the carbonyl-ene reaction is almost always catalyzed by lewis acids in synthetic applications. a stepwise or a largely asynchronous mechanism has been proposed for the catalyzed reaction based on kinetic isotope effect studies. nonetheless, cyclic transition states are frequently invoked to interpret diastereoselectivity. in a seminal review in the early 1990s, mikami and colleagues proposed a late, chair-like transition state, which could rationalize many observed stereochemical results, including the role of steric bulk in diastereoselectivity: