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.     

Optimized scale up of 3-pyrimidinylpyrazolo[1,5-a]pyridine via Suzuki coupling; a general method of accessing a range of 3-(hetero)arylpyrazolo[1,5-a]pyridines

We have developed an improved synthesis of 3-(hetero)aryl pyrazolo[1,5-a]pyridines (such as 3-(2,5-dichloropyrimidin-4-yl)pyrazolo[1,5-a]pyridine (8)) via an optimized synthesis and Suzuki coupling of 3-pyrazolo[1,5-a]pyridine boronic ester 10. These conditions are applicable to both high throughput chemistry and large scale synthesis of these medicinally important compounds. The scope of this chemistry has been further extended to include the synthesis and coupling of a novel boronic ester, 3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine (43).     

Diverse synthesis of pyrimido[1,2-a]benzimidazoles and imidazo[2,1-b]benzothiazoles via CuI-catalyzed decarboxylic multicomponent reactions of heterocyclic azoles,aldehydes and alkynecarboxylic acids

We have developed a straightforward approach to diverse synthesis of 2,3-, 2,4-disubstituted pyrimido [1,2-a]benzimidazoles, 2,4,10-trisubstituted 2,10-dihydropyrimido [1,2-a]benzimidazoles and 2,3-disubstituted imidazo [2,1-b]benzothiazoles via multicomponent reactions (MCRs) of heterocyclic azoles, aldehydes with easily storable and handling alkynecarboxylic acids. In the presence of a catalytic amount of CuI and K₂CO₃, the pyrimido [1,2-a]benzimidazole or imidazo [2,1-b]benzothiazole scaffold could be rapidly constructed through a 6-endo-dig or 5-exo-dig cyclization, respectively. The preliminary mechanistic study suggested that the formation of 2,3- disubstituted pyrimido [1,2-a]benzimidazoles, which completes the assembly of the scaffold and its C-3 position functionalization in one pot, undergoes a novel cascade process involving a decarboxylation, A [3] coupling, 6-endo-dig cyclization, nucleophilic addition and dehydration.     

Synthesis of seven- and eight-membered [1,2-a] alicyclic ring-fused benzimidazoles and 3-aziridinylazepino[1,2-a]benzimidazolequinone as a potential antitumour agent

Azepino and azocino[1,2-a]benzimidazoles were obtained either by treatment of 1-nitrophenyl-2-azacycloalkanes via a one-pot catalytic hydrogenation/acetylation or by treatment of the acetamides generated in the latter reaction with performic acid. This represents the first facile synthesis of eight-membered [1,2-a] alicyclic ring-fused benzimidazoles. 3-Methoxy-azepino[1,2-a]benzimidazole was elaborated to the novel potential cytotoxin, 3-(N-aziridinyl)-7,8,9,10-tetrahydro-6H-azepino[1,2-a]benzimidazole-1,4-dione. The synthesis included clarification of the reactivity of methoxy-substituted benzimidazoles towards nitration.     

Combinatorial synthesis of 4-oxo-4H-imidazo[1,5-a]quinoxalines and 4-oxo-4H-pyrazolo[1,5-a]quinoxalines

A combinatorial synthetic route yielding imidazo[1,5-a]quinoxalines and pyrazolo[1,5-a]quinoxalines is described. The use of 2-fluoroaniline and 1H-imidazole-4-carboxylic acid, respectively, 1H-pyrazole-3-carboxylic acid in the Ugi-reaction (U-4CR) followed by a nucleophilic aromatic substitution (S_NAr) affords the imidazo- as well as the pyrazolo-[1,5-a]quinoxaline moiety in good yield and high diversity.     

Pd-catalyzed intramolecular cyclization of pyrrolo-2-carboxamides: regiodivergent routes to pyrrolo-pyrazines and pyrrolo-pyridines

Treatment of N-alkyl-N-allyl-pyrrolo-2-carboxamides with catalytic amounts of palladium derivatives gave regioselectively intramolecular cyclizations to generate bicyclic pyrrolo-fused structures. Pyrrolo[1,2-a]pyrazin-1-ones were achieved in high yields by an amination reaction, while pyrrolo[2,3-c]pyridin-7-ones and pyrrolo[3,2-c]pyridin-4-ones were obtained by an oxidative coupling process.     

Synthesis of 1,3-diarylated imidazo[1,5-a]pyridines with a combinatorial approach: metal-catalyzed cross-coupling reactions of 1-halo-3-arylimidazo[1,5-a]pyridines with arylmetal reagents

The halogenation of 3-arylimidazo[1,5-a]pyridines was carried out with iodine, bromine, N-chlorosuccinimide, and 1-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate as halogenating agents to give selectively halogenated products 1-halo-3-arylimidazo[1,5-a]pyridines in good to excellent yields. Kumada-Tamao-Corriu cross-coupling of the obtained 1-iodo-3-arylimidazo[1,5-a]pyridines and aryl Grignard reagents led to 1,3-diarylated imidazo[1,5-a]pyridines in good to excellent yields. Suzuki-Miyaura cross-coupling of the 1-bromo-3-phenylimidazo[1,5-a]pyridine and p- or m-methoxycarbonylphenylboronic acids furnished the coupling product in respective yields of 91% and 61%. The obtained 1,3-diarylated imidazo[1,5-a]pyridines showed a wide variety of fluorescent emissions in a wavelength range of 449-533 nm with improved quantum yields compared to monoarylated ones.     

Syntheses of indolo[3,2,1-d,e]phenanthridines and isochromeno[3,4-a] carbazoles: palladium catalyzed intramolecular arylation via C-H functionalization

A new synthetic route has been developed for the preparation of indolo[3,2,1-d,e]phenanthridines and isochromeno[3,4-a]carbazoles via palladium catalyzed intramolecular biaryl coupling reactions. The coupling reactions proceeded smoothly and in high yields under ligand-free conditions with the catalytic system Pd(OAc)₂/Cs₂CO₃/TBAB. Under optimized reaction conditions no halogen-reduced products were observed.     

Synthesis and properties of cyanomethyl derivatives of imidazo[1,2-<Emphasis Type="Italic">a</Emphasis>]pyridine,imidazo[1,2-<Emphasis Type="Italic">a</Emphasis>]pyrimidine,and imidazo[2,1-<Emphasis Type="Italic">b</Emphasis>]thiazole

2-Cyanomethyl derivatives were obtained of imidazo[1,2-a]pyridine, imidazo[1,2-a]pyrimidine, and imidazo[2,1-b]thiazole, and their reactivity was investigated by an example of imidazo[1,2-a]pyridine: It was subjected to nitration, bromination, azo coupling and nitrosation. Acylation of the methylene group effected by amino acids esters with a subsequent addition of the amino group to the cyano group resulted in the formation of 5-amino-4-imidazo[1,2-a]-pyridin-2-yl-1-phenyl-1,2-dihydro-3H-pyrrol-3-one and 2-amino-1-ethyl-3-imidazol[1,2-a]pyridin-2-yl-4(1H)-quinolinone.     

Metal free coupling of heteroaryl N-tosylhydrazones and thiols: Efficient synthesis of sulfides

A metal free coupling of heteroaromatic N-tosylhydrazones with thiols is presented. A convenient synthetic route to synthesize heteroaryl N-tosylhydrazones is also showed. Valuable thioethers with pyrroles, pyridines, thieno[2,3-b]pyridines, imidazo[1,2-a]pyridines, and 6H-thieno[2,3-b]pyrroles derivatives were synthesized in good yields. This coupling reaction can be carried out in a one-pot fashion and scaled up to the gram scale by using heteroaryl aldehydes, without the need to isolate the N-tosylhydrazone.     

A convenient new synthesis of fused 1,2,4-triazoles: the oxidation of heterocyclic hydrazones using copper dichloride

A series of 1,2,4-triazoles have been prepared by oxidative intramolecular cyclization of heterocyclic hydrazones with copper dichloride. General applicability of this simple transformation was confirmed by the synthesis of moderate to high yields of 1,2,4-triazolo[4,3-a]pyridines, 1,2,4-triazolo[4,3-a]pyrimidines, 1,2,4-triazolo[4,3-b]pyridazines, 1,2,4-triazolo[4,3-a]phthalazines, and 1,2,4-triazolo[4,3-a]quinoxalines. A 1,2,4-triazolo[4,3-e]purine-6,8(7H)-dione was obtained in a lower yield.     

Grignard reagent-promoted 6-endo-dig cyclization: instantaneous synthesis of original dipyrido[1,2-a:3′,4′-d]imidazole

Dipyrido[1,2-a:3′,4′-d]imidazole derivatives can be readily synthetized from various 3-alkyne-2-cyanoimidazo[1,2-a]pyridines via an efficient Grignard reagent-promoted 6-endo-dig cyclization of nitrile to alkynes. A previous optimization of the Sonogashira coupling reaction at C(3) of the 2-cyanoimidazo[1,2-a]pyridine was necessary as this coupling reaction is known to be largely influenced by the nature of the 2-substituent.     

Rhodium-Catalyzed Oxidative Annulation of 2- or 7-Arylindoles with Alkenes/Alkynes Using Molecular Oxygen as the Sole Oxidant Enabled by Quaternary Ammonium Salt

Developing an efficient catalytic system using molecular oxygen as the oxidant for rhodium-catalyzed cross-dehydrogenative coupling remains highly desirable. Herein, rhodium-catalyzed oxidative annulation of 2- or 7-phenyl-1H-indoles with alkenes or alkynes to assemble valuable 6H-isoindolo[2,1-a]indoles, pyrrolo[3,2,1-de]phenanthridines, or indolo[2,1-a]isoquinolines using the atmospheric pressure of air as the sole oxidant enabled by quaternary ammonium salt has been accomplished. Mechanistic studies provided evidence for the fast intramolecular aza-Michael reaction and aerobic reoxidation of Rh(I)/Rh(III), facilitated by the addition of quaternary ammonium salt.     

Microwave assisted synthesis of 2,3-dihydro-4H-benzo[4,5]thiazolo[3,2-a]furo[2,3-d]pyrimidin-4-ones and 6,7-dihydro-5H-furo[2,3-d]thiazolo[3,2-a]pyrimidin-5-ones using Mn(OAc)3

2-Hydroxy-4H-benzo[4,5]thiazolo[3,2-a]pyrimidin-4-one 2a and 7-hydroxy-5H-thiazolo[3,2-a]pyrimidin-5-one 2b, were obtained in high yields under mild conditions from the cyclization reactions of bis-(2,4,6-trichlorophenyl) malonate and 2-aminobenzothiazole or 2-aminothiazole, respectively. A new class of compounds, 2,3-dihydro-4H-benzo[4,5]thiazolo[3,2-a]furo[2,3-d]pyrimidin-4-ones and 6,7-dihydro-5H-furo[2,3-d]thiazolo[3,2-a]pyrimidin-5-ones, were synthesized via the microwave assisted radical addition of compounds 2a and 2b to various alkenes using manganese(III) acetate. A preliminary acetylcholine esterase (AchE) inhibition test of compound 4e showed excellent (92%) inhibitory potential, comparable with the standard drug Donapezil®.     

Cyclisation of 3-alkenylpyrido[1,2-a]pyrimidines to furo[2,3-d]pyrido[1,2-a]pyrimidines

3-Alkenylpyrido[1,2-a]pyrimidines react under mild conditions to give novel tricyclic furo[2,3-d]pyrido[1,2-a]pyrimidines in high yields. The cyclisation takes place in the presence of an acid catalyst. The product yield is affected by the type and the strength of the acid used. Exceptionally high yields were obtained when an organic acid like trifluoromethanesulfonic acid and trifluoroacetic acid were used. On the other hand, sulfuric acid gave the best results of the inorganic acids examined.     

An efficient access to 2,3-diarylimidazo[1,2-a]pyridines via imidazo[1,2-a]pyridin-2-yl triflate through a Suzuki cross-coupling reaction-direct arylation sequence

An efficient method to prepare 2,3-diarylimidazo[1,2-a]pyridines is described. The procedure involves a Suzuki cross-coupling reaction followed by a direct arylation at position 3. Imidazo[1,2-a]pyridin-2-yl triflate was identified as a suitable coupling partner, permitting access to a variety of highly functionalized 2,3-diarylimidazo[1,2-a]pyridines.     

Pd-catalyzed Suzuki/Sonogashira cross-coupling reaction and the direct sp3 arylation of 7-chloro-5-methyl-[1,2,4]triazolo[1,5-a]pyrimidine

A rapid and efficient method is reported for the synthesis of the [1,2,4]triazolo[1,5-a]pyrimidine motif. Palladium catalyzed Suzuki and Sonogashira cross coupling reactions on 7-chloro-5-methyl-[1,2,4]triazolo[1,5-a]pyrimidine were performed. The direct sp³ arylation of compounds resulting from the Suzuki reaction was then carried out.     

Cu(I)/Pd(II)-Catalyzed Intramolecular Hydroamidation and C-H Dehydrogenative Coupling of ortho-Alkynyl-N-arylbenzamides for Access to Isoindolo[2,1-a]Indol-6-Ones

An efficient, atom-economic and one-pot synthesis of isoindolo[2,1-a]indol-6-ones via CuI/Pd(OAc)₂-catalyzed intramolecular hydroamidation of alkynyl group, and C-H dehydrogenative coupling of ortho-alkynyl-N-arylbenzamides has been developed. This transformation occurs with the use of oxygen as the oxidant, and water is the only by-product. The reaction shows a high tolerance to a variety of functional groups, and affords isoindolo[2,1-a]indol-6-ones in good to high yields.     

Reaction of 3-oxo-N-phenylbutanethioamide with 5-amino-1H-1,2,4-triazoles

Reactions of 3-oxo-N-phenylbutanethioamide with 3-substituted 5-amino-1H-1,2,4-triazoles in acetic acid led to the formation of 5-methyl-7,8-dihydro[1,2,4]triazolo[1,5-a]pyrimidine-7-thiones, 7-methyl-5,8-dihydrol[1,2,4]triazolo[1,5-a]pyrimidine-5-thiones and 7-methyl-5-phenylamino[1,2,4]triazolo[1,5-a]pyrimidines. The structure of the products was proved by the ¹H and ¹³C NMR spectra, X-ray diffraction data, and chemical transformations.     

Synthesis and ring transformations of 1-amino-1,2,3,9a-tetrahydroimidazo[1,2-a]indol-2(9H)-ones

Heating 1-carbamoylmethyl-2,3,3-trimethyl-3H-indolinium chloride in the presence of hydrazine bishydrate produces regioselectively the five-membered heterocycle 1-amino-1,2,3,9a-tetrahydro-9,9,9a-trimethylimidazo[1,2-a]indol-2(9H)-one. The assignment of the structure is based on extensive ¹H, ¹³C and ¹⁵N NMR spectroscopic studies. No ring-chain tautomerism of the 1-amino-1,2,3,9a-tetrahydroimidazo[1,2-a]indol-2(9H)-one was observed to open-chain hydrazides or the corresponding six-membered 1,2,10,10a-tetrahydro[1,2,4]triazino[4,3-a]indol-3(4H)-one. Further transformations of 1-amino-1,2,3,9a-tetrahydroimidazo[1,2-a]indol-2(9H)-one were performed by treatment with aromatic aldehydes, acid chlorides and isocyanates giving access to 40 novel hydrazones, N,N′-diacylhydrazines, N-acyl-N′-carbamoylhydrazines and 1,3,4-oxadiazoles.