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

Abstract

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.

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

“…Surprisingly, the reactions, particularly when the electron-deactivating substituents (i.e., F and Cl) flanked at para -position to the aldehyde group of the ortho -formylphenyl acrylate esters ( 2d and 2f ) with 3g gave the isomeric products 4dg + 4′dg (1:2) and 4fg + 4′fg (1.5:1) in combined yields 68% and 74%, respectively, in which the double bond has been isomerized within the indene moiety; however, it is worth mentioning that the external arene 3g attack seemed to be highly regioselective and reacted at aldehyde carbon of the bicyclic intermediate K (Scheme ). Indeed, this is in good agreement with our earlier report . The evidence of the veratrole 3g attacked at aldehyde-carbon of the ortho -formylphenyl acrylate esters ( 2d and 2f ) and certainty of the skeleton structure of the double bond isomerized product 4′dg was also supported by the X-ray crystal study of the molecule 4′dg (CCDC-2295030).…”

“…Based on the earlier reports ,,− ,− , and our recent work, a plausible reaction mechanism for constructing products 4ag , (±) 5ag , and (±) 7ag was presented in Scheme . Initially, the acid (i.e., p -TSA•H 2 O or TfOH) might initiate the CHO group of cinnamate ester 2a and lead to the hydroxyindene-ester intermediate I through intramolecular cyclization by the double bond of acrylate moiety via the protonated G and cyclic carbocation H intermediates.…”

“…Though a number of efficient reports exist in the literature, there would still be various possible ways to synthesize indene derivatives using well-organized and effective one-pot procedures. Therefore, due to our research interest in the synthesis of valuable carbocyclic molecules under acid-promoted cyclization reactions, − it was envisioned that the reaction between cinnamate esters and external arenes could deliver the cyclized products. Thus, herein, we describe Bro̷nsted acid-mediated efficient synthesis of monoarylindene esters, biarylindane esters, and indeno­[ a ]­indenones starting from simple ortho -formylcinnamate esters and external arenes in one-pot, which enables formation of two, three, and four new C–C bonds, respectively, as displayed in Scheme e.…”

Acid-Promoted Domino Access to Substituted Benzo[b]carbazoles

“…The ortho -formyl-cinnamate/ ortho -keto-cinnamate esters 5 were prepared from ortho -bromobenzaldehydes 1 or 1-( ortho -bromophenyl)­alkan-1-one 3 according to the literature report . To an oven-dried sealed tube equipped with a magnetic stirring bar were added ortho -bromobenzaldehydes 1 or 1-( ortho -bromophenyl)­alkan-1-one 3 (0.5 mmol, 92.5–116.7 mg), Pd­(OAc) 2 (3 mol %, 4 mg), P­( ortho -tolyl) 3 (6 mol %, 9 mg), acrylates 4 (1.5 mmol, 129–192 mg), triethylamine (1.5 mmol, 152 mg), and toluene (2 mL).…”

“…More importantly, developing strategies compatible with protecting group-free conditions is indispensable in organic synthesis. Hence, in continuing our research curiosity to synthesize valued carbo-/heterocyclic molecules via acid-catalyzed cyclization strategies, − it was anticipated that a suitable acid would be able to trigger the domino reaction between readily available cinnamate esters and indoles to furnish benzo­[ b ]­carbazoles. Thus, herein, we defined Bro̷nsted acids-induced effectual synthesis of benzo­[ b ]­carbazole esters starting from simple ortho -formyl (or ortho -acyl) cinnamate esters and indoles in a one-pot domino process.…”