Magnesium enolates diastereofacial selectivity

An interesting class of chiral enolates are allenyl enolates. These ambident nucleophiles bear an axis of chirality. Krause and coworkers have found that an axis to center chirality transfer takes place in the aldol reaction of chiral magnesium allenyl enolate with pivalic aldehyde . The aldol reaction proceeds with good diastereofacial selectivity if... 

The major and minor products obtained in aldol reactions of chiral aldehyde (168 equation 109) are not those predicted by Cram s rule, presumably because the lithium cation is chelated by the alkoxy and aldehyde oxygens, leading to a rigid six-meml red intermediate that undergoes attack primarily from its unsubstituted face. " Similar behavior, with somewhat higher diastereofacial selectivity (5 1), is seen with the magnesium enolate (equation 50). 

Chiral acetate (204) shows excellent diastereofacial selectivity and has obvious utility as a reagent for asymmetric aldol reactions. As shown in equation (122), reaction of (204) with benzaldehyde provides diastereomers (205) and (206). As shown in Table 23, entry 1, the diastereoselectivity is 83% if the lithium enolate is formed in the conventional manner and the aldol reaction is carried out in THF at -78 C. A significant improvement is obtained by using the magnesium enolate (Table 23, entry 5), and diastereoselectivity of up to 98% is obtained by the use of very low reaction temperatures (Table 23, entries 10-13). 

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