A, jS-unsaturated ketones

The photochemical addition of simple olefins to a,j -unsaturated ketones is a reaction of great current interest. The steroidal A -20-ketone system is especially prone to cycloaddition under mild conditions. Sunder-Plassmann et irradiated 3j5-acetoxypregna-5,16-dien-20-one (67) in the presence... 

In the reaction of simple 2-alkenals and a./J-unsaturated ketones with 2-butenylmagnesium chlorides essentially no diastereoselectivity is achieved it is improved by sterically highly demanding substituents adjacent to the carbonyl group43. 

Obtemperanskaya and Kareva25 determined a, /J-unsaturated ketones and sulphones by adding sodium sulphite to the sample in p-dioxan. After 5-10 min they titrated with standard sulphuric acid potentiometrically or to thymolphthalein ... 

Fig. 11. Interaction diagram. a,/J-Unsaturated ketone, and olefins (/ and s stand for large and small respectively)...

Enolization of a,/J-unsaturated ketones, e.g. 19, under strong acid conditions leads to a mixture of homoannular and heteroannular A2,4- and A4,6-dienes (e.g. 20 and 21 see equation 6)42. 

Although the acylcobalt tetracarbonyls react with hydroxide ion under phase-transfer conditions, in the presence of alkenes and alkynes they form o-adducts rapidly via an initial interaction with the ir-electron system. Subsequent extrusion of the organometallic group as the cobalt tetracarbonyl anion leads to a,(J-unsaturated ketones (see Section 8.4). In contrast, the cobalt carbonyl catalysed reaction of phenylethyne in the presence of iodomethane forms the hydroxybut-2-enolide (5) in... 

In cyclic a,j -unsaturated ketones such as 2-oxo-2,3,4,5,6,7,8,10-octahy-dronaphthalene, where the reduction may lead to two stereoisomers, hydro-... 

Examples of alkylations of dienolates derived from a,/J-unsaturated ketones by base-catalyzed proton abstraction38 38" are shown for enolate 32. Here it is also evident that the presence of a 1,3-interaction greatly influences the diastereoselectivity. 

The regiochemistry of ketene iminium salt cycloadditions can also differ from ketene cycloadditions. Whereas reaction of styrene with a series of ketene iminium salts gave 3-phenyI-cyclobutanones7 (60-70% yield) similar to the regiochemistry of ketene cycloadditions, reaction with a series of acrylates and a,/J-unsaturated ketones gave cyclobutanones 5 with regiochemistry opposite to what would be expected from electrostatic considerations of ketene cycloadditions.s... 

In a,(J-unsaturated ketones, nitriles, and esters (e.g., 119), the y hydrogen assumes the acidity normally held by the position a to the carbonyl group, especially when R is not... 

Begin not with the ketone itself, but with an a,(J-unsaturated ketone in which the double bond is present on the side where alkylation is desired. Upon treatment with lithium in liquid NH3, such a ketone is reduced to an enolate ion. When the alkyl halide is added,... 

Aldol reaction. The reagent (I) adds in a 1,4-fashion to an a,/J-unsaturated ketone lo give an aluminum enolate, which undergoes aldol condensation with an aldehyde. The adduct is converted into an a-substituted-a./l-unsaturated ketone on sulfoxide elimination.1... 

Cyclopropanatioru The ylide I reads with a,/j-unsaturated ketones to form cyclopropanecarboxylic acids in good yield. The reaction with ketones gives glycidic esters 60% yield). 

Selective reduction. NaHTe reduces a./J-unsaturated ketones, aldehydes, esters, and lactones at room temperature to the corresponding saturated carbonyl compounds in high yield. It is not satisfactory for reduction of enediones. 

The most frequently encountered members of this group are A4-3-keto steroids. This is clearly a result of the biological importance of a number of compounds in this series. Among the remaining a,/J-unsaturated ketones the A16-20-ketones (available by degradation of sapogenins) are often encountered because of their utility as intermediates for further transformations. 

An acetoxyl group at C-21, if present, is retained and an a,/J-unsaturated ketone system in ring A does not interfere. 

Some non-cyclic thioacetals have been used sometimes, particularly those bearing phenylthio groups or some special feature such as an extra electron-withdrawing group. The corresponding carbanion can then be used for a 1,4 addition on an a,(J-unsaturated ketone as reported [282],... 

The mechanism by which a, /j-unsaturated ketones (see Scheme 16), jS-keto esters, and uracil derivatives react with iodine in the presence of bis(tctra-w-butylammonium) peroxydisulfate (318) in acetonitrile to give the appropriate iodinated products in good yields is unclear.289 The mechanism may involve the cleavage of (318) to give an n-butylammonium sulfate radical, which can react to fonn a cationic iodine radical and sulfate anion the substrate then reacts with the iodine radical to form an iodine-bridged intermediate. 

In contrast to all investigated a,/J-unsaturated ketones, cyclopentadienones like 497 react with 149 and 473 exclusively to give the spiro compounds 498 i.e. the nominal [2 + 2] cycloadduct, in 61 and 63% yield (equation 164)243. 

UV irradiation of a cis bicyclic a,/J-unsaturated ketone in diethyl (+)-tartrate afforded trans ketone 113 enriched in the (—)-enantiomer, with an estimated optical purity of0.5-l%281. 

A retrosynthetic analysis of a,/J-unsaturated ketones leading to various methods of synthesis is outlined in Section 5.18.2, p. 798. These methods are equally applicable to aromatic aldehydes. Aromatic aldehydes condense with aliphatic or mixed alkyl aryl ketones in the presence of aqueous alkali to form a,[i-unsaturated ketones (the Claisen-Schmidt reaction). 

The quaternary salts produced by the Mannich reaction undergo a number of conversions useful in synthesis. For example, although stable at room temperature they eliminate an amine hydrochloride on heating to yield an a,/J unsaturated ketone (e.g. the conversion of dimethylaminopropiophenone to phenyl vinyl ketone, Expt 6.147). 

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