Acid-promoted Cyclization Research Articles

Strategies to access the [5-8] bicyclic core encountered in the sesquiterpene, diterpene and sesterterpene series.

Terpene compounds probably represent the most diversified class of secondary metabolites. Some classes of terpenes, mainly diterpenes (C20) and sesterterpenes (C25) and to a lesser extent sesquiterpenes (C15), share a common bicyclo[3.6.0]undecane core which is characterized by the presence of a cyclooctane ring fused to a cyclopentane ring, i.e., a [5-8] bicyclic ring system. This review focuses on the different strategies elaborated to construct this [5-8] bicyclic ring system and their application in the total synthesis of terpenes over the last two decades. The overall approaches involve the construction of the 8-membered ring from an appropriate cyclopentane precursor. The proposed strategies include metathesis, Nozaki-Hiyama-Kishi (NHK) cyclization, Pd-mediated cyclization, radical cyclization, Pauson-Khand reaction, Lewis acid-promoted cyclization, rearrangement, cycloaddition and biocatalysis.

Acid-Promoted Redox-Annulation toward 1,2-Disubstituted-5,6-dihydropyrrolo[2,1-α]isoquinolines: Synthesis of the Lamellarin Core.

An efficient synthesis of a variety of 1,2-disubstituted-5,6-dihydropyrrolo[2,1-α]isoquinoline derivatives via an acid-promoted cyclization reaction between 1,2,3,4-tetrahydroisoquinoline (THIQ) and substituted α,β-unsaturated aldehyde derivatives is reported. This cycloaddition allows access to structurally diverse multisubstituted dihydropyrrolo[2,1-α]isoquinolines in moderate to good yields, which was the core scaffold of marine natural alkaloid lamellarins.

Divergent Reactivity of Triaryldivinyl Ketones: Competing 4π and Putative 6π Electrocyclization Pathways.

This work describes an acid-promoted cyclization of triaryldivinyl ketones containing a thiophene moiety in the α-position. Two cyclization pathways are accessible: one a 4π-Nazarov cyclization and the other we propose to proceed through a 6π electrocyclic mechanism. The relative proportion of products from these divergent pathways is affected by reaction conditions and steric bulk in the substrate. We present experimental and computational evidence that when using HCl in dioxane, the 4π-conrotatory electrocyclization is more favorable, whereas GaCl3 in methylene chloride shifts the chemoselectivity toward a putative 6π-disrotatory electrocyclization. DFT calculations suggest that a complex interplay between kinetic and thermodynamic factors is implicated in the chemodivergent behavior.

Acid-Promoted Iso-Nazarov Cyclization of Conjugated trans-Dienones and Dienals for the Synthesis of 2-Cyclopentenones.

The acid-promoted cyclization of all-trans linearly conjugated dienones and dienals constitutes a synthetic strategy for the construction of 2-cyclopentenones.

Lewis acid-promoted cyclizations of o-alkyloxyphenyl-substituted ynamides to construct 2-amidobenzofurans

Lewis acid-promoted cyclizations of o-alkyloxyphenyl-substituted ynamides for the construction of medicinally relevant 2-amidobenzofurans are described. A series of 3-acyl or 3-unsubstituted 2-amidobenzofurans could be effectively constructed.

Stereoselective Synthesis of Polysubstituted Tetrahydropyranones via Acid-Promoted Cyclization of β-Silyl-γ-ethylidene-γ-butyrolactones with Aldehydes and Ketones.

β-Silyl-γ-ethylidene-γ-butyrolactone upon one-pot treatment with aldehydes and ketones in the presence of Lewis acids underwent a tandem Hosomi-Sakurai/Prins cyclization to give polysubstituted tetrahydropyranones stereoselectively. Various aldehydes and ketones can be used in this reaction to produce the corresponding tetrahydropyranones. The optical purity of the starting γ-butyrolactone was substantially retained in the resulting tetrahydropyranones.

Rapid synthesis of azepinoindole derivatives from tryptamine sulfonamides and bromoallyl sulfones via an acid-mediated cyclization and rearrangement.

The azepinoindole framework present in natural alkaloids such as subincanadine F, ibogaine and catharanthine was formed in a novel acid-promoted cyclization-rearrangement of tryptamine-derived N-sulfonyl enamines. The latter were conveniently prepared via a cesium carbonate mediated formal vinylic substitution reaction of 2-bromoallyl sulfones (allenyl sulfone surrogate) and tryptamine sulfonamides. The azepinoindole forming cyclization-rearrangement involves the initial generation of a six-membered tetrahydro-β-carboline derivative. The steric bulk of the N-sulfonyl group on tryptamine plays an important role in deciding the reaction outcome.

An Expedient Route to 9-arylmethylanthracene Derivatives via Tandem Ni-catalyzed Alkene Dicarbofunctionalization and Acid-promoted Cyclization-aromatization.

We report a nickel-catalyzed one pot synthesis of 9-arylmethylanthracene motifs, which find applications in medicinal and material chemistry. In this synthesis, we apply three component alkene dicarbofunctionalization of 2-vinylaldimines with aryl iodides and arylzinc reagent to generate a 1,1,2-diarylethyl scaffold, which then undergoes an acidpromoted cyclization followed by aromatization to furnish 9-arylmethylanthracene cores. With the new method, a number of differently-substituted 9-arylmethylanthracene derivatives can be synthesized in good yields.

An efficient and practical synthesis of 2,4-substituted pyrido[4,3-d]pyrimidin-5(6H)-ones

We describe an efficient synthesis of 2,4-substituted pyrido[4,3-d]pyrimidin-5(6H)-ones, which involves the acid-promoted cyclization of cyano enamine 15 to afford 2,4-bis(methylthio)pyrido[4,3-d]pyrimidin-5(6H)-one 17 as a key intermediate. Selective displacement of the 4-methylthio group by a wide range of anilines followed by oxidation of the 2-methylthio group and subsequent substitution by amines enabled the synthesis of a variety of 2,4-disubstituted pyrido[4,3-d]pyrimidin-5(6H)-ones.

Concise Synthesis of the ABC-Ring System of the Azafluoranthene, Tropoisoquinoline and Proaporphine Alkaloids: An Olefin Hydroacylation/Pomeranz–Fritsch Cyclization Approach

A straightforward approach toward a decorated cyclopenta[ij]isoquinoline embodying the ABC-ring system characteristic of the azafluoranthene (triclisine), tropoisoquinoline (pareitropone) and proaporphine (prodensiflorin B) alkaloids, is reported. The synthetic ­sequence entailed a novel 40% KF/Al2O3-mediated hydroacylation of a 2-allyl-benzaldehyde derivative, obtained in two steps from isovanillin, through O-allylation and Claisen rearrangement to assemble the AC-ring system. This was followed by an O-methylation and a reductive ­amination of the resulting indanone with aminoacetal. A modified Pomeranz–Fritsch cyclization was next implemented to install ring B, through sulfonamidation, followed by acid-promoted cyclization and ­final desulfonylation in situ.

Acid-promoted cyclization of 2-propynolphenols leading to 4-tosyloxy-2H-chromenes

A novel and efficient p-TsOH-promoted cascade cyclization of 2-propynolphenols is developed to give 4-tosyloxy-2H-chromenes in moderate to efficient yields. It is noted that p-TsOH acts not only as a promoter in this reaction, and also as the sulfonate source. Importantly, the obtained 4-tosyloxy-2H-chromenes can be used as versatile intermediates for the synthesis of various functionalized compounds through diverse metal-catalyzed cross-coupling reactions. Moreover, this method can be enlarged to gram scale.

Synthesis of Fluorinated Acridines via Sequential Micellar Buchwald–Hartwig Amination/Cyclization of Aryl Bromides

Fluorinated unsymmetrical acridines are efficiently prepared by means of a tandem micellar Buchwald–Hartwig amination followed by an acid-promoted cyclization. The overall process is advantageous with respect to previously described protocols both in terms of efficiency and sustainability. The role of the cosolvent in the amination step is highlighted, demonstrating that rather than resorting to highly expensive catalysts, Buchwald–Hartwig aminations can be straightforwardly carried out by tuning the reaction site polarity.

Expedient Entry to 1-Aminoadamantane Derivatives via Aza-Prins Cyclization of 7-Methylenebicyclo[3.3.1]nonan-3-one Oximes

An efficient synthesis of 1-aminoadamantane (amantadine) derivatives is described. This synthesis features acid-promoted aza-Prins cyclization of 7-methylenebicyclo[3.3.1]nonan-3-one oximes, which are readily prepared from 1,3-adamantanediol via a Grob fragmentation and the subsequent oximation, to give various 3-substituted 1-(alkoxyamino)adamantanes. After the reduction of alkoxyamines, not only 3-substituted 1-aminoadamantanes, but also chiral 2,5-disubstituted derivatives were obtained in good yields.

Polycyclic Aromatic N-Ethoxycarbonyl Thioamide S-Oxides and Their Triflic Acid Promoted Cyclization to Fluorescent Thiophene Imine-Fused Arenes.

We present the synthesis of a series of polycyclic aromatic-N-ethoxycarbonylthioamide S-oxides and their triflic acid-promoted cyclization to thiophene imine-fused arenes having 2H-naphtho[1,8-bc]thiophen-2-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, 4H-pyreno[1,10-bc]thiophen-4-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, 4H-pyreno[1,10-bc]thiophen-3-imine, 3H-pyreno[10,1-bc]thiophen-3-imine, and 4H-peryleno[3,4-bc]thiophen-4-imine cores. The proposed reaction mechanism involves the intermediacy of a novel type of electrophilic sulfur species, namely protonated iminosulfenic acid or iminosulfenium cations. These species may attack the peri- or ipso-position of the arene, leading in some cases to regioisomeric products. The reaction affords in high yields novel polycyclic fluorophores emitting in the range of 500-606 nm with quantum yields of 0.025-0.64. Comparison with the parent arenes reveals that the fused iminothiophene moiety brings about significant bathochromic shifts of the absorption and emission bands.

Synthesis, crystal structure, and FET characteristics of thieno[2,3-b]thiophene-based bent-thienoacenes

Two novel bent-shaped thienoacenes, naphtho[2,3-b]naphtho[2′,3′:4,5]thieno[3,2-d]thiophene (bent-DNTT) and anthra[2,3-b]anthra[2′,3′:4,5]thieno[3,2-d]thiophene (bent-DATT) were synthesized from thieno[2,3-b]thiophene and their corresponding aromatic anhydrides by three steps: Friedel–Crafts acylation, acid-promoted cyclization, and reductive aromatization. The structural curvature improved the solubility of these thienoacenes in organic solvents. The bent-DNTT based FET device was fabricated by the spin-coating method. The device exhibited p-type characteristics with a mobility of 5.1×10−5cm2V−1s−1. Its thin-film structure was fully characterized as an edge-on orientation with large intermolecular orbital coupling.

Acid-promoted cyclization of 2,4-diaryl-1,1,1-trifluorobut-3-en-2-oles and their TMS-ethers into CF3-indenes.

2,4-Diaryl-1,1,1-trifluorobut-3-en-2-oles and their TMS-ethers in H2SO4 at room temperature in just 2 min are quantitatively cyclized into 1-aryl-3-trifluoromethyl-1H-indenes. The reaction proceeds through an intermediate formation of the corresponding CF3-allyl cations, which are cyclized regioselectively at the allyl carbon atom most remote from the CF3-group. The obtained CF3-indenes in solution of EtOAc in the presence of silica gel at room temperature over 4 h are quantitatively isomerized into 3-aryl-1-trifluoromethyl-1H-indenes.

ChemInform Abstract: Exploring the Reactivity of N‐Alkynylated Sulfoximines: Acid‐Catalyzed Cyclizations.

N-Alkynylated sulfoximines undergo acid-promoted cyclization processes under mild reaction conditions. The transformations proceed in short reaction times affording sulfoximidoyl-functionalized naphtho[2,1-b]thiophenes or pyrrolo[1,2-a]quinolines in up to excellent yields.

Copper-Catalyzed Regioselective Cascade Alkylation and Cyclocondensation of Quinoline N-Oxides with Diazo Esters: Direct Access to Conjugated π-Systems.

An inexpensive copper-catalyzed cascade regioselective alkylation, followed by cyclocondensation of quinoline N-oxides with α-diazo esters has been achieved successfully to provide heteroarene-containing conjugated π-systems. The developed method is simple, straightforward, and economical with a broad range of substrate scope. The dual role of copper catalyst in the C-H bond functionalization and in Lewis acid-promoted cyclization was explored.

Exploring the Reactivity of N-Alkynylated Sulfoximines: Acid-Catalyzed Cyclizations.

N-Alkynylated sulfoximines undergo acid-promoted cyclization processes under mild reaction conditions. The transformations proceed in short reaction times affording sulfoximidoyl-functionalized naphtho[2,1-b]thiophenes or pyrrolo[1,2-a]quinolines in up to excellent yields.

Theoretical study of the regioselective cyclization of enaminones in the construction of benzofurans and indoles.

A theoretical study was undertaken regarding the regioselective Lewis acid-promoted intramolecular cyclization of novel enaminones 1-3 leading to the corresponding benzofurans 4-5 and indoles 6. The density functional theory (DFT) and hard and soft acids and bases (HSAB) principle provided data to describe the electronic effects of the substituents in the reactivity of the benzene ring and the enaminone moiety. The condensed and local Fukui functions for nucleophilic and electrophilic attacks of the reactants accounted for the experimentally observed preference, in regard to precursors 1-3, of the cyclization between the C6' carbon (rather than the C2' carbon) of the benzene ring and the C3 center of the enaminone moiety.