Oxime from aldehydes and ketones

Synthesis from Aldehydes and Ketones. Treatment of aldehydes and ketones with potassium cyanide and ammonium carbonate gives hydantoias ia a oae-pot procedure (Bucherer-Bergs reactioa) that proceeds through a complex mechanism (69). Some derivatives, like oximes, semicarbazones, thiosemicarbazones, and others, are also suitable startiag materials. The Bucherer-Bergs and Read hydantoia syntheses give epimeric products when appHed to cycloalkanones, which is of importance ia the stereoselective syathesis of amino acids (69,70). 

A small number of examples is available for the synthesis of E and Z isomers of oximes. In many cases, E isomers were obtained either from the Z forms (by the hydrochloride method) or isolated by column chromatography. Often, the reagents that have been used for oximation of aldehydes and ketones also catalyze the interconversion of Z and E isomers. The rate of equilibration of a mixture of Z and E isomers and the position of the equilibrium is temperature-dependent ". In 2001, Sharghi and Sarvani reported a convenient method for controlling the stereochemistry of the reaction of hydroxylamine hydrochloride with aldehydes or ketones in the solid state. The highly stereoselective conversion of aldehydes and ketones to their corresponding oximes... 

This compound may be used to form oximes of aldehydes and ketones without alkali because the aqueous solution has a pH of about 5 at which the maximum rate of formation of oximes occurs. It is prepared conveniently from the chloride as follows ... 

Functional groups containing the C = N bond, such as hydrazones, oximes and diazo compounds, can also be used in the synthesis of fluorinated derivatives and these reactions are the subject of this section. The reaction products in these procedures are usually the corresponding g cw-difluoromethylene compounds. Since oximes and hydrazones arc obtained from aldehydes and ketones, these reactions can be considered as an alternative to the direct transformation of a carbonyl group into the difluoromethylene group. As indicated in Section... 

This class of compounds has been prepared by (1) condensation of a-hydroxy / -oximes with electrophilic carbonyl derivatives (Scheme 13 and Equation 55) (2) condensation of l,l,l-trifluoroalkane-2,3-dione 3-oximes with aldehydes and ketones (Equation 62) and (3) Reaction of hydroxylamines with ketals (Equation 56). From these methods, the most convenient seems to be (2), because the yields of the reactions are generally high, and the starting materials are easily available. 

Oximes can be prepared from aldehydes and ketones (Section 16.8B). 

Although the subject will arise again in Chapter 10, it is worthwhile considering at this point the rearrangement of oximes derived from aldehydes and ketones. It will be recalled (item 4, Table 9.4) that the reaction of hydroxylamine hydrochloride (H0NH2 HC1) with these carbonyl compounds yielded the possibility of )- and... 

Aldehydes and ketones may frequently be identified by their semicarbazones, obtained by direct condensation with semicarbazide (or amino-urea), NH,NHCONH a compound which is a monacidic base and usually available as its monohydrochloride, NHjCONHNH, HCl. Semicarbazones are particularly useful for identification of con jounds (such as acetophenone) of which the oxime is too soluble to be readily isolated and the phenylhydrazone is unstable moreover, the high nitrogen content of semicarbazones enables very small quantities to be accurately analysed and so identified. The general conditions for the formation of semicarbazones are very similar to those for oximes and phenylhydrazones (pp. 93, 229) the free base must of course be liberated from its salts by the addition of sodium acetate. 

Experiments.—Being a primary hydrazide (of carbamic acid), semicarbazide reduces ammoniacal silver solutions and Fehling s solution. It reacts readily with aldehydes and ketones with the elimination of water and formation of semicarbazones, which, since they are more easily hydrolysed than are phenylhydrazones and oximes, are to be preferred to the latter for purposes of separation and purification of carbonyl compounds. Shake an aqueous solution of the hydrochloride (prepared as described above) with a few drops of benzaldehyde, isolate the semicarbazone and purify it by recrystallisation from alcohol. Melting point 214° decomp. Benzaldehyde semicarbazone is decomposed into its constituents by gentle warming with concentrated hydrochloric acid. 

The use of mediators to improve reactivity or selectivity in nitrone cycloaddition chemistry begins with the nitrone generation step. As is well known, the N-alkyla-tion of oximes provides one of the most direct and convenient synthetic routes to N-alkylated nitrones from readily available aldehydes and ketones. Electrophilic mediators have been employed to activate alkenes for N-alkylation, both in intramolecular and intermolecular reactions. They include activation of the internal alkene function by the action of (a) strong nonmetallic electrophiles such as phenyl-selenenyl sulfate (159), and (b) metallic catalysts such as Ag(I) (160) and Pd(II) ions... 

The most general method for synthesizing amines involves the reduction of oximes and imine derivatives obtained from aldehydes or ketones (see Sections 5.5.2 and 4.3.11). By catalytic hydrogenation or by LiAltLj reduction, while 1° amines are prepared from oxime or unsubstituted imine, 2° amines are obtained from substituted imine. Unsubstituted imines are relatively unstable, and are reduced in situ. 

It may be noted that aldehydes and ketones can usually be obtained from the corresponding oximes, phenyl-hydrazones, semi-carbazones, etc., by hydrolysis with dilute mineral acids. 

Functionalization of the carbon radical resulting from cyclization of an aminium radical is an important step for synthetic chemists in order to obtain the desired product directly or to provide a handle for further transformations. Radical reactions of A-chloroalkenylamines (Section III,B) lead to /3-chloro pyrrolidines, which are prone to rearrangement to give piperidines. Reactions of N-nitroso alkenylamines lead to 8-nitroso pyrrolidines and, if an a-hydrogen is present, ultimately to oximes of aldehydes or ketones. Advantages of the latter transformation are the formation of stable substituted pyrrolidines and the utility of the oxime moiety in regard to further transformations. 

You should already be familiar with approximately half of the reactions listed in Table 9.2 from your introductory class. Moreover, you have probably tried to prepare an oxime, a phenylhydrazone, a 2,4-dinitrophenylhydrazone, or a semicarbazone. These compounds serve as crystalline derivatives with sharp and characteristic melting points for identifying aldehydes and ketones and for distinguishing them. When spectroscopic methods for structure elucidation were not available, such a means of identification was very important. 

Dipolar cycloadditions have been used successfully in assembling a number of these ring systems. Nitrile oxides, generated from a-chloro oximes and base, add to a wide range of aliphatic and aromatic aldehydes and ketones, but not to esters (Scheme 42), yielding... 

A general synthesis of quinazoline 1,3-dioxides and their 1,2-dihydro derivatives was devised by Taylor and Bartulin. In this synthesis o-hydroxyl-aminobenzaldehyde oxime condensed with aldehydes and ketones to yield 2,2-disubstituted 1,2-dihydro-l-hydroxyquinazoline 3-oxides (63). When the dihydro compounds (63 = H) derived from aldehydes or from formalde-... 

Oxidation of oximes and semicarbazones. Aldehydes and ketones can be regenerated from their oximes or semicarbazones by oxidation with excess ceric ammonium nitrate at—40 to 0° in methanol, acetonitrile, or acetic acid. Yields are in the range 70-90%.4... 

Various reagents have been used for subtracting aldehydes and ketones. Thus, to remove aldehydes and ketones from a mixture with alcohols and epoxides, Osokin et al. [66] used a packing containing diatomite impregnated with an aqueous solution of sodium hydroxide (6%) and a solution of hydroxylammonium chloride in polyethylene glycol (20%). Aldehydes and ketones were converted into oximes and retained by the stationary phase, whereas alcohols and epoxides were not retained by this packing. 

It is preparatively important that aldehydes and ketones can be regenerated by hydrolytic processes from their nitrogenous condensation products such as oximes, phenylhydrazones, semicarbazones, and SchifF bases. 

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