The Vilsmeier–Haack reaction with pyrrole and 1-methylpyrrole has been modified to provide a direct synthesis of the corresponding 2-nitriles. Chlorosulfonyl isocyanate may be used to prepare the same compounds, but less satisfactorily. The latter reagent does enable the synthesis of pyrrole-2,4-dicarbonitrile in one step. This dinitrile may be reduced to pyrrole-2,4-dicarboxaldehyde.
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Barnett et al. (1980) studied this question.