Preparation of P-Nitro Alcohols

The Henry reaction is catalyzed in homogeneous solution with various catalysts, as shown in Eq. 3.1. 

tetramethylguanidine (TMG),P(RNCH2CH2)3N (PAP), KF, n-Bu4NF, Al203, Al203-KF, amberlyst A-21, amberlite pgy 

Dendritic molecules with a single triethylene amine core surrounded by hyperbranched polyether sectors catalyze the nitro-aldol reaction between aromatic aldehydes and nitroalkanes (Eq. 3.5).15 The activity of the catalysts decreases when the generation number increases. No significant changes in stereo-control are observed on passing from lower- to higher-generation dendrimers. 

Fluoride ion is effective as a base for the Flenry reaction, and potassium fluoride in isopropanol (Eq. 3.6)16 and tetrabutylammonium fluoride in THF (Eq. 3.7)17 have been widely used. 

various kinds of bases are effective in inducing the Henry reaction. The choice of base and solvent is not crucial to carry out the Henry reaction of simple nitroalkanes with aldehydes, as summarized in Table 3.1. In general, sterically hindered carbonyl or nitro compounds are less reactive not to give the desired nitro-aldol products in good yield. In such cases, self-condensation of the carbonyl compound is a serious side-reaction. Several modified procedures for the Henry reaction have been developed. 

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Table 3.1. Preparation of p-nitro alcohol by the Henry reaction...

Dehydration of P-nitro alcohols provides an important method for the preparation of nitroalkenes. Because lower nitroalkenes such as nitroethylene, 1-nitro-1-propene, and 2-nitro-l-propene tend to polymerize, they must be prepared carefully and used immediately after preparation. Dehydration with phthalic anhydride is the most reliable method for such lower nitroalkenes.42,43 Such lower nitroalkenes have been used as important reagents for Michael acceptors or dienophiles in the Diels-Alder reaction, which will be... 

Conjugated nitroalkenes are isomerized to allylic nitro compounds under basic conditions. Reactions of a,P-unsaturated nitro compounds with aldehydes under basic conditions lead directly to y,8-unsaturated P-nitro alcohols (Eq. 3.24). 3 This reaction is very useful for preparing allylic nitro compounds. 

An experimentally simple procedure for stereoselectively preparing P-nitro alcohols has been developed. The alkyl nitronates, formed by the action of n-butyllithium on nitroalkanes in THF solution, react with aldehydes in the presence of isopropoxytitanium trichloride at room temperature to give the P-nitro alcohols enriched in the anri-diastereoisomers (Eq. 3.71).112... 

The base-catalyzed joint reaction of nitroalkenes with thiophenol in the presence of aldehydes gives y-phenylthio-P-nitro alcohols in one pot (Eq. 4.5).8 The joint reaction of nitroalkenes with thiols and a,p-unsaturated nitriles (or esters) has also been achieved. (Eq. 4.6).9 P-Nitro sulfides thus prepared show unique reactivity toward nucleophiles or tin radicals. The nitro... 

Jager and co-workers have prepared various amino sugars by the reduction of the corresponding P-nitro alcohols with H2 and Pd/C, as exemplified in Eq. 6.49 (see Chapter 3).95... 

Stereoselective preparation of ( )-allyl alcohols via radical elimination from anti-j-phenylthio-P-nitro alcohols has been reported.154 The requisite anti-P-nitro sulfides are prepared by protonation of nitronates at low temperature (see Chapter 4), and subsequent treatment with Bu3SnH induces anti elimination to give (E)-alkenes selectively (see Eq. 7.112). Unfortunately, it is difficult to get the pure yyw-P-nitro sulfides. Treatment of a mixture of syn- and anti-P-nitrosulfides with Bu3SnH results in formation of a mixture of (E)- and (Z)-alkenes. 

In contrast to the parent substance of the series, p-tolylhydroxyl amine, which is prepared by the reduction with zinc dust of p-nitro-toluene in boiling alcohol, is a quite stable compound. 

Poly(vinyl p-nitrobenzoate) was prepared by the esterification of poly(vinyl alcohol) with p-nitrobenzoyl chloride (2). Poly(p-nitrophenyl acrylate) was obtained by bulk-polymerization of p-nitro-phenyl acrylate (3). Ethylene-vinyl p-nitrobenzoate copolymer... 

The resurgent interest in the synthetic utility of 1-nitro-alkenes is evidenced by the variety of recent reports concerning their preparation. " Thus, the dehydration of 2-nitro-alcohols with sodium hydride or dicyclohexylcarbo-di-imide, the dehydrohalogenation of a-halogeno-oximes, and the nitro-selenylation of alkenes followed by oxidative deselenylation all provide 1-nitro-alkenes in moderate to excellent yield. In particular, these methods are all applicable to the synthesis of conjugated cyclic nitro-alkenes and complement those procedures previously reported for the preparation of both cyclic and acyclic nitro-alkenes (Vol. 3, p. 175 Vol. 4, p. 183 Vol. 5, p. 196 Vol. 6, p. 208). 

Potential precursors to stereoselective INOC and ISOC reactions (e.g., 195 and 196, respectively) have been prepared via stereoselective conjugate additions of several allylic alcohols (e.g., 194,X = 0) and an allylic thiol (e.g., 194,X = S) to a chiral (E)-nitro alkene (e. g., 193) that was derived from (P)-2,3-isopropylidene... 

Ono and Kamimura have found a very simple method for the stereo-control of the Michael addition of thiols, selenols, or alcohols. The Michael addition of thiolate anions to nitroalkenes followed by protonation at -78 °C gives anti-(J-nitro sulfides (Eq. 4.8).11 This procedure can be extended to the preparation of a/jti-(3-nitro selenides (Eq. 4.9)12 and a/jti-(3-nitro ethers (Eq. 4.10).13 The addition products of benzyl alcohol are converted into P-amino alcohols with the retention of the configuration, which is a useful method for anri-P-amino alcohols. This is an alternative method of stereoselective nitro-aldol reactions (Section 3.3). The anti selectivity of these reactions is explained on the basis of stereoselective protonation to nitronate anion intermediates. The high stereoselectivity requires heteroatom substituents on the P-position of the nitro group. The computational calculation exhibits that the heteroatom covers one site of the plane of the nitronate anion.14... 

The addition of alkoxides to 2-nitro-l-phenylthio-l-alkenes affords P-nitro-aldehyde acetals.276 The reaction of the same nitroalkenes with amines gives nitroenamines.270 They are important intermediates for organic synthesis and are generally prepared by the reaction of nitroalkanes with triethylorthoformate in the presence of alcohols or secondary amines.2"1 0 The methods of Eqs. 4.20 and 4.21 have some merits over the conventional methods, for variously substituted (3-nitro-aldehydes acetals or nitroenamines are readily prepared by these methods. 

Primary nitro compounds are good precursors for preparing nitriles and nitrile oxides (Eq. 6.31). The conversion of nitro compounds into nitrile oxides affords an important tool for the synthesis of complex natural products. Nitrile oxides are reactive 1,3-dipoles that form isoxazolines or isoxazoles by the reaction with alkenes or alky nes, respectively. The products are also important precursors for various substrates such as P-amino alcohols, P-hydroxy ketones, P-hydroxy nitriles, and P-hydroxy acids (Scheme 6.3). Many good reviews concerning nitrile oxides in organic synthesis exist some of them are listed here.50-56 Applications of organic synthesis using nitrile oxides are discussed in Section 8.2.2. 

Ono and coworkers have extended the radical elimination of v/c-dinitro compounds to P-nitro sulfones151 and P-nitro sulfides.138,152 As P-nitro sulfides are readily prepared by the Michael addition of thiols to nitroalkenes, radical elimination of P-nitrosulfides provides a useful method for olefin synthesis. For example, cyclohexanone is converted into allyl alcohol by the reaction shown in Eq. 7.110. Treatment of cyclohexanone with a mixture of nitromethane, PhSH, 35%-HCHO, TMG (0.1 equiv) in acetonitrile gives ahydroxymethylated-P-nitro sulfide in 68% yield, which is converted into the corresponding allyl alcohol in 86% yield by the reaction with Bu3SnH.138 Nitro-aldol and the Michael addition reactions take place sequentially to give the required P-nitro sulfides in one pot. 

The properties and reactions of amino-alcohols, obtained largely by hydrolysis of naturally occurring alkaloids, were investigated primarily for the purposes of structural analysis and the preparation of physiologically active derivatives. Many authors have described acylation of pyrrolizidine alcohols with benzoyl chloride and acetic anhydride (see, e.g., refs. 83 and 101). Trachelanthamidine benzoate and p-aminobenzoate were prepared especially for testing of their physiological activity.102 The p-aminobenzoate was obtained by treatment of trachelanthamidine with p-nitrobenzoyl chloride and subsequent reduction of the nitro group with iron in 20% acetic acid. The compound exhibited an anesthetic activity close to that of cocaine. 

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