General Metohds for Synthesizing 2,4-Dicaylpyrroles and their Precursors Containing One or Two Masked Groups

A thorough study of the synthesis of 2,4-diacylpyrroles by direct acylation of pyrrole and 2- and 3-acylpyrroles is reported. Among these, Friedel–Crafts acylation of 3-acylpyrroles is the most general and advantageous method because it utilises easily accessible starting materials. In addition it is always regiospecific and readily provides 2,4-diacylpyrroles containing identical or different acyl groups in very high yields (81–100%) under mild conditions. Alternative procedures concern the synthesis of precursors of 2,4-diacylpyrroles containing one or two acyl groups masked by a 1,3-benzodithiolyl or 1,3-benzoxathiolyl group and subsequent hydrolysis with HgO–35% aq. HBF4Me2SO. Overall yields are always good (52–60%). Indirect acylation constitutes a secure complement to direct acylation when it is necessary to operate in the presence of protected acyl groups.