Cycloadditions of N‐Aryl‐C‐(trifluoromethyl)nitrilimines with dimethyl fumarate and maleate

The cycloadditions of N‐aryl‐C‐(trifluoromethyi)nitrilimines 1, generated in situ from various precursors, with dimethyl fumarate and maleate are described. N‐Aryltrifluoroacetohydrazonoyl bromides 2 reacted with fumarate in the presence of triethylamine to give the trans‐4,5‐dimethoxycarbonylpyrazolines 4 whereas the reactions with maleate afforded the 4,5‐dimethoxycarbonylpyrazoles 5 as well as 4. In the reactions with the chloride 6 under the more drastic conditions, both fumarate and maleate gave the pyrazole 5a. From thermolysis of the oxadiazaphosphole 8, the trans‐pyrazoline 4a was formed in both cases of fumarate and maleate. The preparation of the cis‐pyrazoline 9 was attempted from 8 and maleic anhydride but 9 was found to be so unstable as to be epimerized under the mild conditions.