Reactions of cyclic oxalyl compounds, 37. Substituent effects on the site of nucleophilic attack at 1H‐pyrrole‐2,3‐diones

Reactions of 1H-pyrrole-2,3-diones 2a-i with N- and O-nucleophiles proceed by initial attack at C-2 or/and C-5. Consequently, earlier results suggesting structures of type A (attack at C-3) have to be revised (5, 13, 17, 18b). In general, electron-withdrawing substituents at C-4 favor attack at C-5 with formation of 2-pyrrolones 5, 6, 10, 13, 17, and 20. Alternatively, attack at C-2 leads to α-oxobutenoic acid derivatives 3, 7, 11, 12, 15, and 18. IR and 13C-NMR spectroscopic techniques were employed to differentiate between the possible isomeric addition products formed. The structure of methyl α-oxobutenoate 15 was established by an X-ray crystallo-graphic analysis.