Formation of 3,4-Diarylpyrrole- and Pyrrolocoumarin Core of Natural Marine Products via Barton-Zard Reaction and Selective O-Demethylation

A metal-free approach to 3,4-diarylpyrrole-2-carboxylate and pyrrolocoumarin cores of lamellarins and related natural products based on Barton-Zard reaction of nitrostilbenes with ethyl isocyanoacetate was developed. In the case of diarylpyrrole-2-carboxylates with a 3-(o-methoxyphenyl) fragment, treatment with 1 equiv. of. BBr3 resulted in selective O-demethylation of the ortho-methoxy group, while other methoxy groups in the molecule remained intact. The resulting 3-(2-hydroxyphenyl)pyrrole-2-carboxylates underwent base-induced lactonization to form the target pyrrolocoumarins.