The Role of Amine in the Mechanism of Pentathiepin (Polysulfur) Antitumor Agents

A computational and experimental study is presented, which provides the first evidence that amine has an opportunity to engage in bonding with pentathiepin to promote its decomposition. The study provides mechanistic insight into the process that gives rise to pentathiepin biological activity. Primary or secondary amine will allow for an intramolecular addition to the pentathiepin ring at the nearest sulfur (S1). In contrast, tertiary amine adds reversibly to S1, because nitrogen cannot lose its positive charge by deprotonation. This precludes the amine promotion step. An energetically low-lying process is characterized, corresponding to S3-loss triggered by nucleophilic activation with a primary or secondary amine. Pentathiepin desulfurization via S3-unit transfer is supported by a trapping study with norbornene. That the amine may confer an enhanced reactivity in the natural products varacin, 1, and lissoclinotoxin A, 2, adds to the understanding of the pathway for pentathiepin activation and may provide new design concepts that have potential applications for this class of biocides.