Bromonium Ion Induced Transannular Oxonium Ion Formation-Fragmentation in Model Obtusallene Systems and Structural Reassignment of Obtusallenes V-VII

Ring-closing metathesis was used to construct the strained 11-membered ring of obtusallenes II (and IV). Bromonium ion induced transarmular oxonium ion formation-fragmentation gave the macrocyclic carbon skeleton of obtusallene VII with a bromine atom at C-13, in line with a previously published hypothesis. An additional brominated [5.5.1]bicyclotridecane adduct that must arise from a bromonium ion induced transarmular oxonium ion formation-fragmentation could also be isolated, suggesting that this adduct represents the core of an as yet undiscovered natural product. An authentic sample of obtusallene V was studied by NMR spectroscopy, and the position of the halogens at C-7 and C-13 was reassigned on the basis of a C-13 NMR chlorine induced isotopic shift. This revised structure was subsequently confirmed by X-ray crystallography. These findings allow us to confidently conclude that the structures of obtusallenes VII and VI should also be reassigned.