Regio- and Stereoselectivity in the Ene Reaction of N-Phenyl-l,2,4-triazoline-3,5-dione with, α,β-Unsaturated Carbonyl Substrates

N-Phenyltriazoline-3,5-dione reacts with, α,β-unsaturated ketones, esters, and lactones 1a-1 to give ene adducts 2a-1. The reactions usually proceed in good yield with high regioselectivity and, where possible, high stereoselectivity. Ene substrates capable of adopting an s-cis conformation show much greater reactivity. A variety of mechanistic interpretations is considered.