Open hydrazone structures

Reaction of hydrazine 165 with a number of monosaccharides give hydrazones 166 (92BCJ546). Their acetylations gave peracetyl derivatives 167. The same reactions were extended to the /V-methyl and the 7-methyl derivatives of 165 (93 JPROO). That compounds 166 exist as open-chain structures was based on a 2D NMR study (93MI1) (Scheme 35). 

The base-catalyzed, / -elimination reaction of D-mannose phenyl-hydrazone is consistent with the acyclic structure for the phenylhydra-zone in solution. However, the small proportion of a nitroxide radical observed on treatment of the phenylhydrazone with a strong base may indicate the existence also of a fractional proportion in a cyclic structure in equilibrium with the open-ring structure, as was suggested by Blair and Roberts (43). The hydrazino moiety required for nitroxide-radical formation could be derived from the cyclic form of D-mannose phenylhydrazone in solution. 

Fischer represented the hydrazones and osazones as open chain structures one sees at once, however, other structural possibilities since mutarotation occurs in solution in many cases. Unless this is due to decomposition, some form of tautomeric equilibrium must be postulated, and Haworth pointed out that cyclic structures must be considered Behrend and Lohr recognized the possibility of the existence of syn and anti forms, and it has also been suggested that there may be an equilibrium between hydrazone and azo structures in the osazones. 

The structure of sugar phenylosazones, derivatives first prepared by Fischer in 1884, has remained undefined. Besides Fischer s open-chain structure, a tautomeric azo-hydrazone structure and ring structures have been proposed. Although work in the last two decades involving ultraviolet spectra studies - and examination of the acetylated - and... 

Information valuable for the interpretation of the structure of the hydrazones is provided (209) by the reaction of a-methylphenylhydrazine with tetra-O-acetylgalactopyranose (I), tetra-O-acetylgalactofuranose (II), and a de/i /do-penta-0-acetylgalactose (III). As all the hydrazones formed are converted to the same penta-O-acetylgalactose methylphenylhydrazone (IV) when acetylated, it appears that these hydrazones have open-chain structures. 

Simple oxaziridines and diaziridines do not absorb in the near UV. Lack of absorption was one argument to distinguish between true three-membered ring structures and unsaturated open chain isomers like nitrones or hydrazones. 

The discussion of the structure of the nitrones and the hydrazones received less attention. With the increased application of physical methods to structural problems, the three-membered ring structures for these compounds lost much of their attraction. The problem of the structure of the nitrones was satisfactorily solved with the open-chain A -oxide formulation. The compounds originally designated as diaziridines (2) were partly reformulated with the open-chain hydra-zone structures and partly were left without a. satisfactory proof of structure. 

Certain flavone hydrazones 119 give the fused system 120 [but the alternative (and more likely) 2-(2,4-dinitrophenyl) structure is not excluded by the evidence presented] on treatment with ethanolic alkali, presumably by initial ring opening of the heterocyclic ring, followed by ring closure with displacement of bromine.117... 

Extension of the NMR studies described in Section 58.2.3.3(i)(b) to the compounds (136 R = N02 and R = Bu ) led the authors to conclude that one Na- and two No-coordinated cobalt complexes were produced. These were assigned the partial structures (137), (138) and (139), which involve coordination of the ligand in its azo form and in the two alternative hydrazone forms. In general (Section 58.2.3.2), azophenols exist exclusively in the azo form and serious doubts must exist regarding structure (139). Similarly, in view of the reported equilibration between the various isomers in solution, assignment of discrete tautomeric structures must be open to question. As-... 

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