Synthesis, Characterization, and Reactivity of Complex Tricyclic Oxonium Ions, Proposed Intermediates in Natural Product Biosynthesis

Reactive intermediates frequently
play significant roles in the
biosynthesis of numerous classes of natural products although the
direct observation of these biosynthetically relevant species is rare.
We present here direct evidence for the existence of complex, thermally
unstable, tricyclic oxonium ions that have been postulated as key
reactive intermediates in the biosynthesis of numerous halogenated
natural products from Laurencia species.
Evidence for their existence comes from full characterization of these
oxonium ions by low-temperature NMR spectroscopy supported by density
functional theory (DFT) calculations, coupled with the direct generation
of 10 natural products on exposure of the oxonium ions to various
nucleophiles.