Entropy versus tether strain effects on rates of intramolecular 1,3-dipolar cycloadditions of N-alkenylnitrones

Intramolecular 1,3-dipolar cycloadditions of two N-alkenylnitrones are studied by means of density functional theory calculations. Cycloaddition of an acyclic 4-hexenylnitrone led to the expected isoxazolidine in 46% yield, but a 4-cycloheptenylnitrone did not react. Calculations of the transition states for cycloaddition indicate that although the cycloheptenyl nitrone has a more favorable activation entropy, the strain associated with distortion of the tethering groups into the required boat conformation disfavors the reaction of the cyclic substrate over the acyclic substrate by 8.7 kcal/mol.