Acid-Mediated Domino Cyclization of ortho-Formyl Cinnamate Esters: Synthesis of Substituted Indene/Indane Esters and Indeno[a]indenones

A Bro̷nsted acid-driven protocol to access substituted monoarylindene esters, biarylindane esters, and indeno[a]indenones from simple ortho-formylcinnamate esters and external arenes has been revealed. Remarkably, this single-pot process enabled the construction of two, three, and four new C-C bonds in building monoarylindene esters, biarylindane esters, and indeno[a]indenones, respectively, under metal-free and mild reaction parameters via triggering the inactive cinnamate ester moiety. In addition, the present strategy is investigated with widespread substrate scope.

Acid-Mediated Cascade Cyclization Pathway to Indeno[2,1-c]chromen-6(7H)-ones

Developing mild and effective synthetic strategies for producing significant molecules starting from readily available starting materials is indispensable in organic synthesis. Herein, we present a triflic acid-driven dual cyclization pathway to produce functionalized indeno[2,1-c]chromen-6(7H)-ones from simple 2-formyl (or 2-acyl) cinnamate esters and phenols. Notably, this protocol enabled the construction of two C-C bonds and one C-O bond under metal-free reaction conditions via the activation of the unreactive ester moiety in a single pot. The isolation of intermediate indenol-ester might suggest self-intramolecular cycloaddition by the proximate double bond of the enoate ester with the o-carbonyl moiety, followed by an electrophilic attack with phenol and a subsequent cyclocondensation pathway. In addition, the photophysical properties have also been examined.

Synthesis of a versatile 1H-indene-3-carboxylate scaffold enabled by visible-light promoted Wolff rearrangement of 1-diazonaphthalen-2(1H)-ones

Herein, we have developed a sequential visible-light-promoted Wolff rearrangement of 1-diazonaphthalen-2(1H)-ones, followed by capturing the in situ generated ketene intermediates with various alcohols, producing diverse 1H-indene-3-carboxylates in moderate to good yields under mild reaction conditions. The broad substrate scope, high functional group tolerance, and robust conditions make the resulting derivative a versatile platform for the synthesis of plenty of bioactive molecules.

Preparation of 2,3-Dibromo-1H-indenes and Tetrabromodihydro-s-indacenes as Synthetic Building Blocks

The acid-induced intramolecular cyclization of 1,1-disubstituted 3-aryl-2,3-dibromoallylalcohols affords 2,3-dibromo-1H-indene derivatives. This method is also applicable to the preparation of tetrabromodihydro-s-indacenes. The thus obtained multi-brominated compounds can serve as versatile synthetic building blocks to obtain a variety of indene and indacene derivatives, as demonstrated by the synthesis of dialkylmethylene-bridged oligo(phenylenevinylene)s, which feature attractive photophysical properties.

Brønsted acid catalyzed annulations of ketene dithioacetals: synthesis of 3-aryl coumarins and indenes

PTSA catalyzed divergent synthetic routes toward 3-aryl coumarins and indenes were developed using ketene dithioacetals. These transformations are transition-metal and oxidant free, proceed under mild conditions, and provide quick access to these important structural motifs.

Cp*Co(III)-Catalyzed Ketone-Directed ortho-C-H Activation for the Synthesis of Indene Derivatives

A weakly coordinating, carbonyl-assisted C-H activation of aromatic systems with α,β-unsaturated ketone and subsequent aldol condensation has been developed using a Cp*Co(CO)I₂ catalyst. The developed method is the first example of indene synthesis by cobalt-catalyzed C-H activation. In addition, the reaction requires mild reaction conditions and easily accessible starting materials, and it shows excellent functional group compatibility.

Co(III)-Carbene Radical Approach to Substituted 1H-Indenes

A new strategy for the catalytic synthesis of substituted 1H-indenes via metalloradical activation of o-cinnamyl N-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co(III) carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1H-indene derivatives. The cheap and easy to prepare low spin cobalt(II) complex [Co(II)(MeTAA)] (MeTAA = tetramethyltetraaza[14]annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt(II) complex, and for the first time catalytic activity of [Co(II)(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1H-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C(sp(2))-H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Co(III)-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) reveal the radical nature of the reaction.

One-pot C–C/C–O bond formation: synthesis of spirocyclic lactones

An efficient and practical method for the synthesis of novel spiro(tri or tetra)cyclic lactones via the formation of C–C and C–O bonds in one-pot is presented.

Iron(iii) halide or iodine-promoted synthesis of 3-haloindene derivatives from o-alkynylarene chalcones

The treatment of o-alkynylarene chalcones with ferric halides/iodine affords 3-haloindene derivatives in good yields.

FeCl3 mediated synthesis of substituted indenones by a formal [2+2] cycloaddition/ring opening cascade of o-keto-cinnamates

A novel FeCl₃ mediated formal [2+2] cycloaddition/ring opening cascade of o-keto-cinnamates was developed for the synthesis of indenones. The reaction tolerates a broad range of functional groups, including bromide, chloride, amide, acid and ester groups.

Hg(OAc)2 mediated highly regio- and/or diastereoselective allylic tert-acetylation of olefins

We have developed a novel, highly regio- and/or diastereoselective tertiary allylic acetylation of geminally disubstituted olefins using Hg(OAc)₂.

Superacid mediated intramolecular condensation: facile synthesis of indenones and indanones

Superacid promoted intramolecular acylation is described for the synthesis of indenones.