Asymmetric carbonyl olefinations

Methods for the formation of carbon-carbon double bonds in an asymmetric manner through non-Wittig-type reactions [12,13] have also been reported in recent years, including asymmetric induction by reactions with chiral sulfoxides [13], sul-fones (Julia olefination) [14], sulfoximides [12a, 15], or selenides [16] Pd-catalyzed allylic nucleophilic substitutions [17], as well as asymmetric deprotonation [18]. In the context of the topic of asymmetric carbonyl olefination, some of these asymmetric transformations are beyond the scope of this chapter, although a few of the transformations closely related to the Wittig-type reactions will be discussed in a later part of this chapter. 

Scheme 7.1. The first report of asymmetric carbonyl olefination.

Optically Active Phosphorus or Arsenic Reagents Used in Asymmetric Carbonyl Olefination... 

Scheme 7.5. Three types of chiral phosphorus reagents employed in asymmetric carbonyl olefination.

In a similar manner, asymmetric carbonyl olefination of meso-dicarbonyl compounds was extended to metallic arene or diene complexes [37], such as // -diene Fe or // -arene Cr complexes, to form planar complexes with high enantiomeric bias (Scheme 7.8). Since both complexation and decomplexation of these optically active compounds occur readily, these olefinic complexes are effective as stereocontrollers due to the presence of bulky metal tricarbonyl groups, and serve as useful reactants for obtaining optically active compounds of central chirality by appropriate chemical transformation. 

Scheme 7.8. Asymmetric carbonyl olefinations to give planar chiral alkenes.

Two chiral phosphonic acid derivatives 19a,b, containing a stereogenic phosphorus atom connected to a mercaptoisoborneol moiety, were prepared as a mixture, and were then chromatographically separated. Their ability in asymmetric carbonyl olefination was examined in the reaction with 4-tert-butylcyclohexanone la [56). The two lithium carbanions reacted with the carbonyl group of the substrate to give opposite enantiomers 90a, although no remarkable degree of asymmetric induction was observed (up to 16% ee). 

Scheme 7.19. Asymmetric carbonyl olefinations through kinetic resolution.

Asymmetric carbonyl olefinations through parallel kinetic resolution. 

Scheme 7.26. Some examples of the application of asymmetric carbonyl olefination to natural product synthesis.

Asymmetric Carbonyl Olefinations Without Usage of Optically Active Phosphorus Reagents 329... 

When the achiral phosphonate 182 was reacted with the ketone la in the presence of Sn(II) triflate and N-ethylpiperidine, the chiral diamine 183 was shown to act as a good asymmetric inducer in generating the tetrasubstituted dissymmetric alkene 184 with a high level of enantiomeric excess [35j, 102]. In order to achieve high levels of asymmetric induction, stoichiometric amounts of ligands in relation to the external chiral source were required in all of the aforementioned asymmetric carbonyl olefinations. 

Asymmetric Carbonyl Olefination by Non-Wittig-Type Routes... 

Asymmetric carbonyl olefination methods by routes other than the Wittig and related olefination reactions are available, and some precedents belonging to this... 

By using the same lithium salt (S)-201, asymmetric elimination to give 205 with high diastereotopic differentiation without loss of chirality was reported [15] (Scheme 7.32). This asymmetric carbonyl olefination allowed the selective synthesis of both the (Z)- and ( )-alkenylsulfoximides 208 and 210, which are useful... 

Scheme 7.31. Miscellaneous examples of asymmetric carbonyl olefinations (1).

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