Oxonium ions bicyclo

As shown in Scheme 78, the transient bicyclo[3.3.0]oxonium ylide 255 that was generated from a THF-substituted diazoketone was first protonated by acetic acid to the corresponding bicyclo[3.3.0]oxonium ion, which then provided the ring expansion product <1996CC1077>. In the presence of the weakly acidic MeOH, the ylide underwent a concerted [3+2] cycloreversion to a ketene intermediate to form the ring cleavage product <2004JOC1331>. 

By using this method, they determined the rate constant of the propagation of endo-and exo-2-methyl-7-oxabicydo[2.2.1]heptaneZ1,22 and endo- and exo-2-tert-butyl-7-oxa-bicyclo[2.2.1]heptaneZ5. In these systems the polymerization proceeds via SN2 reaction between monomer and the propagating oxonium ion as was the case for the monocyclic ethers. They achieved the first case of a quantitative comparison of the polymerization reactivity between monocyclic and bicyclic ethers in the polymerization of exo-2-methyl-... 

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