Interaction of β-Lactam Carbenes with Aryl Isonitriles: An Unprecedented Rearrangement of 2-Azetidinonylidene Indoles to δ-Carbolinones

The reaction of β-lactam carbenes with aryl isonitriles proceeded in a novel [2 + 2] fashion to give high yields of 2-azetidinonylidene indoles 4, which underwent an unprecedented rearrangement to furnish 4-arylimino-δ-carbolin-2-ones 5 in almost quantitative yields. Acid catalyzed rearrangement and the subsequent hydrolysis of 2-azetidinonylidene indoles 4 produced two types of δ-carbolin-2,4-diones 10 and 11, respectively, in good to excellent yields. The photophysical study showed that both δ-carbolin-2,4-diones 10 and 11 are highly fluorescent with the fluorescent quantum yields being up to 0.43.