Acyclic systems diastereofacial

Diastereofacial selection in nucleophilic additions to acyclic aldehydes and ketones is of major importance in synthetic organic chemistry." The greater conformational freedom of acyclic systems makes predictions as to the stereochemical outcome of such reactions more difficult than when the carbonyl group is contained within a cyclic framework. 

Facing the challenge of synthesizing the antibiotic erythromycin A la, Woodward s group took advantage of a cyclic system to achieve diastereofacial selectivity, the so-called cyclic approach.3 This approach was taken to deal with the common problem of low diastereoselectivity associated with acyclic substances. 

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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