Synthesis and Characterization of New Formylpyrazolones and Schiff Bases

The reaction of 1‐benzyl‐ and 1‐(2‐phenylethyl)‐ 3‐methyl‐5‐pyrazolone with POCl 3 and DMF affords N,N ‐dimethylaminothylidene derivatives that in acid or basic medium can be transformed into the corresponding aldehydes at carbon 4 of the heterocycle. The dimethylenamine can also be more easily obtained by reacting pyrazolones with the dimethyl acetal of N,N ‐dimethylformamide. Starting from the aldehydes or the N,N ‐dimethylenamines, it is possible to obtain new enamines. The advantages of using dimethylenamines are the absence of the enol as an undesired byproduct and that there is no need to prepare the aldehyde to obtain Schiff bases. Overall, the yields can be classified as acceptable. The aldehydes can be described as 5‐hydroxypyrazole‐4‐carbaldehydes in chloroform solutions, but in solid phase they are dicarbonyl compounds. The Schiff bases exhibit an enamine structure in chloroform solution and in solid phase.