Tetrahedron 2012, 68, 3165-3171
From the condensation between 1,1-dimethoxyethene and anhydrides, synthetically useful beta,beta-dimethoxye-alpha,beta-unsaturated ketones were prepared. Upon addition of hydrazine, followed by io- dination, 4-iodinated 3-methoxypyrazoles were obtained. The occurrence of a side compound also provided insights in the scope of this synthesis. In a second part, 1-(4-chlorophenyl)-3,3-dimethoxyprop- 2-en-1-one was obtained from 1,1-dimethoxyethene and 4-chlorobenzoylchloride. The subsequent ad- dition of hydrazine or phenylhydrazine led to 5-(4-chlorophenyl)-3-methoxy-1H-pyrazole or 1-phenyl- 5-(4-chlorophenyl)-3-methoxypyrazole in unprecedented 64 or 54% overall yield. Unexpectedly, addition of 2-pyridylhydrazine led to the 2-([1,2,4]triazolo[4,3-a]pyridin-3-yl)-1-(4-chlorophenyl) ethanone. This led us to design original conditions, which led to the target 1-(2-pyridyl)-5-(4-chlorophenyl)-3- methoxypyrazole in a 39% overall yield. Additional examples are provided, starting from various carboxychlorides
