A stereocontrolled formal total synthesis of (±)-thienamycin

The stereocontrolled synthesis of a key intermediate 20 for the preparation of (±)-thienamycin 1 is described. The key steps in the synthesis are the formation of the β-lactam ring by cyclization of the amide 5 via a complete S_N2 mechanism, and the stereospecific conversion of the azetidinone 5 to the amide (trans-11) which have the correct relative configurations at three contiguous chiral centres. The mechanism of the conversion of the azetidinone 16E to the N-free azetidinone 17 and the selenide compound 18 is presumed.