Efficient routes to 2-arylhydrazono-3-oxoalkanonitriles, 1,2,3-and 1,2,4-triazoles, and 3-aminoisoxazole utilising the oximes of the title hydrazones are reported. The behaviour of the oximes on pyrolysis in the gas phase and by flash vacuum is analysed. X-ray crystallography were used to confirm the structure of the triazole 10b and the 3-oxoalkanonitrile 12f.

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In the gas phase, both N -t -butylthioacetamide and N-acetylthioacetamide undergo unimolecular firstorder elimination reactions for which log A = 11.58 s-' and 10.64 s-', and €, = 16.45 kJ mol-' and 117.1 5 kJ mol-', respectively. The results are in accord with a reaction pathway involving a cyclic six-membered transition state, and show each compound to be more reactive than its oxygencontaining analogue. At 600 K, the statistically corrected reactivity ratios: t-butyl thioacetate (1 ) / t-butyl acetate (2); N-acetylthioacetamide (3)/diacetamide (4); and N -t -butylthioacetamide (5)/N-t-butylacetamide (6) are 83, 173, and 1 404, respectively. The above rate factors are consistent with the tenet that as C,-X bond fission becomes less rate-contributing in these electrocyclic reactions, so attack by the C=Y bond upon the P-hydrogen atoms becomes more important. Thus, whereas t-butyl acetate at 600 K is some 68 700 times more reactive than N-t-butylacetamide, t-butyl thioacetate is only 4 060 times more reactive than N -t -butylthioacetamide.The gas-phase thermal elimination of esters, amides, and anhydrides containing P-hydrogen atoms are thought to involve pathways in which the transition states approximate to cyclic six-membered structures (Scheme 1).

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