Stereodifferentiation: consonant dissonant

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... 

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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