Thermal rearrangement of 7-methylbicyclo[3.2.0]hept-2-ene: An experimentalprobe of the extent of orbital symmetry control in the [1,3] sigmatropic rearrangement

The gas-phase thermal rearrangement of ero-7-methylbicyelo[3.2.0]hept-2-ene yields almost exclusively 5-methylnorbornene products. Inversion (i) of co nfiguration dominates this [1,3] sigmatropic shift although same retention (r) is also observed. Because the [1,3] migration can only occur suprafacia lly (s) in this geometrically constrained system, the si/sr ratio of 7 obse rved for the migration of C-7 in exo-7-methylbicyclo[3.2.0]hept-2-ene indic ates that the orbital symmetry rules are somewhat permissive for the [1,3] sigmatropic migration of carbon.