Thermally induced cyclization of the anti-alkenyl oximes E-7a,b and E-17a,b affords cyclic α-alkoxycarbonylnitrones 8 and the 6,7-bicyclic nitrones 18, respectively. The syn-oximes Z-7b and Z-17b react via an alternate pathway to give exclusively the fused isoxazolidine derivatives 10b and 19b, respectively. These oximes are configurationally stable at high temperatures with the energy barrier to isomerization being significantly greater than that to cyclization/cycloaddition. Neither the tert-butyl derivative 7c nor the ε-alkenyl oxime 7d share this characteristic and in these cases the products of thermal activation are independent of the geometry of the starting oxime. For 7c the energy barriers to oxime rotation and cyclization or cycloaddition are sufficiently close to allow all three reactions to proceed. With 7d, cis–trans isomerization and cyclization are the only observed reactions.