Effect of a methoxy substituent on the vinylcyclobutane carbon migration.

Over the temperature range 250-300 degrees C, 8-exo-methoxybicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-exo- and 5-endo-methoxybicyclo[2.2.2]oct-2-enes, 2a and 2b, respectively, with a clear preference for the si product: si/sr = 3.2. Both 1a and its 8-endo epimer 1b experience appreciable epimerization and fragmentation. A long-lived intermediate with weakly interacting diradical centers, one of which is stabilized by a methoxy substituent, can account for all such observations.