Henry reaction stereoselectivity

STEREOSELECTIVE HENRY REACTIONS AND APPLICATIONS TO ORGANIC SYNTHESIS... [Pg.51]

The Henry reactions of A, ALdibenzyl-L-phenylalaninal with nitroalkanes using 1.2 equiv of tetrabutylammonium fluoride (TBAF) as the catalyst proceed in ahighly stereoselective manner, as shown in Eqs. 3.82 and 3.83. This reaction provides rapid and stereoselective access to important molecules containing 1,3-diamino-2-hydroxypropyl segments, which are cenhal structural subunit of the HIV protease inhibitor amprenavir (in Scheme 3.21). [Pg.63]

Moderate stereoselectivity (80 20) was observed in the potassium fluoride catalyzed Henry reaction of nitromethane with isopropylidenc-D-glyceraldehyde (10). The major product could be separated from the mixture by fractional crystallization9. [Pg.635]

Henry reactions of readily accessible 1-deoxy-l-nitroaldoses (e.g., 1) with a variety of aldehydes proceed with a high degree of stereoselection. The crude products are hydrolyzed with loss of the nitro group to generate higher ketoses8. [Pg.637]

In general, the Henry reaction gives a mixture of diastereomers and enantiomers. The lack of selectivity is due to the reversibility of the reaction and the easy epimerization at the nitro-substituted carbon atom. Existing reviews have hardly mentioned the stereochemistry of the Henry reaction. Recently, Shibasaki has found that the modification of the Henry reaction can control the stereochemistry to give (3-nitro alcohols with high diastereo- and enantio-selectivity.6 In Section 3.3, the progress of the stereoselective Henry reaction and its application to biologically active compounds are discussed. [Pg.30]

P-Nitro alcohols can be hydrogenated to the corresponding amino alcohols with retention of configuration the stereoselective Henry reaction is a useful tool in the elaboration of pharmacologically important P-amino alcohol derivatives including chloramphenicol, ephedrine, norephedrine, and others. Some important P-amino alcohols are listed in Scheme 3.11.107... [Pg.51]

Bicyclic trimethylsilyl nitronates undergo stereoselective Henry reactions with benzalde-hyde in the presence of fluoride ion to give cyclic hemiacetals in good yield with high diastereo-selectivity (95% ds) (Eq. 3.68).110... [Pg.54]

The stereoselective intramolecular Henry reactions have been reported by Seebach. The Michael addition of doubly deprotonated acetyl acetaldehyde to l-methylenedioxyphenyl-2-nitroethene followed by subsequent intramolecular nitro-aldol cyclization leads to the diastereomerically pure cyclohexanone derivative, where the nitro and OH groups are cis as shown in Eq. 3.73.114 This reaction is applied to the synthesis of l-desoxy-2-lycorinone as shown in Eq. 3.74.115... [Pg.56]

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). [Pg.381]

Few of these studies (460, 462) dealt with the Michael reaction one study (461) with the Henry reaction. The efficiency, stereoselectivity, and enantiose-lectivity of this process are rather high. The mechanism of the transformations is poorly known. Presumably, the chiral cation should shield the Si surface of nitronate, thus providing the Re approach of the substrate. In addition, the approach of the reagents, resulting in generation of syn isomers, is considered less favorable than the approach yielding anti isomers. [Pg.615]

Lemaire et al. have developed a efficient fructose-1,6-bisphosphate aldolase (FBPA)-mediated synthesis of aminocyclitol analogs of valiolamine [34], This one-pot route involves the formation of two C—C bonds where four stereocenters are created. The first C—C bond formation reaction is catalyzed by the aldolase, coupling DHAP to nitrobutyraldehydes the other one is the result of a highly stereoselective intramolecular Henry reaction occurring on the intermediate nitroketone under acidic conditions during the aldolase-catalyzed reaction and phytase-catalyzed phosphate hydrolysis coupled step (Scheme 4.13). [Pg.70]

Ooi has recently reported application of chiral P-spiro tetraaminophosphonium salt 37 as a catalyst for the highly enantio- and diasterioselective direct Henry reaction of a variety of aliphatic and aromatic aldehydes with nitroalkanes (Scheme 5.51) [92]. Addihon of the strong base KO Bu generates in situ the corresponding catalyhcally active triaminoiminophosphorane base A. Ensuing formation of a doubly hydrogen-bonded ion pair B positions the nitronate for stereoselective addition to the aldehyde. This catalyst system bears many similarities to guanidine base catalysis. [Pg.109]

The amines 3 reported by Morao and Cossio also constitute neutral dendritic catalysts without metal sites they consist of a simple amine core functionalised with Frechet dendrons (Fig. 6.44) [74]. Such amines can catalyse the nitroaldol or Henry reaction [75] between aromatic aldehydes and nitroalkanes. Whereas neither the yield nor the stereoselectivity (syn/anti 1 1) of the reaction of p-nitro-benzaldehyde with nitroethane was found to change on use of different generations of dendritic catalysts, a distinctly negative dendritic effect was observed in the reaction of benzaldehyde with 3-nitro-l-propanol. Catalysts 3 a and 3 b gave... [Pg.236]

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