Diastereogenic Addition of Crotylmetal Compounds to Aldehydes

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Abstract
In an ideal synthesis, the formation of the CC bonds as well as the creation of the chiral centers and of the final functionality would be carried out simultaneously in each step. Accordingly, stereoselective CC bond forming reactions are required. Instead of a pair of diastereomers, a synthesis should yield only one of the two possible diastereomers. Such selectivity is often achieved in the addition of crotylboron, ‐aluminum, ‐tin, ‐titanium, and ‐chromium compounds to aldehydes. Depending on whether one starts from E‐ or Z‐crotyl compounds, one diastereomeric adduct is formed preferentially. In this case the newly formed CC bond carries a methyl substituent. However, the synthetic goal is often a compound carrying an oxygen function at the site of the new CC bond. Compounds of this type can be obtained through addition of heteroatom‐substituted allylboronic acid esters or analogous reagents to aldehydes.