Synthesis of Substituted 2-Arylindanes from E-(2-Stilbenyl)methanols via Lewis Acid-Mediated Cyclization and Nucleophililc Transfer from Trialkylsilyl Reagents

Dihydro-10H-indeno[1,2-b]benzofurans and Tetrahydroindeno[1,2-c]isochromenes via Stereoselective Intramolecular Carbocation Cascade Cyclization

Nazarov cyclization of the (E)-(2-stilbenyl)methanols under the catalysis of p-TsOH immobilized on silica (PTS-Si) proceeded to give the corresponding indanyl cation with the exclusive trans relationship at the two newly formed adjacent stereogenic centers. The ensuing intramolecular nucleophilic addition by the MOM-protected phenol (m = 0) or benzyl alcohol (m = 1) furnished the Indane-fused benzofuran [5/5] or isochroman [5/6] system, respectively, with the exclusive cis stereocontrol at the two-carbon ring junction. Thus, in a single step, from nonchiral starting materials, the intramolecular cascade carbocation cyclization (CCC) furnished the [5/5] or [5/6] oxygen-containing Indane fused-ring systems in moderate to good yields with excellent stereoselectivity on all three contiguous stereogenic centers.

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.

Sequential Knoevenagel Condensation/Cyclization for the Synthesis of Indene and Benzofulvene Derivatives

Sequential Knoevenagel condensation/cyclization leading to indene and benzofulvene derivatives has been developed. The reaction of 2-(1-phenylvinyl)benzaldehyde with malonates gave benzylidene malonates, cyclized indenes, and dehydrogenated benzofulvenes. The product selectivity depends on the reaction conditions. The reaction with piperidine, AcOH in benzene at 80 °C for 1.5 h gave a benzylidene malonate in 75% yield as a major product. The reactions with piperidine, AcOH in benzene at 80 °C for 17 h and with TiCl₄-pyridine at room temperature gave an indene derivative in 56 and 79% yields, respectively, as a major product. The reaction with TiCl₄-Et₃N gave a benzofulvene in 40% yield selectively. Indene was transformed to a benzofulvene derivative using the reagents TiCl₄-Et₃N and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The reaction of variously substituted aryl derivatives with dimethyl malonate gave indene and benzofulvene derivatives. The reactions of 2-(1-phenylvinyl)benzaldehyde with Meldrum's acid or malononitrile also gave cyclized compounds in the suitable sequential or stepwise conditions. Furthermore, the reaction of 2-arylbenzaldehydes has been investigated. The limitation and scope have been described. The reaction mechanism of the cyclization steps has been examined by DFT calculations.

Synthesis of Benzoazepine Derivatives via Azide Rearrangement and Evaluation of Their Antianxiety Activities

A new synthetic method for the construction of benzoazepine analogues has been developed employing ortho-arylmethylbenzyl azide derivatives as precursors using an azide rearrangement reaction. In this work, 14 benzoazepine compounds were successfully synthesized in moderate to excellent yields. All synthetic benzoazepines were evaluated for their cytotoxicity against normal human kidney cell line (HEK cell). The results showed that compound 18c had the lowest cytotoxicity (IC₅₀ = 65.68 μM) among tested compounds, which was comparable with the antianxiety drug diazepam (IC₅₀ = 87.90 μM). Based on the cytotoxicity results, five benzoazepine analogues (compounds 18c, 18h, 18j, 18n, and 18p) were selected to determine the antianxiety effect on stressed rats using elevated plus maze (EPM) and open field test (OFT) methods. Interestingly, compound 18c showed better anxiolytic activity than diazepam without a sedative effect by showing superior hyperlocomotor activity. Therefore, this discovery could pave the way for drug development to treat patients with anxiety disorder.

Highly Regioselective Tandem Reaction of Ene-Yne-Oxazolones Induced by H-Phosphonates: Construction of Phosphinylindane Derivatives

A highly regioselective divergent approach for the phosphine-containing indane/indene derivatives from the ene-yne-oxazolone precursors was reported. An insight into the reaction mechanism involving the phospha-1,4-addition followed by 5-exo-dig ring closure with a concomitant C-P/C-C bond formation was also proposed. This promising protocol utilized H-phosphonate as the phosphonating reagent in a silver-catalyzed or base-mediated cascade cyclization to construct the corresponding phosphorylated spiroindenoxazolones and amidoindenes, respectively, in good yields (up to 88% yield).

Stereoselective Convergent Synthesis of Tetrahydro-5 H-benzo[ c]fluorene via Nine-Membered Ring-Closing Metathesis and Transannular Acid-Mediated Cyclization/Nucleophilic Addition

The diene methyl ethers or acetates, constructed from the Li-Br exchange/addition reactions of 2-vinylbenzaldehydes and 2-(but-3-en-1-yl)bromoarenes followed by etherification or acetylation of the corresponding alcohols, smoothly underwent the ring-closing metathesis (RCM) by using Hoveyda-Grubbs II as a catalyst to provide the corresponding benzannulated ( Z)-cyclononenes as single products in good yields (up to 75%). The ensuing one-pot acid-mediated transannular cyclization/nucleophilic addition at C7 furnished the corresponding tetrahydro-5 H-benzo[ c]fluorenes as single stereoisomers with the exclusive cis stereochemistry at the ring junction (C5-C6) and trans at the site of nucleophilic attack (C6-C7) on the three contiguous stereogenic centers in good to excellent yield (up to 94%). The developed strategy was general; the reaction conditions were compatible with hydride, azide, and electron-rich aromatics as nucleophiles. In addition, various methoxylated benzannulated cyclononene acetates could be employed as substrates. Thus, tetrahydro-5 H-benzo[ c]fluorenes could be prepared in four steps from appropriately substituted bromoarenes and benzaldehydes in good yields (up to 56%) with excellent stereo- and regio-control.

Stereocontrolled Construction of 1-Vinylindanes via Intramolecular Cyclization of o-Cinnamyl Chalcones

In this paper, a concise route for the synthesis of 1-vinylindanes is described, including (i) NaBH₄-mediated reduction of o-cinnamyl chalcones and (ii) sequential BF₃·OEt₂-mediated intramolecular annulation of the resulting alkenols. The plausible mechanism is proposed and discussed herein. This protocol provides highly effective stereocontrolled cinnamyl-enone cross-coupling to construct three contiguous trans- trans stereocenters and one ( E)-configured alkenyl or styryl group.

One-Pot Cannizzaro Cascade Synthesis of ortho-Fused Cycloocta-2,5-dien-1-ones from 2-Bromo(hetero)aryl Aldehydes

An intramolecular Cannizzaro-type hydride transfer to an in situ prepared allene enables the synthesis of ortho-fused 4-substituted cycloocta-2,5-dien-1-ones with unprecedented technical ease for an eight-ring carboannulation. Various derivatives could be obtained from commercially available (hetero)aryl aldehydes, trimethylsilylacetylene, and simple propargyl chlorides in good yields.