Semisynthetic Studies Establish a Role for Conjugate Halide Exchange in the Formation of Chlorinated Pyrroloiminoquinones and Related Alkaloids

Two novel pyrroloiminoquinone alkaloids, 6-chlorodamirone A and 6-bromodamirone A, have been identified for the first time from the marine sponge Latrunculia sp. (order: Poecilosclerida: family Latrunculiidae), sourced from Western Australia. Alongside these new compounds, seven previously known metabolites were also isolated. Despite being obtained in submilligram quantities, the structures of these natural products were successfully elucidated using high-resolution mass spectrometry and nuclear magnetic resonance spectroscopy. To confirm the structures of these newly discovered alkaloids, a semisynthetic approach was employed starting from the more abundant metabolite, damirone A, additionally, single crystal X-ray crystallography was used to validate our structural proposals. The semisynthetic studies suggest that the chlorinated alkaloids are likely formed through a nonenzymatic conjugate halide substitution reaction rather than an enzymatic process. This reactivity parallels that observed in related metabolites, such as the caulibugulones B and C. Furthermore, a biomimetic cascade reaction was attempted to synthesize the spirodienone moiety characteristic of the discorhabdin alkaloids, inspired by the nucleophilic substitution observed in the tricyclic damirone A system. Albeit unsuccessful, these findings provide valuable insight into the reactivity of halogenated pyrroloiminoquinones under various conditions.