Lewis Acid‐Assisted Ene Cyclization of 2‐Azetidinone‐Tethered Enals: Synthesis of Enantiopure Carbacepham Derivatives

Diastereocontrolled Lewis acid-catalyzed preparation of enantiopure carbacepham derivatives have been developed starting from 2-azetidinone-tethered enals. The BF3.Et2O-promoted reaction of alkenylaldehydes 1 and 16 is effective as carbocyclization protocol to afford 4-substituted 5-hydroxycarbacephams or 3-substituted 4,5-dihydroxycarbacephams, respectively, by a type I carbonyl-ene reaction, while the BF3.Et2O or SnCl4-mediated type II carbonyl-ene cyclization of alkenylaldehydes 2 furnishes 3-methylene 5-hydroxycarbacephams along with the corresponding 3-halo 5-hydroxycarbacepham. The stereochemical outcome of these carbonyl-ene cyclizations leading to carbacepham derivatives can be explained in terms of six-membered, cyclic chair-like transition-state models. The formation of halocarbacepham derivatives is proposed to proceed by a stepwise mechanism.