“…The guanidine functional group is a structural motif frequently found in marine natural products that provides them with potent biological activities . In fact, the interest in this type of compounds has increased dramatically in recent years, as it has been shown that the incorporation of this moiety into a given molecule can lead to a wide diversity of applications, among which the development of new drugs, organocatalysts, and even new molecular materials can be highlighted. The most remarkable natural compounds within this group are those presenting a cyclic guanidinium, such as saxitoxin ( I ), tetrodotoxin ( II ), or crambescin A ( III ) (Figure ).…”