Formylation of pyrrolo-[1,2-<Emphasis Type="Italic">a</Emphasis>]pyrazines

The formylation of pyrrolo[1,2-a]pyrazines containing alkyl, aryl, or hetaryl substituents in positions 1 and 6 of the heterocycle has been studied. It has been shown that formylation of 1-phenyl-and 1-(2 thienyl)pyrrolo[1,2-a]pyrazine occurs selectively at the α-position of the pyrrole ring. In all of the remaining examples the reaction course depends on substituent, reagent ratio, and reaction time. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 88–93, January, 2008.     

Acylation of pyrrolo[1,2-a]pyrazines

The acylation of pyrrolo[1,2-a]pyrazines with acetic anhydride and the acid chlorides of various carboxylic acids has been studied. It has been shown that pyrrole[1,2-a]pyrazines are selectively acylated at the α-position of the pyrrole ring when it is free. Products of the condensation of 1-methylsubstituted pyrrolo[1,2-a]pyrazines have been obtained for the first time in the process of acetylation. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, 263–272, February, 2008.     

Pyrrolodiazines. 6. Palladium-catalyzed arylation, heteroarylation, and amination of 3,4-dihydropyrrolo[1,2-a]pyrazines

The palladium-catalyzed Suzuki cross-coupling reaction of arylboronic acids and 6-bromo- and 6,8-dibromo-3,4-dihydropyrrolo[1,2-a]pyrazines afforded 6-substituted and 6,8-disubstituted 3,4-dihydropyrrolo[1,2-a]pyrazines in good yields. Stille and Negishi coupling reactions have been used to prepare 6-heteroaryl-substituted derivatives in moderate yields by employing heteroaryl halides and 6-metalated 3,4-dihydropyrrolo[1,2-a]pyrazines as reaction partners. A variety of cyclic secondary amines have also been incorporated at position C-6 of 6-bromo-1-phenyl-3,4-dihydropyrrolo[1,2-a]pyrazine in the presence of the palladium catalyst Pd(2)(dba)(3) in conjunction with BINAP as ligand. This amination reaction is one of the few reported examples of such a palladium-catalyzed transformation on a pyrrole ring, although the reaction could not be extended to less nucleophilic amines.     

Nitration of Pyrrolo[1,2-<Emphasis Type="Italic">a</Emphasis>]pyrazines

We have synthesized nitro derivatives of pyrrolo[1,2-a]pyrazines using a nitrating mixture and acetyl nitrate. We have obtained the products of oxidation of the side chain of 1,6-substituted pyrrolo[1,2-a]-pyrazines. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1579–1586, October, 2005.     

Acylation of 3,4-Dihydropyrrolo[1,2-a]pyrazines

The direction of trifluoroacetylation with trifluoroacetic anhydride of 3,4-dihydropyrrolo[1,2-a]pyrazines containing an alkyl or aralkyl substituent in position 1 depends on both the structure of the 3,4-dihydropyrrolo[1,2-a]pyrazine starting materials and on the ratio of reagent:substrate. It may lead to both mono- and disubstituted products. Trifluoroacetylation of 1-methyl-3,4-dihydropyrrolo[1,2-a]pyrazines occurs at the methyl group. Acetylation of 3,4-dihydropyrrolo[1,2-a]pyrazines leads only to N-acetyl-substituted reaction products.     

Investigations in the imidazole field

It has been established that various inorganic and organic bases can be used for the cyclization of 1,2-dialkyl-3-(-oxoalkyl[aralkyl])benzimidazolium halides into derivatives of pyrrolo[1,2-a]benzimidazole. The action of strong bases on quaternary benzimidazolium salts gives O-betaines—intermediates in the closure of the pyrrole ring.For part XL, see [1].     

Pyrrolodiazines. 2. Structure and Chemistry of Pyrrolo[1,2-a]pyrazine and 1,3-Dipolar Cycloaddition of Its Azomethine Ylides

A new synthesis of the pyrrolo[1,2-a]pyrazine system from pyrrole is described. In light of the ab initio calculations carried out on this heterocyclic system some of its basic chemistry was investigated and included electrophilic substitution, addition of organolithium reagents, metalation with lithium diisopropylamide and subsequent reaction with electrophiles, and formation of salts by quaternization of the nonbridgehead nitrogen. N-ylides obtained from these salts undergo 1,3-dipolar cycloaddition with suitable dipolarophiles to give dipyrrolo[1,2-a]pyrazines, pyrazolo[1,5-a]-pyrrolo[2,1-c]pyrazines, and heterobetaines. Examples of intramolecular 1,3-dipolar cycloadditions are also reported.     

1-(Trifluoromethyl)-3,4-dihydropyrrolo-[1,2-<Emphasis Type="Italic">a</Emphasis>] pyrazines: synthesis and reactions with O- and N-nucleophiles

The method of synthesis has been developed and reactions of 1-(trifluoromethyl)-3,4-dihydro-pyrrolo[1,2-a]pyrazines with O- and N-nucleophiles have been studied. It was found that reactions of nucleophiles with 1-(trifluoro-methyl)pyrrolo[1,2-a]pyrazines transforms the trifluoromethyl group to amide and amidine groups together with aromatization of the pyrrolo[1,2-a]pyrazine system as a result of the formal elimination of a molecule of hydrogen fluoride.     

A new method for the synthesis of pyrrolo[1,2-a]pyrazines and pyrrolo[1,2-a] quinoxalines

A new method is proposed for the synthesis of pyrrolo[1,2-a]pyrazines and pyrrolo[1,2-a]-quinoxalines. By the alkylation of sodium derivatives of 2-acylpyrroles with -bromo carbonyl compounds or their acetals and subsequent treatment of the reaction products with ammonium acetate in acetic acid, a number of derivatives of pyrrolo[1,2-a]pyrazine, including the first member of the class, pyrrolo[1,2-a]pyrazine itself, have been obtained. Similarly, from 2-benzoylpyrrole and the dimethyl ketal of -bromocyclohexanone was obtained 4-phenyltetrahydropyrrolo[1,2-a]quinoxaline, which readily dehydrogenates in the presence of Raney nickel to form 4-phenylpyrrolo[1,2-a]quinoxaline.Translated from Khimiya Geterotsiklicheskikh Soedinenii, Vol. 6, No. 8, pp.1048–1050, August, 1970.     

Formylation of 3,4-Dihydropyrrolo[1,2-a]pyrazines

Formylation of 3,4-dihydropyrrolo[1,2-a]pyrazines containing alkyl or aryl substituents at position 1 has been studied under the conditions of the Vilsmeier reaction. The direction of the reaction depends on the structure of 3,4-dihydropyrrolo[1,2-a]pyrazine starting materials. Formylation of 1-methyl-substituted 3,4-dihydropyrrolo[1,2-a]pyrazines occurs at the methyl group.     

Formal fusion of a pyrrole ring onto 2-pyridyl and 2-pyrimidyl cations: one-step gas-phase synthesis of indolizine and its derivatives

Two ortho-hetarynium ions, the 2-pyridyl and 2-pyrimidyl cations, react promptly with 1,3-dienes in the gas phase by annulation, formally by fusion, onto the ions of a pyrrole ring. This novel reaction proceeds through an initial polar [4 + 2+] cycloaddition across the C[triple bond]N+ bond, followed by fast ring opening, a [1,4-H] shift, and finally a recyclization that results in a contraction of a six- to a five-membered ring and dissociation by the loss of a methyl radical. For the 2-pyridyl cation, this reaction yields ionized indolizines (pyrrolo[1,2-a]pyridines), while for the 2-pyrimidyl cation, it gives ionized pyrrolo[1,2-a]pyrimidines. The annulation reaction, performed in the rf-only collision quadrupole of a pentaquadrupole (QqQqQ) mass spectrometer, occurs readily with both 1,3-butadiene and isoprene, and is thermodynamically and kinetically favored as predicted by ab initio calculations. Ortho-hetarynium ions and 1,3-dienes provide, therefore, the two building blocks for the efficient one-step gas-phase synthesis of ionized bicyclic pyrrolo[1,2-a]pyridine (indolizine) and pyrrolo[1,2-a]pyrimidine, as well as their analogues and derivatives.     

Regioselective synthesis of pyrimido[1,2-a][1,3,5]triazin-6-ones via reaction of 1-(6-oxo-1,6-dihydropyrimidin-2-yl)guanidines with triethylorthoacetate: observation of an unexpected rearrangement

A novel thermal rearrangement, involving pyrimidine ring opening and subsequent ring closure leading to recyclization of the system, was identified in the reaction of (6-oxo-1,6-dihydropyrimidin-2-yl)guanidines 3 (where NR(1)R(2) = NH(2), NH alkyl, NH aralkyl, NHCH(2)Ph(R)) with triethyl orthoacetate, affording 4-substituted-2-methyl-6H-pyrimido[1,2-a][1,3,5]triazin-6-ones 6 and their ring opened products. However, no such rearrangement was observed with (6-oxo-1,6-dihydropyrimidin-2-yl)guanidines 3 bearing a tertiary amino or anilino substituent (i.e. where NR(1)R(2) = N(CH(3))(2), indoline, morpholino, NHAr). As expected, 2-substituted-4-methyl-6H-pyrimido[1,2-a][1,3,5]triazin-6-ones 4 were obtained as the final products. Experimental structural determination and theoretical studies were carried out to get an understanding of the observed thermal rearrangement. In addition, an attempt to obtain similar pyrimido[1,2-a][1,3,5]triazin-6-ones using N,N-dimethylacetamide dimethyl acetal (DMA-DMA) as one carbon inserting synthon had furnished triazine ring annulated product 14 bearing N,N-dimethyl enamino substituent at position 4 as a result of further reaction with a second molecule of DMA-DMA.     

Studies in the imidazole series

By quaternization of 1-alkyl(aryl, aralkyl)-2-methyl-4, 5-diarylimidazoles with -bromo ketones and by subsequent cyclization of the quaternary salts of imidazole, a number of arylpyrrolo[1,2-a]imida~ zoles were obtained. A study was made of the effect of different bases on the process of cyclization and the yield of pyrroloimidazole derivatives. The structure was established of certain intermediate products of the reaction involving the closure of the pyrrole ring.For Part XXXVI, see [11].     

Novel routes to 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines and 5,6,9,10,11,11a-hexahydro-8H-pyrido[1,2-a]pyrrolo[2,1-c]pyrazines

Condensation reactions of benzotriazole and 2-(pyrrol-1-yl)-1-ethylamine (1) with formaldehyde and glutaric dialdehyde, respectively, afforded intermediates 2 and 6. Subsequent nucleophilic substitutions of the benzotriazole group in 2 and 6 with Grignard reagents, sodium cyanide, and sodium borohydride gave 1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazines 3a-e, 4, 5 and 5,6,9,10,11,11a-hexahydro-8H-pyrido[1,2-a]pyrrolo[2,1-c]pyrazines 7a-c, 8, 9, respectively, in good yields.     

Synthesis of azepino[1,2-<Emphasis Type="Italic">a</Emphasis>]benzimidazoles and imidazo[1,2-<Emphasis Type="Italic">a</Emphasis>]azepines

Fusion of 4-bromo-1,3-diphenyl-2-buten-1-ones (γ-bromodypnones) with 1,2-dimethyl-1H-benzimidazole and further treatment of the reaction product with a base (morpholine) gives 7,9-diaryl-5-methyl5,10-dihydroazepino[1,2-a]benzimidazol-11-ium bromides. The reaction of γ-bromodypnone with 1-alkyl-2-methyl-1H-imidazoles in benzene at 25 °C gives quaternary azolium salts. Upon heating their solutions in alcohol in the presence of K₂CO₃ the latter cyclize to 1-R-6,8-diaryl-1,5-dihydroimidazo[1,2-a]azepin-4-ium bromides or 1-R-6,8-diaryl-1H-imidazo[1,2-a]azepines depending on the nature of the substituent in the benzene rings and the substituent at the N(1) atom of the imidazole.     

Diversity-oriented decoration of pyrrolo[1,2-a]pyrazines

Diversity-oriented synthesis of a chemical library based on a pyrrolo[1,2-a]pyrazine core is described by using palladium-catalyzed direct C6 arylation of pyrrolo[1,2-a]pyrazines with various aryl bromides. The starting materials, pyrrolo[1,2-a]pyrazines, were easily synthesized by the base-mediated N-alkylation of pyrrole-2-carboxaldehyde with several 2-bromoacetophenones followed by dehydrative cyclization with incorporation of nitrogen by the action of ammonium acetate. Introduction of other functional groups on this chemical scaffold is also discussed herein.     

One-pot synthesis of pyrrolo[1,2-a]pyrazines via three component reaction of ethylenediamine, acetylenic esters and nitrostyrene derivatives

An effective route to pyrrolo[1,2-a]pyrazines is described via reaction of ethylenediamine,acetylenic esters and nitrostyrene derivatives in the presence of 20 mol%of sulfamic acid.     

Synthesis,Structure, and Biological Activity of [2.2]Paracyclophane Derivatives. 8. α-Pyridyl([2.2]paracyclophan-4-yl)-phenylmethanol: Structure of the Complex with Cu(II) Chloride and Intramolecular Cyclization

The reaction of 2-benzoylpyridine with 4-([2.2]paracyclophanyl)lithium or of 4-benzoyl[2.2]paracyclophane with 2-pyridyllithium gave α-pyridyl([2.2]paracyclophan-4- yl)phenylmethanol. X-ray analysis has been used to study the molecular and crystalline structure of its complex with Cu(II) chloride. It was found that this triaryl-substituted methanol undergoes an intramolecular cyclocondensation in refluxing formic acid and involves the pyridine ring and the cyclophane substituent. Heterocyclization at the ortho-position of the latter gives 10-phenyl[2.2]paracyclophano[4,5-b]indolizine and cyclization at the pseudo-gem-position the 1-phenyl-1,1a-dehydro-6-aza[3.2.2](1,2,5)-6H-cyclophano[1,2-a]pyridine. The compounds prepared have luminescent properties. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 864–873, June 2005.     

Reactions of 2,3-Dioxopyrrolo[2,1-<Emphasis Type="Italic">a</Emphasis>]isoquinolines with Active N-Nucleophiles

It is shown that 2,3-dioxopyrrolo[2,1-a]isoquinolines react readily with aliphatic diamines and hydroxyamine with opening of the dioxopyrrole ring and formation of the corresponding bisenaminoketoamides and hydroxamic acids. Reaction with the thiosemicarbazide and hydrazides of aromatic acids proceeds without opening of the pyrrole ring at the ketone carbonyl. Derivatives of hexahydropyridazine are formed when compounds with carboxyethyl groups at position 1 react with hydrazine. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, 1383–1387, September, 2005.     

Investigations in the imidazole series LXXIII. Synthesis of pyrrolo[1,2-a]benzimidazole derivatives from 2-alkyl(aralkyl)benzimidazoles

The synthesis of 2-alkyl(aryl)- and 2-aryl-3-alkyl(aryl)-4-acylmethyl-substituted pyrrolo-[1,2-a]benzimidazoles was accomplished by the reaction of 2-alkyl(aralkyl)benzimidazoles with -haloketones and subsequent cyclization of the resulting 1,3-(diacylmethyl)-2-alkyl-(aralkyl)benzimidazolium halides.See [1] for communication LXXII.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 403–404, March, 1972.