Intramolecular 4+2 cycloaddition of thieno[2,3-e][1,2,4]triazines: routes towards condensed thieno[2,3-b]pyridines

Synthetic approaches towards new condensed thienopyridine ring systems including furo[2,3-b]thieno[3,2-e]pyridines, bisthieno[2,3-b:3′,2′-e]pyridines, 5H-chromeno[2,3-b]thieno[3,2-e]pyridines, 5H-benzo(f)chromeno[2,3-b]thieno[3,2-e]pyridines have been achieved by application of intramolecular 4+2 cycloaddition reactions of suitably designed thieno[2,3-e][1,2,4]triazines tethered with alkene or alkyne terminals.     

Synthesis of substituted thiazolo[4,5-b]pyridines and other annulated heterocycles via SN2→Thorpe-Ziegler→Thorpe-Guareschi domino reactions

A new combinatorial method for the preparation of substituted thiazolo[4,5-b]pyridines, which utilizes cyanoacetamide, heterocumulenes (isothiocyanates, carbon bisulfide), and ethyl-4-chloroacetoacetate in a new S_N2→Thorpe-Ziegler→Thorpe-Guareschi domino reactions has been developed. The obtained thiazolo[4,5-b]pyridines were then used together with aldehydes and malononitrile in another Knoevenagel reaction→Michael reaction→hetero-Thorpe-Ziegler domino reaction for the synthesis of substituted 4,6-dihydro-5H-pyrano[2,3-d]thiazolo[4,5-b]pyridines.     

New method for the synthesis of substituted thieno[3,2-b]pyridines and 5H-pyrano[2,3-d]thieno[3,2-b]pyridines derived from them

A new highly selective method was developed for the synthesis of substituted thieno[3,2-b]-pyridines based on the domino reaction of monopotassium salt (rather than dipotassium one) of carbamoylcyanodithioacetic acid with ethyl 4-chloroacetoacetate. Substituted 5H-pyrano[2,3-d]-thieno[3,2-b]pyridines were synthesized based on these thieno[3,2-b]pyridines.     

A facile synthesis of 2,3-disubstituted furo[2,3-b]pyridines

In a three-step sequence starting from readily available starting materials, 2,3-carbon disubstituted furo[2,3-b]pyridines can be accessed in good yields and purity. Furo[2,3-b]pyridines bearing ester, amide and ketone groups at the 2-position can be prepared with a variety of aryl and alkyl groups at the 3-position.     

Synthesis of 4-aza analog of ramelteon: a novel tricyclic 1,6,7,8-tetrahydro-2H-cyclopenta[d]furo[2,3-b]pyridine derivative as melatonin receptor ligand

The 4-aza analog of ramelteon (−)-1, a novel tricyclic 1,6,7,8-tetrahydro-2H-cyclopenta[d]furo[2,3-b]pyridine derivative, was synthesized via the intramolecular inverse electron demand Diels-Alder reaction followed by fluoride-induced desilylation-cyclization.     

A concise synthesis of functionalized 2,3-dihydrofuro[3,2-b], [3,2-c], and [2,3-b]pyridines

A strategy for the efficient and rapid one-pot synthesis of 2-aryl-2,3-dihydrofuro[3,2-b], [3,2-c], and [2,3-b]pyridines from readily available o-nitropicolines and aromatic aldehydes is described. The key transformation involves reaction of o-nitropicolines with aromatic aldehydes in the presence of TBAF and Hünig's base giving rise to functionalized products having molecular complexity suitable for further manipulation.     

A simple and convenient approach to the Friedländer synthesis of pyrano[2,3-b]pyridines

SnCl₂·2H₂O mediated efficient synthesis of pyrano[2,3-b]pyridines was achieved by the Friedländer reaction of 2-amino-3-cyano-4H-pyrans with cyclopentanone/cyclohexanone under solvent-free condition.     

Efficient synthesis of 5,6-dihydro-8H-[1,2,4]thiadiazino[6,5,4-de]phenanthridine 4,4-dioxide and 5,6-dihydro-8H-[1,2,4]-thiadiazino[6,5,4-ij]thieno[2,3-c]quinoline 4,4-dioxide

A new efficient and versatile synthesis to obtain different substituted 5,6-dihydro-8H-[1,2,4]thiadiazino[6,5,4-de]phenanthridine 4,4-dioxide and 5,6-dihydro-8H-[1,2,4]-thiadiazino[6,5,4-ij]thieno[2,3-c]quinolone 4,4-dioxide was developed. The four cyclic systems are achieved by a three-step synthesis proceeding under mild conditions in high yields.     

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.     

Synthesis and functionalisation of 1H-pyrazolo[3,4-b]pyridines involving copper and palladium-promoted coupling reactions

A convenient route to novel 3-iodo-1H-pyrazolo[3,4-b]pyridines via iododediazonation of 3-amino-1H-pyrazolo[3,4-b]pyridines, which are obtained by copper-catalysed cyclisation of 2-chloro-3-cyanopyridine with hydrazines. We describe also efficient coupling reactions from 3-iodo derivatives with various reagents according to Suzuki, Heck, Stille, and Sonogashira conditions.     

Synthesis of substituted 2-amino-3-cyano-7,9-dimethyl-4H-pyrano[2",3":4,5]thieno[2,3-b]pyridines

4,6-Dimethyl-2H-thieno[2,3-b]pyridin-3-one reacts with 2-aryl-1,1-dicyanoethylenes or an aromatic aldehyde/ketone (cyclohexanone and piperidone derivatives) and malononitrile to give substituted 2-amino-3-cyano-7,9-dimethyl-4H-pyrano[2",3":4,5]thieno[2,3-b]pyridines.     

Five-membered 2,3-dioxoheterocycles: LXXXVII. [4+2]-cycloaddition of alkyl vinyl ethers and 3,4-dihydro-2H-pyran to 4,5-diaroyl-1 H-pyrrole-2,3-diones

1-Aryl-4,5-diaroyl-1H-pyrrole-2,3-diones react with alkyl vinyl ethers and 3,4-dihydro-2H-pyran affording 1,4-diaryl-7a-aroyl-6-ethoxy-7,7a-dihydropyrano[4,3-b]pyrrole-2,3(1H,6H)-diones and 1,4-diaryl-9baroyl-1,7,8,9,9a-hexahydro-5aH-pyrano[3??,2??:5,6]pyrano[4,3-b]-pyrrole-2,3-diones respectively.     

Antimony trichloride catalyzed three-component reaction of urea, aldehydes and cyclic enol ethers: a novel route to 4-arylhexahydrofuro[2,3-d]pyrimidin-2(3H)-ones

Antimony trichloride efficiently catalyses diastereoselective three-component reaction of urea, aromatic aldehydes and 2,3-dihydrofuran or 3,4-dihydro-2H-pyran leading to 4-arylhexahydrofuro[2,3-d]pyrimidin-2(3H)-ones and 4-arylhexahydro-1H-pyrano[2,3-d]pyrimidin-2(8aH)-ones, respectively in good yield.     

Synthesis of new condensed thieno[2,3-b]pyridines containing pyrimidine and imidazole rings

Two different convenient methods have been developed for the synthesis of condensed thieno[2,3-b]pyridines. The intramolecular cyclization of 3-amino-2-hydrazinocarbonyl-7,7-dimethyl-7,8-dihydo-5H-pyrano[3,4-e]-thieno[2,3-b]pyridine was carried out to give 7,7-dimethyl-2-oxo-1,2,6,7-tetrahydro-9H-pyrano[3′,4′-e]-imidazo[4″,5″∶2,3]thieno[5,4-b]pyridine, which is the first representative of a new heterocyclic system.     

Reduction of indolo[2,3-b]quinoxalines

Reduction of indolo[2,3-b]quinoxalines with zinc in the presence of an anhydride gave N,N-diacyl trapped 6,11-dihydroindolo[2,3-b]quinoxalines in 43-92% yields. When the reduction with zinc was performed in TFA/TFAA, an unexpected ring opened product was isolated in 49% yield. The structure of this product could be identified as 1,2-dihydro-1-trifluoroacetyl-3-[(2-trifluoroacetylamino)phenyl]quinoxaline.     

Synthesis and transformations of 3-(1H-pyrrol-1-yl)thieno[2,3-b]pyridines

Reactions of 2,5-dimethoxytetrahydrofuran with 3-aminothieno[2,3-b]pyridines afford a number of substituted 3-(1H-pyrrol-1-yl)thieno[2,3-b]pyridines. The possibility of the reaction and the yield of the product are determined by the character of a substituent in position 2 of thieno[2,3-b]pyridine. The Curtius rearrangement of 2-acylazido-3(1H-pyrrol-1-yl)thieno[2,3-b]pyridines yields 4,5-dihydropyrido[3",2":4,5]thieno[2,3-e]pyrrolo[1,2-a]pyrazin-4-ones. The molecular and crystal structures of ethyl 4-methoxymethyl-6-methyl-3-(1H-pyrrol-1-yl)thieno[2,3-b]pyridine-2-carboxylate were determined by X-ray diffraction analysis.     

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.     

Synthesis of furo[2,3-c]-2,7-naphthyridine derivatives via domino heterocyclization reaction

2-Amino-4-cyanomethyl-6-dialkylamino-3,5-pyridinedicarbonitriles were found to react with substituted oxiranes yielding 5,6-diamino-8-dialkylamino-1,2-dihydrofuro[2,3-c]-2,7-naphthyridine-9-carbonitriles. The oxirane ring was shown to be opened selectively from the unsubstituted side and further cyclization occurred with participation of 3-CN, but not 5-CN of the starting pyridines. The furonaphthyridines obtained were converted into 2-dialkylamino-5-methyl-9,10-dihydro-4H-furo[2,3-c]pyrimido[4,5,6-ij]-2,7-naphthyridine-1-carbonitriles and 2-dialkylamino-5,6,9,10-tetrahydro-4H-spiro{furo[2,3-c]pyrimido[4,5,6-ij]-2,7-naphthyridine-5,1′-cyclohexane}-1-carbonitriles by treatment with acetic anhydride and cyclohexanone, respectively. The structure of prepared compounds was confirmed unambiguously by X-ray crystallographic study.     

Regioselective formylation of pyrazolo[3,4-b]pyridine and pyrazolo[1,5-a]pyrimidine systems using Vilsmeier-Haack conditions

Regioselective formylation behavior has been found in the reaction of pyrazolo[3,4-b]pyridines and pyrazolo[1,5-a]pyrimidines via Vilsmeier-Haack conditions. While the 4,5- and 6,7-dihydro derivatives afforded pyrazolo[3,4-b]pyridine-5-carbaldehydes and 4,7-dihydropyrazolo[1,5-a]pyrimidine-3,6-dicarbaldehydes, respectively, the aromatic analogs rendered the pyrazolo[1,5-a]pyrimidine-3-carbaldehyde only, and no reaction took place at the pyrazolopyridine derivatives.     

Efficient synthesis of 2- and 3-substituted-2,3-dihydro [1,4]dioxino[2,3-b]pyridine derivatives

A versatile new approach for the synthesis in three steps of 2-substituted-2,3-dihydro[1,4]dioxino[2,3-b]pyridines B via a Smiles rearrangement using easily available reagents is described. A study illustrating the influence of experimental conditions on the progress of the reaction is reported.