Diastereofacial selectivity chiral electrophiles

Although the results are easily rationalised in the case of the a-alkylation (attack of the electrophile at the Re face, i.e., attack from the less hindered a face), in the aldol condensation it is somewhat more difficult to rationalise and several factors should be considered. According to Evans [14] one possible explanation for the diastereofacial selection observed for these chiral enolates is illustrated in Scheme 9.14. In the aldol reactions, the more basic carbonyl group of the aldehyde partner interacts with the chelated boron enolate 45 to give the "complex" A which may... 

When the electrophile is chiral, besides simple stereoselection a second type of diastereoselectivity, termed diastereofacial selectivity , is possible. This sort of diastereofacial preference, qualitatively predictable by Cram s rule for asymmetric induction or one of its more modem descendants, " is typical for additions to chiral aldehydes. 

Similarly, the reaction of an achiral aldehyde with a chiral enolate leads to some degree of diastereofacial selectivity and the two diastereomeric products are not produced in equal amounts. Further enhancement to the selectivity is possible using a chiral aldehyde with a chiral enolate. When both the nucleophile and the electrophile contain a chiral centre then double stereodifferentiation (or double asymmetric induction) can occur. Most of the studies in these areas use chiral, non-racemic enolates, which are discussed in the next section. 

Several factors must be considered in selecting a crotyl metal or allyl metal reagent for use in an acyclic stereoselective synthesis. First, it is necessary that the new stereocenters generated in concert with the new C—C bond (Scheme 1) be formed with a high degree of stereoselectivity. This is the problem of simple diastereoselectivity. Two diastereomeric products may be produced, and in this chapter Masa-mune s synlanti nomenclature system will be used to describe them. Second, the issue of diastereofacial selectivity is encountered if the aldehyde (or other C=X reaction partner) is chiral. This is a problem of relative diastereoselectivity, and four products may be produced in the reactions of the crotyl oiganome-tallics (Scheme 2). The diastereofacial selectivity issue is also critical in the reactions of allyl metal reagents and chiral C=X electrophiles. 

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