Asymmetric Synthesis of Optically Active Spirocyclic Indoline Scaffolds through an Enantioselective Reduction of Indoles

Asymmetric 1,4-Reduction of Pyrano[2,3-b]indoles with Hantzsch Ester by Chiral Phosphoric Acid

A highly regioselective and enantioselective transfer hydrogenation of pyrano[2,3-b]indoles with Hantzsch ester has been successfully realized using spiro-chiral phosphoric acid, affording a series of optically active 1,4-reductive adducts, 4,9-dihydropyrano[2,3-b]indoles, in 34-99% yields with 61-99% ee.

Chiral phosphoric acid-catalyzed transfer hydrogenation of 3,3-difluoro-3H-indoles

A convenient and efficient method for the synthesis of optically active difluoro-substituted indoline derivatives starting from the corresponding 3H-indoles by chiral phosphoric acid-catalyzed transfer hydrogenation was developed. Using Hantzsch ester as the hydrogen source under mild reaction conditions, the target products can be obtained with excellent yield and enantioselectivity.

Electrosynthesis of Spiro-indolenines via Dearomative Arylation of Indoles in Batch and Continuous Flow

An electrosynthesis of spiro-indolenines in batch and continuous flow was achieved through dearomative arylation of indoles with good functional group compatibility. User-friendly undivided cells were used under catalyst- and oxidant-free conditions. Moreover, the use of a flow electrolysis cell gave high daily productivity and excellent scale-up potential under less supporting electrolyte and higher substrate concentration conditions.

Manganese-Catalyzed Asymmetric Hydrogenation of 3H-Indoles

The asymmetric hydrogenation (AH) of 3H-indoles represents an ideal approach to the synthesis of useful chiral indoline scaffolds. However, very few catalytic systems based on precious metals have been developed to realize this challenging reaction. Herein, we report a Mn-catalyzed AH of 3H-indoles with excellent yields and enantioselectivities. The kinetic resolution of racemic 3H-indoles by AH was also achieved with high s-factors to construct quaternary stereocenters. Many acid-sensitive functional groups, which cannot be tolerated when using a state-of-the-art ruthenium catalyst, were compatible with manganese catalysis. This new process expands the scope of this transformation and highlights the uniqueness of earth-abundant metal catalysis. The reaction could proceed with catalyst loadings at the parts per million (ppm) level with an exceptional turnover number of 72 350. This is the highest value yet reported for an earth-abundant metal-catalyzed AH reaction.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

Photoinduced [3+2] Annulation of Alkene with o-Iodoanilines: An Expedient Approach to Indolines

A highly regioselective [3+2] cyclization of alkenes with 2-iodoanilines under the irradiation of UV light is described. This general, metal-free strategy facilitates the direct preparation of 2-mono-/disubstituted indolines as well as spiroindolines through alkene carboamination in one step. Mechanistic studies suggested that the photochemical protocol proceeded via a radical pathway.

Facile synthesis of chiral indolines through asymmetric hydrogenation of in situ generated indoles

A concise synthesis of chiral indolines has been developed through intramolecular condensation, deprotection and palladium-catalyzed asymmetric hydrogenation in a one-pot process with up to 96% ee. A strong Brønsted acid played an important role in both the formation of indoles and asymmetric hydrogenation process.

Oxidative N-Heterocyclic Carbene Catalyzed Dearomatization of Indoles to Spirocyclic Indolenines with a Quaternary Carbon Stereocenter

An efficient method for the asymmetric intramolecular dearomatization of indoles by using oxidative N-heterocyclic carbene catalysis is demonstrated. Valuable optically active spirocyclic indolenines bearing an all-carbon quaternary stereocenter are obtained in excellent yields and with excellent enantioselectivity. The starting indoles are readily prepared and the reactions proceed through an intramolecular indole 3-acylation with an in situ generated acyl azolium intermediate to form a spirocyclic ketone moiety.