Diazomethane adds to ethoxyacetylene to give a 96: 4 mixture in favor of 4-ethoxypyrazole (3) which had been identified as the only product in a previous study. This result contrasts the behavior of ethyl vinyl ether which gives 3-ethoxy-1-pyrazoline (2). Transition structures for the four possible regioisomers are determined by MNDO-PM3 calculations. The regioselectivity is explained on the basis of the PM3 calculations and their perturbational ...
