A Theoretical Proposal for the Synthesis of Carbapenems from 4‐(2‐Propynyl)azetidinones Promoted by [W(CO) 5 ] as an Alternative to the Ag + ‐Assisted Process

The synthesis of carbapenems from 4‐(2‐propynyl)azetidinones assisted by both Ag + and [W(CO) 5 ] was theoretically investigated by using the B3LYP/6‐31+G(d)‐LANL2DZ level, taking into account the effect of solvent by the PB‐SCRF model implemented in Jaguar. According to our results, the silver‐assisted cyclization is a concerted process for which the low yield experimentally observed could mainly stem from the alkaline hydrolysis of the β‐lactam ring. This process is very efficiently catalyzed by Ag + , making it competitive with the formation of the carbapenem. The cycloisomerization of 4‐(2‐propynyl)azetidinone promoted by [W(CO) 5 ] is proposed as an alternative synthetic strategy to obtain the carbapenem. The endo cycloisomerization is by far the most favorable one. When the process is assisted by [(thf)W(CO) 5 ], although the main product is the carbapenem, the formation of a carbene complex represents a certain competition. The presence of a Me 3 N molecule from the very start of the reaction causes an important catalytic effect considerably reducing the energy barriers corresponding to the H atom transfers and rendering a very efficient process. Moreover, this catalytic action determines the evolution of the system through only one mechanistic route which produces the carbapenem, hindering the formation of the carbene. Therefore, the cycloisomerization of 4‐(2‐propynyl)azetidinone promoted by [(Me 3 N)W(CO) 5 ] constitutes an interesting alternative to the silver‐assisted cyclization.