Regioselective synthesis of flavone derivatives via DMAP-catalyzed cyclization of o-alkynoylphenols

A catalytic amount of DMAP promoted cyclization of o-alkynoylphenols via a 6-endo cyclization mode leading to flavone derivatives in high yields without forming 5-exo cyclized aurone derivatives. Utilizing this method, methoxy substituted flavone and alkyl substituted γ-benzopyranone derivatives were synthesized.     

Tetrabutylammonium chloride-triggered 6-endo cyclization of o-alkynylisocyanobenzenes: an efficient synthesis of 2-chloro-3-substituted quinolines

A highly efficient one-pot synthesis of 2-chloro-3-substituted quinolines has been developed by tetrabutylammonium chloride-triggered 6-endo cyclization of o-alkynylisocyanobenzenes, which are generated in situ by dehydration of the corresponding N-(2-ethynylphenyl)formamides.     

tert-BuOK-mediated carbanion-yne intramolecular cyclization: synthesis of 2-substituted 3-benzylbenzofurans

A mild, efficient, and regioselective carbanion-yne intramolecular cyclization mediated by t-BuOK for the synthesis of 2-substituted 3-benzylbenzofurans is developed. It was started from o-iodophenol (1), based on O-alkylation, and the Sonogashira reaction in sequence to produce 2-(2-phenylethynylphenoxy)-1-arylalkanones (5). An intramolecular carbanion-yne 5-exo-dig cyclization reaction of 5, which was mediated by t-BuOK, yielded title benzofurans in good yields.     

Synthesis of a novel pyrrolo-[3,2-c]quinoline N-oxide by aza-Baylis-Hillman adduct of o-nitrobenzaldehyde

A new and high yielding approach for the synthesis of a novel pyrrolo-[3,2-c]quinoline N-oxide is described. The key step consisted in the palladium-catalyzed reductive cyclization of an uncommon 3-ketopyrrole derivative of o-nitrobenzaldehyde, obtained in a straightforward manner through an aza-Baylis-Hillman/ring closing metathesis/aromatization reaction. A deoxygenation reaction of this novel pyrrolo-[3,2-c]quinoline N-oxide afforded a new substituted pyrrolo-[3,2-c]quinoline analogue.     

Asymmetric reactions of axially chiral amides: use of removable ortho-substituents in radical cyclizations of o-iodoacrylanilides and N-allyl-N-o-iodoacrylamides

Radical cyclizations of enantiomerically enriched o-iodoacrylanilides and N-allyl-o-iodoanilides bearing a removable ortho-substituent such as trimethylsilyl or bromine provide oxindoles and indoles in good yields and with good to excellent levels of chirality transfer from the N-Ar axis to the new stereocenter. Transition state models for the chirality transfer are suggested. Chemoselectivity of the radical cyclization in favor of the iodine in the case of 2-iodo-6-bromo-N-allylacrylamides has been exploited for the synthesis of chiral pyrroloquinolinones by a one-pot sequence of 5-exo and 6-endo cyclizations.     

Highly efficient one-pot synthesis of 1-substituted-1,2,3,4-tetrahydropyrazino[1,2-a]indoles

A practical and general one-pot synthesis of 1-substituted-10-methyl-1,2,3,4-tetrahydropyrazino[1,2-a]indoles is described. The approach uses 2-(3-methyl-1H-indol-1-yl) ethylamine, benzotriazole and aldehydes in the presence of catalytic amount of acid catalysts (AlCl₃, ZnCl₂, ZnBr₂, p-TsOH, CH₃SO₃H) and proceeds in high yields via iminium cation intramolecular cyclization. The mechanism of the observed intramolecular cyclization reaction has been investigated theoretically by means of PM3 semiempirical method and results were consistent with the experimental results.     

Domino synthesis of 2-arylbenzo[b]furans by copper(II)-catalyzed coupling of o-iodophenols and aryl acetylenes

A wide range of 2-arylbenzo[b]furans are synthesized through domino intermolecular C_(aryl)-C_(alkynyl) bond formation followed by intramolecular C_(alkynyl)-O bond forming cyclization via copper(II)-catalyzed coupling of o-iodophenols and aryl terminal acetylenes. This method requires neither expensive palladium catalyst nor oxophilic phosphine ligands, can tolerate different functional groups. The methodology is successfully utilized in formal synthesis of β-amyloid aggregation inhibitor 5-chloro-3-[4-(3-diethylaminopropoxy)benzoyl]-2-(4-methoxyphenyl) benzofuran.     

Palladium-catalyzed amination and cyclization to heteroannellated indoles and carbazoles

New ortho-bromodiarylamines in the benzo[b]thiophene series were prepared by palladium-catalyzed amination, either in the benzene or in the thiophene ring. These were submitted to palladium-catalyzed cyclization, under different required conditions, to give several differently substituted thieno[3,2-c] or [2,3-b]carbazoles and indolo[3,2-b]benzo[b]thiophenes. This constitutes a novel synthetic route to both tetracyclic systems.     

Gold(I)-catalyzed stereoselective cyclization of ortho alkynyl benzaldehyde chromium complexes with nucleophiles

Gold(I)-catalyzed cyclization of o-alkynyl benzaldehyde chromium complexes gave stereoselectively 1-anti- and syn-functionalized 1H-isochromene chromium complexes, respectively, depending on the nature of nucleophiles. Enantiomerically pure trans- and cis-1,3-dimethylisochromans were stereoselectively prepared from a single planar chiral o-(1-propynyl)benzaldehyde chromium complex.     

Synthesis of nitrogen-containing heterocycles using exo- and endo-selective radical cyclizations onto enamides

The effect of positional change of the carbonyl group of enamides on Bu₃SnH-mediated alkyl radical cyclization leading to five-, six-, seven-, and eight-membered nitrogen-containing heterocycles was examined. A 5-exo cyclization is generally preferred over a 6-endo ring closure in systems having an alkyl radical center on the enamide-acyl side chain, whereas enamides having an alkyl radical center opposite to the acyl side chain predominantly gave 6-endo cyclization products. These results suggest that the exo or endo selectivity of radical cyclization onto the alkenic bond of enamides can be controlled by positional change of the carbonyl group. For an understanding of these selectivities, heat of formation for each transition state was calculated. 6-endo-Selective radical cyclization was applied to the radical cascade, enabling a concise synthesis of a cylindricine skeleton. A 7- or 8-endo alkyl radical cyclization, however, predominated over a corresponding 6- or 7-exo ring closure regardless of the positional change of the carbonyl group of enamides.     

Synthesis of 6H-isoindolo[2,1-a]indol-6-ones through a sequential copper-catalyzed C-N coupling and palladium-catalyzed C–H activation reaction

A series of 6H-isoindolo[2,1-a]indol-6-ones were synthesized through one-pot sequential coupling reactions, which were comprised of a copper-catalyzed C–N coupling cyclization and a palladium-catalyzed C–H activation course. General chemicals benzoyl chlorides and o-gem-dibromovinyl anilines were employed as the starting substrates.     

A straightforward synthesis of indazolo[3,2-a]isoquinolin-6-amines

4-Substituted 1-bromoisoquinolin-3-amines were subjected to Suzuki coupling with o-nitrophenylboronic acid to yield 1-(2-nitrophenyl) isoquinolinamines, which participated in Cadogan cyclization with triethyl phosphite under microwave irradiation in a sealed vial to yield fluorescent indazolo[3,2-a]isoquinolin-6-amines. The new compounds were also functionalized by transformation of the amino group.     

Efficient synthesis of substituted thieno[3,2-e]indoles

Efficient cyclization reactions of 5-aminobenzothiophene derivatives with internal and terminal acetylenes, giving 7- and 8-substituted thieno[3,2-e]indoles, are described. The reaction of 4-iodo-5-(methylsulfonamido)benzothiophene with terminal alkynes gave 7-substituted thienoindoles using general Sonogashira reaction conditions. Reaction of 5-amino-4-iodobenzothiophene with internal acetylenes, using Larock's heterocyclization reaction conditions, gave 7,8-disubstituted thieno[3,2-e]indoles.     

Facile and efficient synthesis of polyfunctionalized benzofurans: three-component coupling reactions from an alkynylsilane, an o-hydroxybenzaldehyde derivative, and a secondary amine by a Cu(I)-Cu(II) cooperative catalytic system

The combination of 5 mol % Cu(OTf)₂ and CuCl in the presence of DMAP effectively catalyzed a three-component coupling reaction involving an alkynylsilane, an o-hydroxybenzaldehyde derivative, and a secondary amine. The reaction proceeded via intramolecular 5-exo-dig cyclization, resulting in direct synthesis of the corresponding benzofuran derivatives in moderate to excellent yields.     

Studies in pyrimidine-annulated heterocycles: unusual formation of spiroheterocyclic compounds from acid catalyzed reaction of enol ether

A number of spiroheterocycles are synthesized in moderate yield by treating 5-aryloxymethylene-6,7,8-trihydropyrido[2,3-d]pyrimidines-2,4(1H,3H)diones with H₂SO₄ in chloroform-methanol-water. These spiroheterocycles are also regioselectively synthesized by tri-n-butyltin hydride induced radical cyclization of 5-(o-bromoaryloxymethylene)-6,7,8-trihydropyrido[2,3-d]pyrimidines-2,4(1H,3H)diones.     

An efficient approach to isoquinoline via AgNO3-promoted 6-endo-dig cyclization followed by oxidative elimination of o-alkynylarylaldimines and its application in fluoride recognition

A novel 6-endo-dig cyclization followed by oxidation/elimination of o-alkynylarylaldimines with 4-hydroxybenzylamine was developed for preparation of isoquinolines. The intermediates of this tandem reaction were monitored by mass spectroscopy (MS) to confirm the reaction pathway. This methodology was further applied to the design and synthesis of a novel ratiometric chemosensor for determination of fluoride.     

Ligand-, copper-, and amine-free one-pot synthesis of 2-substituted indoles via Sonogashira coupling 5-endo-dig cyclization

Results of the optimized conditions for the one-pot synthesis of 2-substituted indoles via palladium acetate catalyzed tandem Sonogashira coupling 5-endo-dig cyclization at room temperature under ultrasonic irradiation and standard stirred conditions are described. Electron-donating and electron-withdrawing groups present in both coupling partners were well tolerated under these mild conditions. A copper-, ligand- and amine-free condition is an important feature of this protocol. Significant enhancement of reaction rates was observed for the reactions employing ultrasonic irradiation.     

A new synthetic method for the 2H-[1,2,3]thiadiazolo[5,4-b]indoles

The systematic study of oxidative cyclization of 3-hydrazono-1,3-dihydroindole-2-thiones has been carried out and a series of new 2H-[1,2,3]thiadiazolo[5,4-b]indoles has been prepared. The elaborated reaction represents an efficient method for the synthesis of fused 1,2,3-thiadiazoles.     

Facile synthesis of regio-isomeric naphthofurans and benzodifurans

Naphtho[1,2-b]furans 1a-f, naphtho[2,1-b]furans 2a-f, benzo[1,2-b:5,4-b′]difurans 3a-b, benzo[1,2-b:4,5-b′]difurans 4a-b, and benzo[1,2-b:4,3-b′]difurans 5a-b were synthesized by base-catalyzed cyclization reaction of the corresponding o-alkoxybenzoylarene derivatives. The o-alkoxybenzoylarenes were obtained from the etherification reaction of the o-hydroxybenzoylarenes, which were prepared either by the reaction of methoxyarenes with benzoyl chloride in the presence of aluminum chloride or by photo-Fries rearrangement of aryl benzoates.     

Synthesis of 9-arylamino- and (Z)-9-arylimino-9H-pyrrolo[1,2-a]indoles by reactions of 2-(pyrrol-1-yl)benzaldehydes with aryl amines

Heating mixtures of 2-(pyrrol-1-yl)benzaldehydes and aryl amines under argon afforded 9-arylamino-9H-pyrrolo[1,2-a]indoles, via cyclization of the resulting 2-(pyrrol-1-yl)benzaldimine intermediates. Heating in the presence of oxygen afforded (Z)-9-arylimino-9H-pyrrolo[1,2-a]indoles, which were successfully hydrolyzed with hydrochloric acid to give pyrrolo[1,2-a]indol-9-ones.