Osazones, phenyl-, formation preparation

Osazone formation. The preparation of glucosazone has already been given (p. 137). It may be carried out on a small scale by either of the following methods, according as (a) the phenyl hydrazine base, or (Z>) one of its salts, is used. 

Phenylhydrazones are best prepared in dilute acetic acid solution use of more concentrated acetic acid can lead to formation of 1-acetyl-1-phenyl-hydrazine, m.p. 130-131° (corr.) if there is a neighboring hydroxyl group, which can lead to osazone formation, and if this is to be avoided, then any acidity is unfavorable and the reaction should be carried out as near the neutral point as possible. Condensation usually occurs in the cold, and almost always on short warming on the water-bath. A solvent is often unnecessary. <%,/ -Unsaturated aldehydes and ketones may cyclize to pyrazole derivatives on reaction with phenylhydrazine. 

The structure of the osazone derived from dehydroascorbic acid remains in dispute. Recently (Vol. 13, p. 95) it was described as the phenylhydrazine-phenylazo-structure (34), but it is now claimed that the bis(hydrazone) formula (35) fits the n.m.r. and u.v. data more satisfactorily. The structure of the bi-cyclic oxidation product of (35) has been shown to be a 3,6-anhydride (36) by n.m.r. and m.s. A n.m.r. study of the formation of dehydroascorbic acid hydrazones revealed that the two 2-phenylhydrazides (37) were formed initially and subsequently underwent dehydration to give the two rotationally isomeric 2-phenyIhydrazones. Reaction of 6-bromo-6-deoxy-isoascorbic acid (D-ery thro) with phenyl hydrazine afforded the cyclized bis(hydrazone) (38). Treatment of D-threo-ascorbic acid bis(phenylhydrazone) with caustic soda afforded the cyclized product (39). The corresponding D-erythro- and L-threo-isomers were also prepared and the side-chains have been modified in various ways. 

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