Osazones, phenyl-, formation

Phenylhydrazine on exposure to light slowly darkens and eventually becomes deep red in colour salts of the base share this property but to a lesser degree, the sulphate and acetate (of the common salts) being most stable to light. Phenylhydrazine is largely used in organic chemistry to characterise aldehydes and ketones as their phenyl-hydrazones (pp. 342, 345), and carbohydrates as their osazones (pp. 136-140). It is readily reduced thus in the process of osazone formation some of the phenylhydrazine is reduced to aniline and ammonia. On the... 

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. 

The carbonyl group (and adjacent alcohol) oxidizes with excess phenyl hydrazine (PhNHNH2) to form an osazone (see Figure 16-15). Osazone formation is very important in determining the relationship between various monosaccharides. For example, both D-glucose and D-mannose produce the same osazone, so they re epimers. Epimers differ by only one chiral center, which osazone formation destroys. 

The reaction of glucose with an excess of phenylhydrazine (phenyl-diazane) is particularly noteworthy because two phenylhydrazine molecules are incorporated into one of glucose. Subsequent to the expected phenyl-hydrazone formation, and in a manner that is not entirely clear, the —CHOH— group adjacent to the original aldehyde function is oxidized to a carbonyl group, which then consumes more phenylhydrazine to form a crystalline derivative called an osazone, or specifically glucose phenylosazone ... 

Several mechanisms34 have been proposed for this reaction. That of Weygand,36 in which an Amadori rearrangement is proposed, has considerable merit.37 Illustrations of unusual osazone formation are described by Bonner and Drisko.38 When phenyl /S-D-xylopyranosyl sulfone (XXII) or /J-D-glucopyranosyl sulfone (XXIV) is oxidized by periodic acid, a dialdehyde oxidation product (XXIII or XXV), which is susceptible toward further oxidation, is obtained. The reaction of XXIII or XXV with phenylhydrazine yields glyoxal phenylosazone and benzenesulfinic acid. Surprisingly, both XXII and XXIII react with phenylhydrazine to form D-xylosazone and D-glucosazone, respectively. 

Osazone and (p-nitrophenyl)hydrazone formation without desul-fonylation has been accomplished in aqueous ethanolic acetic acid with 3,6-anhydro-5-0-p-tolylsulfonyl-D-glucofuranose. With 6-0-(me-thylsulfonyl)- and 6-O-p-tolylsulfonyl-D-galactose in methanol at room temperature for 2 days, the corresponding (2,4-dinitro-, (N-benzyl-, and (2,5-dichloro-phenyl)hydrazones were readily obtained. How-... 

The oxidation of n-glucose phenylosazone by periodic acid has been shown to yield l-phenyl-4-phenylhydrazono-5-pyrazolone by the intermediate formation of the 1,2-bis(phenylhydrazone) of mesoxaldehyde. The reaction proceeds normally as if the osazone were acyclic it must be presumed, therefore, that there is an equilibrium between the cyclic (XII) and acyclic forms in the aqueous-ethanolic solutions employed,... 

An instance of osazone formation in which substitution takes place on the second and third carbon atoms (in l-desoxy-n-psicose) has been reported. This case is of interest since the possibility arises that with other ketoses, as psicose, 2,3- as well as 1,2-osazones might be produced side by side. The commonest ketose, D-fructose, gives n-glucose phenyl-osazone with such extreme ease, that it is just possible the phenomenon may have escaped observation. 

Of these proposed structures, only that of the n-glucose l-(A -methyl-V -phenyl)-2-(A -phenyl)osazone, suggested by Voto6ek and Vondrafek for mixed osazone B, is theoretically unsuited for formazan formation since... 

According to Micheel, osazone formation starts from an aldose and requires an Amadori rearrangement, so that his scheme may not account for the reaction in the ketose series. It may be concluded that the Fischer mechanism is not valid, and that further studies are needed in order to solve the problem of interaction between ketoses and substituted phenyl-hydrazines. 

Ingles heated a solution of n-glucose, sodium sulfite, and bisulfite, removed the cations, and steam-distilled the residue. The product, free of carbonyl-bisulfite addition compounds, was chromatographed on an ion-exchange resin, giving a sulfonic acid. This acid yields a crystalline brucine salt and phenyl- and (2,4-dinitrophenyl)-osazones. The osazones consume 1 mole of periodate per mole, liberating 1 mole of formaldehyde but no formic acid. The (2,4-dinitrophenyl)osazone also forms a diacetate. The acid is oxidized by sodium hypoiodite, taking up 1 mole of oxidant per mole. From these reactions and the possible reaction mechanisms for its formation, structure (41), a 3,4-dideoxy-4-sulfo-n-hexosulose, was proposed for the sulfonic acid. ... 

Acetylation.— Acetylation of n-arabino- and n-li/xo-hexulose phenyl-osazones results in the formation of crystalline tetraacetates. Wolfrom and coworkers have established an analytical technique that differentiates between iV-acetyl and 0-acetyl groups the latter were determined by the procedure of Kunz and Hudson. They were thus able to show that all four acetyl groups are 0-acetyl groups. This discovery eliminated the possibility of cyclic structures, which would have required three 0-acetyl groups and one A-acetyl group. The infrared spectra of osazone acetates also support the acyclic structure, as these compounds show only one carbonyl band, namely, that of the 0-acetyl groups. 

Diels and coworkers have shown that, when n-arafemo-hexulose phenyl-osazone is oxidized with oxygen (or air) in an alkaline medium, a dehydro-osazone is obtained which possesses two hydrogen atoms fewer than the parent osazone. On acetylation, the compound gives a triacetate, and its hydrazine residues could not be removed with p-nitrobenzaldehyde, suggesting their involvement in ring formation. The authors supported this view by the findings that dehydro-osazones cannot be obtained from methylphenylosazones or from the Diels anhydro-osazone (which, at that time, was believed to be a pyrazole compound). 

Formation of Pyrazoles from Bis(hydrazones).—Mesoxaldehyde bis(phenylhydrazone) (193), obtained by periodate oxidation of saccharide osazones (192) is readily cyclized in the presence of acids to give l-phenyl-4-phenylazo-pyrazole (195).162 365 Hexulose phenylosazones (192) are also disproportionaled in the presence of acidic salts of carbonyl reagents, such as hydroxylamine hydrochloride, to give l-phenyl-4-phenylazo-pyrazolin-5-one (196). The reaction probably proceeds via mesoxalic acid 1,2-bis(phenylhydrazone) (194).365 The hydroxalkyl derivatives of 196 are produced from dehydroascorbic acid bis(phenylhydrazone) by treatment with base to open the lactone ring and permit the conversion of 197 to 199.351,366 Another type of pyrazole that is formed by dehydrating osazones with acetic anhydride is discussed later under anhydroosazones (see Schemes 45,53). 

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. 

In the controversy on the mechanism of osazone formation, it was found that glucose-f-t is converted into the phenylosazone and the phenyl-osotriazole without loss of tritium." This finding was taken as evidence against formation, by the Amadori rearrangement, of an intermediate having two hydrogen atoms on C-1 as in Weygand s scheme" B. 

The methylene group at C2 will not be oxidized to a carbonyl (or a hydroxyl) and the sugar will form a phenyl-hydrazone but not an osazone. Osazone formation is specific to a-hydroxy carbonyl compounds. 

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. 

---