3‐substituted 4,5‐dihydroxycarbacephams, respectively, by a type I carbonyl‐ene reaction, while the BF3⋅Et2O or SnCl4‐mediated type II carbonyl‐enecyclization of alkenylaldehydes 2 furnishes 3‐methylene 5‐hydroxycarbacephams along with the corresponding 3‐halo 5‐hydroxycarbacepham. The stereochemical outcome of these carbonyl‐enecyclizations leading to carbacepham derivatives can be explained in terms of six‐membered