Steric and Conformational Control of the Regioselectivities in the Ene Reaction with Trisubstituted Cycloalkenes:  Comparison of the Enophiles Singlet Oxygen, Triazolinedione, and Nitrosoarene

The nitrosoarene ene reaction with the cycloalkenes 1−3 and E-4 proceeds in high twix regioselectivity to afford the hydroxylamine ene products 1a−4a (twix) and 1b−4b (twin, except far E-4 twix). Steric interactions in the enophile attack are responsible for the skew trajectory of the nitrosoarene enophile. For Z-1-methylcyclooctene (Z-4), twin abstraction dominates, caused by conformational constraints (transannular interactions) in the hydrgogen-atom abstraction. The balance between these steric and conformational factors dictates the regioselectivity in the ene reaction