Recent advances of tryptanthrin and its derivatives as potential anticancer agents

Tryptanthrin is one of the well-known natural alkaloids with a broad spectrum of biological activities and can act as anti-inflammatory, anticancer, antibacterial, antifungal, antiviral, antitubercular, and other agents. Owing to its potent anticancer activity, tryptanthrin has been widely explored for the therapy of various cancers besides being effective against other diseases. Tryptanthrin with a pharmacological indoloquinazoline moiety can not only be modified by different functional groups to achieve various tryptanthrin derivatives, which may realize the improvement of anticancer activity, but also bind with different metal ions to obtain varied tryptanthrin metal complexes as potential anticancer agents, due to their higher anticancer activities in comparison with tryptanthrin (or its derivatives) and cisplatin. This review outlines the recent advances in the syntheses, structures, and anticancer activities of tryptanthrin derivatives and their metal complexes, trying to reveal their structure-activity relationships and to provide a helpful way for medicinal chemists in the development of new and effective tryptanthrin-based anticancer agents.

Electron-Deficient Fluoroarene-Mediated Synthesis of Trifluoromethyl Ketones from Carboxylic Acids

Fluoroarene-mediated trifluoromethylation of carboxylic acids for the synthesis of trifluoromethyl ketones is disclosed. The fluoroarene activates the acid group and generates the fluoride source in situ for the trifluoromethylation reaction. The present protocol is safe and metal-free, operates under mild reaction conditions, and does not require any external additives to generate trifluoromethyl anion. The current transformation provides good functional group tolerance and also delivers 92% and 88% yields of trifluoromethyl ketones in batch and continuous flow, respectively.

A highly efficient metal-free selective 1,4-addition of difluoroenoxysilanes to chromones

A highly efficient metal-free selective 1,4-addition reaction of difluoroenoxysilanes to chromones was developed using the low-cost and readily available HOTf as the catalyst, which is a facile and straightforward method to access valuable C2-difluoroalkylated chroman-4-one derivatives. Interestingly, the products could be readily converted to the difluorinated bioisostere of the natural product (S)-2,6-dimethylchroman-4-one and a difluorinated benzo-seven-membered heterocycle via the Schmidt rearrangement reaction. In addition, the in vitro anti-proliferative activities of these synthesized derivatives against human colon carcinoma cells (HCT116) revealed that compound 3g exhibited potent inhibitory effect on HCT116 cancer cells with an IC₅₀ value of 6.37 μM, representing a novel lead compound for further structural optimization and biological evaluation.

Enantioselective Synthesis of Acyclic Stereotriads Featuring Fluorinated Tetrasubstituted Stereocenters

Elaboration of enantioenriched complex acyclic stereotriads represents a challenge for modern synthesis even more when fluorinated tetrasubstituted stereocenters are targeted. We have been able to develop a simple strategy in a sequence of two unprecedented steps combining a diastereoselective aldol-Tishchenko reaction and an enantioselective organocatalyzed kinetic resolution. The aldol-Tishchenko reaction directly generates a large panel of acyclic 1,3-diols possessing a fluorinated tetrasubstituted stereocenter by condensation of fluorinated ketones with aldehydes under very mild basic conditions. The anti 1,3-diols featuring three contiguous stereogenic centers are generated with excellent diastereocontrol (typically >99 : 1 dr). Depending upon the precursors both diastereomers of stereotriads are accessible through this flexible reaction. Furthermore, from the obtained racemic scaffolds, development of an organocatalyzed kinetic resolution enabled to generate the desired enantioenriched stereotriads with excellent selectivity (typically er >95 : 5).

Synthesis of 6-Alkynyl-6-hydroxyindoloquinazolinone Scaffolds via Copper-Catalyzed Alkynylation of Tryptanthrins

We report a copper-catalyzed direct alkynylation reaction of tryptanthrins with terminal alkynes under mild reaction conditions. The developed method provides an array of synthetic building blocks of 6-alkynyl-6-hydroxyindoloquinazolinone compounds in moderate to good yields with varied functional group compatibility. Furthermore, the obtained adducts can be smoothly converted into versatile building blocks via hydrogenation, hydration, and further Sonogashira coupling transformations.

Switching of Enantioselectivity in the Cu-Catalyzed Asymmetric Decarboxylative Aldol Reaction of Tryptanthrin with β-Keto Acids: An Unexpected Counteranion Effect

Cu-bisoxazoline-catalyzed enantioselective decarboxylative aldol reaction of tryptanthrin with aryl-substituted β-keto acids is developed, providing a straightforward approach to deliver a series of phaitanthrin A analogues. Both enantiomers of the products can be obtained with good to high enantioselectivity in the presence of a single chiral ligand by simply changing the copper salts. Based on the X-ray crystallographic analysis of chiral Cu(II)-bisoxazoline complexes, the tentative stereochemical models are presented to account for the observed counteranion-induced switching in enantioselectivity.

Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

Synthesis of 6-substituted indoloquinazolinones from arynes and 2-acyl-4-quinazolinones: a transition-metal free C–N and C–C bond formation strategy

A versatile transition-metal free synthetic strategy has been developed for the direct synthesis of 6-substituted indoloquinazolinones from 2-acyl-4-quinazolinones and aryne precursors. This cascade strategy proceeds via successive C–N and C–C bond formation in a single reaction vessel.

HClO4 catalysed aldol-type reaction of fluorinated silyl enol ethers with acetals or ketals toward fluoroalkyl ethers

A highly efficient metal-free catalytic aldol-type reaction of fluorinated enol silyl ethers and acetals/ketals toward functionalized fluoroalkyl ethers is developed by using less than 1.0 mol% HClO₄ (70 wt%, aq.).

A Sc(OTf)3 catalyzed Mukaiyama–Mannich reaction of difluoroenoxysilanes with unactivated ketimines

The first example of a Mukaiyama–Mannich reaction of difluoroenoxysilanes with unactivated ketimines catalyzed by Sc(OTf)₃ is described.

Catalytic selective mono- and difluoroalkylation using fluorinated silyl enol ethers

This feature article highlights our recent achievements in catalytic selective mono- and difluoroalkylation using fluorinated silyl enol ethers (FSEEs).

A general and efficient Lewis acid catalysed Mukaiyama-aldol reaction of difluoroenoxysilanes and ketones

We report a general and highly efficient Mukaiyama-aldol reaction of ketones and difluoroenoxysilanes. While the reaction of aryl ketones worked efficiently in the presence of Bi(OTf)₃, that of aliphatic ketones required the use of Sc(OTf)₃. In addition, Sc(OTf)₃ was capable of achieving excellent 1,2-selectivity in the corresponding reaction of α,β-unsaturated ketones. This method provides a facile access to differently substituted β-hydroxy α,α-difluoro ketones, versatile synthons for difluomethylated tertiary alcohols.

CBr4 promoted intramolecular aerobic oxidative dehydrogenative arylation of aldehydes: application in the synthesis of xanthones and fluorenones

A simple and practical carbon tetrabromide promoted intramolecular aerobic oxidative dehydrogenative coupling reaction has been developed to provide a straightforward ring closure protocol for 2-aryloxybenzaldehydes to furnish xanthones. The reaction was performed under metal-, additive- and solvent-free conditions with good tolerance of functional groups. The present method is also applicable to the synthesis of fluorenones by using 2-arylbenzaldehydes as substrates. Preliminary studies of the reaction mechanism indicated that the reaction may proceed through a radical pathway.

Copper-catalysed difluoroalkylation of aromatic aldehydes via a decarboxylation/aldol reaction

A copper-catalysed tandem decarboxylation/aldol reaction of simple aromatic aldehydes with 2,2-difluoro-3-oxo-3-arylpropanoic acid under mild conditions.

Organocatalytic enantioselective Mukaiyama–Mannich reaction of fluorinated enol silyl ethers and cyclic N-sulfonyl ketimines

The first catalytic asymmetric Mukaiyama–Mannich reaction of fluorinated silyl enol ethers and ketimines is developed, allowing highly enantioselective synthesis of benzosultam based β-fluorinated C^α-tetrasubstituted α-amino acid derivatives.

Catalyst-free synthesis of α1-oxindole-α-hydroxyphosphonates via phospha-aldol reaction of isatins employing N-heterocyclic phosphine (NHP)-thiourea

A catalyst-free phospha-aldol reaction of isatins with NHP-thiourea has been developed and the critical role of the Brønsted acid in this transformation is unveiled.

A highly efficient Mukaiyama–Mannich reaction of N-Boc isatin ketimines and other active cyclic ketimines using difluoroenol silyl ethers catalyzed by Ph3PAuOTf

A Ph₃PAuOTf catalyzed highly efficient reaction of difluoroenoxysilanes and active ketimines is reported, which represents the first Au(i)-catalyzed Mukaiyama–Mannich reaction.