Direct access to marine pyrrole-2-aminoimidazoles, oroidin, and derivatives, via new acyl-1,2-dihydropyridin intermediates.

[reaction: see text] A short synthesis of the C(11)N(5) oroidin derivatives is reported. The key step of the strategy is a one-pot oxidative bromine-mediated addition of protected guanidines to the N-acyl-1,2-dihydropyridines 9a-c. The new N-acyl-1,2-dihydropyridines were prepared directly from pyridine and pyrrole-2-carbonyl chloride by reduction with borohydride reagent in one step.