Intramolecular Photochemical Cycloaddition Reactions of N-indoles and N-(ω-Alkenoyl)indoles

Article abstract of DOI:10.1021/jo00085a022

Ultraviolet light irradiation of N-(but-3'-enoyl)indole (3i), N-(pent-4'-enoyl)indole (3j), and N-(hex-5'-enoyl)indole (3k) affords intramolecular photocycloadducts 17a-19a in which the termini of the side-chain alkene have become bonded to the 2- and 3-positions of the indole ring.The regiochemistry of the addition of the alkene is the opposite to that obtained in the corresponding intermolecular reaction of an N-acylindole with a monosubstituted alkene.The length of the methylene linkage between the tethered alkene and the N-acyl activating group in these N-alkenoylindoles affects the quantum efficiency of intramolecular cycloaddition as well as the ability of the reaction to compete with intermolecular cycloaddition in the presence of added cyclopentene.In contrast, the N-(ω-alkenyloxycarbonyl)indoles 3a-e are relatively photostable, apparently because they are frozen in an unreactive conformation for the duration of the excited state lifetime.Compounds 3a-e are, however, capable of undergoing intermolecular photocycloaddition in the presence of added alkenes and thay also photodimerize.These reaction pathways dominate when lower energy wavelengths (λ>300 nm) of ultraviolet light are used; irradiations performed at higher energy wavelength (254 nm) give predominantly photo-Fries rearrangement products.