Oximes stereochemistry

The oxime nitrogen can also participate in cycloamination reactions to give pyrroles. Thus, treatment of oxime esters such as 185 with Pd(Ph3P)4 readily affords 186 [129]. The pentafluorophenyl group is necessary for good results otherwise a Beckmann rearrangement can unfavorably enter the picture. The oxime stereochemistry makes no difference on the outcome of the reaction. In addition to 186, pyrroles 187 and 188 were also prepared in this study (among others) [129]. 

Perreira M, Kim EJ, Thomas CJ, Hanover JA. Inhibition of 0-GlcNAcase by PUGNAc is dependent upon the oxime stereochemistry. Bioorg. Med. Chem. 2006 14 837-846. 

Additional insight into the competition between the various cyclic transition states is provided by a recent study of the reactions of crotylboronates (22) and (23) with the two isomers of oxime silyl ether (24 Scheme 8). The stereoselectivities of these reactions were found to be independent of the geometry of (24), both isomers of which were shown to be conflgurationally stable under the reaction conditions. Since the oxime stereochemistry defines the site of coordination to the boron atom, it seems likely that the (Z)-oxime isomer reacts preferentially through the chair-like transition state (28), while the ( )-oxime reactions proceed preferentially via boat-like transition state (27). Evidently, the chair-like arrangement... 

Again, data for c/s-heterovinylene sets were first correlated with the extended Hammett equation by Charton and Charton (73). There are three sets of data extant in the literature, all for oximes. The sets studied are set forth in Table XXVII (sets 274 through 27-6). Results of the correlations and values of pj are reported in Tables XXVIII and XIX, respectively. Of the three sets studied, two did not give significant correlation. The stereochemistry of the antt-acetyl ketoximes is discussed by Charton and Charton (73). It would seem that the localized effect is predominant in the case of the c/s-heterovinylene sets. 

It is important to note that the E isomer of the oxime predominates over the Z isomer in all the cases 21 a-f and the preferred stereochemistry for the isoxa-zolines 22a-f is trans. 

The unsaturated oximes 224 (see Table 21) were readily prepared by AT-alkyl-ation of allyl amines with a-bromoketones or O-silyl-a-bromoaldoximes. Heating the oximes 224 in toluene under an argon atm at 110 -180 °C smoothly led to isoxazolidines 225 in good yields with cis ring junction stereochemistry. Even when three stereocenters were generated, as in 225 g-1, a single stereoisomer... 

Oximinoimidazolides can be obtained from oximes and CDI with retention of stereochemistry heating in octane or nonane under anhydrous conditions leads to their conversion to imidazolylimidates [123]... 

Oximes Amines can be prepared by the catalytic hydrogenation— hydrogenolysis of oximes over nickel or noble metal catalysts. Nickel is used usually in the presence of ammonia. Noble metals are used under mild conditions. The stereochemistry of the reaction depends on the circumstances. On Ra-Ni the trans-2-alkylcyclohexylamine (41) was the main product,529 whereas on palladium the cis product (42) was produced (Scheme 4.137).530... 

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. 

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]. 

NMR parameters, such as chemical shifts and coupling constants, have been extensively investigated through the use of organic compounds that exhibit restricted rotation, such as oximes. NMR data are routinely used in determination of the stereochemistry of organic compounds and rigid strucmres, such as oxime conformers, can help in the interpretation of many of the physical and chemical properties that are associated with effects of lone pairs on different types of systems that contain nitrogen. 

The last few years have seen the usual dramatic increase in the number of publications in this area, coming from an impressive array of groups using the full range of NMR techniques, and NMR data have been used to determine the stereochemistry of rather complex systems containing hydroxylamines, oximes and hydroxamic acids, but systematic studies have been concentrated mainly on oximes. 

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... 

Both oximes (10) and their ester (11) or ether derivatives can be used in the classical Beckmann rearrangement and the reaction usually proceeds under acidic or neutral conditions (although basic conditions may also be used). In sharp contrast, only 0-oxime esters can be used as starting materials for the Neber rearrangement and basic conditions are always necessary. The Neber rearrangement is not stereospecific, as the stereochemistry of the starting material E or Z) does not influence the outcome of the reaction. In... 

The formation of a very electrophilic intermediate 258 from 256 and 257 is proposed (equation 78). The hydroxyl group of the oxime adds to 259, giving a reactive cationic species 260 that rearranges and affords the nitrile 261 (in the case of aldoxime, equation 79), or the amide 262 upon hydrolytic workup (equation 80). The conversion of 260 to the nitrilium ion should occur through a concerted [1,2]-intramolecular shift. This procedure can be applied in the conversion of aldoximes to nitriles. It was observed that the stereochemistry of the ketoximes has little effect on the reaction, this fact being explained by the E-Z isomerization of the oxime isomers under the reaction conditions. 

However, the stereochemistry of the oxime cannot be easily controlled and this may be a drawback for the synthetic utility of the Beckmann rearrangement. When a mixture of oximes is obtained from the ketone and when the isomerization of the oxime cannot be prevented during the rearrangement reaction, a mixture of amides is obtained. In other less favourable cases, the intended oxime cannot be obtained and the wrong amide will result from the rearrangement reaction. 

The synthesis of chiral, non-racemic a-amino acids remains an interesting field of investigation and the synthesis of fully protected a,a-disubstituted a-amino acids 331 via the Beckmann rearrangement of tosylated oximes was achieved (equation 123). As expected, the migrating group was able to retain the original stereochemistry and good yields and excellent enantioselectivities were observed ... 

Electrochemical reductions of camphor oxime (R = Me) and norcamphor oxime (R = H) at a mercury cathode proceed with a high degree of diastereoselectivity (equation 3) . The products are in fact of opposite stereochemistry to those formed in dissolving metal (sodium-ethanol) reductions of the oximes. 

The reaction of aqneons green Np, or its bine ammonia complex, with colorless dimethylglyoxime (DMG) to form a vibrantly red precipitate of a 1 2 metaLDMG complex demonstrates an example of precise stereochemistry and oxime deprotonation in what is perhaps the archetypal analytical metal dioxime reaction (equation 1). This transformation certainly intrigued both authors early in their education. It is interesting to note that DMG is an excellent example of highly specific reagent because under the same reaction conditions only yellow palladium chelate is also precipitated. 

Further details of the photoaddition of N-nitrosopiperidine to a-pinene have been published.The claim of a-fenchen-6-one oxime formation (Vol. 7, p. 43) has been retracted the product is optically active carvone oxime. The stereochemistry and conformations of amino-oximes derived from a-pinene nitroso-chloride have been examined. ... 

---