Intramolecular oxime–acyl attack: new routes to 1,2,4-triazine 4-oxides and 1,2,3-triazoles
A number of 3-substituted 1,2,4-triazine 4-oxides and their deoxy-analogues have been synthesised via an intramolecular condensation between neighbouring oxime and acylhydrazone functions. The condensation takes place only in molecules of rigidly favourable geometry, such as the oxime-acylhydrazones of 1,2-naphthaquinone, in which oxime and acyl group are rigidly held in positions sterically favourable for interaction. In molecules of less rigid geometry, oxime-acylhydrazones derived from benzil, for example, ring-closure does not take place. The exact nature of the product of cyclisation depends on the structure of the acyl group. Thus oxime-guanyl-hydrazones yield 3-amino-1,2,4-triazine 4-oxides, and oxime-semicarbazones cyclise to the analogous 3-hydroxycompounds. On the other hand, the 3-amino-triazine oxides formed on cyclisation of oxime-thiosemicarbazones undergo deoxygenation concurrently with formation, and the ultimate products are the analogous deoxy-compounds. The ring-closure process, which is formally an addition–elimination reaction at acyl carbon, is subject to catalysis by either acid or base, or both.
Replacement of the acyl group in the oxime-acylhydrazone by structures which bear an electron-deficient atom other than carbon causes a considerable modification in the chemistry of the compounds. Thus oxime-OO′-di-phenylphosphorohydrazones undergo a base-induced ring-closure to 1,2,3-triazoles while the analogous oxime-toluene-p-sulphonylhydrazones decompose spontaneously with the formation of the related 1-hydroxy-1,2,3-triazoles. Mechanisms are suggested for the ring-closure processes discussed.