Stereoselectivity reaction

Most radicals located on saturated bonds are jt-radicals with a planar configuration and may be depicted with the free spin located in a p-orbital (1). Because such radical centers are achiral, stereochemical integrity is lost during radical formation, A new configuration will be assumed (or a previous configuration resumed) only upon reaction. Stereoselectivity in radical reactions is therefore dependent on the environment and on remote substituents. 

Keywords sulfoxides, /refera-Diels-Alder reaction, stereoselectivity, sulfoxide mediator... 

Bloch R., Mandville G. Novel Strategies for the Use of Retro Diels-Alder Reactions in Stereoselective Synthesis Recent Res. Dev. Org. Chem. 1998 2 441-452 Keywords retro-Diels-Alder reactions, stereoselective synthesis... 

Keywords chiral boron reagents, Diels-Alder reaction, stereoselective... 

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

In 2005, Nguyen et al. reported the first example of asymmetric cyclopropa-nation of olefins with EDA mediated by a combination of a (salen) ruthenium(II) catalyst and a catalytic amount of a chiral sulfoxide (Scheme 6.7). These authors proposed that the mechanism explaining the asymmetric induction involved the axial coordination of the chiral sulfoxide to the ruthenium centre as the key induction step in the reaction stereoselectivity. 

Syed, J., Forster, S. and Effenberger, F. (1998) Application of the Blaise reaction stereoselective synthesis of (4R)-tert-butyl 3-amino-4-trimethylsilyloxy-2-alkenoates from (R)-cyanohydrins. Tetrahedron Asymmetry, 9, 805-815. 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

Sridharan V, Avendano C, Menendez JC (2008) New findings on the cerium(IV) ammonium nitrate catalyzed Povarov reaction stereoselective synthesis of 4-alkoxy-2-aryl-l,2,3,4-tetrahydroquinoline derivatives. Synthesis 1039-1044... 

The well-known and widespread phenolic oxidations3 and the related nonphenolic oxidations4-6 for the formation of biaryl linkages are not restricted to the classic cases ofintermolec-ular coupling reactions. There are also examples of intramolecular coupling reactions with the option of performing the reaction stereoselectively, provided that some electronic requirements are fulfilled ... 

One reason for an otherwise apparently excessive interest in Co(trien)X2+ systems is the use of ds-Co(OH)(trien)(OH2)2+ in the hydrolysis of amino acid esters, amino acid amides and peptides785 to form cis-px- and cis-/J2-Co(trien)(aa)2+ (aa = amino acid) complexes.16 In principle, a peptide could be degraded in a stepwise manner and each amino acid residue successively characterized. By the introduction of a chiral center into the backbone of the trien moiety, it was hoped to make such reactions stereoselective. Consequently, while fully A-alkylated trien systems have been widely investigated for M11 central ions, the C-alkylated trien systems have been almost exclusively the reserve of the Co111 chemist (Table 11). 

Let us now define three terms that refer to reactions stereoselective, stereospecific and stereoelectronic. There has been a difference of opinion about the use of the first two we shall use the definitions suggested by Zimmerman2 and now adopted by most authors. 

The terms stereoselective and stereospecific properly refer only to reactions in which diastereomerically different materials may be formed or destroyed during the course of the reaction. Stereoselective reactions are all those in which one diastereomer (or one enantiomeric pair of diastereomers) is formed or destroyed... 

In 2008, the same authors reported the synthesis of polyfunctionalized /V-alkyl-[S-lac tains with high stereoselectivity in an efficient manner performing the same reaction with allyl bromide and heteroarylidene N-alkyl-amines. Interestingly, by modulating the type of alkyl group linked to the nitrogen atom, it is possible to influence the reaction stereoselectivity [164]. 

In addition to introducing energy into reactors via heating, or removal of heat generated during a reaction, some synthetic processes need to be conducted at reduced temperatures in order to prevent decomposition of unstable intermediates or to preserve reaction stereoselectivity. 

The reaction stereoselectively affords the 2 -R isomer as the major product (d.e. 33%). The absolute configuration of the 2 carbon was determined by NOESY analysis, in particular consistent n.0.e.s between H-l and H-l b and between H-l a and H-2 protons were observed. Treatment of 16 with zinc and acetic acid, afforded 17, that was transformed in the C-fiructoside 18 (Scheme 6). Compound 18 can be considered as an a- or a P-C-fructoside, in which both the hydroxymetyl arms can be further manipulated. An interesting example of this is the formation of the new spiro structure 20, as reported in Scheme 6. 

Bach, T. (1996) N-acyl enamines in the Paterno-Buchi reaction stereoselective preparation of 1,2-amino alcohols by C-C bond formation. Angewandte Chemie, International Edition in English, 35 (8), 884-886. 

The Stereoselectivity of 1,3-Dipolar Cycloadditions. There is no endo mle for 1,3-dipolar cycloadditions like that for Diels-Alder reactions. Stereoselectivity, more often than not, is low, as shown by the reactions of C,/V-diphenylnitrone—both regioisomers 6.238 and 6.239 (R=C02Et) from the reaction with ethyl acrylate are mixtures of exo and endo isomers, only a little in favour of the exo product. Similarly, the reactions of methyl crotonate with nitrones favour the exo product 6.242 over the endo 6.243. In contrast, other reactions are endo selective, as in the cycloaddition 6.244 of an azomethine ylid to dimethyl maleate giving largely the endo adduct 6.245. 

When the unsaturated moiety is prochiral, the reaction stereoselectivity may deeply affect the nature of the product. In the reduction of carbonyl Mannich bases containing a chiral center, the formation of a diasteieomeiic mixture of aminoalcohols (Fig. 113) is to be expected. The relative amounts of isomers arc determined by steric hindrance of the asymmetric center as well as by the nature of the reducing agent. On varying the reaction conditions, it is often possible to affect the diastercomeric ratio of the aminoalcohols produced, so as to obtain the predominance of either isomer. This can be particularly relevant in the synthesis of pharmacologically active substances. 

Only very few reactions of this type have been reported in the literature. All of them are intermolecular in nature, and only carbonyl groups have thus far successfully served as EWGs attached to the C—C double bond. Of course, a chiral medium, preferably a chiral catalyst, is required to render the addition reaction stereoselective. Enzymes have been used for this purpose. 

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