Thermal reactions of 8-methylbicyclo[4.2.0]oct-2-enes: competitive diradical-mediated [1,3] sigmatropic, stereomutation, and fragmentation processes

reaction: see text] At 275 degrees C, 8-exo-methylbicyclo4.2.0]oct-2-ene (1a) undergoes a 1,3] sigmatropic rearrangement to 5-methylbicyclo2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is only marginally favored over the forbidden sr product; that is, si/sr is 2.4. Accompanying the 1,3] shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no 1,3] product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.