Carbocations, stability crystallography

Ionizations of furfuryl alcohols in trifluoroacetic acid solutions lead to the formation of fliranoxonium ions. These resonance-stabilized carbocations have been used in (4 + 3)-cycloaddition reactions with 1,3-dienes. Erker and coworkers have found that 2,5-dimethylthiophene (182) undergoes a novel trimerization reaction in solution of hexanes and CF3S03H. The tetracyclic product (183) is formed in high yield and its structure was confirmed by X-ray crystallography. A mechanism is proposed for the conversion, invoking C(2) protonated 2,5-dimethylthiophene as the initial eleetrophilic species. Subsequent intermediates are sulfur-stabilized carbocations and dications. 

---