Synthesis of N-substituted prodigiosenes

Abstract

Prodigiosenes bearing N-methyl or N-benzyl groups in each of the three pyrrole rings have been prepared by coupling either a bipyrrole with a pyrrolecarbaldehyde or a bipyrrolecarbaldehyde with an α-free pyrrole. Synthesis via the bipyrrole route can lead to fragmentation of the products through further attack by the bipyrrole. In contrast to the demethyl series, the salts of 11-methylprodigiosenes exist as E/Z mixtures. In the base form only Z isomers were detected. A number of novel pyrroles (30b, c, d) are accessible by a regioselective Grignard reaction on 3-bromo-2-bromomethylpyrroles.