Enolates potassium hexamethyldisilazide

Spirocyclic oxindole 60 was synthesized by [3,3]-sigmatropic rearrangement of the Af-phenyl-O-acylhydroxamic acid 58 (equation 19). The potassium enolate formed by treatment of 58 with potassium hexamethyldisilazide at low temperature rearranged to 59, which easily cyclized to the spirocyclic oxindole 60. Spirooxindoles were previously synthesized by Wolff and Taddei. The spirooxindole 60 was formed in 51% yield from cyclohexanecarboxylic acid after heating the preformed lithium salts of phenyl hydrazide 61 to 205-210 °C. 

Enol carbonates Di-f-butyl dicarbonate, 94 Potassium hexamethyldisilazide, 257 Enol esters Enol acetates Acetic anhydride, 93 Bis(7i5-cyclooctadienyl)ruthenium(II), 35... 

Enolate hydroxylation (cf. 11, 108).2 Enolates of ketones or esters are oxidized by this oxaziridine to a-hydroxy carbonyl compounds. Yields are highly dependent on the base they are highest with potassium hexamethyldisilazide. Yields are generally higher than those obtained with the Vedejs reagent (MoOPH, 8, 207). 

The most general protocol (eq 10) has therefore been based on potassium hexamethyldisilazide (KHMDS) as the base addition of trisyl azide at —78 °C and then acetic acid at the same temperature. Excellent levels of diastereoselectivity are ohsCTved with most substrates and the method has been used widely in the enantioselective synthesis of a-amino acids from chiral iniides. Chemoselective azidation of an imide enolate in the presence of an ester function has been demonstrated (eq 11). The product distribution is nevertheless finely balanced, as discovered with the respective dimethyl and dihenzyl ethers of the 3,5-substituted phenylacetyl imide (3) (eq 12). ... 

Ester Enolates. Enantiomerically pure amino acids may ultimately be prepared via stereospecific ester enolate generation using an oxazolidine chiral auxiliary (eq 2). Moderate diastere-oselectivity is observed using potassium hexamethyldisilazide. 

Related thermodynamic enolization control has been observed using metallated hexamethyldisilazide to give the more substituted bromomagnesium ketone enolates. Metallation reactions of HMDS to yield Li, K, and Na derivatives are well known and the resulting nonnucleophilic bases have found extensive applications in organic synthesis (see Lithium HexamethyldisUaade, Potassium Hexamethyldisilazide, Sodium Hexamethyldisilazide). ... 

In a study on the electrophilic azide transfer to chiral enolates, Evans found that the use of potassium bis(trimethylsilyl)amide was crucial for this process. The KN(TMS)2 played a dual role in the reaction as a base, it was used for the stereoselective generation of the (Z)-enolate (1). Reaction of this enolate with trisyl azide gave an intermediate triazene species (2) (eq 4). The potassium counterion from the KN(TMS)2 used for enolate formation was important for the decomposition of the triazene to the desired azide. Use of other hindered bases such as Lithium Hexamethyldisilazide allowed preparation of the intermediate triazene however, the lithium ion did not catalyze the decomposition of the triazene to the azide.This methodology has been utilized in the synthesis of cyclic tripeptides. 

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