Double stereodifferentiating crotylation

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... 

Scheme 1-7. Double stereodifferentiation in the Lewis acid-promoted crotylation of (S)-2-ben-zyloxypropanal with chiral allylic silanes.

Allylations. Double stereodifferentiation in the crotylation of aldehydes permits the construction of polypropionate chain segments of natural products. The diastereoselection and absolute stereochemistry are determined by the local chiralities of the two components. 

A few examples showcasing double stereodifferentiation phenomena are outlined below. In the case of crotylation of a-alkoxy-substituted aldehyde 72, Roush observed a reversal of facial selectivity with either enantiomer of the chiral ( )-crotylboronate reagent 29 (Scheme 5.13) [48]. Similarly, Brown found that the pinene-derived crotyl boranes 76 and 77 provide access to all four stereotriads 78-81 with impressive stereoselectivity with use of either the ( )- or the (Z)-crotyl reagent (Scheme 5.14) [77]. 

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