Oxime cyclic

Note that the acetonitrile oxide cyclooligomers (e.g. 13) are not true oxime derivatives. Such derivatives have been prepared from biacetyl, however . Derivatives related to 14, below, were prepared and found not to be good complexing agents. They were, nevertheless, capable of phase transferring either sodium or potassium permanganate into dichloromethane. 

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I.4.I.5.2. Cyclic Oxime Ethers 1.4.1.5.2.1. Substrate-Induced Diastereoselection... 

The single known N-oxide of 1,2,3-dithiazole 132 was isolated from cyclic oxime 133 and sulfur monochloride (1985TL189 Scheme 66). 

A reaction with seven-membered cyclic oximes proceeded similarly to give cyclopenta-l,2,3-dithiazoles 136 and 137 (1993JCS(P1)769 Scheme 69). For chlorination, up to 15 equivalents of sulfur monochloride were used and polychlorination was assisted by N-chlorosuccinimide. 

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. 

Opposite stereoselectivity in reduction of cyclic oximes (e.g. 88, equation 61) can be achieved with silane-trifluoroacetic acid . ... 

It was also found that the rearrangement of cyclic oximes into lactams in the presence of cyanuric chloride is markedly facilitated by the use of l,l,l,3,3,3-hexafluoro-2-propanol (HFIP) as solvent ... 

For cyclic oximes, ring cleavage can take place by hydrolysis of the resulting imi-dates 456. This strategy was used to obtain optically pure octanoic acid derivative 457 (equation 196). 

Beckmann fragmentation is frequently applied to cyclic oximes resulting in a ring-cleavage reaction. Normally, a nitrile-alkene compound is obtained from the oxime and further transformation is usual. 

It was observed that a-amino oximes 511, when treated with sodium borohydride in boiling acetonitrile, produced the expected fragmentation products 513 in moderate to good yields (31-87%) (equation 225). The use of the hydride-induced fragmentation in cyclic oximes leads to amino nitrile compounds, as a result of the reduction of the immonium salt intermediates 512. Careful selected oxime structures showed that the reaction time increases when the stabihty of the immonium intermediate decreases, showing the importance of the mesomeric assistance. 

A relatively unexplored method to prepare a pyridine ring by thermolysis of O-allyl cyclic oximes (593) was used in the synthesis of some unsymmetrical bridged terpyridines and in the synthesis of the E-ring in modified steroids" (equation 261). 

D-Xylose oxime (XVIII) is also transformed into the nitrile even by mild acetylation. The configuration of carbon atoms 2, 3 and 4 is the same as in D-glucose oxime (XIX), but the latter reacts in a different form, giving almost exclusively the cyclic oxime upon mild acetylation. At higher temperatures the acetylated nitrile is produced. 

The reaction proceeds similarly with seven-membered cyclic oximes to give cyclohepta-1,2,3-dithiazoles (127) and (129) (Scheme 22) <93JCS(P1)769>. To effect chlorination up to 15 equivalents of S2CI2 are needed polychlorination is assisted by NCS. The noncyclic a,)S-unsaturated oxime (130)... 

Isoxazolines as cyclic oximes are reduced with initial cleavage of the N—O bond followed by reduction of the C=N bond. 3-Carboxyisoxazol-ines (174) are thus generally reduced to y-hydroxy-ot-amino acids, which may lactonize (175)268 [Eq. (106)]. 

The Beckmann rearrangement of cyclic oximes results in lactams. This is exemplified in Figure 14.42 with the generation of e-caprolactam, the monomer of nylon-6. The nitrilium ion... 

The Beckmann rearrangement of cyclic oximes results in lactams. This is exemplified in Figure 11.38 with the generation of e-caprolactam, the monomer of nylon-6. The nitrilium ion intermediate cannot adopt the preferred linear structure because it is embedded in a seven-membered ring. Therefore, in this case the intermediate might better be described as the resonance hybride of the resonance forms A (C=N+ triple bond) and B (C+=N double bond). The C,N multiple bond in this intermediate resembles the bond between the two C atoms in benzyne that do not carry H atoms. 

XII. CYCLIC OXIME ETHERS AS HETEROCYCLIC SYSTEMS WITH... 

Different heterocyclic systems can undergo cleavage reaction that leads to a linear carbon skeleton with functional groups. The easier to get those systems, to modify them and to break them in several ways, the more their diversity. Two systems of cyclic oxime ethers that were studied in this connection are 2-isoxazoline and 1,2-oxazine. The former, which was obtained by 1,3-dipolar cycloaddition of a nitrile oxide, and a substituted double bond114, can be cleaved in several ways, in which the latent functions are revealed. One example in which a,/Lenoximes and enones114 are formed is shown in equation 43. 

Stereoselectivity of Reductions of Cyclic Oximes and Oxime Derivatives... 

Systematic investigations of the stereochemical outcomes of reductions of cyclic oximes and their derivatives have been sparse the available information suggests that results sometimes parallel those obtained in other imine reductions (Sections 1.2.2.2.3, 1.2.2.3.3 and 1.2.2.4.2), but differences are also noted. A collection of examples in which the stereochemistry was determined is presented in Table 15. Thus, with cyclohexyl systems enhanced equatorial attack (compared to ketones) leading to axial amines... 

Entry 9 in Table 15 illustrates another synthetically useful stereocontrolled reduction of cyclic oxime ethers (isoxazolines) to alicyclic amino alcohols using LAH. The stereoselectivity obtained is further enhanced by the incorporation of a 4a-hydroxy group which, upon reduction, affords almost entirely the erythro isomer (equation High diastereoselectivity in the reductive cleavage of isoxazolines has... 

The reductive cleavage of the N—O bond of oximes is a reaction which has been widely used by synthetic chemists. When accompanied by reduction of the C=N bond the reaction leads to the formation of primary amines. Reactions of this type are described in Chapters 1.2-1.8, this volume. The reductive cleavage of cyclic oximes, particularly isoxazoles and 4,5-dihydroisoxazoles, has been used as a key step in several target syntheses of natural products. These cleavage reactions are covered in Chapter 3.8, this volume however, some of the methods which clearly bring about cleavage of the N—O bond before reduction of the C=N bond are also described in this section. The products of these reactions are A-unsub-stituted imines. Unless the imines are sterically protected or unless special precautions are employed in the work-up, the isolated products are usually carbonyl compounds formed by hydrolysis (Scheme 19). Reductive hydrolysis reactions of this type are also included here. 

Isoxazolines may be regarded as cyclic oximes the reduction of 3,5-diphenylisoxazo-line (Xlla) with LiAlH4 gave 95 5 erythro threo l,3-diphenyl-3-aminopropanol (Xllla) [96], whereas reduction with Na(Hg) [96,97] in methanol yielded the ratio 2 3 of erythro threo Xllla, as in Eq. (5). Similar results were obtained with 3-phenyl-5-methylisoxazoline (Xllb), whereas 5-phenyl-3-methylisoxazoline (XIIc) was not reducible. 

The stereochemistry of the amines obtained on hydrogenation of cyclic oximes is dependent on the pH of the reaction medium in the same way as the formation of alcohols from cyclic ketones, which was discussed in Chapter 18. In... 

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