Oximes tautomerism

Nitroso-oxime tautomerism was reported to occur in compounds 266-268 [76AHC(S1), pp. 436,443,452],... 

Nitroso-Oxime Tautomerism. This equilibrium lies far to the right, and as a rule nitroso compounds are stable only when there is not a hydrogen. 

The nitro form is much more stable than the aci form in sharp contrast to the parallel case of nitroso-oxime tautomerism, undoubtedly because the nitro form has resonance not found in the nitroso case. Aci forms of nitro compounds are also called nitronic acids and azinic acids. 

The diastereomeric ratio is high only for ( )-disubstituted alkenes and norbornene, but low for cyclic and (Z)-alkenes, reflecting the order of stability of the intermediate complexes. The loss of stereochemical purity is probably due to base-catalyzed nitroso-oxime tautomerization during the reduction. However, the use of diisobutylaluminum hydride, before adding lithium aluminum hydride, allows (3/ , 4S )-3,4-hexanediamine to be obtained from (Z)-3-hexene with d.r. [(3R, 4S )IQR AR )] 90 10. 

Heating 3-nitroso-2-phenyl-l//-indole with excess phosphoryl chloride or tosyl chloride in sulfolane at 200 C for 1 h, followed by dilution with water, produces 2-phenylquinazolin-4(3//)-one in 90 or 68% yield, respectively. The ring expansion proceeds via the second-order Beckmann rearrangement of the imino-oxime tautomeric form of 3-nitroso-2-phenyl-l//-indole. °... 

Intramolecular oxime-olefin cycloaddition (lOOC) is a useM approach to iV-unsubstituted isoxazolidines. However, high temperatures are usually necessary to induce oxime tautomerization to nitrone. Tamura et al. reported the generation of iV-boranonitrones 107 by treatment of 0-/-butyldimethylsilyloximes 106 with two equiv of BF3.0Et2 at room temperature. Under the reaction conditions, the intermediate 107 smoothly underwent intramolecular cycloaddition to afford 108 after extractive work-up <05JOC10720>. 

Die Nitrosierung ist auf elektronenreiche 1,3-Thiazole beschrankt, die in 2-Position eine Amino-, Hydroxy- oder Mercapto-Gruppe tragen (vgl. Bd. X/3, S. 58). Die 5-Nitroso-Produkte liegen haufig als Oxim-Tautomere vor. 

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