One-pot two-step tandem reactions for selective synthesis of pyrrolo[2,1-a]isoquinolines and dihydro-, tetrahydro-derivatives

A sequential one-pot two-step tandem reaction for selective and efficient synthesis of pyrrolo[2,1-a]isoquinoline and its dihydro- and tetrahydro-derivatives has been developed. The tandem reactions of isoquinoline, α-halogenated methylene compounds, aromatic aldehydes, and cyanoacetamide firstly give tetrahydropyrrolo[2,1-a]isoquinolines as main products. The corresponding pyrrolo[2,1-a]isoquinolines and dihydropyrrolo[2,1-a]isoquinolines can be obtained directly by controlling oxidation with DDQ. The mechanism of this tandem reaction involved the 1,3-dipolar cycloaddition of isoquinolinium ylide as the key step. A unique elimination of the amido group preferring to the cyano group has been observed.     

A new approach towards the synthesis of pyrrolo[2,1-a]isoquinolines

Reaction of 5-bromo-2-methyl-8-nitro-1,2,3,4-tetrahydroisoquinoline with activated alkynes affords stable tetrahydropyrrolo[2,1-a]isoquinolin-4-ium ylides. Further reactions of ylide 2 gives access to substituted dihydropyrrolo[2,1-a]isoquinolines in good yields.     

Synthesis of substituted benzimidazo[2,1-a]isoquinolines and its condensed analogues using Pd(0)-catalyzed cyclization/C-H activation

An efficient route for the synthesis of benzimidazo[2,1-a]isoquinolines and its condensed analogues has been developed via the palladium-catalyzed cyclization/C-H activation of N-allyl and N-methallyl derivatives of benzimidazoles.     

Solvent-free reaction between acenaphthoquinone, various benzils and ammonium acetate: synthesis of 9,10-diaryl-7H-benzo[d,e]imidazo[2,1-a]isoquinolin-7-ones

A simple synthesis of benzo[d,e]imidazo[2,1-a]isoquinolines is described. Heating a mixture of acenaphthoquinone, a benzil, and ammonium acetate under solvent-free conditions afforded 9,10-diaryl-7H-benzo[d,e]imidazo[2,1-a]isoquinolin-7-ones in good to excellent yields.     

Synthesis of annulated dihydroisoquinoline heterocycles via their nitrogen ylides

3,4-Dihydro-6,7-dimethoxyisoquinoline-1-acetonitrile reacts with some α-bromoketones in dry benzene to give the corresponding isoquinolinium salts, which undergo intramolecular cyclization to give pyrrolo[2,1-a]isoquinolines. Cross-coupling of the latter compounds with some aryldiazonium chlorides resulted in the formation of 3-arylhydrazonopyrrolo[2,1-a]isoquinolines, 3-arylazopyrrolo[2,1-a]isoquinolines and 3-aryl-1,2,3-triazolo[5,1-a]isoquinolines, respectively. The structures of the products were established on the basis of their elemental and spectral analyses as well as X-ray single crystal studies.     

Synthesis of pyrrolo[2,1-a]isoquinolines from activated acetylenes, benzoylnitromethanes, and isoquinoline

Isoquinoline reacts smoothly with aroylnitromethanes in the presence of dialkyl acetylenedicarboxylates and dibenzoylacetylene to produce pyrrolo[2,1-a]isoquinolines in good yields. Quinoline and benzothiazole also react in a similar way. Pyridine and N-methylimidazole catalyze the reaction between nitromethane derivatives with electron-deficient acetylenes to produce highly functionalized isoxazoles.     

Selective synthesis of benzo[4,5]imidazo[2,1-a]isoquinolines via copper-catalyzed tandem annulation of alkynylbenzonitriles with 2-Iodoanilines

An efficient copper-catalyzed cascade cyclization reaction for selectively synthesizing a variety of benzo[4,5]imidazo[2,1-a]isoquinoline derivatives has been developed. The reaction features the formation of three different CN bonds in sequence. In the presence of Cu(OAc)₂ and KO^tBu, o-alkynylbenzonitriles and 2-iodoanilines proceeded smoothly to obtain the corresponding benzo[4,5]imidazo[2,1-a]isoquinolines in moderate to good yields.     

First total syntheses of the 1,2,3,4-tetrahydronaphtho[2,1-f]isoquinolines annoretine and litebamine

We describe here the first total synthesis of the two natural 1,2,3,4-tetrahydronaphtho[2,1-f]isoquinolines, annoretine and litebamine, from [2-(2-styrylphenyl)ethyl]methylcarbamic acid ethyl esters. The key steps were the Bischler-Napieralski cyclization to form the isoquinoline unit and photocyclization of the resulting 2-methyl-5-styryl-3,4-dihydro-2H-isoquinolin-1-ones to 2-methyl-3,4-dihydro-2H-naphtho[2,1-f]isoquinolin-1-ones.     

Synthesis of imidazolidin-4-one and 1H-imidazo[2,1-a]isoindole-2,5(3H,9bH)-dione derivatives of primaquine: scope and limitations

The synthesis of imidazolidin-4-one derivatives of primaquine as potential antimalarial agents is described. The target compounds were synthesized in three steps: (i) condensation of (±)-primaquine with N^α-protected amino acids, (ii) removal of the N^α-protecting group, and (iii) reaction of the N-acylprimaquine with a carbonyl compound: acetone, three cyclic ketones and veratraldehyde. Using 2-formylbenzoic acid in the third step afforded 1H-imidazo[2,1-a]isoindole-2,5(3H,9bH)-diones. All products were isolated in good to excellent yields. Whereas imidazolidin-4-ones were formed as mixtures of all possible diastereomers in equal amounts, 1H-imidazo[2,1-a]isoindole-2,5(3H,9bH)-diones were produced in a stereoselective fashion. The compounds hydrolyse very slowly (t_1/2 5-30 d) in pH 7.4 buffer to release primaquine. These primaquine derivatives are being submitted to biological assays, and preliminary results of their antimalarial activity are quite encouraging.     

Synthesis of the dibenzopyrrocoline alkaloid skeleton: indolo[2,1-a]isoquinolines and related analogues

The indolo[2,1-a]isoquinoline and pyrrolo[2,1-a]isoquinoline nuclei have been synthesized from N-benzylindole or ethyl 1H-indol-1-ylacetate and N-benzylpyrrole precursors, respectively. Firstly, at C-2 of either the indole or pyrrole nucleus, aromatic rings containing a carbonyl substituent ortho to the newly formed biaryl axis were introduced using the Suzuki-Miyaura coupling reaction. Thereafter, under basic conditions the nucleophile that formed at the acidic methylene protons of the N-benzylindole, ethyl 1H-indol-1-ylacetate or N-benzylpyrrole intermediate reacted with the internal aromatic carbonyl to yield (after the expulsion of water) the title compounds. For example, exposure of ethyl 2-(2-(2-formylphenyl)-1H-indol-1-yl)acetate to potassium tert-butoxide resulted in the formation of ethyl indolo[2,1-a]isoquinoline-6-carboxylate.     

7,8,9,10-Tetrahydropyrrolo[2,1-a]isoquinolines in the search for new indolizine derivatives

7,8,9,10-Tetrahydropyrrolo[2,1-a]isoquinolines were obtained by 1,3-dipolar cycloaddition reactions of their corresponding N-ylides with olefinic (acrylonitrile) and symmetrical or non-symmetrical acetylenic dipolarophiles (methyl/ethyl propiolate, dimethyl acetylenedicarboxylate). Also, stable 5,6,7,8-tetrahydroisoquinolinium dicyanomethylide was isolated and characterized by X-ray diffraction analysis.     

Synthesis of benzoindoloquinolizines via a Cu(I)-mediated C–N bond formation

An effective synthesis of the multi ring-fused benzoindoloquinolizines has been accomplished by Cu(I)-mediated and MW-assisted C–N_amide bond formation of benzo[a]quinolizin-4-ones. The deamination of tetrahydro-2H-pyrido[2,1-a]isoquinolines was also studied and was found to give benzoquinolizines. The benzo[a]quinolizin-4-ones were prepared based on the annulations of C-1 substituted 3,4-dihydroisoquinolines and azlactones.     

Reaction of <Emphasis Type="Italic">N</Emphasis>-aryl-3-oxobutanethioamides with 2-amino-1,3-thiazole and 2-amino-1,3-benzothiazole

N-Aryl-3-oxobutanethioamides react with 2-amino-1,3-thiazole (2-amino-1,3-benzothiazole) in acetic acid to give mixtures of 7-methyl-5H-[1,3]thiazolo[3,2-a]pyrimidine-5-thione (2-methyl-4H-pyrimido-[2,1-b][1,3]benzothiazole-4-thione) and 5-arylimino-7-methyl-5H-[1,3]thiazolo[3,2-a]pyrimidines (4-arylimino-2-methyl-4H-pyrimido[2,1-b][1,3]benzothiazoles), whose ratio depends on the nature of the aryl substituent in the initial butanethioamide.     

Polarized ethylene. V. Synthesis of 1-substituted indolizine,pyrazolo[1,5-a]pyridine,and their related compounds using methoxyethylene derivatives

The reaction of pyridinium or isoquinolinium N-ylides with methoxy-substituted ethylenes gave the corresponding indolizine and pyrrolo[2,1-a]isoquinoline derivatives bearing acetyl, aroyl, cyano, ester group at the 1-position in one step. Pyrazolo[1,5-a]pyridine, pyrazolo[1,5-a]quinoline, and pyrazolo[5,1-a]isoquinolines were also synthesized in good yields from the corresponding aromatic N-imines and methoxyethylene derivatives.     

Ionic liquid-catalyzed and microwave-assisted syntheses of pyrrolizine- and indolizinedione derivatives

3H-Pyrrolo[2,1-a]isoindole-2,5-diones and isoindolo[2,1-a]quinoline-5,11-diones were synthesized by intramolecular cyclization of N-[2-oxo-3-(triphenyl-??⁵-phosphanylidene)propyl]- and N-[2-(triphenyl-??⁵-phosphanylidene)acetyl]phthalimides, respectively, in the presence of ionic liquid ([bmim][BF₄], 10 mol %) as catalyst or under microwave irradiation.     

Synthesis and some properties of N-unsubstituted pyrrolo[2,1-c]-1,3-diazacycloalkano[1,2-a]pyrazinones

Reactions of methyl 2-(2-formyl-1H-pyrrol-1-yl)alkanoates with unsubstituted aliphatic 1,2-, 1,3-, and 1,4-diamines gave N-unsubstituted pyrrolo[2,1-c]-1,3-diazacycloalkano[1,2-a]-pyrazinones. Some of them show ring-chain tautomerism. Transformations of these compounds led to a number of novel heterocyclic systems: 2,10-dihydro-3H,5H-imidazo[1,2-a]-pyrrolo[1,2-d]pyrazines, 2,3,4,11-tetrahydro-6H-pyrrolo[1??,2??:4,5]pyrazino[1,2-a]pyrimidines, 1,2,3,5,6,10b-hexahydroimidazo[1,2-a]pyrrolo[2,1-c]pyrazines, 1,3,4,6,7,11b-hexahydro-2H-pyrrolo[2??,1??:3,4]pyrazino[1,2-a]pyrimidines, and 2,3,4,5,6,7-hexahydro-1H-pyrrolo[2,1-c]-[1,4,7]triazacycloundecin-8(9H)-one.     

Multicomponent reactions of pyridines, α-bromo carbonyl compounds and silylaryl triflates as aryne precursors: a facile one-pot synthesis of pyrido[2,1-a]isoindoles

Multicomponent reactions (MCRs) involving pyridines, α-bromo ketones, and silylaryl triflates as aryne precursors were investigated. The reactions could also be extended to isoquinoline or α-bromo ethyl acetate. Substituted pyrido[2,1-a]isoindoles or isoindolo[2,1-a]isoquinolines could be obtained from this routine, which may have potential applications in antitumor drugs and fluorescent material fields.     

Indolopyridines containing a bridgehead heteroatom. 12. Synthesis and reduction of 12-arylidene-5,6-dihydroindolo[2,1-a]isoquinolinium trifluoroacetates

Condensation of 5,6-dihydroindolo[2,1-a]isoquinoline with aromatic aldehydes in trifluoroacetic acid afforded 12-arylidene-5,6-dihydroindolo[2,1-a]isoquinolinium trifluoroacetates. Hydrogenolysis of these salts on rhenium heptasulfide at elevated temperature and hydrogen pressure yielded indolo[2,1-a]isoquinolines, while reduction with sodium borohydride gave 12-arylmethylindoloisoquinolines. Photoluminescence was found for some indolo[2,1-a]isoquinolines.     

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.     

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.