Selective formation of 1,3,4‐trisubstituted and 3,4‐disubstituted trans‐β‐lactams from zinc enolates and imines

A new route for the high yield synthesis (better then 90%) of exclusively trans ‐β‐lactams (azetidin‐2‐ones) is reported which involves the 1:1 reaction of an α‐aminoacid ester zinc enolate with an appropriate imine. The reaction can be carried out as a ‘one‐pot’ synthesis as has been demonstrated for the synthesis of trans ‐3‐diethylamino‐4‐phenyl azetidin‐2‐one (93% yield). The novel zinc enolates have most likely a Z‐geometry as a result of intramolecular chelate coordination. Evidence has been obtained that in the first step these zinc enolates react with the imine in a highly diastereoselective manner providing the threo aldolate which in a subsequent step undergoes ring closure to the azetidin‐2‐one.