Nitroalkenes, reduction

Alkenes. See also Nitroalkene reduction alkane reduction, 32-45... 

Nitroalkene reduction, 102-103 3-Nitrobenzylamine, imine to amine reduction, 135... 

Formic acid is a good reducing agent in the presence of Pd on carbon as a catalyst. Aromatic nitro compounds are reduced to aniline with formic acid[100]. Selective reduction of one nitro group in 2,4-dinitrotoluene (112) with triethylammonium formate is possible[101]. o-Nitroacetophenone (113) is first reduced to o-aminoacetophenone, then to o-ethylaniline when an excess of formate is used[102]. Ammonium and potassium formate are also used for the reduction of aliphatic and aromatic nitro compounds. Pd on carbon is a good catalyst[103,104]. NaBH4 is also used for the Pd-catalyzed reduction of nitro compounds 105]. However, the ,/)-unsaturated nitroalkene 114 is partially reduced to the oxime 115 with ammonium formate[106]... 

The Michael adclidon of a nitrogen-centered nucleophile to nitroalkenes affords compounds that may serve as precursors of vicinal chamines, since the nitro group can be reduced to an amino function by reduction The very convenient method for the preparation of 1,2-chamines is developed by the adchdon of O-ethyihydroxylamines to nitroalkenes followed by redncdon with H-, in the presence of PckC fEq 4 24 ... 

This procedure Is applied to synthesis of 1,3-diamines by the addition ofmetdlated rerr-biityl iV.iV-dimethylcarbamate to nitroalkenes and subsequent reduction fEq. 4.84. ... 

A new selective reduction of nitroalkenes into enamides has been carried out by a combina-m of iron powder, a carboxylic acid, and the corresponding anhydnde fEq 6 64)... 

The Michael addition of the carbanions derived from esters to nitroalkenes followed by reductive cycLizadon has been used extensively for the preparation of pyrrondin-3-ones fEq 1076 This strategy is used for synthesis of the carbaaole alkaloid staurosporine aglycon CK-352c ... 

Nitroalkenes are good dienophiles and the variety of transformations available for nitro groups makes them versatile intermediates.65 Nitro groups can be converted to carbonyl groups by reductive hydrolysis, so nitroethylene can be used as a ketene equivalent.66... 

Chiu et al. developed the first example of a reductive intramolecular Henry reaction induced by Stryker s reagent (Scheme 30) [53]. The conjugate reduction of keto-nitroalkenes with [Ph3PCuH]6 (150 mol%) triggers spontaneous nitro-aldol reaction at - 40 °C to produce (f-hydroxy nitro compounds in moderate yield. 

Nitroalkenes prepared from aromatic aldehydes are especially useful for natural product synthesis. For example, the products are directly converted into ketones via the Nef reaction (Section 6.1) or indoles (Section 10.2) via the reduction to phenylethylamines (Section 6.3.2). The application of these transformations are discussed later here, some examples are presented to emphasize their utility. Schemes 3.3 and 3.4 present a synthesis of 5,6-dihydroxyindole66 and asperidophytine indole alkaloid,67 respectively. 

The reduction of nitroalkenes with ZnBH4 in 1,2-dimethoxyethane (DME) gives the corresponding oximes or nitroalkanes depending on the structure of nitroalkenes. a-Substituted nitroalkenes are reduced to the oximes, whereas those having no a-substituents afford the... 

Very selective reduction of nitroalkenes into the corresponding nitroalkanes is achieved using NaCNBH3 in the presence of the zeolite H-ZSM 5 in methanol (Eq. 3.48).78... 

Amino alcohols like (iS )-prolinol react with nitroalkenes very rapidly with very high facia] selectivity.31 Rapid and stereoselective reduction of the nitro function is essential for the conversion of the products to 1,2-diamine derivatives with the retention of the configuration. Samarium diiodide is recommended in the stereoselective reduction of thermally unstable 2-aminonitroalkanes to give a range of useful 1,2-diamines (Eq. 4.26).32... 

The conversion of primary or secondary nitro compounds into aldehydes or ketones is normally accomplished by use of the Nef reaction, which is one of the most important transformations of nitro compounds. Various methods have been introduced forthis transformation (1) treatment of nitronates with acid, (2) oxidation of nitronates, and (3) reduction of nitroalkenes. Although a comprehensive review is available,3 important procedures and improved methods published after this review are presented in this chapter. The Nef reaction after the nitro-aldol (Henry reaction), Michael addition, or Diels-Alder reaction using nitroalkanes or nitroalkenes has been used extensively in organic synthesis of various substrates, including complicated natural products. Some of them are presented in this chapter other examples are presented in the chapters discussing the Henry reaction (Chapter 3), Michael addition (Chapter 4), and Diels-Alder reaction (Chapter 8). 

Iodotrimethylsilane generated in situ from chlorotrimetylsilane and sodium iodide effects the reduction of nitroalkenes into ketones at 0 °C. This method is useful for the conversion of nitro steroids or nitro terpenoids to the corresponding ketones (Eq. 6.24).43... 

The conversion of nitroalkanes to ketoximes can be achieved by the reduction with Zn in acetic acid,112 or Fe in acetic acid.113 Nitroalkenes are direcdy reduced into saturated ketoximes by these reagents, which are precursors for ketones (see Section 6.1.4 Nef reaction). Reduction of 3-O-ace-ty lated sugar 1 -nitro-1 -alkenes with Zn in acetic acid gives the corresponding 2,3-unsaturated sugar oximes in high yield, which is a versatile route to 2,3-unsaturated sugar derivatives (Eq. 6.58).114... 

The conversion of nitroalkenes into the oximes can be achieved by electrochemical reduction (Eq. 6.60).11[Pg.176]

Reaction of the salts of primary and secondary alky Initio compounds with diborane in THF solution at 25 °C yields the corresponding hydroxylamines.117 Kabalka has reported the reduction of nitroalkenes to hydroxylamines or amines with a variety of borane and borohydride reagents (Eq. 6.61).118... 

Denmark and coworkers have found that methylaluminum bis (2,6-di-tert-butyl-4-methyl-phenoxide) (MAD) or methylaluminum bis(2,6-diphenylphenoxide) (MAPh) is effective as the Lewis acid promoter for cycloaddition of 2,2-disubstituted 1-nitroalkenes (Eq. 8.100).158 Other Lewis acids such as SnCl4, TiCl4, and TiCl2(Oi-Pr)2 fail to promote the cycloaddition of 2,2-disubstituted 1-nitroalkenes. The products are converted into 3,3-disubstituted pyrrolidines via hydrogenolysis.158 Reductive cleavage of N-0 bonds produces oxime hemiacetals, which are further reduced to amido aldehydes and finally to pyrrolidines. This reaction provides a useful synthetic method for pyrrolidines, which is discussed later. 

A convenient and general method has been developed for the synthesis of alkylpyrroles starting from ketones and nitroalkenes via reduction of the intermediate acetic nitronic anhydride as shown in Eq. 10.1. Ketone enolates react with a variety of nitroalkenes to yield the Michael adducts, lithium nitronates, which are trapped with acetic anhydride to give the corresponding acetic nitronic anhydrides. The acetic nitronic anhydrides are easily converted into alkylpyrroles by reduction with Zn(Cu).3... 

The Michael addition to nitroalkenes followed by cyclization provides a general method for the synthesis of various pyrroles. The reaction of nitroalkenes with acetoacetate followed by reduction with Zn in acetic acid provides another route to 2-methyl-3-pyrrolecarboxylates (Eq. 10.8).10... 

A novel synthetic approach toward the AB -ring system of 9-azasteroids using the Diels-Alder reaction of nitroalkene and subsequent reductive cyclization has been shown (Scheme 10.24).132... 

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