| Abstract
| - At 275 °C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangementto 5-methylbicyclo[2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is onlymarginally favored over the forbidden sr product; that is, si/sr is 2.4. Accompanying the [1,3] shiftare significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yieldsno [1,3] product, experiences primarily direct fragmentation and secondarily epimerization. Adiradical intermediate can account for all such observations.
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