Thermal Reactions of 8-Methylbicyclo[4.2.0]oct-2-enes: Competitive Diradical-Mediated [1,3] Sigmatropic, Stereomutation, and Fragmentation Processes
作者:Xavier S. Bogle、Phyllis A. Leber、Lynne A. McCullough、David C. Powers
DOI:10.1021/jo051505g
日期:2005.10.1
At 275 degrees C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,31 sigmatropic rearrangement to 5-methylbicyclo[2.2.2]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 [1,31 shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no [1,31 product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.