Acetylations enols with ketones, acetyl chloride

Enol acetates are readily prepared from saturated ketones by reaction with acetic anhydride alone or diluted, in the presence of acetyl chloride ... 

Enol esterification with acetyl chloride-acetic anhydride gives the A -trien-3-acetate, but reaction with isopropenyl acetate or with hot acetic anhydride-pyridine gives A " -trien-3-acetates. " Since A"" -3-ketones react with Girard reagents, these linear dienones can be separated from A ""-3-ketones. ... 

Ketene thioacetals.1 The enolate (1) of 2-carbomethoxy-l, 3-dithiane does not normally react with aldehydes or ketones, but a 1 1 mixture of trimethyl acetyl chloride and an aldehyde reacts in THF to form the pivaloxy ester 2. This product undergoes decar-boalkoxylation (9, 283 11, 301) when heated in dry DMF with excess Lil to form ketene thioacetals (3). 

Enol acetylation. An a-halo-a-arylacetophenone such as (1) cannot be converted into the enol acetate by reaction with isopropenyl acetate, and conversion into the sodium enolate by sodium methoxide followed by treatment with acetyl chloride gives low yields. However, Cooper and Owen28 found that, if the ketone is treated with dry methanol and sodium hydride and the mixture let stand until evolution of hydrogen has ceased and then treated with acetyl chloride, the enol acetate is obtained in yield of50-70%. 

The Mannich and related reactions provide one of the most fundamental and useful methods for the synthesis of P-amino ketones or P-amino esters. Three-component Mannich-type reactions of aldehydes, amines, and silyl enol ethers have been developed. With the development of green solvent systems, this reaction was also examined in a fluorous phase using perfluorinated rare earth metal salts including Sc(0S02C8Fi7)3 [5]. A characteristic point of this system is that it can be reused many times without reloading a new catalyst. There are also many reports on other scandium-catalyzed Mannich reactions. For example, Sc(OTf)3 was found to be an efficient catalyst for the three-component Mannich-type reactions of aromatic aldehydes, ketones, and nitriles in the presence of acetyl chloride (Scheme 12.3) [6]. 

Activation of C-X Bonds. Even more important than carbonyl activation, ZnBr promotes substitution reactions with suitably active organic halides with a variety of nucleophiles. Alkylation of silyl enol ethers and silyl ketene acetals using benzyl and allyl halides proceeds smoothly (eq 13). Especially useful electrophiles are a-thio halides which afford products that may be desulfurized or oxidatively eliminated to result in a,p-unsaturated ketones, esters, and lactones (eq 14). Other electrophiles that have been used with these alkenic nucleophiles include Chloromethyl Methyl Ether, HC(OMe)3, and Acetyl Chloride... 

Another illustrative example of a solvent-dependent keto/enol tautomerism of a monocarbonyl compound is given by 4-[(pent-l-ylthio)acetyl]-A-(4 -chlorobenzyl)-pyridinium chloride. In the HBA solvent dimethyl sulfoxide, the enol predominates (84 cmol/mol), while in the HBD solvent water it exists as the ketone (47 cmol/mol) and its hydrate (40 cmol/mol) [250]. Recrystallization of this pyridinium salt from a DMSO/ ethyl acetate mixture yields the pure enoUc form, with one molecule of DMSO per molecule of enol present in the crystal lattice. 

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