Reactivity magnesium enolates

Acyl imidazolides are more reactive than esters but not as reactive as acyl halides. Entry 7 is an example of formation of a (3-ketoesters by reaction of magnesium enolate monoalkyl malonate ester by an imidazolide. Acyl imidazolides also are used for acylation of ester enolates and nitromethane anion, as illustrated by Entries 8, 9, and 10. (V-Methoxy-lV-methylamides are also useful for acylation of ester enolates. 

The chapter is organized under the headings Preparation and Reactivity of magnesium enolates. 

Different protocols have been tested to prepare enolates from /1-aryl-a-iodoketones. Reactions nsing EtsB/PhsSnH in benzene, EtsB in benzene or ether and w-BuLi in ether failed to provide the corresponding enolates. Alternatively, the use of EtMgBr succeeds in generating reactive magnesium enolates from a-iodo ketones. In most cases, the formation of the enolate in THF is cleaner than that in EtiO (equation 8). 

Interestingly, even methyl 2-bromo- or 2-chloro-2-cyclopropylideneacetate reacts cleanly with various Grignard reagents in a 1,4-addition without any chlorine-metal exchange side reactions (equation 35). These in situ generated magnesium enolates are particularly reactive with aromatic aldehydes . 

Preparation and reactivity of magnesium enolates TABLE 12. Stereochemistry of cyclopentanones... 

Other examples including the compared reactivity of sodium and magnesium enolates of /3-ketoester, especially in xanthyrone and glaucyrone chemistry, are detailed in the review of L. Crombie . 

---