Aci-nitro esters

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>. 

Oxidation of alcohols. Primary and secondary alcohols are oxidized under neutral conditions by this combination of reagents. An aci-nitro ester is the intermediate, which decomposes slowly when heated in THF (equation I).1... 

Aldehydes and ketones can also be obtained from alcohols via the corresponding aci-nitro-esters. ... 

Carbon-Carbon Bond Formation. Direct coupling of an alcohol with a carbon acid by the use of DEAD/TPP has been limited to only a few examples because of the lack of carbon acids with pAa < 13. If enolizable carbonyl compounds are used, alkylation generally takes place on oxygen. Thus the reaction of 1,3-cyclohexanedione with isopropanol, DEAD, and TPP affords exclusively the O-alkylated product (eq 47). Cyanoacetate reacts at the carbon atom, while diethyl malonate is unreactive. However, the reaction of diethyl malonate with alcohols in the presence of ADDP and BU3P affords C-alkylated products in 3-56% yields (eq 48). Ethyl nitroacetate and nitromalonate oxidize the alcohol via an aci-nitro ester (eq 49). o-Nitroarylacetonitriles of general formula (50) undergo alkylation on carbon (eq 50). ... 

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. 

A second and related consequence in aliphatic nitro compounds is the acidification of the directly bonded CH unit through the attendant stabilization of the derived conjugate bases (5,6). As with all delocalized anions, reprotonation gives rise to tautomers, the original C-nitro compound (I) and the oci-nitro or isonitro form (II), Eq. 2.1. The aci-nitro tautomers are typically present in very minor concentrations, with equilibrium constants (A eq) between 10 and 10 (7). Alkylation of the delocalized anion leads to both a-substituted nitro compounds and the regioisomeric nitronic esters (nitronates). Nitronates were described as early as 1894 (8), however, the first isolated nitronic ester was obtained several years later upon the addition of diazomethane to phenylazonitromethane (1), Eq. 2.2 (9). 

In the presence of sodium or potassium ethoxide, nitric esters react with substances containing an active methylene group to form salts of aci-nitro compounds (Wislicenus [64]) as quoted above in eqn. (59). Esters are hydrolysed on this occasion. 

The intramolecular hydrogen abstraction has been largely developed for the case of the nitro group [22,37], and is well known for some derivatives of o-nitrotoluene, for o-nitrobenzyl alcohols, ethers and esters [38-40] as well as on the acetals of o-nitrobenzaldehyde [41], With these compounds, the nn (singlet or triplet) state abstracts a hydrogen and the biradical relaxes to the aci-nitro form (the first intermediate is actually observed in bicyclic systems where formation of the nitronic acid is sterically hindered) [41], which under basic conditions dissociates to the corresponding anion [42],... 

Glucose esters of 3-N. (endecaphyllins) are responsible for the toxicity of Indigofera endecaphylla for grazing animals. 3-N. functions as suicide substrate of succinate dehydrogenase where the formed 3-nitro-acrylic acid adds to the active thiol group of the enzyme. The compound is formed biosynthetically in Penicillium atrovenetum from L-aspartic acid which is oxidized to the a-nitrosuccinate and then decarboxy-lated to furnish the aci-nitro form of 3-N.. Malonate and N-hydroxymalonamate appear to be intermediates in Indigofera spicata. 

Nitronic ester The result of O-alkylation of the carbanion of the aci tautomer of a nitro compound and having the general formula R2C=N+(0-)0R. 

Properties and Preparation of Nitronates Nitroalkanes exist in equilibrium with a tautomeric form known as a nitronic acid (Scheme 16.19) [103]. The aci-form is usually present in minor concentration with an equilibrium constant of 10 [104]. Salts of nitronic acids, metal nitronates, are formed upon deprotonation of nitroalkanes and are potent nucleophiles [105]. Alkylation of the nitronate salts leads to both a-substituted nitro compounds and the isomeric nitronate esters. 

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