N-Acylpyrazoles

When lithium aluminum hydride reacts with N-acylpyrazoles, the CN bond is hydrogenolyzed and aldehydes are produced in good yield. Ried even recommends this as a method of preparing otherwise inaccessible aldehydes.730,781 Certain labile a-amino aldehydes were... 

Since N-acylpyrazoles have been fully described in (B-76MI40402) only a summary and some new references will be discussed here. Only neutral pyrazolides have been described, but a cationic intermediate (249) is involved in the Olofson and Kendall method of obtaining minor AT-alkylated isomers (70JOC2246). 

The pyrazole ring is resistant to most reducing agents and survives intact when other heterocycles are cleaved. An illustration is provided by the reaction of the pyrazoloimidazolidinone (1) with LAH. Here the imidazolidine unit is sensitized to nucleophilic attack by the presence of a lactam carbonyl group, and the product formed is the monocyclic pyrazole (2). The stability of pyrazoles to reduction has been exploited in a synthesis of aldehydes from acyl halides. Thus N-acylpyrazoles (3) on reduction with LAH produce complexes of the type (4) these when hydrolyzed yield aldehydes and the parent pyrazoles (5). ... 

Reduction of allylic carbonyl derivatives usually leads to the corresponding allylic alcohol however, in some cases, intermediate oxidation states can be obtained. Reduction of amide derivatives such as N-acylpyrazoles results in the formation of an aldehyde in good yield. ... 

N-Condensed N-acylpyrazole ring from cyclic a-diazoketones Ring expansion with ring closure... 

The condensation between 4-acylpyrazol-3-ones 137a,b and n-propylamine to give... 

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