Pyrazoles

Abstract

Although pyrazole (1) was first prepared and described by Pechmann in 1898, it was almost another 60 years before the natural occurrence of a pyrazole derivative was reported (Shinano and Kaya, 1957; Noe and Fowden, 1959). This was the amino acid β-pyrazol-1-yl-L-alanine (2) an isomer of histidine (3). Together with its γ-L-glutamyl peptide (4) it has been found in the seeds and seedlings of many species of the plant family Cucurbitaceae, to which cucumber, watermelon, squash and pumpkin belong. Evidence for the existence of free pyrazole in biological tissue was obtained, indirectly, by Dunnill and Fowden (1963) who heated a benzene extract of cucumber seeds with serine, pyridoxal and aluminium sulphate, and obtained β-pyrazol-l-ylalanine. It had previously been shown that authentic samples of pyrazole readily undergo this Al³⁺/pyridoxal-catalysed reaction with serine, and which simulates the action of the enzyme β-pyrazol-1-ylalanine synthase.