Oxime stereoisomers

Isomers s. a. Oximes, Stereoisomers cw,trans-isomers s. Geospecificity, CIS- and trans-Oxido compounds Isonitrile dihalides... 

Related chemistry was reported by Saeki and Toyota for the racemic S5mthesis of natural product CIS-195A (64), an alkaloid that has been isolated from dendrobatid frog skin extracts tScheme A 1 1 mixture of indanone oxime stereoisomers 62 underwent a... 

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... 

After the formation of tautomeric anions A=A. the anion A a rearranges to give the anion B, which reacts with the second nitroso acetal molecule to form a mixture of stereoisomers of silyl derivative 509a. After desilylation of 509a, oxime 510a is isolated. The reaction with the fluoride anion proceeds at low temperature, whereas the use of triethylamine is efficient only at room temperature. The yield of oxime (510a) is virtually independent of the reaction conditions, whereas the diastereomeric ratio varies substantially. 

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. 

Allylboronates of type 103 react with equivalent amounts of aldoximes 102 (equation 73) giving allylhydroxylamines 104 in good yields. Similar reactions of aldoximes and glyoxylate oxime ethers with allyl bromide and indium also provide hydroxylamines. Additions of substituted allyl boronates to oximes produce mixtures of stereoisomers with ratio highly dependent on the steric size of substituents in both molecules. Addition of allyltri-n-butyltin to aldoxime ether 105 (equation 74) was found to proceed with a considerable diastereoselectivity. 

The term oxime dates to the 19th century, a combination of the words oxygen and imide. Oximes exist as two stereoisomers syn (Z) and anti (E). Aldoximes, except for aromatic aldoximes, exist for the most part as the syn isomer, while ketoximes are obtained as both syn and anti isomers, which can be separated almost completely. Recently, Kolandaivel and Senthilkumar have studied the molecular structure and conformational stability of anti and syn conformers of some aliphatic aldoximes by employing the ab initio and density funetional theory (DFT) methods. 

A stereoselective synthesis of testosterone (261) was advanced by Fukumoto and co-workers (331), where ring B was joined to the C/D part by an intramolecular nitrile oxide cycloaddition. The key nitrile oxide dipole was generated in situ from oxime 258, which in turn was derived from the optically active tetrahydroin-danone 257. Tetracyclic isoxazoline (259) was obtained as a single stereoisomer... 

An alternative and elegant approach to bicyclo[3.3.0]isoxazolidines from alkenyl oximes was developed by Grigg (205) and applied in asymmetric reactions by Hassner et al. (206-209) and others (210). The optically active L-serine derived oxime 130 was proposed to be in a thermal tautomeric equilibrium with the nitrone tautomer 131, which underwent an intramolecular 1,3-dipolar cycloaddition to form the product 132 in 80% yield as a single stereoisomer (Scheme 12.44) (209). 

Nitrones can be generated by Michael reaction of oximes with appropriate conjugated substrates.25 If a generated nitrone has a built-in dipolarophile, cyclization can ensue (Scheme 15). There are three synthetic variations on this theme.25a,b First, the oxime may contain the dipolarophile as in (56). Reaction of (56) with phenyl vinyl sulfone provided a quantitative yield of the tandem product as one stereoisomer. Alternatively, die dipolarophile can reside in the Michael substrate as in (57). Reaction of (57) with cyclohexanone oxime produced two isoxazolidines from competitive cyclization of the intermediate nitrone through six- and seven-membered carbocyclic transition states. It is also possible to carry out an intramolecular Michael addition followed by an intramolecular nitrone cyclization as in thermolysis of (58) to produce a tricyclic isoxazolidine. Very recently several examples of a tandem Diels-Alder, Michael addition, nitrone cyclization sequence have been reported.250... 

A 6-exo-trig cyclization of a pyran derived radical onto an oxime has been employed as the key step in the synthesis of Amaryllidace alkaloid (+)-7-deoxypancratistatin [95JA7289], Treatment of 202 under reductive cyclization conditions gave 203 as a single stereoisomer in good yield. The compatibility of the radical reaction conditions with the dense functionality present in 202 is noteworthy. 

In compounds where nitrogen forms one single and one double bond, the grouping X—N=Y is nonlinear. This can be explained by assuming that nitrogen uses a set of sp2 orbitals, two of which form a bonds to X and Y, while the third houses the lone pair. A it bond to Y is then formed using the nitrogen pz orbital. In certain cases stereoisomers result from the nonlinearity, for example, cis-, and trans-azobenzenes (9-Ia and b) and the oximes (9-IIa and b). These are intercon-verted more easily than are cis- and trans-olefins, but not readily. 

As listed in Table 10, various 0-2,4-dinitrophenyloxiraes of 2-(3-hydroxyphenyl)ethyl ketones 80b-g cyclized to provide quinolin-8-ols 82b-g in moderate to good yield. It is noteworthy that, in all cases, none of the regioisomers such as quinolin-6-ols nor the Beckmann rearrangement products could be detected at all. As both stereoisomers of the oxime react smoothly, their separation is not necessary. 

DeJong, L.P.A., Verhagen, M., Langenberg, J., Hagedom, I., Loffler, M. (1989). The bispyridinium oxime HLd-7, a potent reactivator for acetylcholinesterase inhibited by the stereoisomers of tabun and soman. Biochem. Pharmacol. 38 633-40. 

A procedure for l-alkyl(aryl)-177-4-methylimidazole Af-3-oxides 1539 involves the cyclocondensation of a-amino-oximes, for example, 1537, and orthoesters 1538. Low yields (6-27%) in the cyclization process are due to the predominant Z-stereoisomer around the oxime moiety in 1537 (Scheme 397) <2004S2678>. 

The asymmetric synthesis of aziridines via these routes has been sparingly investigated. Aziridines bearing an A -alkoxy substituent can be resolved into nitrogen stereoisomers. Modest asymmetric induction was observed in the reaction of a chiral 0-alkyl oxime with dimethylsulfonium methylide (equation 41). The analogous reaction with diazomethane was not stereospecific. Earlier work in this area has been reviewed. 

Dependent on the stereochemistry of the oxime sulfonate and the different groups R , R, in many cases only one preferred stereoisomer rearranges. In hydrochloric acid/acetic acid, isomerization gives rise to only one final product. ... 

The synthesis of the bisbenzannelated spiroketal core of the y-rubromycins was achieved by the research team of C.B. de Koning." The key step was the Nef reaction of a nitroolefin, which was prepared by the Henry reaction between an aromatic aldehyde and a nitroalkane. The nitroolefin was a mixture of two stereoisomers, and it was subjected to catalytic hydrogenation in the presence of hydrochloric acid. The hydrogenation accomplished two different tasks it first converted the nitroalkene to the corresponding oxime and removed the benzyl protecting groups. The oxime intermediate was hydrolyzed to a ketone that underwent spontaneous spirocyclization to afford the desired spiroketal product. 

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