Nuclear magnetic resonance study of addition–cyclization involving ethyl thioacetoacetate and α-nucleophiles

Addition of NH2NH2 or NH2OH to ethyl thioacetoacetate to form the corresponding cyclic product, 3-methylpyrazol-5-one or 3-methylisoxazol-5-one proceeds via cyclization of the carbinolamine formed by addition to the β-keto carbon, i.e., cyclization is faster than dehydration of the carbinolamine to form the imine. In contrast the corresponding carbinolamine derived from ethylacetoacetate undergoes dehydration faster than cyclization. By means of 1H nuclear magnetic resonance spectroscopy, it is possible to detect the cyclic carbinolamine as well as another transient and measure their rates of decay. Based on these results, a mechanism is proposed.