a two-step route to selectively produce 5-methyl-2,3-dihydro-1H-indene (abbreviated as methylindan) and tetralin with xylose or the hemicelluloses from agricultural or forestry waste. Firstly, cyclopentanone (CPO) was selectively formed with ~60% carbon yield by the direct hydrogenolysis of xylose or hemicelluloses on a non-noble bimetallic Cu-La/SBA-15 catalyst. Subsequently, methylindan and tetralin were selectively produced with CPO via a cascade self-aldol condensation/rearrangement/aromatization reaction catalyzed by a commercial H-ZSM-5 zeolite. When we used cyclohexanone (another lignocellulosic cycloketone) in the second step, the main product switched to dimethyltetralin. This work gives insights into the selective production of bicyclic aromatics with lignocellulose.

Photoredox-Enabled Deconstructive [5 + 1] Annulation Approach to Isoquinolones from Indanones in Water

We disclose a deconstructive [5 + 1] annulation protocol for the synthesis of isoquinolones through a nitrogen insertion into abundant indanones. This method exploits photoredox-catalyzed ring-opening of oxime esters. The reaction proceeds smoothly with water as the reaction medium and tolerates a range of functional groups on diverse thiophenols, amines, or indanones. Moreover, the representative isoquinolones exhibit promising antifungal activities.

Methylene C(sp3 )-H Arylation Enables the Stereoselective Synthesis and Structure Revision of Indidene Natural Products

The divergent synthesis of two indane polyketides of the indidene family, namely (±)-indidene A (11 steps, 1.7 %) and (+)-indidene C (13 steps, 1.3 %), is reported. The synthesis of the trans-configured common indane intermediate was enabled by palladium(0)-catalyzed methylene C(sp3 )-H arylation, which was performed in both racemic and enantioselective (e.r. 99 : 1) modes. Further elaboration of this common intermediate by nickel-catalyzed dehydrogenative coupling allowed the rapid installation of the aroyl moiety of (±)-indidene A. In parallel, the biphenyl system of (±)- and (+)-indidene C was constructed by Suzuki-Miyaura coupling. These investigations led us to revise the structures of indidenes B and C.

Benzo-Fused-Ring Toolbox Based on Palladium/Norbornene Cooperative Catalysis: Methodology Development and Applications in Natural Product Synthesis

ConspectusBenzo-fused skeletons are ubiquitous in agrochemicals, medicines, natural products, catalysts, and other organic function materials. The assembly of these skeletons in an efficient manner is an actively explored field in organic synthesis. Palladium/norbornene (Pd/NBE) cooperative catalysis is a powerful tool for the expeditious assembly of polysubstituted arenes through bis-functionalization of the ortho and ipso positions of aryl iodides in one operation. Owing to the efforts of Lautens, Catellani, and others, an array of Pd/NBE-promoted annulations for the syntheses of diversified benzo-fused rings have been developed. However, these methods have not been broadly applied in total synthesis yet.Our group is interested in efficient and practical total synthesis of biologically active molecules. In the past 7 years, we have been devoted to the development of new annulation strategies for the assembly of common benzo-fused skeletons through Pd/NBE-promoted reactions of aryl iodides with novel bifunctional reagents. In this Account, we summarize our laboratory's systematic efforts in this direction. First, readily available epoxides and aziridines were exploited as versatile bifunctional alkylating reagents, which enables quick assembly of a series of valuable benzo-fused heterocycles, including isochromans, dihydrobenzofurans, 1,3-cis-tetrahydroisoquinolines (THIQs), 1,3-trans-THIQs, etc. Second, a convergent access to 5-7-membered benzo-fused carbocycles (including indanes and tetrahydronaphthalenes) was developed by Pd/NBE-promoted annulation of aryl iodides with simple olefinic alcohol-containing alkylating reagents. Third, a Pd/NBE-promoted annulation between aryl iodides and cyclohexanone-containing amination reagents was developed for the construction of benzo-fused N-containing bridged scaffolds. Thus, we have established a practical and versatile toolbox for the quick assembly of diversified benzo-fused skeletons. These new annulation reactions are of high chemo-, regio-, and stereoselectivities with good step and atom economy. Moreover, they are able to rapidly increase molecular complexity from simple building blocks. Finally, their synthetic value has been demonstrated by immediate adoption in several efficient total syntheses of medicines and complex natural products. Compared to conventional synthetic logics, the Pd/NBE-promoted annulation toolbox allows the development of highly convergent strategies, which significantly improves the overall synthetic efficiency.We believe the results presented in this Account will have significant implications beyond our research. It can be envisaged that new Pd/NBE-promoted annulations as well as new applications in complex total synthesis will be revealed in the near future.

Palladium-Catalyzed C(sp2)-H Arylation of Peptides Directed by Aspartic Acid

Herein, we report a palladium-catalyzed C(sp²)-H di- or monoarylation of short peptides containing N-terminal benzamide groups using aspartic acid (Asp) as an endogenous directing group. This strategy has the following merits: a broad substrate scope, selective diarylation of peptides, and gram-scale synthesis. Furthermore, this strategy can be successfully utilized to synthesize peptide-peptide conjugates.

One-Pot Synthesis of 2,3-Disubstituted Indanone Derivatives in Water under Exogenous Ligand-Free and Mild Conditions

Diverse 2,3-substituted indanones are accessed in an efficient and robust protocol by a rhodium-catalyzed tandem carborhodium/cyclization and intramolecular proton shift pathway. The reaction is compatible with a broad range of functional internal acetylenes, especially for natural and functionalized alkynes derivatives, affording the desired indanones in good to excellent yields. Remarkably, this reaction features very mild and sustainable conditions using water as the sole solvent and without exogenous ligands. Control studies support that indanone is formed through the intramolecular proton transfer process from the key intermediate indenol.

Manganese-Catalyzed C(sp2)-H Alkylation of Indolines and Arenes with Unactivated Alkyl Bromides

Selective C(sp 2 ) - H bond alkylation of indoline, carbazole and (2-pyridinyl)arenes with unactivated alkyl bromides is achieved using MnBr 2 catalyst in the absence of an external ligand. The alkylation uses a simple LiHMDS base and avoids the necessity of Grignard reagent, unlike other Mn-catalyzed C - H functionalization. This reaction proceeded either through a five- or a less-favored six-membered metallacycle, and tolerated diverse functionalities, including alkenyl, alkynyl, silyl, aryl ether, pyrrolyl, indolyl, carbazolyl and alkyl bearing fatty alcohol and polycyclic-steroid moieties. Alkylation follows a single electron transfer (SET) pathway involving 1e oxidative addition of alkyl bromide and a rate-limiting C-H metalation.

4b-Aryltetrahydroindeno[1,2-a]indenes by Acid-Catalyzed Trans­annular Cyclization of Benzannulated Cyclooctene Alcohols

By starting from two simple building blocks, benzannulated cyclooctenones were obtained in three steps. Subsequent Grignard/aryl lithium addition to the ketone yielded the corresponding tertiary alcohols that underwent stereoselective acid-catalyzed transannular cyclization to provide a cis-fused 5/5 bicyclic indanylindane framework exclusively. Subsequent stereoselective nucleophilic addition to the indanyl cation by hydride, water, or electron-rich aromatics furnished the 4b-aryltetrahydroindano[1,2-a]indenes in good to excellent yields (up to 92%) in the trans-C9–C9a form in up to a >99:1 diastereomeric ratio.

Bifunctional thiourea-catalyzed asymmetric [3 + 2] annulation reactions of 2-isothiocyanato-1-indanones with barbiturate-based olefins

Bifunctional thiourea-catalyzed asymmetric [3 + 2] annulation reactions of 2-isothiocyanato-1-indanones with barbiturate-based olefins have been developed to afford chiral dispiro[indene-pyrrolidine-pyrimidine]s. Through this strategy, the target products could be obtained in good to excellent yields with excellent stereoselectivities. In addition, the synthetic utility was verified through a gram-scale synthesis, one-pot three-component reactions and further transformation experiments of the products.

Reactions of linear conjugated dienone structures with arenes under superelectrophilic activation conditions. An experimental and theoretical study of intermediate multicentered electrophilic species

Reactions of conjugated dienones with arenes under superelectrophilic activation conditions result in the formation of conjugated enones, indanes, and carbocyclic structures.

Palladium-Catalyzed Aminomethylative Oppolzer-Type Cyclization of Enynes: Access to Aminomethylated Benzofulvenes

A novel palladium-catalyzed Oppolzer-type cyclization reaction aided by the aminomethyl cyclopalladated complex has been developed, which provides rapid access to functionalized benzofulvenes with excellent stereoselectivity. The corresponding products can undergo Diels-Alder reaction with maleimides, providing a series of complex polycyclic compounds with excellent regio- and stereoselectivities.

Triflic Acid-Catalyzed Synthesis of Indole-Substituted Indane Derivatives via In Situ Formed Acetal-Facilitated Nucleophilic Addition and 4π-Electron-5-Carbon Electrocyclization Sequence

An efficient protocol for the synthesis of indole-substituted indanes from o-alkenylbenzaldehydes under acetalization conditions has been presented. The cyclization occurs via a nucleophilic addition of indole on the oxacarbenium ion generated from acetal formed under the reaction condition followed by a conrotatory 4π-electrocyclization reaction, which takes care of the exclusive diastereoselectivity observed during the cyclization step. Olefin geometry of o-alkenylbenzaldehyde and the amount of indole play a decisive role in the success of this cyclization process.

Enantioresolution and Binding Affinity Studies on Human Serum Albumin: Recent Applications and Trends

The interaction between proteins and drugs or other bioactive compounds has been widely explored over the past years. Several methods for analysis of this phenomenon have been developed and improved. Nowadays, increasing attention is paid to innovative methods, such as high performance affinity liquid chromatography (HPALC) and affinity capillary electrophoresis (ACE), taking into account various advantages. Moreover, the development of separation methods for the analysis and resolution of chiral drugs has been an area of ongoing interest in analytical and medicinal chemistry research. In addition to bioaffinity binding studies, both HPALC and ACE al-low one to perform other type of analyses, namely, displacement studies and enantioseparation of racemic or enantiomeric mixtures. Actually, proteins used as chiral selectors in chromatographic and electrophoretic methods have unique enantioselective properties demonstrating suitability for the enantioseparation of a large variety of chiral drugs or other bioactive compounds. This review is mainly focused in chromatographic and electrophoretic methods using human serum albumin (HSA), the most abundant plasma protein, as chiral selector for binding affinity analysis and enantioresolution of drugs. For both analytical purposes, updated examples are presented to highlight recent applications and current trends.

Synthesis of 1-Trifluorometylindanes and Close Structures: A Mini Review

This review describes methods for the synthesis of 1-trifluomethylindanes and close structures, which are still quite rare and scarcely available compounds. There are two main approaches to obtain 1-CF3-indanes. The first one is the construction of an indane system from CF3 precursors; the main methods are acid-mediated Friedel–Crafts cyclization, transition metal-catalyzed [3+2] annulation, and free-radical transformations. The second approach is the trifluoromethylation of a ready-made indane core by various CF3 sources, such as Ruppert–Prakash or Togni reagents. Many of these synthetic procedures possess high regio- and stereo-selectivity, allowing the preparation of unique 1-CF3-indane structures. In recent years, great attention has been paid to the synthesis of 1-CF3-indanes, due to the discovery of important biologically active properties for these compounds.

Enantioselective synthesis of 1-aminoindene derivatives via asymmetric Brønsted acid catalysis

We describe a catalytic asymmetric iminium ion cyclization reaction of simple 2-alkenylbenzaldimines using a BINOL-derived chiral N-triflyl phosphoramide. The corresponding 1-aminoindenes and tetracyclic 1-aminoindanes are formed in good yields and high enantioselectivities. Further, the chemical utility of the obtained enantiopure 1-aminoindene is demonstrated for the asymmetric synthesis of (S)-rasagiline.

Hydrogen radical-shuttle (HRS)-enabled photoredox synthesis of indanones via decarboxylative annulation

Hydrogen atom transfer (HAT) process is a powerful and effective strategy for activating C-H bonds followed by further functionalization. Intramolecular 1,n (n = 5 or 6)-HATs are common and frequently encountered in organic synthesis. However, intramolecular 1,n (n = 2 or 3)-HAT is very challenging due to slow kinetics. Compared to proton-shuttle process, which is well established for organic synthesis, hydrogen radical-shuttle (HRS) is unexplored. In this work, a HRS-enabled decarboxylative annulation of carbonyl compounds via photoredox catalysis for the synthesis of indanones is developed. This protocol features broad substrate scope, excellent functional group tolerance, internal hydrogen radical transfer, atom- and step-economy. Critical to the success of this process is the introduction of water, acting as both HRS and hydrogen source, which was demonstrated by mechanistic experiments and density functional theory (DFT) calculations. Importantly, this mechanistically distinctive HAT provides a complement to that of typical proton-shuttle-promoted, representing a breakthrough in hydrogen radical transfer, especially in the inherently challenging 1,2- or 1,3-HAT.

Although hydrogen atom transfer is widely observed in synthetic organic chemistry, intramolecular hydrogen atom transfer between atoms separated by fewer than four bonds is kinetically slow. Here the authors show a method to form indanones, with hydrogen atoms shuttled across short distances by water.

Pd-Catalyzed ipso,meta-Dimethylation of ortho-Substituted Iodoarenes via a Base-Controlled C-H Activation Cascade with Dimethyl Carbonate as the Methyl Source

A methyl group can have a profound impact on the pharmacological properties of organic molecules. Hence, developing methylation methods and methylating reagents is essential in medicinal chemistry. We report a palladium-catalyzed dimethylation reaction of ortho-substituted iodoarenes using dimethyl carbonate as a methyl source. In the presence of K₂CO₃ as a base, iodoarenes are dimethylated at the ipso- and meta-positions of the iodo group, which represents a novel strategy for meta-C-H methylation. With KOAc as the base, subsequent oxidative C(sp³)-H/C(sp³)-H coupling occurs; in this case, the overall transformation achieves triple C-H activation to form three new C-C bonds. These reactions allow expedient access to 2,6-dimethylated phenols, 2,3-dihydrobenzofurans, and indanes, which are ubiquitous structural motifs and essential synthetic intermediates of biologically and pharmacologically active compounds.

Palladium(0)-Catalyzed Enantioselective Intramolecular Arylation of Enantiotopic Secondary C-H Bonds

The enantioselective functionalization of nonactivated enantiotopic secondary C-H bonds is one of the greatest challenges in transition-metal-catalyzed C-H activation proceeding by an inner-sphere mechanism. Such reactions have remained elusive within the realm of Pd⁰ catalysis. Reported here is the unique reactivity profile of the IBiox ligand family in the Pd⁰ -catalyzed intramolecular arylation of such nonactivated secondary C-H bonds. Chiral C₂ -symmetric IBiox ligands led to high enantioselectivities for a broad range of valuable indane products containing a tertiary stereocenter, as well as the arylation of secondary C-H bonds adjacent to amides. Depending on the amide substituents and upon control of reaction time, indanes containing labile tertiary stereocenters were also obtained with high enantioselectivities. Analysis of the steric maps of the IBiox ligands indicated that the level of enantioselectivity correlates with the difference between the two most occupied and the two less occupied space quadrants, and provided a blueprint for the design of even more efficient ligands.

Efficient kinetic resolution in the asymmetric transfer hydrogenation of 3-aryl-indanones: applications to a short synthesis of (+)-indatraline and a formal synthesis of (R)-tolterodine

Efficient kinetic resolution occurs in ATH of racemic 3-arylindanones using (R,R)-or (S,S)-Ts-DENEB catalyst and HCO₂H/Et₃N mixture providing near equal yields of cis-3-arylindanols and unreacted 3-arylindanones with excellent stereoselectivities.

MnBr2-Catalyzed Direct and Site-Selective Alkylation of Indoles and Benzo[h]quinoline

Manganese-catalyzed regioselective C-H alkylation of indoles and benzo[h]quinoline with a variety of unactivated alkyl iodides is reported. Unlike other Mn-catalyzed C-H functionalization, this protocol does not require a Grignard reagent base and employs a simple and inexpensive MnBr₂ as a catalyst. This method tolerates diverse functionalities, including fluoro, chloro, bromo, iodo, alkenyl, alkynyl, pyrrolyl, and carbazolyl groups. The alkylation proceeds through a single-electron transfer pathway comprising reversible C-H manganesation and involving an alkyl radical intermediate.

Copper-catalyzed asymmetric tandem borylative addition and aldol cyclization

A highly enantioselective asymmetric copper-catalyzed tandem conjugate addition/aldol cyclization of electron-deficient olefins with B₂pin₂ was developed, which provided a rapid access to indanes bearing three consecutive chiral stereogenic centers.

Enantioselective hydrogenation of α-ketoesters catalyzed by cinchona alkaloid stabilized Rh nanoparticles in ionic liquid

The heterogeneous enantioselective hydrogenation of α-ketoesters catalyzed by rhodium nanoparticles (Rh NPs) in ionic liquid was studied with the stabilization and modification of cinchona alkaloids. TEM characterization showed that well-dispersed Rh NPs of about 1.96 nm were obtained in ionic liquid. The results showed that cinchona alkaloids not only had good enantiodifferentiating ability but also accelerated the catalytic reaction. Under the optimum reaction conditions, the enantiomeric excess in ethyl benzoylformate hydrogenation could reach as high as 60.9%.

N-Heterocyclic carbene-catalyzed diastereoselective synthesis of sulfenylated indanes via sulfa-Michael-Michael (aldol) cascade reactions

N-Heterocyclic carbene (NHC)-catalyzed diastereoselective synthesis of multisubstituted sulfenylated indanes has been developed. In the presence of 1 mol% NHC, various thiols underwent the sulfa-Michael-Michael cascade reaction with benzenedi(enones) efficiently to form the carbon-sulfur bond and construct sulfenylated indanes in good to excellent yields with high diastereoselectivity. In addition, the NHC-catalyzed sulfa-Michael-aldol cascade reaction between o-formyl chalcone and thiols has also been demonstrated to afford sulfenylated indanes with a free hydroxyl group in moderate yields and good diastereoselectivity.

Synthesis of cyano-substituted carbazoles via successive C-C/C-H cleavage

A highly efficient protocol for the synthesis of polysubstituted cyano carbazoles has been developed. This process is realized through the tandem reactions of Cs2CO3 promoted C-C σ-bond activation of α-cyano ketones followed by Fe-catalyzed selective C-H and/or C-C bond activations. Two of the sp2 C-H bonds are functionalized, and two of the C-C σ-bonds are cleaved involving a 1,2-acyl migration during the reaction process.

A [4 + 1] annulation of ortho-electrophile-substituted para-quinone methides for the synthesis of indanes and isoindolines

Indanes and isoindolines were obtained via a 1,6-conjugated addition/1,4-Michael addition of ortho-electrophile-substituted para-quinone methides with nucleophiles in high yields and with good functional group tolerance.