Magnesium amide enolates

With (Z)-amide enolates and (Z)-thioamide enolates a strong preference for sm-adducts is also observed. In general, boron or zirconium (Z)-enolates of ketones and amides display a higher simple diastereoselectivity in favor of syn-products than the corresponding lithium or magnesium enolates6,7. 

In recent years, a variety of hindered magnesium amides have been used to produce magnesium enolates. The versatility of these bases is now well recognized. Some typical examples are presented below. 

Examples for the preparation of magnesium enolates (or further subsequent reactions of the enolates) by magnesium amides are listed in Table 7. 

The magnesium amides of choice for the preparation of magnesium enolates via met-allation are the Hauser bases, such as 39 and 40, or (bis)amidomagnesium reagents, such as 46 and 47. The reaction has been successfully applied to the preparation of enolates derived from cyclic, acyclic and a-siloxyketones, benzyUc ketones, aldehydes, carboxylic esters and amides, even with the less hindered Hauser bases. 

Substrate Magnesium amide Magnesium enolate or reaction product after trapping procedure of enolates Ref- erence... 

The increasing interest in enolization reactions mediated by magnesium amides led to new investigations for structural features of these reagents . 

The concept of chiral magnesium amides for the preparation of magnesium enolates has been extended to chiral magnesium bis(sulfonamide) complexes as catalysts for the enolization of A-acyloxazolidines ° (equation 63). 

Aldol reactions of simple amide enolates give poor stereoselection. Stimulated by the interest in /3-lactams, the stereochemistry of aldol reactions of chiral magnesium enolates of /3-lactams has been studied . The best results have been obtained with 6,6-dibromopenams 85 (equation 108). After bromine-magnesium exchange with MeMgBr,... 

The chemistry of magnesium bisamides has been reviewed" . They can be used for the regio- and stereoselective formation of enolates", while chiral magnesium amides are applied in asymmetric synthesis for enantioselective enolisations ". 

The chemistry of magnesium bis-amides has been reviewed.21 Magnesium bis-amides have been used for the region- and stereoselective formation of enolates.2 a Enantioselective enolization with chiral magnesium amides has been applied in asymmetric synthesis.23 233... 

Carhon nucleophiles of pXa 10-20 have heen most studied and often work hest with these substrates (eq 1), hut enamines, cyclopentadiene anions, enolates (eq 2), organotin, -thaUium, -zinc, -aluminum, -lithium, and -zirconiums, as weU as Grignards (eq 3) and horates, also add to r-allylpaUadium complexes. Heteroatom nucleophiles also add successfully these include amines (eq 4), amides, azides, magnesium amides, sulfonamides, alcohols, acids (eq 5), nitrites, sulhnic acids, thiols, phosphines, and phosphites. Limited use of transition metal nucleophiles is also known. ... 

The chemistry of magnesium bisamides has been reviewed. They can also be used for the regioselective and stereoselective enolization, and chiral magnesium amides can be advantageously used for enantioselective enolizations. ... 

Other chiral magnesium enolates derived from amides are known to react with aldehydes. For example, the aldol-type reaction of magnesium enolate of —)-trans-2-N,N-diethylacetamide-l,3-dithiolanes-5 -oxide with isobutyraldehyde affords a single diastere-omer in 82%. The relative stereochemistry of the adduct originates from a rigid transition state 87 where the oxygen atoms of the enolate and the aldehyde are coordinated to the magnesium atom. ... 

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