Asymmetric radical cyclization leading to β-lactams: Stereoselective synthesis of chiral key intermediates for carbapenem antibiotics PS-5 and thienamycin
A stereoselective synthesis of β-lactams by 4-exo-trig radicalcyclizations of N-[2,2-bis(phenylthio)ethenyl]-α-bromo amides bearing a chiral inductor on the nitrogen atom has been examined. Bromide 8, upon treatment with Bu3SnH in the presence of AIBN in boiling benzene, gave a mixture of (4S)-2-azetidinone 12a and its (4R)-isomer 12b in a ratio of 71:29 and 69% combined yield. Similar treatment of