On the Origin of Cytotoxicity of the Natural Product Varacin. A Novel Example of a Pentathiepin Reaction That Provides Evidence for a Triatomic Sulfur Intermediate

A density functional theoretical study is presented, which implicates a novel S(3)-cleavage in the decomposition of a pentathiepin. This study predicts an interconversion between a pentathiepin and an open-chain polysulfur ion intermediate from which a key determinant in the chemistry then follows. Expulsion of diatomic sulfur, S(2), is unlikely from the unimolecular collapse of the open-chain polysulfur ion. Instead, S(3) can dissociate due to an unusually long and weak sulfur-sulfur (S4-S5) bond. A mechanistic picture now emerges which predicts that the novel S-S cleavage reaction and the unanticipated S(3) fragmentation are a result of delocalization of the negative charge within the remaining carbon-sulfur fragment. The computed results presented here reveal a new aspect to the chemistry of pentathiepins, that of S(3) unit transfer, which is proposed to have significance in the mechanism of cytotoxicity of the natural product varacin, 1.