Keto-hydrazones

No X-ray crystallographic studies of azo pigment lakes have as yet been reported, but solid state high resolution 13C-NMR spectroscopy was used to demonstrate that P.R.57 1 also exists in the keto-hydrazone form [9]. 

Modest yields of bicyclic 1,2,4-trithiolanes (156) have been obtained from the sulfurization of nonenolizable diketones of type (157) using Lawesson s reagent (Scheme 44) <90JOC2421>. Although the reaction is limited, a similar procedure has provided one of the few routes to 1,3,4-oxadithiolanes (158) from the corresponding keto hydrazone (159) (Scheme 44) <92CC7,93BCJ1714>. 

The oxidation of hydrazine derivatives with diethyl azodicarboxylate is of particular interest because it involves direct hydrogen abstraction. The oxidation of keto hydrazones with lead tetraacetate leads to azoacetates, presumably by a free radical mechanism. 

However, with lead tetraacetate, keto hydrazones may be oxidized to azoacetates according to Eq. (44) [102]. 

Figure 4 Azo compound synthesis and the molecular structure of the hydroxy-azo tautomer showing the positions of the coupler (I) and the amine (II) components. The keto-hydrazone tautomer is shown in equilibrium with the hydroxy-azo tautomer.

An earlier application of 1,4-conjugate additions employing azaallyllithium reagents formed from ketone dimethylhydrazones is Kelly s hydrazone version of the classical Knoevenagel route to substituted pyridine derivatives.71 In this chemistry, a mixed cuprate derived from a dimethylhydrazone lithio anion and copper thiophenoxide was first allowed to react with an a,p-unsaturated ketone. The product enolate can then be acylated or protonated to yield either a diketo hydrazone or a keto hydrazone. Addition of... 

A new route to vicinal amino-dideoxy-sugars has been reported. Thus, catalytic reduction of the keto-hydrazone (135) obtained by treatment of the glycosidulose (134) with alkali gave methyl 3-amino-3,4,6-trideoxy-a -L-n6o-hexopyranoside (Scheme 51). Stevens and his co-workers have extended their work on 4-amino-sugars by synthesizing methyl 4-amino-4,6-dideoxy-a-D-altropyranoside and the isomeric D-idose compound. 3-Amino-3-deoxy-D-altrosides [e.g. (136)] derived from tryptamines have been prepared from methyl... 

Fig. 7 Structure of the dark blue/black reactive azo dye Remazol Black B. The azo/keto-hydrazone groups can be reduced to produce the corresponding colourless amines Para Base and Diamino H-acid.

As seen in the structures in Table 10.6, all the photoconductive azo pigments are o-hydroxy substituted. o-Hydroxy substituted azo compounds are known to undergo hydroxy-azo/keto-hydrazone tauto-merism. For instance, 1-phenylazo-2-naphthol (XII) can be a hydroxy-azo or a keto-hydrazone, depending on the solvent, the substituents and the physical state [208,211]... 

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