Green oxidation of indoles using halide catalysis

Oxidation of indoles is a fundamental organic transformation to deliver a variety of synthetically and pharmaceutically valuable nitrogen-containing compounds. Prior methods require the use of either organic oxidants (meta-chloroperoxybenzoic acid, N-bromosuccinimide, t-BuOCl) or stoichiometric toxic transition metals [Pb(OAc)4, OsO4, CrO3], which produced oxidant-derived by-products that are harmful to human health, pollute the environment and entail immediate purification. A general catalysis protocol using safer oxidants (H2O2, oxone, O2) is highly desirable. Herein, we report a unified, efficient halide catalysis for three oxidation reactions of indoles using oxone as the terminal oxidant, namely oxidative rearrangement of tetrahydro-β-carbolines, indole oxidation to 2-oxindoles, and Witkop oxidation. This halide catalysis protocol represents a general, green oxidation method and is expected to be used widely due to several advantageous aspects including waste prevention, less hazardous chemical synthesis, and sustainable halide catalysis.Published versionThis research was financially supported by Research Grant Council of Hong Kong (16311716, 16303617, 16304618) and National Natural Science Foundation of China (21772167). Dr. J.X. also acknowledged the Doctor Start-up Fund ([2018]28) and the Guizhou Province First-Class Disciplines Project (Yiliu Xueke Jianshe Xiangmu-GNYL [2017]008) from Guizhou University of Traditional Chinese Medicine (China)

Six-membered ring systems: with O and/or S atoms

A large variety of publications involving O- and S-6-membered ring systems have appeared in 2018. The importance of these heterocyclic compounds is highlighted by the large number of publications on the total synthesis of natural oxygen derivatives and of other communications dedicated to natural and synthetic derivatives. Reviews on the ruthenium-catalyzed synthesis of various O-6-membered heterocycles (18SC1551), the enantioselective syntheses of 3,4-dihydropyran derivatives (18CR2080), and strategies for the synthesis of coumarins (18SC1534) have appeared. The synthesis of natural pyran-2-ones and pyran-4-ones starting from alkynes and promoted by gold complexes (18AGE4215) and cyclization strategies of substituted alkynyl acids and esters to afford pyran-2-one and isocoumarin derivatives (18SL1) have been detailed in minireviews. The synthesis of various O-6-membered derivatives was also achieved through ultrasound irradiation (18SC1235) and using 4-hydroxy-6-methylpyran-2-one as a building block in multicomponent reactions (18H(96)381).info:eu-repo/semantics/publishedVersio