Enolates, aluminum magnesium

Lithium, magnesium, and aluminum enolates appear to afford comparable levels of kinetic aldol diastereoselection for a given enolate of defined structure. 

Selective reduction of only one C-F bond in hexafluoroacetone (1) to give enol 2 is accomplished using activated aluminum as a reducing agent.89 90 It has been found in the reduction of hexafluoroacetone by magnesium metal in tetrahydrofuran that the intermediate penta-fluoropropen-2-olate subsequently reacts with a molecule of hexafluoroacetone to yield per-... 

Other Lewis acids have been demonstrated to provide moderate levels of enolization, including Aluminum Chloride, Magnesium Bromide, and Tin(II) Trifluoromethanesulfonate. However, SnCU, Me2 AlCl, and ZrCLt failed to provide detectable enolization. ... 

Aldol-Type Addition. Aldol-type addition of the magnesium enolate of (R)-(+)-7-butyl 2-(p-tolylsulfinyl)acetate, prepared with 7-butylmagnesium bromide, with aldehydes and ketones afforded, after desulfurization with Aluminum Amalgam, p-hydroxy esters in very high diastereoselectivity (eq Two chiral centers are created in the first step with very high diastereoselectivity (mainly one diastereomer is formed). A model M based on the structure of the sulfinyl ester enolate (determined by C NMR) and on electrophilic assistance of magnesium to the carbonyl approach, was proposed to explain and predict the absolute configuration of the two created chiral centers. ... 

In the synthesis of L-acosamine, condensation of the predominant isomer 609 with the magnesium enolate of tert-hutyl acetate gives 610 in 54% yield. Acetylation to 611 (77% yield) and subsequent catalytic hydrogenation in an autoclave (70 Kg/cm ) furnishes 612 as a single isomer. Acid hydrolysis of the ketal, benzoylation under Schotten—Baumann conditions, and lactonization affords 613. Reduction of the lactone to a lactol with diisobutyl-aluminum hydride gives N- benzoyl-L-acosamine (614) as an anomeric mixture. 

As polarization and enolization of carbonyl groups are the crucial steps in these efforts, one is not surprised that oxophilic countercations such as aluminum and magnesium are particularly helpful and that they manage to trigger the in situ enolate formation. 

Probably the oxophilic aluminum compound attacks the carbonyl groups to form 5, which is then reduced to enolate 7. As long as this enolate is not quenched by protonation, one could continue with other transformations in a molecule of this type without touching the 1,3-dicarbonyl moiety. As predicted, this type of enolate formation can also be exercised with magnesium as the countercation, and as an example one notices the dimerization of cyanoacetate to form the P-dicarbonyl system 10 [7]. 

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 transmetallation of alkali enolates 164 (M = Li, Na, K) with metal salts (M Y L, ) is a general method for the preparation of a large variety of enolates 165, provided that is less electropositive than M. It is particularly suitable for such enolates 165 whose reactivity and/or selectivity is tuned by additional ligands L. Thus, a variety of magnesium, boron, aluminum, siUcon, tin, titanium, zirconium, and zinc enolates become readily available (Scheme 2.48) [2c,d]. Usually, the configuration of the enolates is maintained during the transmetallation, but cis-tmns isomerization in the transmetallated enolates occur occasionally. Individual examples will be discussed with their applications in asymmetric syntheses. 

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