And the aldol reaction

XVIII in a yield of 88 to 92%. It is here that the relationship between the Patemo-Buchi photocycloaddition process and the aldol reaction is most readily apparent. 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

This catalyst is readily soluble in ether, and the aldol reaction proceeds with high yield (>95% in most cases) and enantioselectivity (>90% in most cases) in the presence of a catalytic amount of 30. The salicylate counterion probably... 

A rather nice example of enolate anion chemistry involving the Michael reaction and the aldol reaction is provided by the Robinson annulation, a ring-forming sequence used in the synthesis of steroidal systems (Latin annulus, ring). 

Furthermore, this reaction was applied to the aldol reaction of 3-pentanone. When the chiral oxazolidine was prepared from 3-pentanone and the aldol reaction was carried out by the same procedure, the a, 6-anti 6-hydroxy ketones were produced predominantly over the syn-isomer 8i with excellent optical purities.(5) (See Table 2.)... 

In addition to these intermolecular processes, intramolecular versions of the Claisen (Dieckmann) reaction and the mixed Claisen and the aldol reaction (Robinson annulation) are also well known. In all cases the same structural classes of products are formed. 

Enolate ions are formed by treating aldehydes or ketones with a base. An a-proton must be present. Enolate ions can undergo a number of important reactions including alkylation and the Aldol reactions. 

D. The use of chiral oxazaborolidines as enantioselective catalysts for the reduction of prochiral ketones, imines, and oximes, the reduction of 2-pyranones to afford chiral biaryls, the addition of diethylzinc to aldehydes, the asymmetric hydroboration, the Diels-Alder reaction, and the aldol reaction has recently been reviewed.15b d The yield and enantioselectivity of reductions using stoichiometric or catalytic amounts of the oxazaborolidine-borane complex are equal to or greater than those obtained using the free oxazaborolidine.13 The above procedure demonstrates the catalytic use of the oxazaborolidine-borane complex for the enantioselective reduction of 1-indanone. The enantiomeric purity of the crude product is 97.8%. A... 

Similar approaches to that outlined in Scheme 4.79 have been successfully developed to produce a set of highly efficient chiral catalysts for a number of C-C bond-forming reactions, like, for example, the ethynylation of aldehydes, the Diels-Alder reaction, and the aldol reaction,as well as for vicinal hydroxylation of certain alkenes. ... 

Other Applications. Chiral oxazaborolidines derived from ephedrine have also been used in asymmetric hydroborations, and as reagents to determine the enantiomeric purity of secondary alcohols. Chiral l,3,2-oxazaborolidin-5-ones derived from amino acids have been used as asymmetric catalysts for the Diels-Alder reaction,and the aldol reaction. ... 

Enolate anions are an important class of carbanions that appear in a variety of important reactions, including alkylation a- to a carbonyl group and the aldol (reaction 16-34) and Claisen condensation (reaction 16-85) reactions. Metal enolate anions of aldehydes, ketones, esters, and other acid derivatives exist as aggregates in ether solvents, and there is evidence that the hthium enolate of... 

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