Benzofuroxan, syntheses with

The formation of quinoxaline heterocyclic systems is a well-known transformation of benzofuroxanes, which occurs in the presence of /3-dicarbonyl compounds <2001RJ0891, 2003BMC2149, 2003EJM791, 2005JME2019>. For example, the synthesis of quinoxaline 1,4-di-jV-oxides was carried out by reaction of the appropriate benzofuroxane 69 with the corresponding /3-ketoester, using triethylamine as the catalyst (Scheme 15) <2005JME2019>. 

Benzofuroxan reacts with benzo[6]furan-3(2//)-oncs to give a product at an oxidation level lower than the expected di-lV-oxide in a process where benzo[A]furan-3(2//)-one plays the dual role of substrate as well as reductant during the reaction, and redox processes determine the structure of the products,This is a general, one-step synthesis of 2-(2-hydroxyaryl)-quinoxaline 4-oxidcs, c.g. formation of 3. ... 

Reduction of benzofuroxans is usually the most convenient route to benzofurazans and o-quinone dioximes (see Section VI, C). Reduction of 4-nitrobenzofuroxan would seem to be a method of synthesis of 1,2,3-triaminobenzene, while the haloalkoxy-substitution reaction (Section VTT,B) and the rearrangements of Section VIII provide compounds accessible only with difficulty by other methods. Apart from these reactions, the benzofuroxans appear to be of limited synthetic utility. 

There is an extensive literature on the use of 2,1,3-benzoxadiazole 1-oxide [often called benzofuroxan(e) (BFO) (462)] as a substrate for the primary synthesis of quinoxaline 1,4-dioxides and occasionally quinoxaline mono-V-oxides or even simple quinoxalines. Very few substituted derivatives of the parent substrate (462) have been employed in recent years. The general mechanism clearly involves a fission (usually amine-catalyzed) of the oxadiazole ring followed by reaction with an ancillary synthon. The following examples are divided according to the type of synthon employed. 

Another one-step method of similarly limited value for the synthesis of multiply substituted phenazines 84 is the Beirut reaction [80]. It involves the transformation of the easily accessible benzofuroxans 85 together with, in... 

Benzofuroxans are far more stable than simple furoxans and are more favourable for practical applications. There are two standard methods for the synthesis of the benzofuroxan skeleton (1) treating an ortho-nitroarylamine with a mild oxidant like sodium hypochlorite and (2) either heating or irradiating an orf/to-nitroarylazide with UV light. Benzofuroxan itself has also been prepared by treating 1,2-benzoquinone dioxime with alkaline hypochlorite or alkaline potassium ferricyanide solution. ... 

Examples include the synthesis of 3-amino-l,2,4-oxadiazoles starting from 3-acylamino-5-methyl-l,2,4-oxadiazole <2002H811> and 2-aryl-l,2,3-triazoles from l,2,4-oxadiazole-3-ketone arylhydrazones <1999T12885, 2006JOC5616>. Oximes, hydrazones, formamidines, and thioureas of the furazan series also undergo base-catalyzed mononuclear rearrangements <2004RCB1121>. Nucleophilic attack at N(3) takes place in the benzofuroxan series. For example, reaction with secondary amines leads to o-nitroarylhydrazines (Scheme 55). 

Enones react with benzofuroxan in the presence of a primary or secondary amine to afford specific quinoxaline monoxides the course of the reaction depends on the nature of the base. The usefulness of these reactions for synthesis are limited by low yields and the lack of universality for enones as substrates. 

Major variants on this method are the use of o-nitrosoaminobenzenes (Chapter IX) and benzofuroxans (Chapter IV) as substrates for reaction with a two-carbon component as illustrated in Scheme 2. The o-nitroso-aminobenzene-based synthesis has the advantage that it leads to products... 

Six articles cover general aspects of heterocyclic chemistry 1,3-dipolar cycloreversions, syntheses with arylnitrenes and a-metallated isocyanides, and photo-oxygenation of nitrogen heterocycles,while others deal with more specialized subjects, i.e. preparation and use of halogeno-lactones, aspects of the chemistry of furan, 1-hydroxy-indoles, ring-opening of azoles by the action of amines, " the use of 2-chlorobenzoxazolium (1) and other heterocyclic onium salts for dehydration and condensation reactions," the synthesis of monosub-stituted tetrathiafulvalenes (2), cycloadditions of azoles containing three heteroatoms,sydnone imines (3), the conversion of acyl-benzofuroxans into nitro-indazoles (cf. p. 199), and advances in the chemistry of pyrrolizidine " and indolizine." ... 

Instead of 1,2-diaminoarenes, 2-nitroanilines, 1,2-dinitroarenes, or benzofuroxanes (cf p. 253) can be reacted with 1,2-diketones to give quinoxalines in an SnCl2-promoted process, in which SnCh acts both as a reducing agent and as a catalyst for cydoconden-sation [311]. Instead of 1,2-dicarbonyl compounds, phenacyl bromides in the presence of DABCO and atmospheric oxygen have been used for 2-arylquinoxaline synthesis from 1,2-diaminoarene [312]. 

Scheme 5.2 Synthesis of crown ethers with diaminobenzene and benzofuroxane moieties...

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