New efficient access to thieno[3,2-b]pyridine derivatives via regioselective lithiation of 3-methylthiopyridine

The synthesis of thieno[3,2-b]pyridines was achieved using a three-step process allowing the construction of the thiophenic ring with 17-34% overall yields. The key step was the regioselective lithiation-bromination of the 3-methylthiopyridine induced by BuLi-LiDMAE superbase followed by Sonogashira coupling and halogenocyclization producing the fused heterocycles.     

New star-shaped molecules derived from thieno[3,2-b]thiophene unit and triphenylamine

A series of new substituted triphenylamine (TPA) derivatives with alkyl thieno[3,2-b]thiophene and thiophene units were synthesized in a combinatorial manner. Suzuki coupling of a dioxaborolane TPA derivative and 2-bromo-3-nonylthieno[3,2-b]thiophene or Stille coupling of fresh stannyl thieno[3,2-b]thiophene was used. All compounds were characterized by ¹H and ¹³C NMR, HRMS, UV-vis spectrometry and DSC measurements. It was demonstrated that the optical and thermal properties of these materials can be tuned by varying both the conjugation length and thienothiophene and thiophene combination on the TPA branches. Moreover, the measured molar extinction coefficients were increasing from 63,000 (λ_max = 354 nm) to 131,000 L mol⁻¹ cm⁻¹ (λ_max = 428 nm) for TPA-thienothiophenes and TPA-bithiophene thienothiophenes, respectively. Some of them showed molecular glass behavior.     

Synthesis of dihydrochromeno[4,3-b]pyrrolo[3,2-f]quinolines via intramolecular aza Diels-Alder reaction

An efficient and convenient one-pot intramolecular aza Diels-Alder approach for the synthesis of dihydrochromeno[4,3-b]pyrrolo[3,2-f]quinolines has been reported. Particularly valuable features of this methodology include simple execution, inexpensive catalyst, and good product yields.     

2-Benzyliden-2H-thieto[3,2-b]quinoline: a new heterocycle and its rearrangement to 2-phenylthieno[3,2-b]quinoline

Synthesis of the strained 2H-thieto[3,2-b]quinoline ring system is reported for the first time. Treatment of (Z)-2-benzyliden-2H-thieto[3,2-b]quinoline derivatives of this heterocycle with base, at reflux in ethanol, causes a novel rearrangement to 2-phenylthieno[3,2-b]quinolines. Indeed, the one-pot reaction of 2-aminobenzaldehydes and (Z)-2-benzylidenethietan-3-one in refluxing basic ethanol leads directly to 2-phenylthieno[3,2-b]quinolines in good yields.     

Enhanced absorption and fluorescence efficiency of silylethynyl-functionalized oligothiophenes and thieno[3,2-b]thiopahene

Silylethynyl-substituted oligothiophenes and thieno[3,2-b]thiophene were synthesized by the Sonogashira coupling reaction. The absorption and fluorescence maxima of these compounds shifted to longer wavelength, and they exhibited higher absorption coefficients and quantum yields than the corresponding unsubstituted molecules and tert-butylethynyl derivatives. From the DFT calculation, the energy band gap between HOMO and LUMO decreases by the introduction of a silicon atom. Thus, silyl groups can play important role in enhancing the quantum efficiency and decreasing the band-gap of chromophores. Hence, Silylethynyl group can serve as a highly efficient auxiliary for use in optical devices.     

Synthesis and alkylation of indolo[3,2-b]carbazoles

Double Fischer cyclisation was used to prepare indolo[3,2-b]carbazole and its 2,8-di-OMe, OH, Br and F-derivatives. N-monosubstituted derivatives of indolo[3,2-b]carbazole (methyl, hydroxymethyl, dimethylaminoethyl) were obtained starting from 5,11-di-Boc-indolo[3,2-b]carbazole.     

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.     

One pot synthesis of imidazo[2,1-b]thiazoles and benzo[d]thiazolo[3,2-a]imidazoles

A series of imidazo[2,1-b]thiazole and benzo[d]thiazolo[3,2-a]imidazole analogues were synthesized by stirring an equimolar mixture of dibenzoylacetylene with imidazole/thiazole derivatives in toluene or acetonitrile at room temperature. The products were generated in good yields and characterized by standard analytical techniques such as IR, ¹H NMR, ¹³C NMR and mass spectrometry. The structure of products 19, 20, 22, 24 and 25 were also unambiguously confirmed by single crystal X-ray analysis.     

Synthesis of annelated [a]aza-anthracenones and thieno[3,2-g]aza-naphthalenones through ring transformation of 2H-pyran-2-one followed by photocyclization

A concise synthesis of some new classes of heterocycles (4-aryl-11-oxo-1,2,3,11-tetrahydro-1,3a-diaza-cyclopenta[a]anthracen-6-carbonitriles and 5-aryl-12-oxo-1,3,4,12-tetrahydro-2H-1,4a-diazabenzo[a]anthracene-7-carbonitriles) has been developed by the ring transformation of suitably functionalized 2H-pyran-2-one with α-oxoketene cyclic aminals to intermediates (8-aroyl-5-aryl-2,3-dihydro-1H-imidazo[1,2-a]pyridine-7-ylidene)-acetonitriles and (9-aroyl-6-aryl-1,2,3,4-tetrahydropyrido[1,2-a]pyrimidin-8-ylidene)-acetonitriles) followed by their photocyclization either in CHCl₃ or acetonitrile. This reaction was further explored for the synthesis of methyl 4-aryl-11-oxo-1,2,3,11-tetrahydro-1,3a,9-triaza-cyclopenta[a]anthracene-6-carboxylate, 4-aryl-11-oxo-1,2,3,11-tetrahydro-1,3a,9-triaza-cyclopenta[a]anthracene-6-carbonitriles, 5-aryl-12-oxo-1,3,4,12-tetrahydro-2H-1,4a,10-triazabenzo[a]anthracene-7-carbonitriles, 4-aryl-10-oxo-1,2,3,10-tetrahydro-9-thia-1,3a-diazadicyclopenta[a,g]naphthalene-6-carbonitriles and 5-aryl-11-oxo-1,3,4,11-tetrahydro-2H-10-thia-1,4a-diazacyclopenta[b]phenanthrene-7-carbonitriles from the similar reactions.     

Furo[3,2-b]pyridine: a convenient unit for the synthesis of polyheterocycles

An efficient and rapid synthesis of furo[3,2-b]pyridine 1 is described using a one-pot Sonogashira coupling/heteroannulation sequence. Regioselective lithiation of this synthon was performed leading to various 2-substituted furo[3,2-b]pyridines. Some of them were used as substrates for short functional synthesis of polyheterocycles.     

Preparation of new pyrido[3,4-b]thienopyrroles and pyrido[4,3-e]thienopyridazines

Two new types of pyrido-fused tris-heterocycles (1a,b and 2a,b) have been prepared from 3-aminopyridine in five/six steps. A synthetic strategy for the preparation of the novel pyrido[3,4-b]thieno[2,3- and 3,2-d]pyrroles (1a,b) and pyrido[4,3-e]thieno[2,3- and 3,2-c]pyridazines (2a,b) has been studied. The Suzuki cross coupling of the appropriate 2- and 3-thienoboronic acids (3,4) and 4-bromo-3-pyridylpivaloylamide (9) afforded the biaryl coupling products (10,11) in high yields (85%). Diazotization of the hydrolysed (2-thienyl)-coupling product (12) and azide substitution gave the 3-azido-4-(2-thienyl)pyridine intermediate (72%, 14). 3-Azido-4-(3-thienyl)pyridine (15) was prepared by exchanging the previous order of reactions. The desired β-carboline thiophene analogues (1a,b) were obtained via the nitrene by thermal decomposition of the azido precursors (14,15). By optimising conditions for intramolecular diazocoupling, the corresponding pyridazine products (72-83%, 2a,b) were afforded.     

A convenient route to functionalized 3-amino-N-methylfuro[3,2-b]pyridine-2-carboxamides

The synthesis of novel functionalized 3-amino-N-methylfuro[3,2-b]pyridine-2-carboxamides is described from cyanopyridine intermediates. Based on the difference in halogen reactivity, ethyl [(5-bromo-2-chloropyridin-3-yl)oxy]acetate was functionalized by a palladium-catalyzed reaction, before the cyclization to the desired furo[3,2-b]pyridines.     

Short and efficient access to imidazo[1,2-a]pyrrolo[3,2-c]pyridine derivatives

Access to N-protected or N-free imidazo[1,2-a]pyrrolo[3,2-c]pyridine derivatives as potential antiviral compounds was achieved in good yields from N-protected 7-amino-8-halo-2-methylimidazo[1,2-a]pyridines by catalytic coupling of terminal acetylenes under mild conditions using [PdCl₂(PPh₃)₂] or [Cu(Phen)(PPh₃)₂]NO₃.     

A convenient route to functionalized 3-amino-6-bromofuro[3,2-b]pyridine-2-carboxamides

An efficient synthesis of 3-amino-6-bromofuro[3,2-b]pyridine-2-carboxamides is described via the formation of 3-amino-6-bromofuro[3,2-b]pyridine-2-carbonitrile. Functionalization of the amino group at position 3 of the heterocycle will be discussed.     

Synthesis of new benzo[b]thieno fused ring systems via transition metal-mediated cyclisations

The compact synthesis of a new ring fused benzo[b]thieno derivative with an embedded nine-membered ring system via ring closing metathesis methodology is described. The preparation of the novel 11H-benzo[b]thieno[2,3-c]pyrrolo[2,3-a]indol-11-one via palladium-mediated oxidative cyclisation of benzo[b]thien-2-oyl indole derivatives is also reported.     

New synthetic approach to [1]benzopyrano[4,3-b]pyridin-5-one derivatives

A new synthesis of [1]benzopyrano[4,3-b]pyridin-5-ones 4 was developed starting from 3-formyl-coumarin N-functionalized amidines 3. The reaction is based likely on the intramolecular cyclocondensation of the C-α amidinic carbanion in basic medium on the formyl group.     

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.     

Synthesis, photophysical and electrochemical properties and theoretical studies on three novel indolo[3,2-b]carbazole derivatives containing benzothiazole units

Three novel indolo[3,2-b]carbazoles derivatives were successfully synthesized by condensation reaction and structurally characterized by elemental analysis, mass spectrometry and proton nuclear magnetic resonance spectroscopy methods, which belong to donor-π-acceptor systems comprising an indolo[3,2-b]carbazole group as an electron donor and two benzothiazole rings as electron acceptors. The thermal, electrochemical and photophysical properties of the compounds were characterized by thermogravimetric analysis combined with electrochemistry, UV-vis absorption spectroscopy and fluorescence spectroscopy. On the other hand, the geometries, molecular orbitals, charge-injection and transport properties were determined by quantum chemical calculations. The results show that the compounds synthesized exhibit good thermal stability and high fluorescence quantum yields, indicating the potential application as optoelectronic materials.     

New regio-selective method of combinatorial synthesis of substituted thiophenes,thieno[3,2-b]pyridines and other heterocycles via combination of ‘domino’-type reactions

We present a novel combinatorial multicomponent regio-selective approach towards the synthesis of thieno[3,2-b]pyridines and pyridine pyrans. The methodology is based on the ‘domino’-type reaction. The high regio-selectivity in this reaction is gained by the in situ generation of the mono-potassium salt of 2-cyano-1-mercaptoethenethiolate. We also demonstrate that the use of ethyl 2-cyanoacetate in this reaction as a CH-acid leads to the termination of the domino sequence at the Dieckmann condensation step and yields novel ethyl 3-(4-cyano-3-hydroxy-5-(alkylthio)thiophen-2-yl)-3-oxopropanoate.     

Synthesis of 1-benzyl-8,9-dihydroimidazo[4,5-c]pyrrolo[3,2-g]quinolin-4(5H)-one via palladium-catalyzed intramolecular arylation

The synthesis of a novel tetracyclic structure, 8,9-dihydroimidazo[4,5-c]pyrrolo[3,2-g]quinolin-4(5H)-one, has been achieved by a convergent pathway. Coupling of the weakly nucleophilic hindered aromatic amine with 1-benzylimidazole-4-carboxylic acid, 7, afforded the corresponding amide 9 using a DCP/DMF complex; subsequent Heck-type arylation leading to desired tetracyclic molecule imidazo[4,5-c]-pyrrolo[3,2-g]quinolin-4(5H)-one.