Formation of Enaminohydrazones and Oximes

The coupling of enamines with aromatic diazonium salts has been used for the syntheses of monoarylhydrazones of a-diketones (370,488-492) and a-ketoaldehydes (488,493). Cleavage of the initial enamine double bond and formation of the phenylhydrazone of acetone and acetophenone has been reported with the enamines of isobutyraldehyde and 2-phenylpropionalde-hyde. Rearrangement of the initial coupling product to the hydrazone tautomer is not possible in these examples. 

An a-oximino enamine was formed by the reaction of nitrosyl chloride and triethylamine with morpholinocyclohexene 494). Oxime functions have also been introduced into carbonyl conjugated enamines with isoamyl nitrite or NjOj 495). 

The selective bromination of a ketone in the presence of another susceptible functional group was achieved in a diterpene synthesis 240). A competing bromination of an anisole ring could be avoided here through the use of a pyrrolidine enamine derivative for activation of the methylene group adjacent to the carbonyl function. 

The reaction has also been applied to the syntheses of a number of a-bromoaldehydes (497,498) and 2-a-bromoeholestanone (2J). 

The most extensive use of enamine halogenations has, hctwever, been in the attachment of fluorine to the steroid skeleton (499-503). The formation of a ]6-fluoro-17-ketosteroid by the reaction of perchlorofluoride with a 17-enamide has also been reported (504). 

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