The synthesis and chemistry of azolenines. Part 3. Some reactions of 4H-pyrazole quaternary salts

Abstract

The preparation of some novel 4H-pyrazolium salts is described. Unlike the structurally related 2H-imidazolium salts (1), the metho-salts (2) of 4H-pyrazoles show no exchange with deuterium at the N-methyl protons. Exchange occurs readily at a 5-methyl group, anions formed at this site giving a cyanine dye (10) with triethoxymethane and benzylidene derivatives (11) and (12) with 1 mol equiv. of aromatic aldehyde. The activated benzylidene derivatives react further with an excess of aldehyde and base, apparently via an N-ylide intermediate, to give pyrazolo[5,1-b]oxazoles (15), which ring-open in the presence of acid forming N-(2-hydroxyethyl)-4H-pyrazolium salts (13). With sodium borohydride, the metho-salts (2) are reduced to 4,5-dihydropyrazoles (16), which with methyl iodide are quaternised at N-1 rather than N-2.