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3-Substituted 2-oxindoles are important structural motifs found in many biologically active natural products and pharmaceutical lead compounds. Here, we report an enzymatic formation of the 3-substituted 2-oxindoles catalyzed by MarE in the maremycin biosynthetic pathway in Streptomyces sp. B9173. MarE is a homologue of FeII/hemedependent tryptophan 2,3-dioxygenases (TDOs). Typical TDOs usually catalyze the insertion of two oxygen atoms from O2 into an indole ring to generate N-formylkynurenine (NFK)-like products. In contrast, MarE catalyzes the insertion of a single oxygen atom from O2 into an indole ring, to probably generate an epoxyindole intermediate that undergoes an unprecedented 2,3-hydride migration to form 2-oxindole structure. MarE shows substrate robustness to catalyze the conversion of a series of 3-substituted indoles into their corresponding 3- substituted 2-oxindoles. Although containing most key amino acid residues conserved in well-known TDO homologues, MarE falls into a separate new subgroup in the phylogenetic tree. The characterization of MarE and its homologue enriches the functional diversities of TDO superfamily and provides a new strategy for discovering novel natural products containing 3- substituted 2-oxindole pharmacophores by genome mining. [ABSTRACT FROM AUTHOR]

The N-heterocyclic carbene catalyzed [3 + 3] annulation of indolin-2-imines and bromoenals was developed to give dihydropyridinone-fused indoles in good to high yields, which were transformed to α-carbolines with different 2-subsituents by a process of dehydrogenation, tosylation, and palladium catalyzed C-C or C-N coupling reaction. [ABSTRACT FROM AUTHOR]

The ionicity value, which is a key property of protic ionic liquids, was obtained by attenuated total reflection Fourier transform infrared spectroscopy and nuclear magnetic resonance, respectively, for a protic ionic liquid: n-propylammonium acetate. The method of potentiometric titration is found to not be suitable for such a kind of ionic liquid, as the ΔpKa of the compositing acid and base is relatively small. In the IR spectrum, molecular species can be directly observed in the range 1200-1800 cm-1, and the ratio of ionic and molecular species can be quantitatively calculated by the area of characteristic absorption peaks calibrated by a standard curve from NaAc/HAc solutions. The results show that 93% components in n-propylammonium acetate are ionic species. The NMR method was also introduced to test and verify the result. Despite that only one mixed peak can be observed for molecular and ionic species, the observed ¹H chemical shift can be assumed to be the weighted average of them. In this way, the ionicity can be calculated and it fits well with the IR approach. It indicates that, for a protic ionic liquid with a relatively small ΔpKa value, spectroscopic methods such as IR and NMR could be applied to determine the ionicity. [ABSTRACT FROM AUTHOR]