Chiral auxiliaries conjugate reactions

As is the case for aldol addition, chiral auxiliaries and catalysts can be used to control stereoselectivity in conjugate addition reactions. Oxazolidinone chiral auxiliaries have been used in both the nucleophilic and electrophilic components under Lewis acid-catalyzed conditions. (V-Acyloxazolidinones can be converted to nucleophilic titanium enolates with TiCl3(0-/-Pr).320... 

Enantioselective Reactions of Organocopper Reagents. Several methods have been developed for achieving enantioselectivity with organocopper reagents. Chiral auxiliaries can be used for example, oxazolidinone auxiliaries have been utilized in conjugate additions. The outcome of these reactions can be predicted on the basis of steric control of reactant approach, as for other applications of the oxazolidinone auxiliaries. 

As well as organic chiral auxiliaries, organometallic fragments have found some application as chiral auxiliaries in conjugate addition reactions. Particularly noteworthy are chiral molybdenum allyl complexes [69], chiral iron complexes [70], and planar chiral arene chromium species [71]. 

Lewis acid-catalyzed stereoselective addition of crotylsilanes to chiral 74 has been studied in detail111,112. The presence of the chiral auxiliary at C2 (e.g. p-tolylsulfinyl or menthoxy carbonyl group) induces the diastereofacially selective addition of cyclopentenones with crotylsilanes. Thus, ( )-crotylsilane favors the erythro product, whilst (Z)-isomer favors the threo product. High enantioselectivity is observed in both reactions (equation 48). In a similar manner, conjugated addition of allylsilane to 75 proceeds with high efficiency (equation 49)113. Interestingly, the yield and enantiomeric excess of the product is dependent on the amount of TiCL used and the best selectivity... 

Chiral phosphine ligands are the chiral auxiliaries extensively studied for transition metal-catalyzed asymmetric reactions. Most of the ligands were originally designed for asymmetric hydrogenation, but they also worked well in the conjugate addition of organoboronic acids to electron-deficient alkenes.951,952 The rhodium(i) complexes of 496-503953-966 have been successfully utilized for these asymmetric reactions (Scheme 37). 

Some of the other reactions in which N. 0-acetals have been used as chiral auxiliaries include conjugate addition reactions,172 eyelopropanation,211 addition of nucleophiles to cyclic ketones212 and Diels-Alder reactions213... 

In addition to being an efficient chiral controller in a number of stereoselective transformations of chiral acrylates, (i.e. the Diels-Alder reaction, the conjugate reduction, the asymmetric dihydroxylation, and the nitrile oxide cycloaddition ) the bomanesultam (11) has been shown to be an exceptionally efficient chiral auxiliary for stereoselective aldol condensations (eqs eq 3 and eq 4). Depending upon the reaction conditions, A -propionylsultam can produce either the syn or anti aldol product with an excellent diastereoselectivity, Furthermore, good diastereoselectiv-ities are also observed for the corresponding acetate aldol reaction (eq 5), ... 

Related a,p-Unsaturated Esters. Similar a,p-unsaturated esters bearing a heterocyclic chiral auxiliary of a-amino acid origin at the p-position are known and have been utilized in asymmetric synthesis. Effective asymmetric conjugate additions of cuprates to (2), (3), and (5)J epoxidations of (3), and dipolar cycloadditions of (2) have been reported. Although oxazoli-dine (4) is only obtained as an 86 14 equilibrating mixture of stereoisomers, reactions with the lithium (Z)-enolate of methyl N-benzylideneglycinate (see Ethyl N-Benzylideneglycinate) are exclusively diastereoselective. 

Bicyclic oxazolidinones derived from carbohydrates have been used as chiral auxiliaries in conjugate addition reactions [147]. After deprotonation with MeMgBr, the D-galacto-oxazolidin-2-one 178 and the D-g/wc6>-oxazolidin-2-one 179 (Figure 10.17) were A-acylated with... 

SCHEME 10.59 Bicyclic oxazolidinones derived fiom carbohydrates are efficient chiral auxiliaries in conjugate addition reactions of organoaluminum compounds. 

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