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Electrophile-Induced Ring Expansions of β-Lactams toward γ-Lactams

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journal contribution
posted on 28.10.2005, 00:00 by Willem Van Brabandt, Norbert De Kimpe
An efficient and straightforward route toward 3,4-cis-4-isopropenylazetidin-2-ones was developed from 4-(1-chloroalkyl)azetidin-2-ones. Starting from the latter β-lactams, a new synthesis of pyrrolidin-2-ones was achieved. When 4-isopropenylazetidin-2-ones were treated with bromine in dichloromethane, diastereoselective electrophile-induced ring expansions toward 5-bromopyrrolidin-2-ones were performed. Further oxidation of 3-benzyloxypyrrolidin-2-ones with bromine toward 3-bromopyrrolidin-2-ones was also established. When 4-isopropenyl-β-lactams were added to a mixture of NBS and TMSN3, 5-azidopyrrolidin-2-ones were obtained in moderate to high yields.