A-nitro-ketones

Aliphatische Nitro-Verbindungen, besonders mit weiteren flankierenden elektro-nenanziehenden Gruppen wie a-Nitro-carbonsaure-ester9 10, a-Nitro-ketone", vici-... 

Nitration of ketones or enol ethers provides a useful method for the preparation of a-nitro ketones. Direct nitration of ketones with HN03 suffers from the formation of a variety of oxidative by-products. Alternatively, the conversion of ketones into their enolates, enol acetates, or enol ethers, followed by nitration with conventional nitrating agents such as acyl nitrates, gives a-nitro ketones (see Ref. 79, a 1980 review). The nitration of enol acetates of alkylated cyclohexanones with concentrated nitric acid in acetic anhydride at 15-22 °C leads to mixtures of cis- and rrans-substituted 2-nitrocyclohexanones in 75-92% yield. 4-Monoalkylated acetoxy-cyclohexanes give mainly m-compounds, and 3-monoalkylated ones yield fra/w-compounds (Eq. 2.40).80... 

Nitration of the potassium enolates of cycloalkanones with pentyl nitrate81 or nitration of silyl enol ethers with nitronium tetrafluoroborate82 provides a method for the preparation of cyclic a-nitro ketones. Trifluoroacetyl nitrate generated from trifluoroacetic anhydride and ammonium nitrate is a mild and effective nitrating reagent for enol acetates (Eq. 2.41).83... 

Cyclic and acyclic silyl enol ethers can be nitrated with tetranitromethane to give a-nitro ketones in 64-96% yield (Eqs. 2.42 and 2.43).84 The mechanism involves the electron transfer from the silyl enol ether to tetranitromethane. A fast homolytic coupling of the resultant cation radical of silyl enol ether with N02 leads to a-nitro ketones. Tetranitromethane is a neutral reagent it is commercially available or readily prepared.85... 

A novel one-pot synthesis of a-nitro ketones from alkenes has been observed on treatment with trimethylsilyl nitrate-chromium trioxide or a trimethylsilyl nitrate-DMSO reagent system (Eq. 2.44).86... 

The nitro group of a-nitro ketones is readily removed either by treatment with Bu3SnH89 or reduction with LiAlH4 of the corresponding tosylhydrazones (Eq. 3.58).90 Details of denitration are discussed in Section 7.2, and some applications of this process are shown in Schemes 3.5-3.7. 

Construction of the carbon frameworks by using the activating property of the nitro group followed by denitration provides a useful tool for the preparation of various natural products as shown in Schemes 3.5-3.7. For example, (Z)-jasmone and dihydrojasmone, constituents of the essential oil of jasmone flowers, have been prepared as shown in Scheme 3.5.91 Schemes 3.6 and 3.7 present a synthesis of pheromones via denitration of a-nitro ketones.92,93... 

The addition-elimination reaction of hetero-atom-substituted nitroalkenes provides functionalized derivatives of unsaturated nitro compounds.26 Nitroenamines are generally prepared from a-nitro ketones and amines (see Chapter 5 regarding acylation of nitro compounds).26... 

The Michael addition of cyclic a-nitro ketones to acrolein or methyl vinyl ketone followed by reduction of the carbonyl group and treatment with base results in the ring expansion (Eq. 

The Michael addition of a-nitro ketones to a,P-unsaturated ketones followed by radical denitration provides a useful strategy for the preparation of 1,4-diketones.134b 1-Phenylheptane-1,5-dione, isolated from the decayed heart wood of aspens, is prepared by this strategy (Eq. 4.127).174... 

Dianions derived from cyclic a-nitro ketones have been used for the preparation of the natural product phoracanthlide and related macrocyclic lactones (see Scheme 5.2).13 Alkylation of dianion of a-nitro cyclic ketones is followed by radical denitration with Bu3SnH (see Section... 

The acylation of the carbanions derived from nitroalkanes with acyl imidazoles or alkoxy-carbonylimidazoles takes place at the carbon atom to yield a-nitro ketones or a-nitro esters, respectively (Eq. 5.10).21 The lithium salts of nitroalkanes were isolated and used in THF or DMSO in the original procedure. Later, potassium salts generated in situ on treatment with r-BuOK in DMSO are reactive enough to give a-nitro ketones in good yield (Eq. 5.11).22... 

As the nitro group is removed by radical denitration with Bu3SnH, allylic alkylation of a-nitro ketones with allyl carbonates in the presence of Pd(0) followed by denitration with Bu3SnH provides a new regio-selective allylation of ketones under neutral conditions (Eq. 5.55).79... 

Enantioselective allylations of a-nitro ketones and a-nitro esters with allyl acetates are carried out in the presence of 2 equiv of alkali metal fluorides (KF, RbF, CsF) and 1 mol% palladium catalysts prepared in situ from Pd2(dba)3-CHC13 and chiral phosphine ligands. Moderate enantio-selectivity (ca 50% ee) is reported for allylation of cx-nitroketones (Eq. 5.60). The highest selectivity (80% ee) is observed for allylation of the reaction of tert-butyl ester (Eq. 5.61).93... 

Primary nitro ketones, ethyl nitroacetate, and (phenylsulfony l)nitromethane react with alkenes in the presence of Lewis acids to give nitrile oxide cycloaddition.61a Similarly, the reaction of a-nitro ketones with TeCl4 generates the corresponding nitrile oxides, as shown in Eq. 6.36.61b... 

In 1970, a new reaction, the displacement of a nitro group from a-nitro esters, a-nitro nitriles, a-nitro ketones, and a,a-dinitro compounds by nitroalkane salts, was described.3 These displacements, which are exemplified by the reaction presented in Eq. 7.1, take place at room temperature and give excellent yields of pure products. The reaction proceeds via a radical chain mechanism involving one electron-transfer processes as shown in Scheme 7.1 the details of the mechanism are described in a review.1... 

Because the a-nitroketones are prepared by the acylation of nitroalkanes (see Section 5.2), by the oxidation of (3-nitro alcohols (Section 3.2.3), or by the nitration of enol acetates (Section 2.2.5), denitration of a-nitro ketones provides a useful method for the preparation of ketones (Scheme 7.10). A simple synthesis of cyclopentenone derivatives is shown in Eq. 7.66.76... 

Lastly, the radical inter- and intramolecular cyclizations in the presence of one-electron oxidizing agents as a procedure for the synthesis of five-membered cyclic nitronates can be considered. Radical oxidation of a-nitro ketones (19) in the presence of disubstituted olefins under the action of Mn(OAc)3 was documented (72a) (Scheme 3.22, Eq. 1). 

The mediated oxidations of a-nitro ketones and a-nitro amides in the presence of olefins have been studied [29]. In this work, Warsinsky and Steckhan found that the ensuing addition-cyclization sequence led to the... 

A double 5n2 reaction, which proceeds via a Favorskii-like cyclopropanone intermediate, has been proposed to account for the novel stereoretentive replacement of NO2 by OH on reaction of a-nitro ketones (which must bear an acidic hydrogen at the O -position) with aqueous base. ... 

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