Abstract
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 analysis using the program PERVAL. Distortions of the dipolarophiles in the transition structures due to closed-shell repulsions lead to FMO interactions which favor the experimental regiochemistry.
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