Studies on uracils: a facile one-pot synthesis of oxazino[4,5-d]-, pyrano[2,3-d]-, pyrido[2,3-d]- and pyrimido[4,5-d]pyrimidines using microwave irradiation in the solid state

N,N-Dimethyl-5-formylbarbituric acid 1 reacts with maleimide 2 and phenyl isocyanate/phenyl isothiocyanate 4 under microwave-assisted conditions in the solid phase to afford pyrano[2,3-d]pyrimidines 3 and oxazino[4,5-d]pyrimidines 5 in excellent yields. Under identical conditions, N,N-dimethyl-6-amino-5-formyluracil 6 reacts with 2 and 4 to give pyrido[2,3-d]pyrimidine derivative 7 and pyrimido[4,5-d]pyrimidines 8 in high yields.     

Reactivity of the Mitsunobu reagent toward 3-formylchromones: a strategy for the one-pot synthesis of chromeno[2,3-c]pyrazolines and chromeno[2,3-e]tetrazepines

The zwitterionic intermediates generated from dialkyl azodicarboxylates and triphenylphosphine displayed excellent reactivity toward 3-formylchromones to afford chromeno[2,3-c]pyrazolines and chromeno[2,3-e]tetrazepines.     

Novel isocyanide-based three-component reaction: a facile synthesis of substituted 1H-chromeno[2,3-d]pyrimidine-5-carboxamides

A mild and efficient method for the synthesis of 1H-chromeno[2,3-d]pyrimidine-5-carboxamide derivatives via a one-pot, three-component reaction of an isocyanide, barbituric acid, and a salicylaldehyde in the presence of acetic acid in ethanol/water mixture at 75 °C is reported. This high atom economy reaction led to the construction of one benzopyran ring, and one amide group in a single synthetic step.     

Microwave-assisted novel synthesis of amino-thieno[3,2-b]pyridines under solvent-free conditions

In the presence of a catalytic amount of ytterbium(III) triflate and under microwave irradiation, mixtures of 2-amino-3-thiophenecarbonitriles, ketones, and silica gel afforded smoothly the corresponding amino-thieno[2,3-b]pyridine derivatives in one step. A wide variety of ketones were tested under these conditions. The reactions proceeded rapidly and afforded the desired products in good to excellent yields.     

A novel and efficient synthesis of pyrimido[4,5-d]pyrimidine-2,4,7-trione and pyrido[2,3-d:6,5-d] dipyrimidine-2,4,6,8-tetrone derivatives

An efficient and direct procedure for the synthesis of pyrimido[4,5-d]pyrimidine-2,4,7-trione derivatives has been described under microwave-assisted conditions. Reaction of 6-amino-1,3-dimethyluracil with aromatic aldehydes resulted in the formation of pyrido[2,3-d:6,5-d] dipyrimidine-2,4,6,8-tetrone derivatives.     

Synthesis of fused pyrrolo[3,4-d]tetrahydropyrimidine derivatives by proline-catalyzed multicomponent reaction

Novel proline-catalyzed multicomponent reactions (MCRs) for the synthesis of fused pyrrolo[3,4-d]tetrahydropyrimidines 7 and 9 with different substituted patterns have been developed, which provide rapid access to a library of compounds 7 and 9 in medium to excellent yields, by using N-methyl-α-aryl(alkyl)aminomaleimides, amines, and aldehydes as reactants. The catalyst and the ratio of reactants were found to have significant impact on these reactions, and a reasonable mechanism was also proposed.     

A facile approach for the synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes under solvent-free condition

A facile, efficient and environment-friendly protocol for the synthesis of 14-aryl- or alkyl-14H-dibenzo[a,j]xanthenes has been developed by one-pot condensation of 2-naphthol with aliphatic and aromatic aldehydes in the presence of P₂O₅ or InCl₃ as catalysts under solvent-free conditions. The present approach offers the advantages of clean reaction, simple methodology, short reaction time, high yield, easy purification, and economic availability of the catalyst.     

An efficient one-pot synthesis of tetrahydrobenzo[a]xanthene-11-one and diazabenzo[a]anthracene-9,11-dione derivatives under solvent free condition

Indium(III) chloride catalyzed one-pot synthesis of 12-aryl/alkyl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-one and 8,10-dimethyl-12-aryl-8,12-dihydro-7-oxa-8,10-diazabenzo[a]anthracene-9,11-dione derivatives have been achieved by three component cyclocondensation of aldehydes, β-naphthol and cyclic 1,3-dicarbonyl compounds under solvent free condition in high yields. P₂O₅ too has been found as an effective catalyst towards this transformation.     

Application of 2-(2-chloroaroyl)methyleneimidazolidines in domino and multicomponent reaction: new entries to imidazo[1,2-a]pyridines and benzo[b]imidazo[1,2,3-ij][1,8]naphthyridines

A new strategy for the synthesis of tetrahydroimidazo[1,2-a]pyridines and unusual tetrahydrobenzo[b]imidazo[1,2,3-ij][1,8]naphthyridines has been successfully developed by cascade reactions including Knoevenagel condensation, aza-ene reaction, imine-enamine tautomerization, cyclocondensation/oxidation, and intramolecular S_NAr of precursors 2-(2-chloroaroyl)methyleneimidazolidines as new heterocyclic ketene aminals (HKA), which represent a class of polyfunctional scaffolds with four active reaction sites with aromatic aldehydes and malononitrile or ethyl 2-cyanoacetate under mild conditions. In this domino reaction, nine different active sites are involved, and two C-C bonds, two C-N bonds, and two new rings are constructed with all reactants efficiently utilized in the chemical transformation.     

Preparation of novel pyridine-fused tris-heterocycles; pyrido[4,3-e]pyrrolo-/pyrido[4,3-e]furano[2,3-c]pyridazines and pyrido[3,4-b]pyrrolo[3,2-d]pyrrole

Three novel pyrido-fused tris-heterocycles have been prepared based on a Suzuki coupling and subsequent cyclisation approach. Pyrido[4,3-e]pyrrolo[2,3-c]pyridazine (3b, 77%) and pyrido[4,3-e]furano[2,3-c]pyridazine (5b, 76%) were obtained by intramolecular diazocoupling. Successful diazocoupling of furan (5b) is thus reported for the first time by NOBF₄ generation of the diazonium intermediate. N-TIPS-pyrido[3,4-b]pyrrolo[3,2-d]pyrrole (TIPS-4b) was synthesised by thermal cyclisation of pyridyl nitrene in considerably higher yield (71%) than previously experienced from similar cyclisations, due to TIPS-activation.     

One-pot synthesis of aliphatic and aromatic 2H-indazolo[2,1-b]phthalazine-triones catalyzed by N-halosulfonamides under solvent-free conditions

N, N, N′, N′-Tetrabromobenzene-1,3-disulfonamide and poly(N-bromo-N-ethylbenzene-1,3-disulfonamide) were used as efficient catalysts for the one-pot synthesis of aliphatic and aromatic 2H-indazolo[2,1-b]phthalazine-triones in excellent yields from aldehydes, phthalhydrazide, and dimedone at 80-100 °C under solvent-free conditions.     

Synthesis of novel oxindolylpyrrolo[2,3-d]pyrimidines via a three-component sequential tandem reaction

A novel one-pot three-component reaction of 6-amino-uracil, isatin, and acetophenone was accomplished through a programmed pH variation for the synthesis of 5-(2-oxoindolin-3-yl)-1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-dione derivatives. The reaction was conducted in a sequential tandem manner to give the oxindole substituted pyrrolo[2,3-d]pyrimidine products in good to excellent yields. Despite of timing all the processes were carried out in one pot. Most of these novel compounds show narrow to good spectrum of antimicrobial activities in vitro.     

One-pot synthesis of 12H-benzo[b]xanthen-12-ones and naphtha[2,3-b]furans from non-naphthalene substrates

An efficient one-pot synthesis of 12H-benzo[b]xanthen-12-ones via an oxidant initiated and base promoted tandem reaction of 2-(4-(2-haloaryl)-4-hydroxybut-1-ynyl)benzaldehydes has been developed. By using similar strategy, a one-pot synthesis of naphtha[2,3-b]furans from the cascade reaction of 2-(4-hydroxy-but-1-ynyl)benzaldehydes with 2-bromoacetonitrile or α-bromocarbonyl compounds has also been achieved.     

Regioselective synthesis of fused benzopyrazolo[3,4-b]quinolines under solvent-free conditions

New 6,8-dihydro-5H-benzo[f]pyrazolo[3,4-b]quinolines 6 have been obtained in a novel solvent-free three-component reaction involving β-tetralone along with 5-aminopyrazoles 1 and benzaldehydes 2. The isomeric 6,10-dihydro-5H-benzo[h]pyrazolo[3,4-b]quinolines 9 could not be prepared in a similar fashion directly from α-tetralone, but were obtained by the reaction of amines 1 with benzylidene-derivative 10 of α-tetralone in similar conditions. The yields of quinolines obtained via this novel protocol were good and the reaction times varied from few minutes to just few seconds.     

Application of the intramolecular aza-Wittig reaction to the synthesis of pyrido[2,3-d]pyrimidines

Pyrido[2,3-d]pyrimidines are synthesized in a two-step procedure from amides and tetrazolo[1,5-a]pyridine-8-carbonyl chloride. Reaction of the crude imides with triphenylphosphine effects an intramolecular aza-Wittig reaction to afford a variety of substituted pyrido[2,3-d]pyrimidines in good to moderate yields (30-76%).     

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 a novel tricyclic 1,6,7,8-tetrahydro-2H-cyclopenta[b]furo[3,2-d]pyridine derivative,the 5-aza analog of ramelteon

The 5-aza analog of ramelteon 3, a novel tricyclic 1,6,7,8-tetrahydro-2H-cyclopenta[b]furo[3,2-d]pyridine derivative, was synthesized by hetero-Diels–Alder reaction and chemoselective Dieckmann condensation.     

Synthesis of novel 5,6-substituted furo[2,3-d]pyrimidines via Pd-catalyzed cyclization of alkynylpyrimidinols with aryl iodides

A flexible method for the synthesis of 5,6-disubstituted furo[2,3-d]pyrimidine derivatives is described. The key step is a palladium-catalyzed arylative cyclization of alkynylpyrimidinols with various aryl iodides, which gave the title compounds in 36-75% yield.     

l-Proline catalyzed synthesis of densely functionalized pyrido[2,3-d]pyrimidines via three-component one-pot domino Knoevenagel aza-Diels-Alder reaction

Highly functionalized hitherto unreported pyrido[2,3-d]pyrimidines have been concisely synthesized in good yields via l-proline catalyzed one-pot three-component domino coupling of 6-amino-1,3-dimethyluracil, aldehydes, and dialkyl acetylenedicarboxylates under mild conditions for the first time. The MCR process involves Knoevenagel condensation followed by [4+2] cycloaddition reaction. No co-catalyst or activator is required for this MCR. The molecular structures of two representative pyrido[2,3-d]pyrimidines 4a and 4h were confirmed by single crystal X-ray diffraction.     

Pd-catalyzed cross-coupling/heterocyclization domino reaction: facile access to anthra[2,3-b]furan-5,10-dione scaffold

A new facile route to the synthesis of 4,11-dimethoxyanthra[2,3-b]furan-5,10-dione and its novel 2-substituted derivatives was proposed. The developed scheme was based on a Pd-catalyzed cross-coupling/heterocyclization domino reaction of 3-bromo-2-hydroxy-4,11-dimethoxyanthraquinone with terminal alkynes.