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
and a catalytic amount of AIBN in boiling benzene caused 5-exo cyclization of allylic O-stannyl ketyl radicals generated by addition of Bu3Sn· on the amide-oxygen atoms to provide γ-lactams 2a–h after acidic workup. When enamide 1d was treated with Bu3SnH in the presence of AIBN followed by aldehydes 3a–d, sequential radicalcyclization and aldol reactions occurred to afford anti-adducts 4a–d and syn-adducts
Enamides 5, on treatment with Bu3SnH-AIBN, underwent aryl radical cyclization in a 5-exo manner to give 1-[bis(phenylthio)methyl]dihydroisoindoles 6, which were partially desulfurized with Bu3SnH-AIBN to give 1-mono(phenylthio)methyl congeners 7. Formation of 6 from 5 may be explained by the presence of two phenylthio groups at the terminus of the N-vinylic bond of 5, since enamide Sa having no phenylthio group underwent aryl radical cyclization in a 6-endo manner. Compound 7d (R = CF3) was transformed into sulfoxide 16, which was treated with (CF3CO)(2)O and then with 10% NaOH to give a model compound 20 of mappicine ketone (MPK) (1) through aldol condensation of aldehyde 18. An attempt to synthesize MPK using this method with sulfoxide 28 prepared from 25, however, was unsuccessful, and, instead, photochemical cyclization of enamide 38 prepared from 25 furnished MPK.
Synthesis of a model compound of mappicine ketone based on sulfur-directed 5-exo selective aryl radical cyclization onto enamides
Enamides 10, upon treatment with Bu3SnH-AIBN, gave 5-exo aryl radical cyclization products 11, which were partially desulfurized to give 1-substituted dihydroisoindoles 7 and 12. This method was applied to the synthesis of a model compound 4 of mappicine ketone (1). (C) 2001 Elsevier Science Ltd. All rights reserved.