Synthesis of condensed quinoxalines

A novel efficient synthesis of fluorescent, fused quinoxalines was achieved. 6-Triazolylthiazolo[4,5-b]quinoxalines were synthesized by the diazotisation of 6-amino-2-methylthiazolo[4,5-b]quinoxaline and coupling with selected aromatic amines followed by air oxidation. Diazotised aryl amines were coupled with 6-amino-2-methylthiazolo[4,5-d]quinoxaline followed by subsequent air oxidation afforded 1,2,3-triazolo[5,4-f]quinoxalino[2,3-d]thiazoles. 6-Amino-2-methylthiazolo[4,5-b]quinoxaline was condensed with conjugated enol ethers followed by cyclization in dowtherm resulted in thiazolo[4,5-b]quinoxalino[6,5-b]pyridine.     

Synthesis of 1-hydrazinopyridazino[4,5-b]quinoxaline and related compounds

This paper describes the synthesis of 1-hydrazinopyridazino[4,5-b]quinoxaline ( 10 ), tetrazolo[4,3-b]pyridazino[4,5-b]quinoxaline ( 11 ) and some 1,2,4-triazolo[4,3-b]pyridazino[4,5-b]quinoxalines 13 . Starting with 2-ethoxycarbonyl-3-methylquinoxaline 1,4-dioxide ( 1 ), 1,2-dihydro-1-oxopyridazino[4,5-b]quinoxaline ( 5 ) was prepared by three different ways: (a) chlorination of 1 in acetic acid gave 2-ethoxycarbonyl-3-dichloromethylquinoxaline 1,4-dioxide, which reacts with an excess of hydrazine to give about 60% of 5 ; (b) oxidation of 1 with selenium dioxide gave 90% of 2-ethoxycarbonyl-3-formylquinoxaline 1,4-dioxide ( 3 ), which reacts with hydrazine to give 5 (63%); (c) compound 3 was treated with hydrazine to give 1,2-dihydro-1-oxopyridazino-[4,5-b]quinoxaline 1,4-dioxide ( 4 ) (70%), which by reduction with sodium dithionite gave 5 (80%). Compound 5 reacts with phosphorus pentasulfide or the Lawesson reagent to give 1,2-dihydro-1-thiocarbonylpyridazino[4,5-b]quinoxaline ( 9 ), which treated with hydrazine gave 5 (80%). This last compound reacts with nitrous acid to give 11 . Some hydrazones 12 from 10 are described. Heating the aldehyde hydrazones 12a,c,d with dimethylsulfoxide some 1,2,4-triazolo[4,3-b]pyridazino[4,5-b]quinoxalines 13 were obtained. Compound 13a was also obtained in the reaction of 10 with benzoyl chloride. Reaction of 3 with phenylhydrazine gave 1,2-dihydro-1-oxo-2-phenylpyridazino[4,5-b]quinoxaline ( 6 ). Reactions of 5 with acetic anhydride and dimethylsulfate gave, respectively, 1-acetoxypyridazino[4,5-b]quinoxaline ( 8 ) and 1,2-dihydro-1-oxo-2-methylpyridazino-[4,5-b]quinoxaline ( 7 ). All the compounds were characterized by elemental analysis and ¹H-nmr spectra. Compounds 5 and 10 showed antihypertensive activity in rats.     

Synthesis of condensed quinoxalines. Part II

A novel efficient synthesis of fluorescent, fused quinoxalines was achieved. 7-Triazolyl-1,4-dioxino[2,3-b]-quinoxalines were synthesized by the diazotisation of 7-amino-1,4-dioxino[2,3-b]quinoxaline and coupling with selected aromatic amines followed by air oxidation. Diazotised aryl amines were coupled with 7-amino-1,4-dioxino[2,3-b]quinoxalines followed by subsequent air oxidation afforded 1,4-dioxino[2,3-b]quinoxalino-[6,5-d]1,2,3-triazoles. 7-Amino-1,4-dioxino[2,3-b]quinoxaline was condensed with conjugated enol ethers followed by cyclization in dowtherm resulted in 1,4-dioxino[2,3-b]quinoxalino[6,5-b]pyridines.     

Synthesis of 9-(trifluoromethyl)pyrido[1′,2′:1,2]-imidazo[4,5-b]quinoxalines

9-(Trifluoromethyl)pyrido[1′,2′:1,2]imidazo[4,5-b]quinoxalines (9-CF₃-PIQs) were obtained from the cyclization of 2-amino-3-chloro-6-(trifluoromethyl)quinoxaline ( 1a ) with some substituted pyridines. 3-[2-(4-Pyridyl)ethenyl]-9-CF₃-PIQ, one of thus obtained 9-CF₃-PIQs, cyclized with another molecule of 1a to produce the dihydro bis-PIQ-ethene derivative.     

The preparation of the benzo[4,5]cyclohepta[1,2-b]pyrazine and benzo[4,5]cyclohepta[1,2-b]quinoxaline systems

Benzo[4,5]cyclohepta[1,2-b]quinoxaIine 2 , benzo[4,5]cyclohepta[1,2-b]pyrazine 3a and benzo[4,5]cyclohepta[1,2-b]quinoxaline 4 were prepared from 4,5-benzotropolone and 1,2-phenylenediamine, ethylenediamine and 1,2-diaminocyclohexane, respectively. Compound 3a was methylated to 3b .     

Synthesis and Antimicrobial Activity of Novel Furothienoquinoxalines,Pyranothienoquinoxalines and Pyrimidopyranothienoquinoxalines

The reaction of ethyl‐3‐mercaptoquinoxaline‐2‐carboxylate with phenacyl bromide, ethyl chloroacetate, chloroacetonitrile or chloroacetone furnished the corresponding 3‐hydroxy thieno[2,3‐b]quinoxaline. 2‐Cyano‐3‐hydroxythieno[2,3‐b]quinoxaline and 2‐acetyl‐3‐hydroxythieno[2,3‐b]quinoxa line were employed as precursors in the synthesis of some novel furo[2′,3′:4,5]thieno[2,3‐b]quinoxaline, pyrano[2′,3′:4,5]thieno[2,3‐b]quinoxaline and other heterocyclic systems fused with thieno[2,3‐b]quinoxalines. The antibacterial and antifungal activities of some the synthesised compounds were studied.     

Synthesis of pyridazino[3,4-b]quinoxalines and pyrazolo[3,4-b]-quinoxaline by 1,3-dipolar cycloaddition reaction

The pyridazino[3,4-b]quinoxalines 6a,b and pyrazolo[3,4-b]quinoxaline hydrochloride 9 were synthesized by the 1,3-dipolar cycloaddition reaction of 6-chloro-2-(1-methylhydrazino)quinoxaline 4-oxide 5 with dimethyl or diethyl acetylenedicarboxylate and 2-chloroacrylonitrile, respectively. The reaction mechanisms were postulated for the formation of 6a,b and 9 .     

13C-NMR study of 4H-1,3,4-thiadiazino[5,6-b]quinoxalines. A proof for the N5-deuteration in deuteriotrifluoroacetic acid

The carbon signals of the 2-acylamino-4H-1,3,4-thiadiazino[5,6-b]quinoxalines 1a,b , 2-acylamino-4H-1,3,4-thiadiazino[5,6-b]quinoxaline 1,1-dioxides 2a,b , and 2-amino-4H-1,3,4-thiadiazino[5,6-b]quinoxaline 3 in deuteriodimethyl sulfoxide and in deuteriotrifluoroacetic acid were assigned by the nmr (HMBC, HMQC) spectroscopy. The comparison of the carbon chemical shifts in deuteriodimethyl sulfoxide with those in deuteriotrifluoroacetic acid clarified that compounds 1a, 1b , and 3 were deuterized at the N₅-position in deuteriotrifluoroacetic acid, while the 1,1-dioxides 2a,b did not undergo the N₅-deuteration in deuteriotrifluoroacetic acid.     

Synthesis of novel pyridazino[3,4-b]quinoxalines

The pyridazino[3,4-b]quinoxaline 12 was synthesized by the cyclization of the α-arylhydrazonoacyl-hydrazide 11. The reaction of compound 12 with phosphoryl chloride gave pyridazino[3,4-b]quinoxaline 13, whose reactions with sodium azide or cyclic secondary amines provided pyridazino[3,4-b]quinoxalines 14,17 and 18, respectively. The acylhydrazide 15 was also cyclized to pyridazino[3,4-b]quinoxaline 16.     

A comparison of nucleophilic reactions of 3-benzenesulfonyloxyalloxazine and its 1-methyl analog

Reactions of 3-benzenesulfonyloxyalloxazine ( 1a ) and its 1-methyl analog 1b with a number of nucleophilic reagents are reported. Relatively small nucleophiles, such as hydroxide ion, methanol, ethanol, methylamine, hydrazine and hydroxylamine converted 1a to 4-carboxy-s-triazolo[4,3-a]quinoxalin-1(2H)-ones and the corresponding esters or amides. As the size of the amine increased from methylamine to ethylamine, dimethylamine, propylamine and isopropylamine, there were obtained 4-(carboxamido)-s-triazolo[4,3-a]quin-oxalin-1(2H)-ones, (1-carboxamido)imidazolo[4,5-b]quinoxalines and 2,3-bis(ureido)quinoxalines. Sodium hydride or potassium cyanide in hot DMF degraded 1a to imidazolo[4,5-b]quinoxaline. However, methylmer-captide and benzylmercaptide ions attacked the sulfonate group of 1a to form 3-hydroxyalloxazine. 1-Methyl-3-benzenesulfonyloxyalloxazine ( 1b ) reacted with methanol, ethanol, 1-propanol, and to some degree 2-propanol, in the presence of triethylamine to furnish anhydro-1-hydroxy-3-methyl-4-(alkoxycarbonyl)-s-triazolo[4,3-a]quinoxalinium hydroxides. However, sodium methoxide in methanol converted this starting material to a mixture of anhydro-1-hydroxy-3-methyl-s-triazolo[4,3-a]quinoxalinium hydroxide and 1-methyl-3-hydroxyflavazole. A saturated aqueous solution of triethylamine transformed 1b to anhydro-1-hydroxy-3-methyl-s-triazolo[4,3-a]quinoxalinium hydroxide, apparently via the corresponding unstable 4-carboxylic acid. The reactions of 1b with a number of aliphatic amines yielded either amides based on the above mesoionic system or on the 3-carboxamido-2-quinoxalyl semicarbazide structure. The reaction of 1b with potassium cyanide furnished 1-methylimidazolo[4,5-b]quinoxaline. Mechanisms to explain all of the degradations are advanced.     

Synthesis of Imidazo[4,5-b]quinoxaline Ribonucleosides as Linear Dimensional Analogs of Antiviral Polyhalogenated Benzimidazole Ribonucleosides

We have recently found that 2,5,6-trichloro-1-(β-D-ribofuranosyl)benzimidazole (TCRB) and the corresponding 2-bromo analog have better in vitro activities against HCMV than the clinically used agents ganciclovir and foscarnet. These benzimidazole nucleosides act by a unique mechanism, however, their biological target has not been completely identified. As an approach to probing the target, we have designed imidazo[4,5-b]quinoxaline nucleosides as linear dimensional analogs of the benzimidazole nucleosides to study the spatial limitation of the binding site in the target enzyme. A convenient route was developed for the synthesis of 2-substituted 6,7-dichloroimidazo[4,5-b]quinoxalines involving a reaction of 2,3,6,7-tetrachloroquinoxaline with ammonia followed by a ring annulation as the key step. This furnished the versatile heterocycle 6,7-dichloroimidazo[4,5-b]quinoxalin-2-one. Ribosylation of 2-substituted imidazo[4,5-b]quinoxalines was influenced by the functional group at the 2-position and the 2-one compound was found to smoothly undergo ribosylation. The 2-one group of the nucleoside was converted into specifically selected 2-substituted compounds. Evaluation of the compounds for activity against two herpesviruses and for cytotoxicity showed they were less active and/or more cytotoxic than TCRB. We conclude therefore, that the binding pocket on the protein target of TCRB will tolerate some electronic and size changes.     

Synthesis of 2-styrylthiazolo[4,5-b]quinoxaline based fluorescent dyes

A novel efficient synthesis of 2-styrylthiazolo[4,5-b]quinoxaline based fluorescent dyes was achieved by the condensation of 2-alkylthiazolo[4,5-b]quinoxaline with selected 4-N,N-dialkylamino-substituted aryl-aldehydes or hetarylaldehydes in the presence of piperidine or acid anhydride. The coloristic, fluorophoric and dyeing properties of these dyes were studied.     

A convenient synthesis of 1,2,4-triazolo[4,3,2-o,p]-[1,3]diazocino[4,5-b]quinoxalines via a 1,3-dipolar cycloaddition reaction and a ring transformation

The reaction of 6-chloro-2-hydrazinoquinoxaline 4-oxide 5 with triethyl orthoformate gave 7-chloro-1,2,4-triazolo[4,3-a]quinoxaline 5-oxide 6. The reaction of compound 6 with phenyl isocyanate afforded 7-chloro-4-phenylamino-1,2,4-triazolo[4,3-a]quinoxaline 7 , while the reaction of compound 6 with phenyl isothiocyanate resulted in deoxygenation to provide 7-chloro-1,2,4-triazolo[4,3-a]quinoxaline 8. However, the reaction of compound 6 with allyl isothiocyanate effected the 1,3-dipolar cycloaddition reaction, but not deoxygenation, to furnish 9-chloro-4,5-dihydroisoxazolo[2,3-a][1,2,4]triazolo[3,4-c]quinoxalin-5-ylmethylisothiocyanate 9. Moreover, the reduction of compound 9 with iron/acetic acid resulted in ring transformation to give 11 -chloro-7-hydroxy-4-thioxo-4,5,6,7,8,9-hexahydro-1,2,4-triazolo[4,3,2- o,p][1,3]diazocino[4,5-b]quinoxaline 10 , whose acetylation afforded 5-acetyl-11-chloro-7-hydroxy-4-thioxo-4,5,6,7,8,9-hexahydro-1,2,4-triazolo[4,3,2-o,p][1,3]diazocino[4,5-b]quinoxaline 11.     

A convenient synthesis of 5,14-methano-16-oxo-1,5,6-benzoxadiazonino[3,4-b]quinoxalines utilizing a 1,3-dipolar cycloaddition reaction and an intramolecular alcoholysis

The reaction of the hydrazones 5a-c with 2-chloroacrylonitrile produced the 1,2-diazepino[3,4-b]quinoxaline hydrochlorides 6a-c , which were transformed into the 5,6,7,13-tetrahydro-5,14-methano-16-oxo-1,5,6-benzoxadiazonino[3,4-b]quinoxalines 7a-c , respectively. The oxidation of 7a-c with diethyl azodicarboxylate afforded the 7,13-dihydro-5,14-methano-16-oxo-1,5,6-benzoxadiazonino[3,4-b]quinoxalines 8a-c , respectively. Compounds 7a-c and 8a-c were also obtained by a one-pot synthesis from 5a-c and 6a-c , respectively.     

Synthesis of 4-hydrazino-5H-pyrimido[5,4-b]indole and related compounds

This paper describes the synthesis of two 4-amino-5H-pyrimido[5,4-b]indoles 5 , 4-hydrazino-5H-pyrimido[5,4-b]indole 6 , two 1,2,4-triazolo[4,3-c]pyrimido[5,4-b]indoles 8 , and tetrazolo[4,5-c]pyrimido[5,4-b]indole 10 . Starting with ethyl 3-aminoindole-2-carboxylate 1 , 5H-pyrimido[5,4-b]indol-4-one 2 was obtained (80%) by condensing with formamide. Reactions of 2 with phosphorus oxychloride and phosphorus pentasulfide gave respectively, 4-chloro-5H-pyrimido[5,4-b]indole 3 (70%) and 5H-pyrimido[5,4-b]indole-4-thione 4 (80%). Compound 3 reacted with amines (morpholine, piperidine) to give the respective 4-amino-5H-pyrimido[5,4-b]-indoles 5 , and compound 4 reacted with hydrazine to give 4-hydrazino-5H-pyrimido[5,4-b]indole 6 (80%). Two hydrazones of 6 (benzylidene, isopropylidene) 7 were also prepared (90%). Compound 6 reacted with formic and acetic acids to give (65–75%) the respective 1,2,4-triazolo[4,3-c]pyrimido[5,4-b]indoles 8 and with nitrous acid to give tetrazolo[4,5-c]pyrimido[5,4-b]indole 9 (85%). All the new compounds 2 to 9 were characterized by elemental analysis and spectral data (ir, nmr).     

A convenient synthesis of novel pyridazino[3,4-b]quinoxalines

Novel 4-chlorophenylhydrazono-3-oxo-1,2,3,4-tetrahydropyridazino[3,4-b]quinoxalines 10a-c were synthesized by the cyclization of the α-hydrazonohydrazides 8a-c. The chlorination of 10a with phosphoryl chloride afforded 3-chloro-4-[2-(o-chlorophenyl)hydrazino]pyridazino[3,4-b]quinoxaline 12.     

Fused polycyclic nitrogen-containing heterocycles 20. Thiazolo[3,4-<Emphasis Type="Italic">a</Emphasis>]quinoxalines from 4-hydroxy-2-phenyliminothiazolidines and C-substituted 1,2-phenylenediamines

The condensation of 4-hydroxy-3,5-diphenyl-2-phenyliminothiazolidine with 4,5-dimethyl-1,2-phenylenediamine affords 7,8-dimethyl-3-phenyl-1-phenyliminothiazolo[3,4-a]quinox-alin-4(5 H)- one; the condensation with 1,2-phenylenediamines containing different substituents at positions 4 and 5 gives both theoretically possible isomeric thiazolo[3,4-a]quinoxalines, which differ in the distribution of these substituents between positions 7 and 8 in the benzene ring of the quinoxaline system. 3a-Hydroxy-7,8-dimethyl-3-phenyl-l-phenylimino-3,3a-di-hydrothiazolo[3,4-a]quinoxalin4(5 H)- one was isolated and characterized as the intermediate of the reaction giving rise to thiazolo[3,4-a]quinoxaline from 4,5-dimethyl-1,2-phenylene-diamine. This intermediate is a covalent hydrate of the final product.     

Electrochemical properties of pyrido[1′,2′:1,2]imidazo[4,5-b]pyrazine and pyrido[1′,2′:1,2]imidazo[4,5-b]quinoxaline derivatives

The peak potentials (Ep) of 3-substituted pyrido[1′,2′:1,2]imidazo[4,5-b]pyrazine and pyrido[1′,2′:1,2]-imidazo[4,5-b]quinoxaline derivatives are sufficiently correlated with Hammett substituent constant ～_m and with the PM3 calculated LUMO energy levels, and the linear relationship between electron potentials of 9-substituted pyridoimidazoquinoxalines and the LUMO energy levels is also found out.     

A facile synthesis of novel 1,2-diazepino[3,4-b]quinoxalines by a 1,3-dipolar cycloaddition reaction

The 1,3-dipolar cycloaddition reaction of the quinoxaline 4-oxides 4a,b with 2-chloroacrylonitrile gave the 2,3-dihydro-1H-1,2-diazepino[3,4-b]quinoxalines 5a,b , respectively, which were converted into the 2,3,4,6-tetrahydro-1H-1,2-diazepino[3,4-b]quinoxalines 7a,b and 8a,b , respectively.     

A selective synthesis of 2-arylamino-4H-1,3,4-thiadiazino[5,6-b]-quinoxalines and mesoionic triazolo[4,3-a]quinoxaline

The reaction of the 6-chloro-2-(1-methyl-2-thiocarbamoylhydrazino)quinoxaline 4-oxides 3a-d with trifluoroacetic anhydride gave the 2-(N-aryl)trifluoroacetamido-8-chloro-4-methyl-4H-1,3,4-thiadiazino-[5,6-b]quinoxalines 7a-d , respectively, while the reflux of compounds 3a-c in N,N-dimethylformamide afforded the mesoionic triazolo[4,3-a]quinoxaline 4 . Hydrolysis of compounds 7a-d with triethylamine/water provided the 2-arylamino-8-chloro-4-methyl-4H-1,3,4-thiadiazino[5,6-b)]quinoxalines 8a-d , respectively.