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 transannular 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 transannular 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 (13)C 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.
Braddock
D Christopher
DC
Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, UK. c.braddock@imperial.ac.uk
Millan
David S
DS
Pérez-Fuertes
Yolanda
Y
Pouwer
Rebecca H
RH
Sheppard
Richard N
RN
Solanki
Savade
S
White
Andrew J P
AJ
D001966:Bromine / Q000737:chemistry*; D003500:Cyclization; D004988:Ethers, Cyclic / Q000138:chemical synthesis / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D010100:Oxygen / Q000737:chemistry*; D013237:Stereoisomerism
D004988:Ethers, Cyclic; D007477:Ions; C522877:obtusallene; D010100:Oxygen; D001966:Bromine