Controlled Rearrangement of Lactam-Tethered Allenols with Brominating Reagents: A Combined Experimental and Theoretical Study on α- versus β-Keto Lactam Formation

N-Bromosuccinimide (NBS) smoothly promotes the ring expansion of lactam-tethered allenols to efficiently afford cyclic alpha- or beta-ketoamides with good yields and high chemo-, regio-, and diastereoselectivity, through controlled C-C bond cleavage of the beta- or gamma-lactam nucleus. Interestingly, in contrast to the rearrangement reactions of 2-azetidinone-tethered allenols, which lead to the corresponding tetramic acid derivatives (beta-keto lactam adducts) as the sole products, the reactions of 2-indolinone-tethered allenols under similar conditions give quinoline-2,3-diones (alpha-keto lactam adducts) as the exclusive or major products. To rationalize the experimental observations, theoretical studies have been performed.