FACTORS CONTROLLING C=C VS. C=O ATTACK IN CYCLOADDITION OF A 1, 3-DIENE TO AN AMBIDENT DIENOPHILE. DIELS-ALDER REACTION OF 2-PHENYL-Δ²-PYRROLINE-4, 5-DIONES

Abstract

The [4+2] cycloaddition using an enone as a dienophile is on a balance with the normal C=C attack (ene-addition) and the unusual C=O attack (one-addition) of a diene. Steric hindrance on the C=C (caused by such phenomena as restricted free rotation of the substituent on C=C) retards the C=C attack, thus increasing the ratio of one-addition. Lewis acids greatly increase not only the reactivity of C=O but also the ratio of one-addition.