Oxalin

A number of thermally stable polymers have been synthesized, but in general the types of stmctures that impart thermal resistance also result in poor processing characteristics. Attempts to overcome this problem have largely been concentrated on the incorporation of flexible groups into the backbone or the attachment of stable pendent groups. Among the class of polymers claimed to be thermally stable only a few have achieved technological importance, some of which are polyamides, polyimides, polyquin oxalines, polyquinolines, and polybenzimidazoles. Of these, polyimides have been the most widely explored. 

Fluorinated and iodinated derivatives are usually prepared by halogen exchange reactions, although the Baltz-Schiemann reaction has been applied to the synthesis of 2-fluoroquin-oxaline (66JHC435>. 

Finally the preformed 2-formyl-3-aminoquinoxaline (448) gave the pyrido[2,3-f ]quin-oxalin-2-one (449) with activated esters/sodium alkoxide (79ZC422). 

Little work has been reported on pyrazine-2,3-diones, however spectral evidence has been advanced by HonzL to support the dioxo formulation 126. Dioxo structures have been assigned to quin-oxaline-2,3-dione (127) and its 1-methyl analog on the basis of ultraviolet and infrared spectral data, and according to... 

Amino-pyrazines and -pyridazines have been shown to exist predominantly in the amino form by infrared spectroscopic studies (cf. Table VI). Ultraviolet spectral data have been interpreted to indicate that 4-aminocinnoline exists predominantly in the imino form 256, but this conclusion, which was based on comparison of its spectrum with those of cinnolin-4-one and 4-ethoxycinnoline, is probably incorrect. Ultraviolet spectroscopic data strongly support the predominance of amino structures for 2-aminopyrazine (257) and 2-aminoquin-oxaline how ever, the former compound was at first erroneously concluded to exist in the imino form from ultraviolet spectral evidence. Isolation of two isomers of 2-amino-8-dimethylamino-3-methylphenazine, assigned the amino and imino structures 258 and 259, respectively, has been claimed, but it is very unlikely that these assignments are correct. 

The displacement of the two a-chloro atoms of 2,3-dichloroquin-oxalines may be carried out in a stepwise manner (Scheme 3). Thus 2,3-dichloroquinoxaline has been converted into 2-amino-, 2-anilino-,... 

Attempts to isolate 1,4-dihydroquinoxalinc itself were not successful, but the polarographic behavior of quinoxaline and 6-substituted quin-oxalines in buffered aqueous media suggests that in all cases reduction stops at the 1,4-dihydro stage/ - 2,3-Dimethylquinoxaline and 2-d-araho-tetrahydroxybutylquinoxaline show similar polarographic be-havior, ... 

The infrared spectra of twelve di-, tri-, and tetra-substituted quin-oxalines have also been reported " ... 

Many engineering thermoplastics (e.g., polysulfone, polycarbonate, etc.) have limited utility in applications that require exposure to chemical environments. Environmental stress cracking [13] occurs when a stressed polymer is exposed to solvents. Poly(aryl ether phenylquin-oxalines) [27] and poly(aryl ether benzoxazoles) [60] show poor resistance to environmental stress cracking in the presence of acetone, chloroform, etc. This is expected because these structures are amorphous, and there is no crystallinity or liquid crystalline type structure to give solvent resistance. Thus, these materials may have limited utility in processes or applications that require multiple solvent coatings or exposures, whereas acetylene terminated polyaryl ethers [13] exhibit excellent processability, high adhesive properties, and good resistance to hydraulic fluid. 

Morpholino-3-[4-phenyl(thiosemicarbazido)carbonyl]quinoxaline (129) gave 2-morpholino-3-(4-phenyl-5-thioxo-5,6-drhydro-4//-l,2,4-triazol-3-yl)quin-oxaline (130) (2M NaOH, reflux, 3 h 35%), 2-(5-anilino-l,3,4-thiadiazol-2-yl)-3-morpholinoquinoxaline (131, X = S) (98% H2SO4, 0°C, 1 h, then 20°C, 12 h 89%), or 2-(5-anilino-l,3,4-oxadiazol-2-yl)-3-morpholinoquinoxaline (131, X = O) (NaOH, H2O, EtOH, I/Kii dropwise, 5°C 95°C, 4 h 81%) analogs of all three products were made similarly. ... 

Kurasawa et al. carried out some transformations with the a,/3-unsaturated ester-containing tetrazolo[l,5- ]quin-oxaline moiety 88 (Scheme 16) <1994JHC233>. This compound underwent azo couplings with 2-(iV-methyl)-pyrazinyldiazonium salt in the methylene position to yield hydrazones 89 in high yield. 

Auch mit Phenolen bzw. Phenolaten reagiert Benzofurazan-1-oxid unter Bildung von Chin-oxalin-1,4-bis-oxiden, die auch mit Resorcin, Hydrochinon und 1,4-Benzochinon erhalten wer-dcn. Mit einer Mischung aus einem Chinon und Trialkylphosphan werden die blauvioletten 1 -Trialkylphosphoniono-phenazin-5,10-bis-oxid-2-olate312 erhalten. 

New metal complexes other than Ru have also been investigated. These include Fe complexes [117,118], Os complexes [119-122], Re complexes [123], and Pt complexes [124]. A nanocrystalline 2 solar cell sensitized by a square-planar platinum(II) complex containing 4,4 -dicarboxy-2,2 -bipyridine and quin-oxaline-2,3-dithiolate ligands showed an efficiency of 2.6% (Jsc = 6.14 mA/cm2... 

Following extraction/cleanup, quinoxaline-2-carboxylic acid can be detected by electron capture, or mass spectrometric techniques, after gas chromatographic separation on capillary or conventional columns. A prerequisite of quin-oxaline-2-carboxylic acid analysis by gas chromatography is the derivatization of the molecule by means of esterification. Esterification has been accomplished with methanol (419, 420, 422), ethanol (421), or propanol (423) under sulfuric acid catalysis. Further purification of the alkyl ester derivative with solid-phase extraction on a silica gel column (422), thin-layer chromatography on silica gel plate (420), or liquid chromatography on Hypersil-ODS, 3 m, column (423), has been reported. 

Similar rearrangements have been observed on irradiation of quin-oxaline mono- (LXXXIII) and di-X-oxides (LXXXI) in dilute aqueous solution or dilute sulfuric acid,169 of a-9-acridyl-JV -phenylnitrone (LXXXV) moistened with acetone,10 and of LXXXVIT178 forming LXXXII, LXXXIV, LXXXVI, and LXXXVIII, respectively. 

Mixtures of isomeric di-ALoxides are generally obtained when 5(6)-substituted benzofurazan 1-oxides (51) are used in the Beirut reaction52-54 however, only 7-substituted 2-cyano-3-phenylquin-oxaline 1,4-dioxides (52) were isolated from benzoylacetonitrile (PhCOCH2CN).55 The di-AAoxide mixtures obtained from reaction with acetonyl methyl sulfide (MeCOCH2SMe) were analyzed by H nuclear magnetic resonance (NMR) analysis of the mixed hydroxamic esters (53) formed by hydrolysis and methylation of the primary products.52... 

Various peracids have been used for the N-oxidation of quinoxalines, but the most convenient reagent for many 2-substituted and 2,3-disub-stituted quinoxalines is 30% aqueous hydrogen peroxide in acetic acid. With this reagent, oxidation has been carried out at temperatures ranging from 20° to 90°.108-112 For example, 2-methyl-3-phenylquin-oxaline (91) when treated with peroxide and acetic acid at 56° for 14 hours yields a mixture of the 1-oxide (92) and the 1,4-dioxide (93).112... 

Peracetic acid oxidation of 2-carbamoylquinoxaline (94) at 20°-25° gives the monoxides 95 and 96, and at higher temperatures the 1,4-dioxide (97) is isolated in 50% yield, together with a small amount of the 1,4-dioxide of 2-amino-3-quinoxalinone.m However, Hayashi and co-workers report the isolation of only 96 from 94 using monoperphthalic acid in ether <10°.109 In their attempt to correlate the nature of 2-substitution with the formation of 1- versus 4-oxides, they examined the behavior of some 2-alkyl substituted quinoxalines 113,114 2-ethylquin-oxaline gives the 1- and 4-oxides and the 1,4-dioxide, 2-isopropylquin-oxaline yields the 4-oxide and the 1,4-dioxide however, 2-f-butylquin-oxaline only furnishes the 4-oxide because of steric hindrance.114 The N-oxidation of 2-phenyl- and 2-alkyl-3-phenyl-quinoxalines with monoperphthalic acid furnishes the products shown in Table 1.114... 

Tennant and Livingstone have reported the preparation, and some substitution reactions, of l-acetoxy-3,4-dihydro-3-oxo-2-phenylquin-oxalinium perchlorates (e.g., 109). With sodium acetate, 109 gives the 6-acetoxyquinoxalinone 110 via an intermediate tetrahydroquin-oxaline.121... 

Quinoxaline and 2-methylquinoxaline form 1 1 adducts (111) with diphenylcyclopropenone,122 and an analogous pyrrolo[l,2-a]quin-oxaline has been isolated from the reaction of quinoxaline with di-phenylcyclopropenethione.123... 

Methylquinox aline reacts with dimethyl acetylenedicarboxylate to give a mixture of the azepino[l,2-a]quinoxalines 119 and 120, product formation involving migration of an ester group. 2,3-Dimethylquin-oxaline yields the azepinoquinoxaline 121 and a second product, 122, the structure of which was not proved unambiguously and which may... 

Chlorination of 5-hydroxyquinoxaline 1,4-dioxide in methylene chloride gives the 6,8-dichloro derivative, but reaction with N-chlorosuccinimide yields 8-chloro-5-hydroxyquinoxaline dioxide. Bromination in acetic acid gives the 6,8-dibromo derivative.195 15 Side-chain bromination is observed, however, when 2,3-dimethylquinoxaline 1,4-dioxide reacts with bromine in dioxane 2,3-bis(bromomethyl)quin-oxaline 1,4-dioxide is formed.195c... 

Elina and co-workers have extensively investigated the reactions of 2-methylquinoxaline 1,4-dioxide and 2,3-dimethylquinoxaline 1,4-dioxide with aldehydes.206 Under alkaline conditions formalin gives hydroxy-ethyl derivatives, and aryl aldehydes form styrylquinoxalines. As expected, 2,3-dimethylquinoxaline 1-oxide yields 3-methyl-2-styrylquin-oxaline 1-oxide.207... 

The lactone 223 was isolated from 2-benzoyl-3-hydroxymethylquin-oxaline 1-oxide 221 on treatment with alkali and subsequent acidification, probably by intramolecular Cannizzaro reaction of 2-benzoyl-3-quinoxaline carboxyaldehyde (222), which in turn is obtained from 221 by a series of tautomeric shifts and 1,4-elimination.221... 

The UV spectrum of 1-methylquinoxalium iodide in dilute aqueous alkali at pH 10.5 shows absorption maxima at 301 and 340 nm, and in methanolic sodium methoxide, maxima at 304 and 344 nm. The two maxima in aqueous alkali are attributed to the existence of an equilibrium mixture of the pseudobase 251 and the tetrahydroquin-oxaline 252. The pseudobase is the species that gives rise to the longer wavelength absorption maximum at 340 nm. It is formed by nucleophilic attack of hydroxide ion at C-2 in aqueous alkali, and the tetra-hydroquinoxaline is the result of covalent addition of water across the C3-N4 double bond of the pseudobase.251 A similar explanation is advanced for the two maxima observed in methanol with methoxide. 

There are two possible furoquinoxaline systems, the furo[2,3-6]quinoxaline (201) and the furo[3,4-6]quinoxaline (202). Both types are known. The synthesis of furo[2,3-fe]quin-oxalines has been accomplished by ring closure of suitable 1,4-dicarbonyl units (equation 10) (73CHE239,73MIP31700,79MI31702) which may be employed in a masked form, e.g. (203 ... 

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