Dibromoketones, reaction with

In a reverse of the usual role of the a-dibromoketone, (86) undergoes reaction with potassium O-ethyldiselenoxanthate (87) to give the l,3-oxaselenol-3-one (88 61% yield) and the 1,2-oxaselenole (89 30% yield) (78)CZ36l). 

The procedure described here illustrates the preparation of mixed lithium arylhetero(alkyl)cuprate reagents and their reactions with carboxylic acid chlorides,4 These mixed cuprate reagents also react with a,a -dibromoketones,12 primary alkyl halides,4 and a,/3-unsaturated ketones,4 with selective transfer of only the alkyl group. 

Electrochemical reduction of a,a -dibromoketones affords the unstable cyclo-propanone, which is in equlibrium with the dipolar intermediate 22. The cyclopro-panone hemiacetal is isolated in yields of 40 - 85 % from reaction in acetonitrile and methanol at -20 °C [99], The dipolar form can be trapped in a cycloaddition process with furan [100], Reaction with acetic acid leads to the a-acetoxy-ketone.[101]. Unstable three membered heterocyclic rings are intermediate in the reduction of sulphur and phosphorus linked dibromo compounds 23. In these reactions, the heteroatom is extruded leaving ci - and trans-stilbenes as the isolated products [102,103],... 

The synthesis of seven-membered rings from furans and a,a -dibromoketones via oxyallyl intermediates is well known. It was reported that this reaction occurs quite easily when water is used as solvent <97TL8031>. In a study on the influence of steric and electronic effects on a function attached at C-2 of furans in the yield and diastereoselectivity of [4 + 3] cycloaddition reactions with oxyallyl cations it was found that in almost all cases a c/x-diastereospecificity and a high endo diastereoselectivity is obtained <97T11669>. A tandem Peterson olefination-[4 + 3] cycloaddition reaction with furans has been reported <97JOC1578 97TL386l>. For an intramolecular [4 -t- 3] cycloaddition see <97JOC6051>. The first asymmetric [4 + 3]... 

Metals can be activated directly under the form of fine dispersions or colloids by sonication in suspension. Thus, mercury emulsions can be formed in various solvents. Their reactions with a,a dibromoketones were described by Fry et al. (p. 231). The effects of sonication in this system were interpreted as a result of the large surface area of mercury droplets, and comparable results were obtained by high-speed stirring. The electrification of the particles was not envisaged. 

Care should be taken to prevent either the dibromoketone or the cyclopropenone from coming into contact with the skin, as allergic reactions have been observed in several cases. The use of gloves is recommended especially for the bromoketone. The latter product has a wide melting range because it is a mixture of the meso- and d,/-compounds. 

In organometallic chemistry, the use of ultrasound in liquid-liquid heterogeneous systems has been limited to Hg. The emulsification of Hg with various liquids dates to the very first reports on sonochemistry (3,203,204). The use of such emulsions for chemical purposes, however, was delineated by the extensive investigations of Fry and co-workers (205-212), who have reported the sonochemical reaction of various nucleophiles with a,a -dibromoketones and mercury. The versatility of this reagent is summarized in Eqs. (30)-(36). 

To a dry 1-1., three-necked flask fitted with an efficient mechanical stirrer, a low temperature thermometer, and a solid addition assembly (Note 11) is added 97 g. (0.30 mole) of dibromoketone and 700 ml. of anhydrous tetrahydrofuran (Note 12). After the reaction vessel has been... 

The most effective synthesis for thietanones is the eliminative cyclization of halogenated ketones with hydrogen sulfide ions in the presence of bases (Eq. lOb). The reaction of 1,3-dibromoketone derivatives with sodium hydrogen sulfide produced 3-thietanones in association with a five-membered cyclic disulfide (Eq. 10c). 

The reactions of a,a -dibromoketones with iron carbonyls generate oxyallyliron complexes (75). These undergo cycloaddition with cyclopentadiene and furan, but with thiophene only products of electrophilic attack are obtained (78JA1765). Thus the oxyallyliron complex (75 R = Me R = H) reacts with thiophene to produce (76) in 37% yield. 

It is known that the first stage of the reaction of a,/ -dibromoketones with amines is their dehydrobromination leading to ce-bromo derivatives [9, 21]. o-Bromochalcones are mentioned in the literature and also can be used for the synthesis of aziridinyl ketones [8,11, 22, 23, 24, 25]. For example, Khomutov et al. [25] carried out the synthesis of the dodecafluoro derivative 13 by the interaction of the corresponding a-bromoketone 11 with hexylamine 12 (Scheme 1.4). 

In the case of the interaction of [Pg.19]

Another intermediate for which Die Is-Alder trapping provided convincing evidence is the oxy-allyl cation. This compound can be made from a,oc -dibromoketones on treatment with zinc metal. The first step is the formation of a zinc enolate (compare the Reformatsky reaction), which can be drawn in terms of the attack of zinc on oxygen or bromine. Now the other bromine can leave as an anion. It could not do so before because it was next to an electron-withdrawing carbonyl group. Now it is next to an electron-rich enolate so the cation is stabilized by conjugation. 

However, we are not surprised that an enolate ion is formed from a ketone in basic solution. The oxyallyl cation is much more surprising. How can we be convinced that it really is an intermediate There are several alternative ways to make the same intermediate. If basic nucleophiles such as the methoxide ion are avoided and reaction of zinc with an a,a -dibromoketone in a nonnucleophilic solvent like diglyme is used instead, the oxyallyl cation can be trapped in a Diels-Alder reaction. This is tile basis for a good synthesis of seven-membered rings. 

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