Oxime-nitrone isomerization

Interestingly, a facile oxime - Tf-nitrone isomerization in the pyridopyrimi-dine systems 248 has been attributed to the favorable proton transfer from the... 

Rather different conclusions were reached recently by Pas-paleev [79], who suggested that the nature of the first wave, which is observed both with the syn (a)-form and the anti ()S)-form, is determined not only by the oxime - nitrone tautomerization but also by isomerization of the cv- and forms in the bulk of the solution. 

Nitrogen chemical shifts are clearly different for the isomeric structures of nitrones, oximes and nitroso compounds. 

Grigg and co-workers (310) recently examined the 1,3-APT reaction of various aldoximes (270) (R or R = H) with divinyl ketone (Scheme 1.56). While ketoximes 270 (R = R) form a mixture of adducts, 271 and 272 via nitrone 273, the aldoximes selectively afford 272 (as a mixture of endo and exo diastereoisomers). Under the thermal reaction conditions, the oxime starting materials can undergo ( /Z) isomerization, while the nitrone intermediate was expected to be unaffected and the isolated cycloadducts showed no interconversion via cycloreversion. Thus, the increasing selectivity for endo-212 [via ( )-273, R = H] over exo-212 [via (Z)-273, R = H] with the increasing size of the aldoxime substituent was attributed primarily to the inhibition of oxime isomerization by steric clash between R or R and the oxime OH. In contrast, Lewis acid catalysis, in particular by hafnium (iv) chloride, of the cycloaddition of various aldoximes with this dipolarophile gave exo-271 exclusively (216). 

In the past, an important problem was distinguishing nitrones from the isomeric oxime O-ethers113. Among other possibilities the difference in dipole moments2 is also quite... 

Modifying the aldehyde function in the pyrroles 1162 into 1,3-dipoles 1163 (nitrone) and 1167 (nitrile oxide) furnished tricyclic heterocycles 1164 and 1168 via intramolecular 1,3-dipolar cycloaddition reactions (Scheme 226) <2000T3013>. None of the isomeric bridged product 1165 is produced despite the preference for that regiochem-istry in the intermolecular reaction. Generated in situ (from the oximes 1166) nitrile oxides 1167 cyclized spontaneously to the dihydroisooxazole 1168 in quantitative yield at room temperature. 

Oximes and nitrones are mutually isomeric structures [50], [51] which are clearly distinguished by means of their nitrogen chemical... 

This series of rearrangements includes the dithia-Claisen rearrangement mentioned above (Section IV.E.l) as well as the palladium-catalyzed [3,3]-sigmatropic isomerizations of allyl methyl N-aryldithiocarbonimidates 627 (refluxing dioxane, 20 h, 62-90%) (equation 275) and a Pd"-catalyzed tandem 2,3]-sigmatropic shift, followed by 1,3-dipolar cycloaddition which takes place at equilibrium between O-allyl ethers of oximes 628 and the corresponding N-allyl nitrones 629 (equation 276) ... 

Suginome, H., E.Z Isomerization and Accompanying Photoreactions of Oximes, Oxime Ethers, Nitrones, Hydrazones, Imines, Azo and Azoxy Compounds, and Various Applications. In Horspool, W. M., Lenci, F. (eds), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn, CRC Press LLC, Boca Raton, EL, 2004, Chapter 94, pp. 1 55. 

Oximes can participate in cycloaddition reactions after tau-tomerizing to the isomeric nitrone derivative generating the isox-azoline. In early reports this tautomerization was achieved at high temperatures, or in the presence of Pd(II) salts, although this latter approach has some structural limitations. It has been demonstrated that BF3 Et20 can be used to accomplish the same transformation with a wider array of substrates and at substantially lower temperatures via the 0-silyloxime derivatives (eq 64). For comparative purposes, the parent oxime requires a week at reflux in toluene to provide 54% yield. ... 

Suginome, H. E,Z-isomerization and acconpanying photoreactions of oximes, oxime ethers, nitrones, hydrazones, imines, azo- and azo conpounds, and various applications. In CRC Handbook of Organic Photochemistry and Photobiology Horspool, W. M. Lenci, R, Eds. CRC Press Boca Raton, FL, 2004, pp. 1-55, Chapter 94. 

A convenient alternative to the Beckmann rearrangement is reported by Barton 1 Alkyl nitrones, readily available from the corresponding ketones, are smoothly transformed into A-alkylamides by treatment with toluene-/ -sulphonyl chloride, as shown in Scheme 36. Beckmann rearrangement of the oxime of (105) affords the isomeric lactam (106). This reaction is applicable also to saturated ketones. 

The cycloaddition proceeded smoothly, when promoted by acid, to provide ketones 75 and 76. Whereas 76 was surely the kinetic product, acid promoted enolization-isomerization provide 75 as well. Treatment of the mixture with hydroxylamine gave the desired oxime (77) and Borch reduction from the convex face provided 72 The rest of the synthesis proceeded as expected. The intramolecular nitrone cycloaddition gave 70. Methylation of the amine and reduction of the weak N-O bond provided 78. Jones oxidation (Cr in acid) of 78 completed the synthesis of luciduline (55). When a single enantiomer of 74 was used, a single enantiomer of luciduline was obtained. 

The dioximes (and also monoximes) of benzoquinone, naphtha-quinone, and anthraquinone were iuvestigated by Elofson and Atkinson [97], who observed one wave in acidic and alkaline solutions and two waves in neutral solution the smallest wave, of kinetic character, was explained by nitrone — oxime isomerization at the electrode ... 

Z-Isomerization and Accompanying Photoreactions of Oximes, Oxime Ethers, Nitrones, Hydrazones, hnines, Azo- and Azoxy Compounds, and Various Apphcations... 

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