Nitro ring closure, oxidative

Reaction of nitromalon-bis-A -methylanilide (105) with sulfuric acid gives A -methylisatin- -oxime (107) and not 4-methylquinoxalin-3-one 1-oxide (108) as originally suggested. This transformation may involve a Beckmann-type rearrangement of the protonated aci-nitro compound (106) prior to dehydrative ring closure. ... 

The base-catalysed reaction of a-bromo-a,P-unsaturated ketones with aliphatic nitro compounds leads to 2-isoxazoline A-oxides by tandem conjugate addition-ring closure (Scheme 5) <95JOC6624>. A -Acyl-3-isoxazolin-5-ones are transformed into oxazoles by photolysis or by flash vacuum pyrolysis (Scheme 6) <96TL675>. 

Adjacent 2-azido and 3-nitro groups facilitate the elimination of nitrogen followed by ring closure to form the corresponding furazan iV-oxide compound 342 (Equation 77) <1996JOC5801, 2005AGE7089>. 

Amino-5-nitro-2,l-benzisothiazole is manufactured by thiolysis of 2-amino-5-nitrobenzonitrile and oxidative ring closure. With aniline coupling components, shades from navy to greenish blue are obtainable (e.g., 28 and 29). 

The synthesis of furoxans has been discussed in a recent review (81AHC(29)25l), and the material summarized in the following sections is cited therein unless otherwise stated. The principal routes comprise oxidative ring closure of a-dioximes dehydration of a-nitro ketone oximes and dimerization of nitrile oxides. In choosing the method the possibility of equilibration between the 2- and 5-oxides of asymmetrically substituted furoxans, which was described in Section 4.22.3.2.1, must be taken into account. There has been no case of direct oxidation of a furazan to a furoxan. 

This ring closure takes place readily whenever the carbonyl and amino groups occur in the relative positions shown above. Reduction of o-nitro-phenylacetonitrile by stannous chloride produces indole rather than the corresponding amino aldehyde. The synthesis is most useful for the preparation of indole-2-carboxylic acid by reduction of o-nitrophenyl-pyruvic acid with ferrous sulfate and ammonia or with sodium hydrosulfite. The ethyl ester is obtained by a similar reduction with zinc and acetic acid or by catalytic hydrogenation of ethyl o-nitrophenyl-pyruvate over platinum oxide catalyst. ... 

Mild oxidation converts the pyridylethylamine (299) into pyrazolo[l,5-a]pyridine (1) (57JCS4510). The useful ring closure of the amine on to the nitro group of (300) proceeds as shown in Scheme 89. Similar procedures provide the pyridopyrazolobenzimidazole (301) and the pyrazolopyridines of Scheme 90 (73JCS(P1)319). However, under different conditions... 

Cyclization of amidines 1 to the corresponding quinoxalin-3-amine 1-oxides 2 can be readily achieved by reaction with bases. This ring closure occurs via deprotonation of the benzyl group and intramolecular nucleophilic addition to the nitro group giving structurally definite compounds, in contrast to the mono-A -oxidation reaction of the parent hcterocycles (sec Section6.3.2.1.4.1.). ... 

An oxidative ring opening with subsequent ring closure is the result of an attempted nitration of l,3-dimethyl-6,8-dihydroxypyrido[3,2-d]pyrimidine-2,4(177,377)-dione, which gives 1,3-dimethylimidazolidinetrione instead of the desired 7-nitro compound.443... 

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