Indolines (also known as benzene pyrrolidines) constitute an important family of Aromatic heterocyclic compounds with a bicyclic structure, which consists of a six membered benzene ring fused with a five membered nitrogen-containing ring on the side. They are widely abundant in nature, numerous natural products contain the indolines moiety as structural framework. There are two important approaches to synthetize indolines: 1. Dearomatization from noncyclic compounds, but few successful examples have been reported thus far; 2. Reduction from indolinones, while this method requires demanding conditions.
Fig.1. Indoline compounds
Unmodified indolines are slightly water-soluble, because the bicyclic rings bring stable structure but poor solubility, while the secondary amine protons (red region in Fig 1) contribute towards the dissolution in water. Owing to the presence of benzene ring in indoline, which the conjugate group promotes absorption in UV region and brings the property of light sensitive. Most indolines exhibit high boiling point and low melting point, thus they are commonly presented as brown or dark liquid. All indolines possess secondary amine group as a major reaction site, which can be applied in post-modification. They can react with carboxylic acid to produce acid amides and participate in N-methylation to produce tertiary amines. Besides, secondary amine groups are the key active functional groups of modern drugs, advancing the fabrication of many natural products and bioactive molecules with indolines.
Medicine: Indoline compounds are widespread in natural drug molecules and alkaloids (Fig 2). Their derivatives are usually used to prepare medicinal precursors, which possess remarkable pharmacological and physiological activities, such as antibacterial, anticonvulsant, anti-tumor and anti-inflammatory behavior, etc. The bioactivity and solubility of the indoline derivatives can be adjusted by modifying different functional groups to achieve optimal effect.
Fig.2. Representative natural alkaloids with the indoline framework (Chinese Journal of Organic Chemistry 2019, 39, 2705-2712.)
Agricultural chemicals: Owing to the similar chemical structure between indolines and tryptophane, their derivatives can be conducted as precursors of high-efficiency plant growth regulators, fungicides, insecticide and so on.
Fig.3. Example of indoline-containing dye (D-205) (Chem. Commun. 2008, 5194-5196.)
Dye: When the conjugated groups of indolines (bicyclic structure) are large enough, the peak of UV spectrum exhibited red shift to visible range, thus the dye of different colors can be designed via adjusting the degree of conjugation between benzene rings. D-205 is a kind of indolines dye (Fig 3) after design optimization and has been investigated in high-efficiency organic dye-sensitized solar cells (DSCs).