Synthesis of ferrocene annulated trifluoromethylated heterocycles with crispine and lamellarin skeletons

Ferrocene analogues of pyrrolo[2,1-a]isoquinoline alkaloids, 2-(trifluoromethyl)-5,6-dihydroferroceno[g]indolizine and 6-(trifluoromethyl)-8,9-dihydro-6H-chromeno[4,3-b]ferroceno[g]indolizines, were obtained in 44–65% yield via the Grob reaction between 1-nitro-1-(2,2,2-trifluoroethylidene)alkanes or 3-nitro-2-(trifluoromethyl)-2H-chromenes and 1-alkyl-3,4-dihydroferroceno[c]pyridines in 2-propanol at reflux.     

Condensation reaction of 3,4-dibenzoyl-1-methyl-2,5-diphenylpyrrole and -1-phenylpyrazole with methylamine derivatives affording pyrrolo [3,4-c]pyridine and 2H-pyrazolo[3,4-c]- and [4,3-c]pyridines

The reaction of 3,4-dibenzoyl-1-methyl-2,5-diphenylpyrrole ( 1 ) and -1-phenylpyrazole ( 2 ) with methylamines ( 3a-c ) afforded pyrrolo[3,4-c]pyridine ( 4 ), and isomeric 2H-pyrazolo[3,4-c]pyridines ( 5a-c ) and [4,3-c]pyridines ( 6a-c ), respectively.     

New approach to the synthesis of pyrrolo[3,4-<Emphasis Type="Italic">c</Emphasis>]pyridines

Strategies of synthesis of pyrrolo[3,4-c]pyridines were developed based on cyclization of 3-phenacyl-4-ethoxycarbonyl(acetyl)pyrrole derivatives reacting with N-methylformamide or formamide, and also on recyclization of pyrrolo[3,4-c]pyrylium salts under the action of ammonium acetate and hydrazine acetate. The influence of substituents in initial reagents on composition of reaction products was studied.     

Construction of pyrazolo[3,4-b]pyridines and pyrazolo[4,3-c]pyridines by ring closure of 3-acylpyridine N-oxide tosylhydrazones

3-Acylpyridine N-oxide tosylhydrazones give good overall yields of a mixture of pyrazolo[3,4-b]pyridines and pyrazolo[4,3-c]pyridines when treated with an electrophilic additive and an amine base. (Z)-Hydrazones cyclize readily, while (E)-hydrazones fail to react under the reported conditions. The reaction takes place at room temperature, and moderate regiocontrol over the cyclization can be achieved by varying the electrophile/solvent combination.     

Reduction of 4,7-diphenyl-1,2,5-thia(oxa)diazolo[3,4-c]pyridines affording 2,5-diphenyl-3,4-diaminopyridines and ring closure of the diamines to fluorescent azaheterocycles

Reduction of 4,7-diphenyl-1,2,5-thia- ( 1a-i ) and 1,2,5-oxadiazolo[3,4-c]pyridines ( 3a and c-e ) gave 3,4-diamino-2,5-diphenylpyridines ( 2a-g ), which were converted into the fluorescent triazolo[4,5-c]-( 5 ), 1,2,5-selenadiazolo[3,4-c]- ( 6 ), imidazolo[4,5-c]pyridines ( 8 ), and pyrido[5,6-c]pyridines ( 11 ). In the reduction of 3a, c and e , 4,5-dihydro[1,2,5]oxadiazolo[3,4-c]pyridines ( 4a-c ) were obtained.     

A strategy for the synthesis of 2-aryl-3-dimethylaminopyrazolo-[3,4-c]pyridines that utilizes [4+1] cycloaddition reactions of 5-arylazo-2,3,6-trisubstituted pyridines

In order to explore the viability and generality of a recently uncovered [4+1] cycloaddition based strategy for the preparation of pyrazolo[3,4-c]pyridine derivatives, members of a series of 5-arylazo-2,3,6-trisubstituted pyridines were prepared by reactions of 3-oxo-2-arylhydrazonopropanals with 3-oxo-3-phenylpropionitrile. The results show that 3-oxo-3-phenylpropionitrile reacts with hydrazone substrates, which do not contain electron-withdrawing substituents on the N-aryl ring of the arylhydrazone moieties, to efficiently produce 6-aryl-2-phenyl-5-arylazonicotinonitriles. In contrast, 2-amino-6-aryl-5-arylazo-3-benzoylpyridines are generated in reactions of 3-oxo-2-arylhydrazonopropanals, which contain electron-withdrawing substituents on the N-aryl moiety. In the forecasted manner, the 6-aryl-2-phenyl-5-arylazonicotinonitriles undergo smooth reactions with dimethylformamide dimethylacetal (DMF-DMA) that led to formation of a new class of 2-aryl-3-dimethylaminopyrazolo[3,4-c]pyridines. The mechanism for this process involves a [4+1] cycloaddition reaction that takes place through initial nucleophilic addition of dimethylamino)methoxycarbene, generated from DMF-DMA, to the azadiene moiety of the arylazopyridines followed by cyclization of the formed zwitterionic intermediate.     

α-aminoazoles in synthesis of heterocycles: III. 4-trifluoromethylpyrazolo[3,4-<Emphasis Type="Italic">b</Emphasis>]pyridines: Synthesis and structure

Cyclocondensation of N-substituted 5-aminopyrazoles with fluorinated 1,3-diketones yielded 4-trifluoromethyl-substituted pyrazolo[3,4-b]pyridines as the only reaction products. The regiostructure of compounds obtained was proved by ¹H and ¹³C NMR homo- and heteronuclear correlation spectroscopy. Characteristic chemical shifts in the ¹³C NMR spectra of regioisomeric pyrazolo[3,4-b]pyridines were established.     

A synthesis of pyrrolo[3,4-c]pyridines via pyridyne intermediates

A synthesis of pyrrolo[3,4-c]pyridines from pyridyne intermediates is reported together with a study of the reactions of 2-methyl-2H-pyrrolo[3,4-c]pyridine (5) with oxidising and reducing agents.     

Polysubstituted 2,3-dihydrofuro[2,3-b]pyridines and 3,4-dihydro-2H-pyrano[2,3-b]pyridines via microwave-activated inverse electron demand Diels-Alder reactions

Polysubstituted 2,3-dihydrofuro[2,3-b]pyridines and 3,4-dihydro-2H-pyrano[2,3-b]pyridines have been synthesized from 1,2,4-triazines using the inverse electron Diels-Alder reaction. For this purpose, 3-methylsulfanyl-1,2,4-triazines were reacted with several nucleophiles allowing the formation of appropriately substituted alkynes to undergo the intramolecular inverse electron demand Diels-Alder reaction. Sealed-tube microwave activation of the cycloaddition reaction has proved to be very efficient and allowed shorter reaction times. This strategy enabled an efficient synthesis of 3-hydroxy-2,3-dihydrofuro[2,3-b]pyridines and 4-hydroxy-3,4-dihydro-2H-pyrano[2,3-b]pyridines with several points of diversity on the bicyclic scaffold.     

Reactions of malonthioamides and malonamidines with methyl acetylpyruvate as a one-step method to prepare 4-thio- and 4-aminopyrrolo[3,4-c]pyridines

We report an efficient one-step approach to 4-thio and 4-aminopyrrolo[3,4-c]pyridines based on the selective reaction of malonthioamides and malonamidines with methyl acetylpyruvate. Structural proof is given by HMBC and HMQC spectra. A cascade heterocyclization mechanism is proposed for the reactions studied.     

Stereoselective synthesis of highly functionalised tricyclic β-lactams via intramolecular nitrilimine cycloaddition

The novel azeto[2′,1′:1,2]pyrrolo[3,4-c]pyrazole skeleton has been obtained in both racemic and enantiopure forms by means of intramolecular cycloaddition of nitrilimines 6. Fully stereoselective cycloadditions were obtained, giving tricyclic β-lactams 7 as single diastereoisomers with good overall yields.     

An efficient access to 3,6-disubstituted 1H-pyrazolo[3,4-b]pyridines via a one-pot double SNAr reaction and pyrazole formation

A general and efficient synthetic method for the synthesis of biologically important series of 3,6-disubstituted-1H-pyrazolo[3,4-b]pyridines was discovered. 2,6-Difluoropyridine was deprotonated using 1.1 equiv of n-BuLi in THF at <−60 °C, followed by quenching with a variety of Weinreb amides to generate 2,6-Difluoro-3-ketopyridines in high yields. A mild tandem reaction sequence of selective nucleophilic substitution of the 6-fluoride with a variety of nucleophiles, followed by hydrazine substitution of the 2-fluoride and pyrazole formation in a one-pot fashion afforded a series of 3,6-disubstituted-1H-pyrazolo[3,4-b]pyridines in moderate to good yields.     

Reaction of 5-aminopyrazoles with β-dimethylaminopropiophenones. Synthesis of new pyrazolo[3,4-b]pyridines

Several new pyrazolo[3,4-b]pyridines were obtained from the reaction of 5-amino-1-aryl-3-methylpyrazoles 1 with β-dimemylaminopropiophenones 2 in pyridine. The structure elucidation of 4,5-dihydropyrazolo[3,4-b]pyridines 3 is based on nmr measurements and X-ray diffraction. The treatment of compounds 3 with N-bromosuccinimide led to the formation of pyrazolo[3,4-b]pyridines 4 .     

New Synthesis of Pyrano[4,3-<Emphasis Type="Italic">d</Emphasis>]pyrazolo[3,4-<Emphasis Type="Italic">b</Emphasis>]pyridines

A new efficient method has been developed for the synthesis of highly biologically active pyrano-[4,3-d]pyrazolo[3,4-b]pyridines on the basis of Smiles rearrangement of ethyl [(8-alkyl(aryl)-5-cyano-3,3-dimethyl-3,4-dihydro-1H-pyrano[3,4-c]pyridin-6-yl)oxy]acetates. Intermediate acetohydrazides have also been isolated. The proposed procedure is advantageous due to the possibility of avoiding experimentally difficult chlorination stage.     

A mild method for the synthesis of bis-pyrazolo[3,4-b:4′,3′-e]pyridine derivatives

A mild and efficient method for the synthesis of bispyrazolo[3,4-b:4′,3′-e]pyridines from 5-aminopyrazoles and aromatic aldehydes using an inexpensive FeCl₃ catalyst is reported. The reaction temperature was reduced from 220–250?°C to 130?°C compared to conventional methods. A large proportion of the products precipitated directly from the mixture at room temperature. Aliphatic and α,β-unsaturated aldehydes selectively resulted in formation of the corresponding 1H-pyrazolo[3,4-b]pyridine derivatives. A possible domino reaction mechanism was also proposed. Several aryl alkynyl groups were introduced to these tricyclic molecules via the Sonogashira coupling reaction.     

Compounds with bridgehead nitrogen: 45—the 1H NMR spectra and stereochemistry of some dimethylperhydro-oxazolo[3,4-a]pyridines

The positions of conformational equilibria in 1,5-, 1-6- and 1,8-dimethylperhydro-oxazolo[3,4-a]pyridines were determined by ¹H NMR spectroscopy. The cis-(H-5, H-8a)-1,6-dimethyl-perhydro-oxazolo[3,4-a]pyridine. In contrast, r-1,t-6,t-8a-1,6-dimethylperhydro-oxazolo[3,4- a]pyridine preferred the cis-fused conformation. Three of the 1,8-dimethylperhydro-oxazolo[3,4-a]pyridines adopted the trans-fused conformations (with distortion of the system in the case of the r-1,c-8,c-8a derivative) and the r-1,c-8,t-8a-1,8-dimethyl derivative adopted the cis-fused conformation.     

Efficient synthesis of tetrahydro-1H-pyrazolo[3,4-b]pyridines based on the recyclization of N-arylitaconimides with aminopyrazoles

New polyfunctional hydrogenated pyrazolo[3,4-b]pyridines were obtained by the chemoselective reaction between N-arylitaconimides and 5-aminopyrazoles. The transformation is the cascade reaction including Michael addition at the active double bond of the itaconimide followed by the intramolecular transamidation and recyclization.     

Synthesis of pyrrolo[2,3-c]2,7-naphthyridine derivatives by cascade heterocyclization reaction of 2-amino-4-cyanomethyl-6-dialkylamino-3,5-pyridinedicarbonitriles

Alkylation of the title pyridinedicarbonitriles with N-substituted chloroacetamides was found to give 5,6-diamino-8-dialkylamino-2,3-dihydro-2-oxo-1H-pyrrolo[2,3-c]2,7-naphthyridine-9-carbonitriles. The structure of obtained compounds was unambiguously confirmed by X-ray crystallographic study. The heterocyclization reaction proceeded regioselectively involving 3-CN group of the starting pyridines without participation of 5-CN. The reasons of the selectivity were discussed. An interaction of prepared naphthyridine derivatives with acetic acid anhydride and cyclohexanone yielded 2-dialkylamino-6,8,9,10-tetrahydro-5-methyl-9-oxopyrimido[4,5,6-ij]pyrrolo[2,3-c]2,7-naphthyridine-1-carbonitriles and 2-dialkylamino-4,5,6,8,9,10-hexahydro-9-oxospiro{pyrimido[4,5,6-ij]pyrrolo[2,3-c]2,7-naphthyridine-5,1′-cyclohexane}-1-carbonitriles, respectively. All fused 2,7-naphthyridines obtained were derivatives of novel heterocyclic systems.     

Pyrazoles as building blocks in heterocyclic synthesis: Synthesis of pyrazolo [3,4-d]pyrimidine, pyrazolo[3,4-e][1,4]diazepine, pyrazolo [3,4-d][1,2,3]triazine and pyrolo [4,3-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives

Several new pyrazolo[3,4-d]pyrimidine, pyrazolo[3,4-e][1,4]diazepine, pyrazolo[3,4-d][1,2,3]triazine and pyrolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidine derivatives were prepared by the reaction of the corresponding 5-amino-pyrazole-4-carbonitrile derivative with different organic reagents under different reaction conditions. Using IR, ¹H NMR, and mass spectra we have characterized all new compounds.     

Cu(I) substitutions. furo[3,2-b] pyridines,furo[3,2-c] pyridines and 1h-thieno[3,4-b] 2-pyran-l-ones from cuprous acetylides

The reaction of cuprous acetylides with aryl halides bearing a nucleophilic ortho substituent provides a versatile route to heterocyclic substances. The present work portrays the ease with which polyheterosystems can be constructed with this reaction. The synthesis of 2-substituted 7-iodofuro[3,2-c]pyridines, 2-substituted furo[3,2-b]pyridines, and 3-substituted lH-thieno-[3,4-b]-2-pyran-l-ones (thiaisocoumarins) is described. The latter two ring systems have not been previously reported.