Methylglyoxal « phenylhydrazone

As the reaction sequence of Scheme 12-38 can be stopped at the stage of the oo-methylglyoxal phenylhydrazone (12.78), it is possible to synthesize asymmetrically substituted formazanes (12.80, Ar = Ar ) by reacting acetone with one equivalent of a diazonium ion ArNJ under acidic conditions and then coupling the co-methyl-glyoxal phenylhydrazone with Ar NJ in alkaline solution. 

The procedure described is essentially that of Japp and Klingemann.3 Methylglyoxal-u-phenylhydrazone may also be prepared by heating phenylazoacetoacetic acid at 170-180° 9 or by warming ethyl phenylazoacetoacetate with a solution of sodium hydroxide in dilute ethanol. 8 9... 

For example, with benzenediazonium chloride 2-oxocyclopentanecarboxylic add gives 1,2-cyclopentanedione mono(phenylhydrazone),336 and acetoacetic acid gives an 80% yield of methylglyoxal l-(phenylhydrazone).337... 

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