The pyrrole-imidazole alkaloids are a group of structurally unique and biologically interesting marine sponge metabolites. Among them, the cyclic dimers have caught synthetic chemists’ attention particularly. Numerous synthetic strategies have been developed and various biosynthetic hypotheses have been proposed for these fascinating natural products. We discuss herein the synthetic approaches and the biosynthetic insights obtained from these studies.

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

A concise formal synthesis of echinopines A and B is reported. The key [5.5.7] tricyclic intermediate, which has been previously used for the synthesis of echinopine A and B, was assembled using Cr(0)-promoted photochemical [6π + 2π] cycloaddition followed by a radical cyclization step.

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We report herein the synthesis of a fully functionalized B,C,D-ring system of lancifodilactone F. The key transformations involve an arene–olefin meta-photocycloaddition reaction and a palladium-catalyzed oxidative C–C cleavage reaction to establish its B,C-rings.