Toward "E-Ring-Free" Lamellarin Analogues: Synthesis and Preliminary Biological Evaluation

Since the discovery of anticancer properties of a naturally occurring hexacyclic marine alkaloid Lamellarin D, the attempts have been made to prepare its synthetic analogues and elucidate the effects of each structural component on their activity profile. While F-ring-free, A-ring-free and B-ring-open lamellarins are known, E-ring-free analogues have never been investigated. In this work, we developed a facile and straightforward synthetic method toward E-ring-free lamellarin analogues based on the [3+2]-cycloaddition. For the first time, we prepared several pentacyclic lamellarin analogues without E-ring in their structure and assessed their cytotoxicity in a panel of cancer cell lines in comparison with several hexacyclic lamellarins. E-ring-free lamellarins were devoid of cytotoxicity due to their poor solubility in cellular environment.