Saccharide hydrazones formation

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). 

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