A-acetoxyketones

The formation of a-acetoxyketones by oxidation of enamines with thallic acetate has been studied in detail (27) and found to be of preparative value (80 % yields) particularly in five- and six-membered-ring ketone derivatives. Enamines of linear or seven-membered-ring ketones were oxidized also, but at very much slower rates. Enamines of aldehydes with a-hydrogen substituents underwent self-eondensations during the oxidation reactions. Lead tetraacetate was less satisfactory as an oxidizing agent. 

Another useful reagent for reduction of a-acetoxyketones and similar compounds is samarium diiodide.233 Sml2 is a strong one-electron reducing agent, and it is believed that the reductive elimination occurs after a net two-electron reduction of the carbonyl group. 

When this reaction sequence is applied to enol esters or enol ethers, the result is a-oxygenation of the starting carbonyl compound. Enol acetates form epoxides that rearrange to a-acetoxyketones. 

Anodic addition converts enolacetates into a-acetoxyketones or enones, depending on the reaction conditions [135], conjugated dienes into 1,2- or 1,4-dimethoxy alkenes [65], and hydroquinone dimethyl ethers into quinone bisketals [136, 137]. Anodic addition also affords products, some of which are of industrial interest, such as propylene oxide [138a] or 1,4-dimethoxydihydrofuran [138b]. To... 

The preparation of an activated metal for use in an organometallic synthesis can be effected in a separate sonication step. An early example of this was the preparation of finely dispersed mercury for the reduction of a,a -dibromoketone to a mixture of a-acetoxyketones which favours the tertiary product (Eq. 3.6) [71]. The procedure is simple in that the dibromocompound, dissolved in acetic acid is subjected to sonication in a bath in the presence of a small amount of mercury. In this particular case... 

Electrochemical oxidation of enol acetates in an undivided cell gives monomeric products in parallel with the reactions of simple alkenes [47, 48]. Thus, in the reaction of menthol enol acetate 23, the a-acetoxyketone product arises from nucleophilic attack of acetate ion on the radical-cation while the enone product... 

Lead tetraacetate can effect oxidation of carbonyl groups, leading to formation of a-acetoxyketones.177 The yields are seldom high, however. Boron trifluoride can be used to catalyze these oxidations. It is presumed to function by catalyzing the formation of the enol, which is thought to be the reactive species.178 With unsymmetrical ketones, products from oxidation at both a-methylene groups are found.179... 

The reactivity of ketones toward aryllead(iv) triacetates is quite different from the reaction of these ketones with lead(iv) tetraacetate, which gives the a-acetoxyketones. Under the usual conditions for arylation (pyridine, CHCI3), simple ketones remain unaffected. Only ketone enolates and some specially activated ketones have been successfully... 

Rather less common are condensations between a-hydroxyketones, ammonia and an imidate. Equivalent amounts of the hydrochloride salt of the imidate and the or-substituted carbonyl compound (cr-halogeno-and a-acetoxyketones also take part) are heated (40-70°C) in liquid ammonia (3-48 h) at elevated pressures. Yields of imidazoles vary between 10 and 90%, but the requirements of working with liquid anomonia in an autoclave may make this approach unappealing if alternatives are available [42,43]. Examples include 4-hydroxymethyl- (35%), 4-hydroxymethyl-2-phenyl- (73%), 4-hydroxymethyl-2-(p-tolyl)- (68%) and 2-benzyl-4-hydroxymethylimidazoles (85%) [44]. 

More elaborate 4-hydroxy-enals and -enones have been generated in a variety of ways, for example via alkynes or often via epoxides, " it being sometimes unnecessary to isolate the hydroxy-enone, " or via Homer-Wadsworth-Emmons condensation of P-ketophosphonates with a-acetoxyketones. Acetal, thioenolether or terminal aUcyne can be employed as surrogate for the carbonyl group. 1,2,3-Trienyl-... 

Cupric acetate a-Acetoxyketones from a-diazoketones Carbohydrate derivatives s. 3, 182 s. a. Am. Soc. 71, 2668 (1949) (CHsC00)2Cu COCHNs COCHjOAc ... 

Carbonyl reduction. Many substrates have been reduced enantioselectively to give alcohols trifluoromethyl ketones, a-acetoxyketones, pyridinophenones, ethyl a-methylacetoacetate, a-keto acid derivatives," and 3-chloro-2-oxoaIkanoic esters a,/3-Epoxy ketones undergo reduction and hydrolytic ring opening/... 

Enol acetates form epoxides, which are rearranged to a-acetoxyketones ... 

A number of deoxy-sugar derivatives have been prepared from the chiral diols obtained by reduction of a-acetoxyketones (37)-(39)... 

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