Semisynthetic\nStudies Establish a Role for Conjugate\nHalide Exchange in the Formation of Chlorinated Pyrroloiminoquinones\nand Related Alkaloids

Two novel pyrroloiminoquinone alkaloids, 6-chlorodamirone\nA and\n6-bromodamirone A, have been identified for the first time from the\nmarine sponge <i>Latrunculia</i> sp. (order: Poecilosclerida:\nfamily Latrunculiidae), sourced from Western Australia. Alongside\nthese new compounds, seven previously known metabolites were also\nisolated. Despite being obtained in submilligram quantities, the structures\nof these natural products were successfully elucidated using high-resolution\nmass spectrometry and nuclear magnetic resonance spectroscopy. To\nconfirm the structures of these newly discovered alkaloids, a semisynthetic\napproach was employed starting from the more abundant metabolite,\ndamirone A, additionally, single crystal X-ray crystallography was\nused to validate our structural proposals. The semisynthetic studies\nsuggest that the chlorinated alkaloids are likely formed through a\nnonenzymatic conjugate halide substitution reaction rather than an\nenzymatic process. This reactivity parallels that observed in related\nmetabolites, such as the caulibugulones B and C. Furthermore, a biomimetic\ncascade reaction was attempted to synthesize the spirodienone moiety\ncharacteristic of the discorhabdin alkaloids, inspired by the nucleophilic\nsubstitution observed in the tricyclic damirone A system. Albeit unsuccessful,\nthese findings provide valuable insight into the reactivity of halogenated\npyrroloiminoquinones under various conditions.