Nitrones nitroketones

The reduction of y-nitroketone acetals as in Eq. 6.50 v/ith ammonium formate in the presence of Pd/C gives the correspondmg amines in good yields. However, the reduction ofy-nitro ketones are reduced to cyclic nitrones fEq. 6.51. This reduction is far superior to the classical method using Zn/T4HlCl due to improved yield and simple workup. 

Redncdons of y-nitroketones yield cyclic nitrones, which undergo inter- and intramolecular cycloadthdon to various alkenes. The result of adthdon to acrylonitrile is shown in Eq. 8.43, in which a rruxnire of regio- and stereoisomers is formed. - ... 

The addition to alkenes of radicals derived from an a-nitroketone is also catalysed by manganese(III) [30]. During the reaction between a-nitroacetophenone 8 and m-but-2-ene, the stereochemical relationship between the methyl substituents is not preserved. The process terminates with the formation of a nitrone. A related process will generate nitromethyl radicals from nitromethane and these add to benzene to give phenyinitromethane [31],... 

The synthesis of unsymmetrical 1,4-diketones results from the conjugate addition of the nitronate anion (as the acyl anion equivalent) to an a,fi-unsaturated ketone [Method (6)].l47a The intermediate nitroketone is converted into the diketone by reduction with titanium(m) chloride at pH 1, or by ozono-lysis of the nitronate anion147b (cf. Section 5.7.7, p. 599). 

P, P] Seebach and Brock reported the dichlorodiisopropoxytitanium-mediated addition of the trimethylsilyl enol ether of cyclohexanone to /3-nitrostyrenes (83). The initial products generated are nitronic esters 39.1-39.3. Separation followed by fluoride-induced desilylation of these intermediates yields the corresponding syn and anti nitroketones. The results of this study are summarized in Scheme 39 and Table 12. Anti isomers are obtained in moderate diastereomeric excess. Moreover, the method is complementary to the additions of similar substrates by way of their lithium enolates (2) or enamines (vide supra), which provide the syn diastereomers. Further reactions of the intermediate nitronic esters were briefly explored. For example, addition of aldehydes and activated olefins provides stereoselectively the products from nitroaldol and [3 + 2] cycloadditions. 

The photoreactions (X >435 nm in dichloromethane) of tetranitromethane (261) with styrenes (260) to yield nitro-trinitromethyl adducts (269), diastereo-meric oxazolidines (270), nitro ketones (271) and nitronic esters (272) may be rationalised in terms of the interlinked pathways in Scheme 3. SET yields nitrogen dioxide (262), the styrene radical cation (263) and the trinitromethanide ion (264). Addition of nitrogen dioxide (262) to the styrene (260) initiates a radical chain process. Benzylic radical (266) is oxidised by tetranitromethane (261) to the benzylic cation (265). Nitro-trinitromethyl adducts (269) result from coupling of the cation (265) with the anion (264). A competing pathway involves reaction of radical cation (263) with anion (264). C clisation of the resulting radical (268) yields the aminoxyl (267), from which loss of nitrogen dioxide yields the nitronic ester (272), whereas coupling of (267) with the radical (266) forms oxazolidine diastereomers (270). The nitroketones (271) may arise, at least in part, from secondary photolysis of the oxazolidines (270). ... 

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