Asymmetric reactions involving

Difficulties have been encountered in mismatched double asymmetric reactions involving (a-S,S,S)-5, as illustrated by the reaction with 21. 

Asymmetric Reactions Involving Acyl Anion Equivalents... 

Note that in aldol reaction IV (from 31 to 42 in Scheme 7-9), the methodology differs from that used in I, II, and III (see Scheme 7 7). Aldol reaction IV is also a double asymmetric reaction involving the coupling of two structurally... 

Novel asymmetric conjugate-type reactions have been accomplished with Cinchona alkaloid-derived chiral thioureas, including less traditional reactions such as asymmetric decarboxylation [71]. In the following discussion, asymmetric reactions involving nitro-olefms, aldehydes and enones, and imines will be highlighted (Fig. 5). 

This example suggests that veiy high levels of selectivity will be realized in matched double asymmetric reactions involving chiral y-alkoxyallylboronates of general structure 66 and 67 (Figure 19), but also foreshadows potential problems in applications of these reagents in... 

Asymmetric Reactions Involving Enantiotopic and Diastereotopic Hydrogen Atoms... 

Asymmetric hydroboration and conceptually similar reactions involving chiral reagents have been used with great success. Their principal shortcoming is that stoichiometric quantities of chiral compounds must be invested and only rarely can these compounds be recycled. An asymmetric reaction involving a catalyst that is chiral would be a superior way to accomplish an asymmetric synthesis since, with only a small amount of chiral material, large quantities of optically active product could, in principle, be obtained. 

Other examples of asymmetric reactions involving organocatalysts are (i) Diels-Alder reactions, (ii) Michael reactions, (iii) Mannich reactions and (iv) Shi epoxida-tion and organocatalytic transfer hydrogenation. 

The first examples of highly diastereoselective double asymmetric reactions involving chiral allyl metal reagents were obtained in reactions with D-glyceraldehyde acetonide (151 Table 6). Aldehyde (151) displays an 80 20 preference for (154) in reactions with the achiral pinacol allylboronate (144 entry 4),25.ioi selectivity for (154) improves to 96-98% with reagents (-)-(215) and (RJi)-... 

Further examples of asymmetric reactions involving chiral monoterpenoids include the formation of chiral /S-keto-sulphoxides from (-)-menthyl carboxy-lates, " synthesis of diastereomeric tetrahedral molybdenum complexes,the enantioselective hydrosilylation of ketones,and the preparation of the chiral l-amino-2-phenylethylphosphonic acids. 

By stereoselective additions of carbenes or carbene equivalents to alkenes optically pure cyclopropanes are obtained. " In concerted [2+1] cycloadditions the stereochemistry of the alkene is conserved in the products. (Z)-configurated alkenes lead stereospecifically to cis-cyclopropanes. So far, compared to [2+1] cycloadditions involving alkenes bearing the chiral auxiliary, asymmetric reactions involving chiral carbene precursors proved to be less efficient. 

Broad biological potential of enantiopure 1,2-dihydroquinolines prompted the recently reported organocatalytic, asymmetric Petasis reaction, catalyzed by chiral biphenols [31]. Since it was observed that chiral biphenol derivatives XXVI-XX-VIII serve as proficient catalysts for asymmetric reactions involving boronates [32, 33], the authors postulated that they could be used as ligands in multi-component condensation reactions, and the Petasis reaction in particular. 

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