Stereodifferentiation: consonant

As a corollary to the cases above, the aldehyde may also contain a proximal center of asymmetry. In these cases the resident chirality in both the enolate and the aldehyde can influence the generation of new asymmetry in either a mutually cooperative (consonant) or an antagonistic (dissonant) fashion. The consonant or dissonant diastere oface selection imparted to both condensation partners has been referred to as double stereodifferentiation (83,109). This issue becomes important in the lasalocid A aldol bond construction illustrated in eq. [93]. This pivotal aldol condensation has been examined in detail... 

The observation that aldehyde diastereoface selection is interrelated with allylborane geometry has important implications for the related aldol processes. The reactions of (-)-180a and (-)-180b with both enantiomers of aldehyde 181 revealed both consonant and dissonant double stereodifferentiation. For the Cram-selective ( )-crotyl... 

According to another NMR study, the mechanism of bifunctional activation in the asymmetric aza-Morita-Baylis-Hillman reaction (Scheme 7) involves rate-limiting proton transfer (116) in the absence of added protic species155 (in consonance with the report summarized in Scheme 5144), but exhibits no autocatalysis. Addition of Brpnsted acids led to substantial rate enhancements through acceleration of the elimination step. Furthermore, it was found that phosphine catalysts, either alone or in combination with protic additives, can cause racemization of the reaction product by proton exchange at the stereogenic centre. This behaviour indicates that the spatial arrangement of a bifunctional chiral catalyst for the asymmetric aza-Morita-Baylis-Hillman reaction is crucial not only for the stereodifferentiation within the catalytic cycle but also for the prevention of subsequent racemization.155... 

However, one should not always expect to see additivity in such double stereodifferentiation experiments, as is illustrated by the following logic. For equation (129), AG° = +1.8 kcal mol" (1 cal = 4.18 J) that is, an axial methyl group disfavors the conformation on the right by 1.8 kcal mol". In equation (130), the effects of two axial methyl groups are additive, and AG° = 3.6 kcal mol. However, in equation (131), the effects of the two axial methyl groups are not additive, and AG° 3.6 kcal mol". Thus, we should expect that there will be cases in which the ideas of consonant and dissonant double stereodifferentiation will break down. ... 

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