Oximes hindered

Sterically hindered derivatives of isoxazole carboxylic acids have yielded a goodly number of antibiotics. Chlorination of the oxime of the appropriately substituted benzaldehydes (15) leads to the intermediates, ( 16. Condensation of the chloro oximes with ethyl acetoacetate in base gives the esters (17) of the desired isoxazole carboxylic acids. Alternately, the esters... 

Scheme 5 details the synthesis of / -cormorsterone (14) from 17. Oxidative scission of both carbon-carbon double bonds in 17 with ozone, followed by two straightforward operations, furnishes intermediate 38. The stability of the oxime in these systems is noteworthy, and is attributed to its hindered nature. At this juncture, it is instructive to note that substituted cyclopentene rings, like the... 

In a reaction very much like 16-19, oximes can be prepared by the addition of hydroxylatnine to aldehydes or ketones. Derivatives of hydroxylamine, for example, H2NOSO3H and H0N(S03Na)2, have also been used. For hindered ketones, such as hexamethylacetone, high pressures (e.g., 10,000 atm) may be necessary. " ... 

Fry and Newberg 1,2> examined the electrochemical reduction of nor-camphor oxime (109) and camphor oxime (110) to the corresponding amines. The results of this study are shown in Table 3. It is clear from a comparison of these data with those in Table 2 that the electrochemical reduction of oximes 109 and 110 takes a very different stereochemical course from reduction of the corresponding anils 103 and 104. Reduction of oximes apparently proceeds under kinetic control, affords products corresponding to protonation at carbon from the less hindered side of the carbon-nitrogen double bond, and affords the less stable epimeric amine in each case. It is not evident why the stereochemistry of reduction of anils and oximes should differ, however. 

Silylation of AN (528b,c,e) with another silylating agent (Me3SiCl/Et3N) gives poorly separable mixtures of unidentified products. However, the reaction of AN (528a) under these conditions produces the silyl derivative of bis-oxime (533), which can be subjected to desilylation to prepare free bis-oxime (534) (491, 497). The stereoselectivity of the reaction with respect to the new C,C double bond is low (E/Z 1.3 1). Silylation of sterically more hindered nitroalkane (528 d)... 

The 36d-LAH complex was applied to the reduction of ketone oximes and their O-tetrahydropyranyl and O-methyl derivatives to optically active amines (69). Results for a variety of phenyl alkyl and dialkyl ketones are shown in Table 4. The predominant amines formed all were of the S absolute configuration with optical purities up to 56%. The oxime hydroxy group presumably reacts with the less hindered H2 in the 36d-LAH complex (cf. Scheme 6) to form an oxime complex (45), which probably undergoes infermolecular hydride transfert of H2 from a second molecule of the 36d-LAH complex (Scheme 8). Asymmetric reduction with the ethanol-modified 36d-LAH reagent gave amines of R con-... 

Electrochemical reduction of camphor-and norcamphoroxime at a Hg cathode proceeds with a high degree of stereoselectivity to give products of opposite stereochemistry to those formed in the dissolving metal (Na-alcohol) reduction of the oximes. The electrolyses are proposed to proceed by a kinetically controlled attack by the electrode on each oxime from the less hindered side (Fig. 62) [348]. In contrast, the corresponding N-phenyl imines yield products of the same stereochemistry as those isolated from a dissolving metal reduction. Cyclic voltammetry and polarographic data point to RH and intermediates in this case that are proto-nated from the least hindered side [349]. 

The following compounds are unaffected by bis(p-methoxyphenyl) telluroxide dithi-olanes, enamines, aldehydes, ketones, alcohols, pyrroles, indoles, amino acids, aromatic amines, monohydroxyarenes, esters, hindered thiocarbonates, isonitriles, oximes, arylhy-drazones, sulphides, and selenides. ... 

Reduction of chiral ketoximes results in formation of a new stereogenic center. Although mixtures of stereoisomers are generally obtained, kineticaUy controlled reduction of cyclic oximes (e.g. 86, equation 59 and 87, equation 60) with sodium cyanoborohydride can proceed with high diastereoselectivity Stereoselectivity in these reactions closely resembles that of reduction of ketones with complex hydrides featuring attack from the least hindered side. 

Bicyclic derivatives of furazan A-oxide are prepared by nitrile oxide dimerization reaction. Dioxime 272 (R, R = Me) undergoes cyclization to the corresponding 4,4-tetramethylperhydrocycloocta[c]furazan A-oxide 273 (84% yield) by treatment with NaOCl/HaO/CHaCla at 0°C and then refuxing in toluene (equation 117). However, in the cases of sterically less hindered oximes 272 (R = H, Me R = H) only complex mixtures of oligomerization and cyclization products could be obtained ". Interestingly, the reaction of pyridyl oxime -274 with TsCl afforded 1,2,5-oxadiazole 275 as single product (equation 118). On the other hand, the reaction of Z-isomer of oxime 274 leads only to 0-tosylated oxime. ... 

Carboxymethyl)-oximes are normally prepared in a single step from ketones, but the 11-oxo-group in 17j8-hydroxy-5a-androstane-3,ll-dione was too hindered to react with O-carboxymethylhydroxylamine. Instead the 11-oxime, obtained by selective cleavage of the 3,11-dioxime, was treated with alkaline sodium chloro-acetate to prepare the hapten (31) for linking to BSA via C-11. ... 

Two features about steroid oximes should be mentioned since they impact quantification firstly, an 11-carbonyl group is sterically hindered and not derivatized, so steroids containing this moiety have a mass 29 units less than expected. Secondly, each carbonyl can be derivatized two stereospecific ways, termed syn- and anti- . These may or may not be resolved by GC. If they are resolved it is preferable to determine the area of both peaks since the relative amount of syn- and anti- forms may vary between different derivatizations. 

The OH-group in the E-configuration of acetophenone oximes has been proposed to be more accessible than that in the sterically hindered Z-conformers, and thus may be more... 

The kinetics of oxidative deoximation of aldo- and keto-oximes by 2,2/-bipyridinium chlorochromate (back to the parent carbonyl compounds) have been studied in DMSO, where the reaction is found to be first order in both oxime and oxidant.89 The aldoximes proved more reactive, and rates correlated well with the Pavelich-Taft dual substituent equation. Following extension of the study to hindered cases, and to 18 other solvents (analysed by Taft and Swain multi-parametrics), a cyclic intermediate is proposed for the rate-determining step. The same reaction order behaviour is found using the pyridinium version, and again electronic, steric, and solvent effects were examined.90... 

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