Enones, asymmetric catalysis

Whereas the mono- and the S/S-dithioether moieties have been used to date, the 1,3-dithianyl motif was used for the first time in 2005 by Ricci et al. as a new hybrid ligand in asymmetric catalysis. Hence, a series of new chiral oxazoline-1,3-dithianes have been successfully applied to the copper-catalysed conjugate addition of ZnEt2 to enones (Scheme 2.16). The expected products were obtained in almost quantitative yields and enantioselectivities of up to 69% ee. 

For these and similar reactions recently a variety of Lewis acidic aluminium, rare earth metals, and titanium alkoxides have been applied. Alkoxides have the additional advantage that they can be made as enantiomers using asymmetric alcohols which opens the possibility of asymmetric catalysis. Examples of asymmetric alcohols are bis-naphtols, menthol, tartaric acid derivatives [28], Other reactions comprise activation of aldehydes towards a large number of nucleophiles, addition of nucleophiles to enones, ketones, etc. 

Aggarwal, V. K. Epoxide formation of enones and aldehydes, in Comprehensive Asymmetric Catalysis l-lll (eds. Jacobsen, E., Pfaltz, A.,Yamamoto, H.), 2, 679-696 (Springer-Verlag, Berlin Heidelberg, 1999). 

Enders, D., Breuer, K. Addition of acyl carbanion equivalents to carbonyl groups and enones. Comprehensive Asymmetric Catalysis l-lll 1999,3, 1093-1102. 

Dual enantioselective control in the Cu-catalysed conjugate addition of dialkylzinc compounds to acyclic and cyclic enones has been realized in the presence of functionalized A -heterocyclic carbene ligands. " For example, a high degree of asymmetric catalysis (up to 96% ee) has been achieved using a (hydroxy amide)-functionalized azolium salt (19). Most likely the selectivity appears because of the facial selection of the enone, which is a result of steric repulsion between the alkyl group on the azolium ring and the alkyl substituent at the / -position of the enone. 

The asymmetric epoxidation of enones with polyleucine as catalyst is called the Julia-Colonna epoxidation [27]. Although the reaction was originally performed in a triphasic solvent system [27], phase-transfer catalysis [28] or nonaqueous conditions [29] were found to increase the reaction rates considerably. The reaction can be applied to dienones, thus affording vinylepoxides with high regio- and enantio-selectivity (Scheme 9.7a) [29]. 

An a-substituted system (Entry 28) has also been epoxidised in good yield and with moderate stereoselectivity. Although the absolute stereochemistry of the resulting epoxide has yet to be determined, this is the first example of such an enone undergoing asymmetric epoxidation using polyamino acid catalysis. 

The epoxidation of enones using chiral phase transfer catalysis (PTC) is an emerging technology that does not use transition metal catalysts. Lygo and To described the use of anthracenylmethyl derivatives of a cinchona alkaloid that are capable of catalyzing the epoxidation of enones with remarkable levels of asymmetric control and a one pot method for oxidation of the aUyl alcohol directly into... 

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