Aci-Nitro -

Nitrones or aci-nitro esters react with alkenes to give in some cases A/-substituted isoxazolidines and in others 2-isoxazolines. When the intermediate isoxazolidines were observed, a number of procedures transformed them into the 2-isoxazolines. Acrylonitrile and phenyl rzcf-nitrone esters produced an A/-methoxyisoxazolidine. Treatment with acid generated a 2-isoxazole while treatment with base generated an oxazine (Scheme 118) (68ZOR236). When an ethoxycarbonyl nitrone ester was reacted with alkenes, no intermediate isoxazolidine was observed, only A -isoxazolines. Other aci-mtro methyl esters used are shown in Scheme 118 and these generate IV-methoxyisoxazolidines or A -isoxazolines which can be further transformed (72MI41605). 

The trimethylsilyl group has been used to prepare stable aci-nitro esters and these react with alkenes to produce intermediate isoxazolidines which were readily converted into 2-isoxazolines (Scheme 119) (73ZOB1715, 74DOK109, 78ACS(B)ll8>. 

The intermediacy of an aci-nitro compound has been proposed for the sulfuric acid cyclization of o-nitrophenylacetic acid to yield a mixture of 2,1-benzisoxazole and 2,1-benzisoxazole-3-carboxylic acid. The acid does not decarboxylate under the reaction conditions. The proposed aci-nitro intermediate cyclized to an A/ -hydroxy compound which decomposed to the products (Scheme 179) (70JCS(C)2660). 

Metal Fulminates or aci - nitro salts Fluorodinitromethyl compounds... 

The sequence (81 84) has been proposed - to account for this process which involves decomposition of the aci-nitro anion by strong acid. The a-carbonyl group presumably stabilizes the aci-salt and thus could be responsible for inhibiting the normal Nef reaction. A similar transformation has been observed in the case of a 16-nitro-17-0X0 steroid. 

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. ... 

Finally, we can see that, neutral meolecules, either in nitro-type or in aci-nitro-type, are more stable than acid-dissociated anions the anion formation is a high endothermic reaction. The energy difference between neutral molecules and acid-dissociated anions calculated at the MP2/6311+-I-G level is 1539 kJ/mol for nitro-type species, and 1683 kJ/mol for aci-nitro-type species. It is clear that, in these conditions, the acid dissociation of the neutral molecules can hardly occur.in pure nitromethane solutions. It provides another theoretieal support for nitromethane as an ideal model of aprotic solvents. 

In either neutral molecules or acid-dissociated anions, the nitro-type species are more stable than the aci-nitro-type species. The 1,3-intramolecular hydrogen rearrangment is a high barrier process. In the tautomeric system formed via the 1,3-hydrogen shift, the equilibrium is therefore strongly displaced to the side of nitro-type species. 

The salt (possibly an aci-nitro salt) explodes at 154-158°C and is less stable than the di-potassium salt (241-245°C). 

Crude 2-nitrotoluene, containing some hydrochloric and acetic acids, was charged into a vacuum still with flake sodium hydroxide to effect neutralisation prior to distillation. An explosion occurred later. Similar treatments had been used uneventfully previously, using the weaker bases lime or sodium carbonate for neutralisation. It seems likely that the explosion involved formation and violent decomposition of the sodium salt of an aci-nitro species, possibly of quinonoid type. 

This is prepared from 2-nitrochlorobenzene and sodium disulfide in ethanol, and the reaction residue after recovery of the ethanol by steam distillation is known to be of limited thermal stability. On one occasion, a container (atypically with closed vents) of bulked residues totalling over 2 t exploded violently while awaiting collection for disposal. It seems remotely possible that an aci-nitro thioquinonimine species derived from 2-nitrobenzenethiol may have been involved. 

Interesting intramolecular cyclization of 1-nitroalkyl radicals generated by one-electron oxidation of aci-nitro anions with CAN is reported. As shown in Eq. 5.44, stereoselective formation of 3,4-functionalized tetrahydrofurans is observed.62 l-Nitro-6-heptenyl radicals generated by one electron oxidation of aci-nitroanions with CAN afford 2,3,4-trisubstituted tetrahydropyrans.63 The requisite nitro compounds are prepared by the Michael addition of 3-buten-l-al to nitroalkenes. 

However, if protons at the a-carbon atom of the O-alkyl fragment are absent, another process can occur resulting in elimination of olefin and generation of the respective aci-nitro compound (Eq.2). In particular, this anomalous decomposition was found by Nenitzescu and Isacescu (234) for nitronate 74a (Eq. 3). 

The UV spectra of nitronates, which are not functionalized at the a-C atom, have an intense absorption at 230 to 240 nm, which is very similar in characteristics to UV absorption of salts of nitro compounds and solutions of aci-nitro compounds in protic solvents. Since standard alkyl- or silyl nitronates cannot have ionic structures, the presence of the above mentioned absorption in the UV spectra of nitronates, unambiguously confirms, that these compounds have the structures of O-esters. 

Figure 13.2 Biosynthetic pathways of (A) cyanogenic glucosides and (B) glucosinolates. The CYP79s are assumed to catalyze the same reaction in both pathways. It is not known whether the oxime is oxidized to an aci-nitro compound or a nitrile oxide in the glucosinolate pathway.

Protonated aci-nitro forms of C-glycosylnitromethanes that are resistant to the Nef reaction in aqueous acidic media undergo a modified Nef reaction in acidified ethanol. These reaction conditions allowed the... 

Experiment.—Dissolve 1 c.c. of nitromethane in water and test the solution with litmus paper. Then add some phenolphthalein and, drop by drop from a burette, OliV-sodium hydroxide solution. Before a permanent pink colour develops about 2 c.c. of the alkali will be added—a sign that an acid, aci-nitromethane, H2C NOOH, has been formed from the neutral nitromethane. A small sample of this solution gives with ferric chloride a blood-red colour, characteristic of aci-nitro-compounds. The salts of the oci-compound undergo extensive hydrolysis. This is shown by further addition of 0-1 N-alkali which produces a deep red colour. If 10 c.c. of alkali were added and 5 c.c. of 0-1 JV-hydrochloric acid are now run in the solution is decolorised because the liberated oci-compound restricts the hydrolysis of its salt. But the conversion of H2C N02H into H3C.N02 proceeds so rapidly that the red colour reappears in a few moments. 

The nitrolic acids are colourless, hut dissolve in alkalis with a deep red colour because in addition to the chromogenic nitroso-group the aci-nitro-group is formed. The following formula is ascribed to the... 

The remainder of the aci-nitro-compound, dissolved in alcohol, is allowed to stand over night. The solution then neither takes up bromine nor gives the colour reaction with ferric chloride. If a few particles of the substance are left exposed on a watch-glass over night they are transformed into an oil. 

The mechanism of the coupling reaction has been very exhaustively studied. Summarising first what has already been mentioned, it must be noted that the reaction is not confined to the aromatic series, for diazo-compounds condense also with enols and with the very closely related aliphatic aci-nitro-compounds. The final products of these reactions are not azo-compounds, but the isomeric hydrazones formed from them by rearrangement. 

Recently, time-resolved resonance Raman spectroscopic studies47 of the excitation of 2-(2, 4 -dinitrobenzyl)-pyridine 58 and 4-(2, 4 -dinitrobenzyl)pyridine have shown that three transient intermediates are involved they are aci-nitro acid 59, aci-nitro anion 60 and N—H... 

NITROALKYL PEROXONITRATES, NITROARYL COMPOUNDS NITROARYLPHOSPHINIC ACIDS, NITROBENZYL COMPOUNDS C-NITRO COMPOUNDS, A-NITRO COMPOUNDS ac/ -NITROQUINONOID COMPOUNDS, aci-NITRO SALTS POLYNITROALKYL COMPOUNDS, POLYNITROARYL COMPOUNDS POLYNITROAZOPYRIDINES Other relevant entries are ... 

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