Prevention ketones

Attempts to predict, from precedent, the behaviour of any particular alicyclic epoxide with a Lewis acid are complicated by the great diversity of known reactions. The best that can be done is first to consider the reaction of the epoxide in isolation as likely to lead to a ketone, unless special steric or conformational features interfere with the development of the necessary transition state. Consideration must then be given to inductive or electrostatic effects of any substituents in the vicinity of the epoxide, which may prevent ketone formation. 

Both Norrish Type II photoproducts remained incarcerated. Furthermore, the surrounding host prevented ketonization of 105, which requires acid or base catalysis. In fact, the resistance of incarcerated photoenol to ketonize is remarkable. Incarcerated 105 required... 

Acetals hydrolyze under acidic conditions, but they are stable to strong bases and nucleophiles. Acetals areeasily made from the corresponding ketones and aldehydes and easily converted back to the parent carbonyl compounds. This easy interconversion makes acetals attractive as protecting groups to prevent ketones and aldehydes from reacting with strong bases and nucleophiles. 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

Stereoselectivities of 99% are also obtained by Mukaiyama type aldol reactions (cf. p. 58) of the titanium enolate of Masamune s chired a-silyloxy ketone with aldehydes. An excess of titanium reagent (s 2 mol) must be used to prevent interference by the lithium salt formed, when the titanium enolate is generated via the lithium enolate (C. Siegel, 1989). The mechanism and the stereochemistry are the same as with the boron enolate. 

No intennolecular reaction of malonate or /3-keto esters with halides has been reported, but the intramolecular reaction of /3-diketones such as 790 and malonates proceeds smoothly[652,653]. Even the simple ketone 791 can be arylated or alkenylated intramolecularly. In this reaction, slow addition of a base is important to prevent alkyne formation from the vinyl iodide by elim-ination[654]. 

These association reactions can be controlled. Acetone or acetonylacetone added to the solution of the polymeric electron acceptor prevents insolubilization, which takes place immediately upon the removal of the ketone. A second method of insolubiUzation control consists of blocking the carboxyl groups with inorganic cations, ie, the formation of the sodium or ammonium salt of poly(acryhc acid). Mixtures of poly(ethylene oxide) solutions with solutions of such salts can be precipitated by acidification. 

Bode acid catalyzes the air oxidation of hydrocarbons and increases the yield of alcohols by forming esters that prevent further oxidation of hydroxyl groups to ketones and carboxyHc acids (see Hydrocarbon oxidation). 

The acid prevents the formation of insoluble aluminum salts that make separation of ether-water layers difficult. It is helpful in this regard to stir the mixture of water, ketone, and acid for an hour or so before extracting the ketone with ether. 

A condensible blowdown tank, designed on a similar basis to that described above for phenol, may be provided in other services where a conventional condensible blowdown drum would not be acceptable (e.g., due to effluent water pollution considerations). Examples of such cases are methyl ethyl ketone (MEK) and dimethyl formamide (DMF). A suitable absorbing material is specified (e.g., a lube oil stock for MEK water for DMF), and the design must include consideration of maximum permissible operating temperatures to prevent excessive vapor evolution or the boiling of water. 

Sometimes reduction of a ketone by NaBH4 is accompanied by hydrolysis of an ester elsewhere in the molecule. Norymberski found that a 20-keto-21-acetoxy compound with NaBH4 in methanol at 0° for 1 hour gives the 20/ ,21-diol. 50 % aqueous dimethylformamide has been used as the solvent in an attempt to prevent acetate hydrolysis, but sometimes under these conditions the 21-acetoxy group migrates to the 20-position. The rearrangement is favored by addition of the 20-acetate as seeds or by addition of... 

The formation of an epoxyketone (1) is generally favoured when the expected product of oxidation of an allylic alcohol is a cisoid enone. This type of reaction is promoted by acid conditions and may be prevented by using the chromium trioxide-pyridine reagent which gives only the unsaturated ketone (2) corresponding to the starting alcohol. ... 

Halogenation of the double bond usually prevents the satisfactory preparation of vinylogous a-halo ketones by direct reaction of unsaturated ketones with... 

A modification introduced by Joly allows clean dehydrohalogenation of (5a or 5)8) 2,4-dibromo-3-ketones to -3-ketones. This involves the use of an excess of lithium carbonate in DMF, which presumably prevents the... 

The nitrites aie most conveniently prepared from the corresponding alcohols by treatment with nitrosyl chloride in pyridine. The crude nitrites can be precipitated by addition of water and recrystallized from appropriate solvents. However nitrites prepared from carbinols in which the adjacent carbon is substituted by halogen, free or esterified hydroxyl or a carbonyl function are very readily hydrolyzed and must be recrystallized with great care. In general the photolysis gives higher yields if purified and dried nitrites are used which do not contain acids or pyridine, although occasionally the addition of small amounts of pyridine is recommended in order to prevent hydrolysis of the nitrite. Traces of acids do in fact catalyze the thermal decomposition of secondary nitrites to equimolar amounts of alcohol and ketone. ... 

Carbon beds are often used for absorbing vapors in vent systems but absoiption produces heating, and the beds may catch fire, particularly if they are used to absorb ketones, aldehydes organic, acids, and organic sulfur compounds. References 35-37 describe some fires and ways of preventing them. 

Aldehyde or Ketone may be separated from the other constituents by shaking the liquid, which should be free from water, with a saturated solution of sodium bisulphite, and decanting or filtering the liquid residue. If the liquid is soluble in water, like ethyl alcohol, it may piecipitate the bisulphite of sodium. This is prevented by adding a little ether befoie introducing the bisulphite into the liquid. 

Reactions of enamines with selenium dioxide gave low yields of enamino ketones (J8). Aromatization of cyclohexanone derived enamines eould be largely prevented by the use of aeetonitrile as solvent for the reaction. Even then, yields were eonsiderably below the limit of 50%, imposed by the generation of an equivalent of water. 

The rearrangement with ring contraction probably is the most important synthetic application of the Favorskii reaction it is for example used in the synthesis of steroids. Yields can vary from good to moderate. As solvents diethyl ether or alcohols are often used. With acyclic a-halo ketones bearing voluminous substituents in a -position, yields can be low a tcrt-butyl substituent will prevent the rearrangement. 

An unusual solvent system was chosen for the intramolecular reductive alkylation of the masked amino ketone (15). The purpose of the strongly acid system was to prevent cyclization of the deblocked amino ketone to 16, further hydrogenation of which gives the unwanted isomer 17 by attack at the convex face. The desired opposite isomer can be obtained by reduction of 16 with UAIH4 (52). 

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